5/2/2014 Improving Properties of Granular Soil using Waste Materials A Special Study Project on Using Rubber Soil Mixture and its variants in everyday life

Armaan Gupta, armaangupta625@gmail.com 2011A2PS484P, BITS PILANI,

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ACKNOWLEDGEMENT

I would like to thank Dr. Ravi Kant Mittal, for giving me an opportunity to do the project on “Improving properties of granular soil using waste materials” and providing me all support and guidance which made me complete the project on time. I am extremely grateful to him for providing such a nice support and guidance though he had a very busy schedule.

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INTRODUCTION

One of the problems associated with socio-economic development of a country is waste disposal. In engineering and transportation sector one of the wastes generated is scrap tire and it poses serious environmental problem. Majority of them end up in the already congested landfill or becoming mosquito breeding places. Worst when it is burnt. Recent statistics in India indicated more than 100% increase in number of registered vehicle within ten years. Some of the applications of rubber waste in geotechnical area are lightweight fill for embankments and retaining walls, leachate drainage material and alternative daily cover at municipal solid waste landfills, insulating layer beneath roads and behind retaining walls, agriculture - soil aeration and detoxification, golf course green liners, and mats.They found that under repeated loads, the shredded tire chips undergo less plastic strain per cycle with successive load cycles, ranging from 8% strain for first cycle to 0.03% strain for 80th cycle. Drescher concluded that by assuming a constant rate of creep for the period of 60-630 days after completion of load. The results indicate that the mixtures up to 20% coarse grained and 30% fine grained tire- chips can be used above ground water tables where low weight, low permeability and high strength are needed in fills such as highway embankments, bridge abutments and behind retaining structures especially built on weak foundation soils with low bearing capacity and high settlement problems.

Plant roots stabilize soils, through reinforcement of soil in nature, against erosion and failure of deep slopes. Presently, reinforcement is an effective and reliable technique for increasing strength and stability of soils. The technique used today varies in the applications ranging from retaining structures and embankments to surged stabilization and surface drainage systems. In general soilreinforcements can be classified into two major categories (by their stiffness): (1) ideally inextensible and (2) ideally extensible inclusions. The former includes high metal strips and bars, while the latter includes relatively low modulus natural and synthetic fibers, plant roots and polymer fabrics. Soil reinforced with randomly-distributed inclusions is another type of reinforced-soil, which have attracted considerable attraction over past years, such as concrete technology and more recently in soils [ Hoare DJ. 1997].

In this type of soil reinforcement, soil is mixed randomly with discrete small inclusions such as tire shreds, fibers, filaments and small meshes until it become like a homogeneous material. Reuse and recycling of scraped tires is essential to avoid growing stockpiles of discarded tires around the world, e.g. approximately 240 million tires are disposed in United States each year and currently 5 billion tires are stockpiled [Markets for scrap tiers 1991]. Rubber Manufacturers Association estimates that about 4595.7 thousand tons of tires were generated in the U.S. in 2007. At the end of 2007, about 128million scrap tires remained in stockpiles in the United States, a reduction of over 87 percent since 1990. Uses for scrap tires in civil engineering applications are growing recently. There are around One billion tyres (100 crore nos of tyres) which are thrown away every year in India. Some 300 millions (30 crores) of them are recycled or are used as fuel and the rest are sent to landfill.(Rao and Dutta 2006)

Scrap tires are used in the production of paving material which is called rubber modified asphalt and in retaining walls as embankment materials. More recently tires were shredded into smaller pieces

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producing a bulk material, which was used as subgrade fill alone or mixed with granular soil to improve the engineering properties of the soil [Edil TB. 2002]. The main objective of this study is to investigate the feasibility of using shredded waste tires as reinforcement to increase the bearing capacity of soil. Thus a series of laboratory loading tests have been carried out on sand reinforced with randomly distributed tire shreds to determine the effects of shred content and shred aspect ratio on bearing capacity of reinforced soil.

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Improving properties of granular soil using waste materials.

SEISMIC BASE ISOLATION: The stockpiling of scrap tires is a significant threat to our environment. We can utilize scrap tires for applications in infrastructure protection, base isolation of structures. The method involves mixing scrap tires with soil materials and placing the mixtures around building foundations, for vibration absorption. Seismic base isolation can be defined as forming a laterally flexible system between the ground and a structure in order to isolate earthquake-induced seismic forces by increasing the natural period of the structure.Smooth synthetic liners have been proposed underneath the foundation of structures or between soil layers for dissipating seismic energy through sliding and rubber-soil mixtures (RSM) have been proposed around the foundation of structures for absorbing seismic energy and exerting a function similar to that of cushion. (Santucci de Magistris et al., 1999, Tatsuoka et al.,2000). Energy dissipation is the primary mechanism attributing to the reduction of seismic ground shaking. Rubber is known for its excellent energy absorption capability, and hence its uses for vibration control and dampening such as in automotive components have been extensive. Rubber solids and soil particles are complementary in their functions. Comparing with normal soils, soil reinforced with rubber demonstrates a significant increase in shear strength and more importantly a tremendous increase in energy dissipating capability. Although STP (Scrap Tire Pads) application cannot eliminate foundation-basement structural requirements, but it presents advantages such as low technology, no-cost pad, weight reduction, ease of handling, simple shear stiffness adjustment by changing the number of layers, and environmental benefits by recycling scrap tires.

Fig. 1 Typical example of base isolation of a residential building using RSM (Rubber soil Mixture). Types of STP to be used in base isolation materials: STPs are prepared by placing 18 cm x 20 cm cut tread sections layers of car tires on top each other. Larger size STPs can also be formed by placing longer strips of tread sections (integer multiples of tread width) side by side in alternating direction layers or using a woven structure. Properties of STPs are compared with Laminated Rubber Bearing (RB) system. Since, car tires are made from rubber and have steel mesh reinforcement in the tread section. Scrap Tire Pad (STP) is made by placing layers of scrap tire thread sections on top of each other in an attempt to mimic the behavior of RB.(Rao and Dutta 2006)

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COMPRESSION TEST: All STP specimens used in compression tests had four layers and were prepared by using four well-known tire brands. All STP specimens had 180 mm x200 mm bearing area and four layers of tires for a total height of 40–50 mm. The tested RB specimen was a standard 150 mm x 150 mm x 40 mm elastomeric pad, containing a single layer of 3 mm thick steel plate. The specimens were tested under cyclic axial load with gradual increments. STP specimens generally failed between 0.20 and 0.25 vertical strain levels. The STP failure began at about 8 MPa axial stress level and was identified by a series of snapping sounds, while RB started showing signs of failure at about 40 MPa. The results of the STP and RB axial loading tests are similar to each other in shape and behavior. Although the horizontal reinforcement working mechanism is similar for RB and STP lesser amount of horizontal steel inside STPs causes relatively low vertical strength.(ASTM D4254-00, 2000)

Fig. 2 RB steel plate and STP steel cord layout and working mechanism Reversed cyclic loading tests: Large amplitude, reversed cyclic loading tests were conducted in order to obtain the high-strain shear behavior of STP specimens. Specimens were loaded in shear while they were also under axial compression. All STP specimen test results show that slippage is initiated at about 20–25% of the vertical load and at a horizontal displacement in the range of 50–75% of the height. The entire test has performed satisfactorily until 20–25% of the vertical load which would correspond to 0.2 g lateral acceleration. The lateral accelerations developing in a seismically isolated system is expected to be lower than 0.2 g; therefore, STP base isolation system is expected to perform well during earthquakes. Inclined compression tests: This test was used to test STP specimens and compare with the static reversed cyclic loading and free vibration dynamic test results.(Rao and Dutta 2006) The same specimens were used for shear tests, before being tested to failure under compression tests.

Fig. 3.G-STP refers to STP from 'GOOD YEAR' tire Brand Company.

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G-STP graphs in above figure show clear hysteretic behavior when the maximum lateral load was applied till about 15 KN which was 15% of the vertical load. The lateral deformation measured in GSTP test was about 20 mm which is equivalent to 43% drift.(Santucci de Magistris et al., 1999, Tatsuoka et al.,2000). Hypothetical Case Study: STP usage was considered for rural bridge supports. Rural bridges generally have low daily traffic volumes, can have multiple spans, but are usually of simply supported beam type. Total mass of 9 ton/m is calculated for the unit length of the bridge assuming a two lane bridge with five beams having dimensions of 500 mm x 800 mm that supports a 250 mm thick slab. If an average bridge length of 10 m is considered, the worst loading case combined with lane or HS20-44 truckloads would yield a vertical load of 153 KN /pad. This load in turn would generate about 4.2 MPa stress on a 180 mm x 200 mm STP specimen and would be within the axial load limits. The thermal expansion demand of the bridge can be obtained by considering thermal expansion coefficient. A temperature shift of 80 C from -20 C to +60 C would result in a maximum length change of 4.4 mm over 10 m at each support, which is equivalent to about 8% lateral strain of a 58 mm thick STP (5 tire layers) and is within the acceptable limits. Therefore, STPs can also be used as a low-cost alternative for temperature elastomeric bridge support bearings for short to medium span, simply supported rural bridge supports.

Fig. 4 The Fourier amplitude spectra (FAS) of the (a) horizontal and (b) vertical ground accelerations; and the corresponding normalized (c) horizontal and (d) vertical ground acceleration time histories for the Reference scenario (Tsang, 2008). Notes: In each figure, the scenarios of placing RSM and pure sand were plotted.Rubber has been used as base bearings in the past three decades, with much of the developmentbyKelly(2007). The above figure shows that amplitude and acceleration (due to earthquake or any other forces) are significantly reduced in case of RSM as compared to pure sand.

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CONCLUSION:

1- If vertical strength is taken to be more important than horizontal strength (which is unlikely in the case of base isolation since earthquake forces act horizontally), RB (Rubber system) fare better than STP. But for all the rest of cases, STP is better than RB as it is widely available, cheap and its usage is encouraged by government because it will generate waste otherwise. (Thenmozhi and Stalin 2010)

2- In summer, when bridges expand, then also the total strain produced in underlying STP is

within the acceptable limits, so we can use STP in bridges too. 3- The steel present inside STP enhances its strength and it also provides horizontal and shear

stiffness. 4- The study shows that wire mesh inside rubber behaves similar to steel plate usage inside RB.

Using higher number of wire-mesh layers may improve vertical stiffness of RB by reducing amount of bulging between reinforcement layers.

5- STPs may be used for short span bridges (at expansion joints) in developing countries such as the ones in Middle East, Asia, South America, and Africa. STP may also be used to isolate masonry houses located in developing countries.

6- We can mix scrap tires with soil to create RSM (rubber soil mixture). RSM layer decouple the building or structure from ground motions by interposing elements or materials of low stiffness in between. RSM layer modifies the dominant frequency of the incident seismic waves and dissipates the seismic energy of high frequency components in particular. On the other hand, both spherical sliding bearings and geo-synthetic liners limit the transfer of shear across the isolation interface which has a low level of frictional resistance.

7- Tsaang&Nelson(2008) also presented a paper on potential earthquake protection method by placing rubber-soil mixtures (RSM) around foundations (footing or pile) of low-to-medium-rise buildings for absorbing vibration energy and exerting a function similar to that of a cushion. The validity of the proposed method has been shown by a number of numerical simulations using various recorded ground motions. On average, 40-60% reduction in horizontal accelerations at roof and foundation as well as first floor inter-storey drift can be achieved.

8- Rubber has excellent energy absorption capability, rendering its extensive uses for vibration control and dampening such as in automotive components.

9- Moreover, soil reinforced with rubber demonstrates a substantial increase in shear strength compared to normal soils.

10- Tsang's did research on compressibility of tire shreds and found that tyre shred (as well as

RSM) is highly compressible. However, it is shown that the compressibility decreases substantially once the tyre shreds have experienced one load application. For example, it is found that embankment sections composed of tyre shreds that were overlain with a soil cap (in the order of 1 m thick) can significantly reduce the compressibility and deflections. Thus, preloading can be used to eliminate plastic compression once the fill has been constructed. Moreover, it is reported that soil-tyre shred mixtures can be compacted using common compaction procedures.(Thenmozhi and Stalin 2010)

11- We can mix sand and RSM with 75% rubber by volume, which is found to have a satisfactory performance in shear strength, horizontal stiffness compressibility.

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All the writers have the same view on usage of STP in base isolation, as a medium for energy dissipation and other fields too.

LIQUEFACTION:

Liquefaction is a phenomenon in which the strength and stiffness of a soil is reduced by earthquake shaking or other rapid loading. Liquefaction and related phenomena have been responsible for tremendous amounts of damage in historical earthquakes around the world. Liquefaction occurs in saturated soils, that is, soils in which the space between individual particles is completely filled with water. This water exerts a pressure on the soil particles that influences how tightly the particles themselves are pressed together.When liquefaction occurs, the strength of the soil decreases and, the ability of a soil deposit to support foundations for buildings and bridges are reduced

The two most important factors accounting for the occurrence of liquefaction include (1) The cohesiveness and density of the soil deposit and (2) The level of shaking. As the above mentioned isolation method requires partial replacement of the soil materials with RSM. As the density of RSM is reduced from 17.4 kN/m3 (of pure sand) to 9.5 KN /m3, this may lead to a decrease in the shear strength and potentially enhance the possibility of liquefaction occurrence. Preliminary studies have shown that the addition of small quantity of tire chips reduces the cyclic shear strength of RSM. However, there is evidence to show that the shear strength of loose sand becomes greater than that of dense sand with an addition of more than 10% tire chips. Various studies of the engineering properties of RSM have also demonstrated a significant increase in the cohesion intercept. Moreover, rubber normally has higher frictional angles than normal soils and the value 'phi' increases with the percentage of shred content in the mix. In addition, randomly mixing tire chips can reinforce sand, resulting in greater shear strength than that of pure sand at its densest state. Densification can be carried out to reduce the void ratio and thus increase the density in order to minimize liquefaction. (Lee 2010). Tire chips are utilized as liquefaction preventive backfill material. Undrained cyclic shear tests were conducted on tire chips and sand mixed tire chips for various percentages of mixtures, and the liquefaction potentials of the mixtures were evaluated. The best mixing percentage of tire chips was found to be close to 50% by the total volume of sand. Despite the fact that the tire chips reinforced composite backfill has a very low relative density, there was no liquefaction in the backfill. Also, the earth pressure on the wall and its residual displacement could be substantially reduced, implying a good performance of the soil-structure system during earthquake loading. UNDRAINED CYCLIC SHEAR TESTING: The results of this test show that for specimens with sf = 1 (where sf= 1 indicates sample composed of sand only and sf=0 indicate sample composed of tire chips only) and 0.9, the excess pore water pressure accumulated as the cyclic shear loading progresses until the effective stress completely dropped to zero. Moreover, since the amplitude of axial strain suddenly increased at a particular time, it is understood that liquefaction took place. For specimens with sf = 0.8, 0.7 and 0.6, the effective stress did not disappear completely and reached a steady state at the end, with the amplitude of axial strain showing gradual increase. For specimens with sf <= 0.5, the decrease in effective stress with the cyclic loading was controlled, while for specimen with sf = 0 (pure tire chips), the decrease in effective stress was very minimal. Moreover, the relation between deviator stress and axial strain for sf = 0 showed visco-elastic behavior, with large axial strain generated

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during the first cycle of loading followed by small increase in axial strain during subsequent loading. For specimens with sf = 0.3 and 0.5, the amplitude of axial strain appeared to show gradual increase, but in the end, the stiffness was maintained and followed a regular loop. Thus, the undrained cyclic shear behavior of specimens with sf <= 0.5 clearly differed from that of specimens with sf = 0.6~ 0.9, with the characteristics of tire chips dominant when sf <= 0.5.

Fig. 5 EFFECTIVE STRESS PATH Typical results of undrained cyclic shear tests.

Fig. 6. Relation between maximum excess pore water pressure ratio and sand fraction. BACKFILL TEST: Two model backfills were created; one with pure sand and the other one was creating using equal proportion of tire chips and sand by volume. (Rao and Dutta 2006)

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A sinusoidal acceleration was imparted to the both soil-structure system for a period of 1 sec (frequency = 20 Hz). They were applied in several stages starting with 100 Gal and increasing up to 600 Gal at an increment of 100. For a given acceleration, the response accelerations, the dynamic load on the wall, displacement of the caisson, and the pore water pressures at various locations were measured.(ASTM D4254-00, 2000) It can be seen that while the conventional backfill shows the increase of the excess pore water pressure with time, the tire chips reinforced backfill shows almost no increase in the pore water pressure. This is, in spite of the fact that the two backfill were having almost the same relative density (40%). The presence of tire chips in the sand could control the buildup of the excess pore water pressure and thus prevent any liquefaction related damages. It can be inferred from these test results that the tire chips acts as a good earthquake resistant geosynthetic material.

Fig. 7 Residual displacement of the structure with acceleration amplitude. CONCLUSION:

1- The studies show that as the percentage of tire chips in sand increases. The decrease in effective stress becomes slower. When both tire chips and sand are mixed equally in volume, then decrease in effective stress with cyclic loading was controlled. The adopted mixing percentage in this model test was reasonable from the liquefaction and practical point of view.

2- Undrained cyclic shear testing of tire chips and tire chips sand mixtures have demonstrated that pure tire chips have very small stiffness and displacements are easily generated during cyclic shear loading, resulting in visco-elastic stress-strain relation. Moreover, excess pore water pressure was not generated during cyclic loading and liquefaction did not occur. The occurrence of liquefaction was confirmed for specimens with sand fraction greater than 0.5, but for specimens with sand fraction less than 0.5, liquefaction was not clearly observed.

3- Tire chips appear to control the build-up of excess pore water pressure of the mixture during shearing, and for specimens with sand fraction less than 0.7, such effect was remarkable for low values of sand fraction.(Anderson and Stokoe 1978).

4- The test using tire chips as reinforcing agent by mixing it with sand, have revealed that tire chips mixed sand does not undergo liquefaction, if proper mixing percentage is selected. The reinforcing action of the tire chips embedded into the backfill soil could prevent the liquefaction of the backfill. The test results have also demonstrated that the liquefaction prevention measures could substantially reduce the earthquake induced permanent displacement of structures. (Santucci de Magistris et al., 1999, Tatsuoka et al., 2000).

5- Nonlinear Site Response: Nonlinear response behavior can be resulted from soils yielding at moderate to high levels of strains. As stated in Hauksson and Gross (1991), most damage is caused by soft, near-surface ground conditions. Hence, it may be reasonable to deduce that RSM may not be beneficial in reducing the level of ground shaking. However, Trifunac (2003)

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illustrated that buildings on softer soils were damaged to a lesser degree under strong shaking (e.g. peak ground velocity > 200 mm/s) due to energy absorption of incident seismic waves by nonlinear soil response. In fact, soft soils can potentially act as a natural mechanism for passive isolation, especially for near-field earthquakes that are rich in high-frequency wave components. Considering the excellent energy absorption capability of rubber, it is therefore believed that the proposed method should be feasible.

DYNAMIC BEHAVIOUR OF RUBBER SAND MIXTURES (RSM): It is assumed that the percentages of rubber used ranges between 0 and 35% by mixture weight. GO (dynamic shear modulus) values increase whereas DTO (damping ratio) values decrease systematically as the content of rubber decreases and the mean confining pressure increases. Based on the experimental results they proposed an analytical relationship for the estimation of GO , which is expressed in terms of an equivalent void ratio that considers the volume of rubber solids as part of the total volume of voids, along with an analytical relationship for the estimation of DTO. Granulated rubber or tire chips composed of recycled scrap tires exhibit low unit weight of solids, along with low bulk density, high drainage capacity, and high elastic deformability. In addition, physical soils, when mixed with recycled rubber; exhibit, in general, lower unit weight and satisfactory strength characteristics. Recycled rubber materials are commonly used as lightweight construction material in high embankments overlying soft soils and lightweight backfill material in retaining walls and slopes. In addition, due to their high hydraulic conductibility, recycled rubber materials are used as drainage layer at landfills. Only those mixtures that are composed of uniform, fine to medium grained sands and uniform recycled rubber materials (classified as granulated rubber), that exhibit, in general, a ratio of D50,r/D50,s>=1, where D50,r and D50,s are the mean grain size of rubber and sand particles, respectively are studied here(Matthew Oman 2013). Effect of Rubber Content and Specimen’s Size on Small-Strain Shear Modulus and Damping Ratio Figures 1 and 2 show the effect of the mean effective confining pressure and rubber contentpr on the initial shear modulus GO and the initial damping ratio DTO of the sand/rubber mixtures. Figure 1 refers to the saturated 35.7mm x 82.2mm specimens, and Fig. 2 refers to the dry 71.1mm x 142.2mm specimens.

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Fig. 8

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CONCLUSION:

1- As illustrated in these figures, GO values systematically decrease as the rubber content increases, whereas the opposite trend is observed for DTO values. In addition, such as in clean sands, the GO values increase and the DTO values decrease as mean confining pressure increases.

2- Study reported that the increment of rubber content and, in particular, for the rubber contents used herein, the void ratio decreases; that is, mixtures exhibit a more dense fabric of the sand/rubber solid matrix as the rubber content increases. The specimens of this study exhibit the general trend of decreasing void ratio with increasing rubber content.

3- Effect of mean confining pressure on the small-strain shear modulus of clean rubber specimens is significantly lower compared to clean soils, whereas the effect of mean confining pressure on the small-strain damping ratio of clean rubber specimens is relatively negligible(Vucetic and Dobry, 1991).

4- Furthermore,Hazarika et al. (2008), Anastasiadis et al. (2009) and Senetakis et al. (2011a).

reported that dry and saturated sand/rubber specimens of the same geometry (71.1mm x 142.2 mm) exhibit similar values of GO, whereas saturated specimens exhibit slightly higher DTO values compared to dry specimens of the same size due to strain-rate effects.

5. Also specimens of clean rubber materials exhibit significantly lower shear stiffness compared to clean soils (on the order of 1:100). In addition, the increase of the rubber content monotonically leads to a decrease of the shear stiffness and an increase of the damping ratio at small-strain levels.

6. Increase of the initial shear stiffness of the mixtures with increasing mean confining pressure is mainly due to the increase of normal stresses at soil-to-soil and soil-to-rubber interfaces. In addition, the decrease of the initial damping ratio with increasing mean confining pressure is possibly due to the effect of mean confining pressure on the sandy part of the solid skeleton. Furthermore, the decrease of the mixture’s initial shear stiffness with increasing rubber content cannot be sufficiently represented by the void ratio. (Rao and Dutta 2006)

7. In eeq(equivalent void ratio), the volume of rubber solids is considered as part of the total volume of voids, whereas the volume of solid particles that contribute to the stiffness of the sand/rubber matrix is assumed to be the volume of the sandy particles. This is possible because of the small contribution of rubber solids on the shear stiffness of the sand/rubber matrix.

8. The effect of moisture seems to be less important on GO.

9. An increase of the rubber content leads to a reduction of pore water pressure buildup at the same shearing strain amplitude. This trend is clearly shown for rubber content equal to 15 % by mixture weight. These observations are possibly explained by the high deformability of rubber particles, which leads to an expansion of pore water pressure buildup during the

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cycling loading. This general trend of more gradual pore water pressure buildup with increasing rubber content was also reported by Hazarika(2007) who studied similar mixtures.

10. The increase of rubber content in the mixtures increases the rubber-to-rubber interfaces and

thus mixtures gradually transform from sand-like to rubber-like behavior. At relatively high rubber contents (above 15% by mixture weight) the soil /rubber solid matrix is significantly controlled by the synthetic portion. The transformation from sand-like to rubber-like behavior is also affected by the relative size of soil versus rubber solids, expressed as D50, s/D50, r. The increase of rubber-to-rubber interfaces is more pronounced as the ratio D50, s/D50, r increases.

Note: Based on research done by Tsang and others on Seismic base isolation systems, the recommended proportion is mixing sand and RSM with 75% rubber by volume.

Based on research done by Tsang(2008) , Gonghui Wang, M.N. Sheikh(2008) and others on use of tire chips in soil against liquefaction, the recommended proportion is using equal proportion of tire chips and sand by volume.

Based on research done by,Hazarika et al. (2008), Anastasiadis et al. (2009) and Senetakis et al. (2011a) the recommended proportion is percentage of rubber used should be in between 0 and 35% by mixture weight. Almost all writers mentioned above recommend using tire chips for seismic base isolation and against liquefaction. Using tire chips in soil doesn’t changes odor and color of ground water significantly and the change may not be apparent to common man, so there is no worry of rejection of drinking water by common man due to usage of tire chips/shreds in soil. Dynamic properties of granular soils mixed with granulated rubber

In the last two decades many studies were conducted on engineering properties of soil–rubber mixtures by numerous researchers. Some key parameters studied are shear modulus, bearing capacity, permeability, shear strength, poisson’s ratio, compaction characteristics, swelling, and compressibility. A series of conventional direct shear, torsional resonant column, large-scale direct shear and dynamic triaxial tests are carried out by various researchers across the globe to investigate the feasibility of using shredded waste tires, granulated rubber and tire buffings as reinforcement materials to find out their effects on physical properties of sands.

They found that shred content and sand matrix unit weight were the most significant characteristics influencing the shear strength of the mixture. Foose et al. also stated that reference strain can be used to normalize the shear modulus into a less scattered band for granulated rubber/sand mixtures. shear modulus and damping values of sand-tire mixtures were higher than those measured for tire buffings only, and tire buffings addition to sand increased the initial slope of shear stress-displacement curve. Attomreported that the addition of shredded waste tires increased both the angle of internal friction and the shear strength of the sands(Matthew Oman 2013).

Mechanical behaviour of tire chip–sand mixtures and the usefulness of optimizing the size of waste tire shreds on shear strength were investigated ..Gotteland et al. showed that the percentage mass and unit weight had effective influence on maximum shear strength. Ghazavi and Sakhifound that,

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for a given width of tire rectangular shreds, there is solely a certain length, which gives the greatest initial friction angle for sand-tire shred mixtures.

A series of laboratory tests were conducted on Ottawa sand by mixing ground rubber of similar size and reinforcing the effects of the newly devised ‘‘Tirecell‘‘ made from treads of waste tires, in sand. Pamukcu and Akbulutshowed a simultaneous increase in both the shear modulus and the damping ratio of the sand specimens up to an optimum volume proportion of the rubber. Yoon et al. found that the bearing capacity increased and the settlement reduction was the highest at the lowest density of sand, and the reinforcing effect of sand was obtained when the embedded depth was within 1.0B, where B was the loading width. (Robinson and Sharon 2000)

A series of experimental works performed on the effects of the addition of shredded waste tires, compacted rubber fiber and waste polymer materials on the properties of clayey soils . The test results of Attom et al. showed that increasing the amount of shredded waste tires will increase the shear strength and decrease plasticity index, maximum dry density, permeability, swelling pressure, swell potential, and the compression index of the clayey soil. The findings of Tsang(2008) and Baykal showed that the peak strength of the composite is comparable to or greater than that of clay alone when tested at confining pressures below 200–300 kPa. Above this threshold, the presence of inclusions tended to degrade the strength of the clay. (Robinson and Sharon 2000)clearly showed a significant improvement in the shear parameters (c and phi) of the treated soils.

Engineering demand for modelling of behaviour of earth structures formed of waste materials will continue to increase. Due to the lightweight and high capacity of rubber in damping energy, it can be used for seismic force reduction and absorption of earthquake vibration in various Civil Engineering structures.

Fig. 10 The particle size distribution for granular soil and granulated rubber. The soil tested was river type granular soil excavated from a huge mine. Based on ASTM D 3999-91, specimens shall be cylindrical and the largest particle size shall be smaller than 1/6 the specimen diameter; thus, regarding the mould dimensions, (diameter and height were 15 and 30 cm, respectively), 100 percent of the used materials must be finer than 25 mm in diameter. Considering above figure , it is understood that this limitation is fully observed.(Cabalar 2011)

The coefficients of uniformity and curvature were determined 77.77 and 1.29, respectively based on ASTM D 2487-10 . The percentage passing sieve number 200 was 8.5%. The values of sand

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equivalent and plasticity index of the tested soil determined based on ASTM D 2419-02and ASTM D 4318-00 and were 51 and 4%, respectively. Based on Unified Soil Classification System , the tested soil was well-graded gravel with clay, GW-GC. The specific gravity of the tested soil was found to be 2.65 at a temperature of 20 1C using ASTM Test Method for Specific Gravity of Soils (D 854-02) .The above stated information are summarized in below table

The granulated rubber used in the test samples was composed of waste tires that had been mechanically chopped using multiple chopping steps and sieved with mesh numbers of 3/800 , No.4, No.10, No.40 and No.100. The particles passing sieved 3/800 and remaining on sieve No.100 were used. These particles had non-spherical shapes with dimensions, from 0.15 to 9.5mm.Theparticlesizedistribu-tion curve for the granulated rubber is shown in above figure. The specific gravityofthegranulatedrubberwasfoundtobe1.1ata temperature of20 1C according to ASTM Test Method for Specific Gravity of Soils (D 854-02) . Generally, the specific gravity of granulated rubber varies between 1 and 1.36 . The difference depends on the existence of steel wire particles. In this research, the steel wire particles were extracted by a magnet. (Rao and Dutta 2006)

Specimen preparation procedure:

Based on the standard laboratory procedures ASTM D4253-00 and ASTM D4254-00, relative density expresses the degree of compactness of tested mixture with respect to the loosest and the densest conditions. ASTM D 4253-00suggests that, for some soils containing between 5 and 15% fines, the use of impact compaction may be useful in evaluating appropriate maximum unit weight. By this consideration, the soil was mixed with 0, 8, 10 and 14% granulated rubber by weight with the same grain size distribution. Then, to determine the maximum dry unit weight and optimum water content, the compaction tests were carried out based on ASTMD1557-02e1. The optimum water content, maximum and minimum dry unit weights, and specific gravities of the mixtures are presented in below table.

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Test procedure :

All the samples were tested with a large-scale cyclic triaxial apparatus manufactured by WykehamFarrance in United Kingdom. The samples diameters and heights were 15 and 30 cm, respectively. Based on ASTM D 3999-91 , clauses 10.3, 10.4, and 10.5, all the samples were

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saturated, and pore water pressure parameter, B, reached over 95%. Then, each sample was consolidated iso tropically under 50, 100, 200 and 300 kPa pressures, Hazarika et al. (2008), Anastasiadis et al. (2009) and Senetakis et al. (2011a).

In each test, forty cycles with the same deviator stress amplitude and the same confining pressure were applied on the specimens under stress controlled condition and 1 Hz frequency. In every cycle, 100 data were obtained by data logger and saved by computer program.

Test results and discussion: Kokusho suggested that the effect of the number of cyclespractically disappears when the stress application is repeated for more than 10 cycles. Therefore, the 11th cycle output data were used for calculation procedures and diagram layout.

Considering the grain size distribution curves of granular soil and granulated rubber, it is clear that the granulated rubber grains accommodate amongst the soil grains, and since the grains of granulated rubber have more flexibility than the soil grains, the dependency of mixture function on the behaviour ofgranulated rubber is expected. Based on the tested materials, the following discussions are made on the mixture behaviours.(Anderson and Stokoe 1978).

Discussion on shear modulus :

The shear modulus-shear strain amplitude curves for the soil-granulated rubber mixtures are given. the confining pressures of 50, 100, 200, and 300 kPa and 0.0, 8, 10, and 14% of granulated rubber additive. The results indicate that at a constant cofining pressure, with an increase in granulated rubber percentage, shear modAlso, by replacement of the soil grains with granulated rubber themixture becomes softer and the shear modulus decreases. It can be observed that, for the soil with or without rubber, with an increase in cofining pressure, shear modulus increases. This phenomenon is due to increasing intergranular friction as a result of an increase in confining pressure which leads to an increase in stiffness.Figure also shows that, with an increase in granulated rubber percentage, the effect of confining pressure on shear modulus decreases. This coincides with the results obtained by Feng . This result was predictable by comparing the stiffness of rubber and soil grains.

who worked on sand-granulated rubber. They showed that, in soil–rubber mixtures containing high percentage of rubber, the material tends to behave more elastic, and also the influnce of confining pressure on the material’s stiffness is insignificant.(perlea 1998)

Discussion on damping ratio:

The damping ratio-shear strain amplitude curves for soil-granulated rubber mixtures are presented in or the confining pressures of 50, 100, 200, and 300 kPa and 0.0, 8, 10, and 14% addition of granulated rubber. The results show that, with an increase in rubber inclusion, damping ratio decreases for the confining pressures of 50 and 100 kPa, while this trend is reversed for the confining pressures of 200 and 300 kPa. This discrepancy may be described in this way that, in low confining pressures (50 and 100 kPa), with an increase in rubber inclusion and due to the high elastic deformation capacity of rubber grains, the elastic strain increases and causes damping ratio to

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decrease. However, for high cofining pressures (200 and 300 kPa), the rubber grains are pressed and become inflexibe, which leads to an increase in the relative displacement of the grains during the application of deviator stress and finally to an increase in bothplastic strain and damping ratio.

The damping ratio-shear strain amplitude curves for soil-granulated rubber mixtures are presented in Fig. 5 for 0.0, 8, 10, and 14% of granulated rubber additive and the confiningpressures of 50, 100, 200, and 300 kPa. It can be observed that, for the soil without rubber inclusion, with an increase in confining pressure, damping ratio decreases. The explanation for this behaviour is that, with an increase in intergranular friction which is due to an increase in confining pressure, plastic strain decreases. Thus, the width of hysteresis loop decreases and causesthe damping ratio to decrease.

It also shows that, for the soil with granulated rubberinclusion, with an increase in confining pressure, damping ratioincreases and is reversed for the soil without granulated rubberinclusion. This discrepancy may be described in such a way that,in the soil with rubber inclusion, increasing confining pressure causes the rubber grains to be compressed and become more inflexible; hence, during the application of deviator stress, the slippage of the soil grains on each other increases. So the plastic strain increases and cuases damping ratio to increase.(perlea 1998)

Furthermore, the results indicate that, not only does the damping ratio increase with an increase in confining pressure for 8, 10 and 14% rubber inclusion, but also the influence of confining pressure on damping ratio increases as rubber inclusion increases(Vucetic and Dobry, 1991). This phenomenon can be explained in a way that, when higher percentage of rubber is used, the mixture has more plastic strain at high confining pressures and more elastic strain at low confining pressures. Therefore, it makes a higher difference between damping ratios at different confining pressures due toincreasing rubber inclusion.

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Fig. 11

Fig. 12 Damping ratio versus shear strain amplitude for the confining pressures of 50, 100, 200 and 300 kPa and the variation of granulated rubber percentages (Matthew Oman 2013).

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Fig. 13 Maximum shear modulus versus granulated rubber percentage for the variation of the confining pressures.(Robinson and Sharon 2000)

Fig. 14 Reference shear strain versus granulated rubber percentage for the variation of confining pressures.

Conclusions:

In the present research a series of large-scale consolidated undrained cyclic triaxial tests was carried out using granular soils-granulated rubber mixtures. Based on the tested materials, the following conclusions are made:

1. Shear modulus decreased with an increase in rubber inclusion for all the confining pressures. 2. Shear modulus increased with an increase in the confining pressure for any percentage of

rubber inclusion. 3. Damping ratio decreased with an increase in rubber inclusion at 50 and 100 kPa confining

pressures. However, for 200 and 300 kPa confining pressures, the results were vice versa. 4. Damping ratio in granular soil-granulated rubber mixtures increased as the confining

pressure increased. This was the reverse for granular soil without rubber inclusion. 5. A model was introduced to predict Gmax for various confining pressures and rubber

inclusion percentage. (Robinson and Sharon 2000) 6. A model was established to evaluate the normalized shear modulus (G/Gmax) versus shear

strain amplitude (g) for various confining pressures and rubber inclusion percentage.

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7. For a given percentage of rubber, with an increase in confining pressure, the values of G/Gmax increased.

8. At a given confining pressure, the values of G/Gmax increased with an increase in rubber inclusion percentage.(perlea1998)

Seismic isolation by rubber–soil mixtures for developing countries

This Paper proposes a promising seismic isolation method particularly suitable for developing countries,which makes use of rubber–soil mixtures. Apart from reducing the level of shaking in the horizontaldirection, the distinctive advantage of the proposed method is that it can also significantly reduce theshaking level of vertical ground motion, to which an increasing attention has been paid in the earthquakeengineering community. On the other hand, the use of scrap tires as the rubber material can provide analternative way to consume the huge stockpile of scrap tires all over the world. Moreover, the low cost ofthis proposed seismic protection scheme can greatly benefit those developing countries where resourcesand technology are not adequate for earthquake mitigation with well-developed, yet expensive, techniques.(ASTM D4254-00, 2000) A seismic isolation system is defined as a flexible or sliding interface positioned between a Structure and its foundation for the purpose of decoupling the horizontal motions of the ground from the horizontal motions of the structure, thereby reducing earthquake damage to the structure and its contents. Rubber has been used as base bearings in the past three decades, with much of the developmentby Kelly since 1976. Laminated rubber bearing is currently the most commonly adoptedsystem due to the strength requirement in the vertical direction to support the full weight of thebuilding. Another type of seismic isolation technique is typified by the sliding system, such as thefriction–pendulum system. It limits the transfer of shear across the isolation interface. At present,owing to the tremendous cost of implementing base isolation technique, applications can only beseen in structures with critical or expensive contents.(Anderson and Stokoe 1978). The main intention of this invention is to provide low-cost seismic protection method for residential buildings in developing countries and economically distressed areas within the United States. This method requires minimal treatment on the scrap tires, rendering it a very convenient method of seismic isolation. Experiments resulted in an acceleration response of structure that could be reduced by more than 70% at low periods. However, one potential problem is that the structure has to be detached from the ground, which is practically not favourable in particular for small residential houses. The feasibility of this method to large structures is also questionable. There is an increasing interest in applying seismic isolation technology to public housing, schools and hospitals in developing countries where the replacement cost due to earthquake damage could be significant. This paper proposes an alternative seismic isolation scheme particularly suitable for developing countries, making use of rubber–soil mixtures (RSM). It is emphasized that there is no intention to claim that the proposed method can replace the well-established and commonly adopted isolation system (Lee 2010). The method has been demonstrated through a series of numerical simulations. As well as other well-known seismic isolation systems, it can greatly reduce the level of horizontal shaking. In addition, it can significantly reduce the level of vertical shaking, to which increasing attention has been paid to the earthquake engineering community. A parametric study has also been carried out

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to test the robustness of the proposed system. Use of rubber: Energy dissipation is the primary mechanism accounting for the reduction of shaking level in the proposed isolation method. Rubber has excellent energy absorption capability, rendering its extensive uses for vibration control and dampening such as in automotive components. Hence, the use of rubber is the key component in this method(Vucetic and Dobry, 1991). The first use of rubber for earthquake protection of building structure was in Macedonia in 1969. A three-storey concrete structure was constructed on large blocks of unreinforced natural rubber. However, the building would bounce and rock backwards and forwards in an earthquake. In light of this practical problem, the proposed use of RSM could avoid bouncing and rocking to happen, as the rubber solids could be reinforced by normal soil materials. This concept of reinforcing rubber is actually similar to the commonly adopted laminated rubber bearing. Also, the use of pure rubber is not recommended for RSM.The dynamic properties of RSM have been investigated by Feng and Sutter (2000). In fact, rubber solids and soil particles are complementary in their functions. Soil reinforced with rubber demonstrates a substantial increase in shear strength compared with normal soils , and more importantly a tremendous increase in energy dissipating capability. More details of the Engineering properties of rubber-reinforced soils can be found in later sections.(Anderson and Stokoe 1978). Use of scrap tires It is generally believed that recycled rubber will play an important role in base isolation in the near future, and scrap tire is potentially a suitable source of material for the proposed method. The durability of tires is also guaranteed, for instance, they are termite proof, fireproof and do not outgas once they are buried. The potential environmental effects have been discussed in the Discussion section. Scrap tire stockpile has been a significant disposal problem. Citing the United States as an example, since the banning of the disposal of used tires in sanitary landfills, the stockpile has grown up rapidly at a rate of around 300 million tires per year. It has been a hot topic among engineering community to find new beneficial ways to recycle and reuse the huge stockpile, notably among those are the uses as fuel in power plants and for asphalt mixtures in pavement construction. Moreover, many scrap tires are exported to foreign countries to be reused as rethreads or fuel. Unfortunately, not all exported tires were reused or recycled, but ended up with a disproportionate amount of tires, in addition to their own internally generated scrap tires. Hence, the proposed seismic protection method presented in this paper provides a promising way to reduce the huge Stockpile, especially that each project could use up a large volume of tires.

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Fig. 15 Shear modulus degradation curves

Fig. 16 damping curves adopted

Nonlinear site response It is well recognized that soils yield at moderate to high levels of strains and give rise to non linear response. There was a consensus that damage was mostly caused by soft, near-surface ground conditions, as stated by Hauksson and Gross. Hence, it might be reasonable to postulate that RSM may not be beneficial in reducing the shaking level. However, Trifunac and Todorovska showed that buildings on softer soils were damaged to a lesser degree, because the energy absorption of incident seismic waves by nonlinear soil response would lead to a reduction of the destructive power of the strong motion. This finding has been further confirmed in a later study. Hence, soft soils can potentially act as a natural mechanism for passive isolation, especially for near-field earthquakes that are rich in high-frequency wave components. Evidences have already been shown in the simulations presented in the form of FAS from which significant reduction in amplitudes could be observed in high-frequency range. It is therefore believed that seismic isolation using RSM should be a feasible method,considering the excellent energy absorption capability of rubber. (Edil TB, 2002) oil resonance effects Earthquakes produce seismic waves with a wide spectrum of frequencies. If a certain seismic wave

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component with high energy matches the natural frequency of the surface geological deposits, the interaction could potentially amplify the level of shaking, commonly referred to as soil resonance. As the proposed method requires replacement of a certain thickness of surface geological deposits by RSM, which would significantly modify the stiffness (and in turn the natural frequency) of the materials beneath the structure, the potential harmful effects should not be neglected. Although this problem could not be observed in the finite element modelling in the previous section, further investigation has to be carried outby Feng and Sutter (2000). On the other hand, if the natural frequency of the site can be adjusted, with specific design of the configuration and properties of the RSM layer, to a frequency that does not coincide with that of the incident seismic waves, the level of shaking can then be further reduced, in addition to energy dissipation by RSM, which is basically the underlying philosophy of the traditional base isolation system (Matthew Oman 2013). Liquefaction Liquefaction is defined as the state when saturated sandy soil loses shear strength and effective stresses are reduced as a result of increased pore water pressure. The two most important factors attributing to the occurrence of liquefaction are the cohesiveness and density of the soil deposit and the level of shaking . As the method proposed in this paper requirespartial replacement of the soil materials by RSM, it concerns whether it would enhance the potential of liquefaction during earthquakes.(Anderson and Stokoe 1978). Various studies of the engineering properties of RSM demonstrate a substantial increase in the cohesion intercept (commonly referred to as the c-value) . Moreover, rubbers generally have higher frictional angles (commonly referred to as the phi value) compared with normal soils and shown to be increasing with the percentage of shred content . It is mentioned in the previous section that the density of RSM is reduced from 17.4kN/m3 (of pure sand) to 9.5kN/m3, which would result in a decrease in the shear strength and potentially enhance the occurrence possibility of liquefaction. However, it is shown that an addition of more than 10% tire chips into loose sand results in shear strength that is greater than that of the dense sand . It showed clearly that randomly mixing tire chips can reinforce sand to shear strength that is greater than the strength of pure sand at its densest state. Furthermore, densification works can be performed to reduce the void ratio and, hence, increase the density. Regarding the ground shaking intensity, it is evident from the previous section that the damping effects of RSM could significantly reduce both the peak and root-mean-square ground accelerations. Thus, the probability of liquefaction occurrence should be reduced. Nevertheless, other remedial measures against liquefaction could be implemented during the construction process. Ground settlement It is well known that tire shred (as well as RSM) is highly compressible. However, it is shown that the compressibility decreases substantially once the tire shreds have experienced one load application. Thus, preloading can be used to eliminate plastic compression once the fill has been constructed. Bosscheret al. found that embankment constructed with pure tire shreds settled slightly more than that constructed with soils. However, embankment sections composed of tire shreds that were overlain with a soil cap (in the order of 1 m thick) can significantly reduce the compressibility and deflections and perform equally well as those constructed with soils.

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Moreover, it is reported that tire shreds and soil–tire shred mixtures can be compacted using common compaction procedures, such as dynamic compaction method by dropping a heavy weight from a high position, vibratory tamper method by the oscillating force coupled to a vibrating base plate, and so forth (Matthew Oman 2013). Surface compaction (e.g. rolling and ramming) is the cheapest and the simplest one among available ground improvement methods, rendering it suitable particularly for developing countries. It is the process of increasing the soil unit weight by forcing the soil particles into a tighter state and reducing air voids by the addition of either static or dynamic forces . In the case of cohesion less soils, compaction leads to higher density and higher internal frictional angles. For cohesive soils, the compaction process leads to closer particle arrangement and more cohesion. Environmental effects There is an increasing interest in the applications of recycled rubber in civil engineering. However, the long-term environmental issues, such as ground water contamination and impact on local ecology, have been the subjects in intense debates. It was shown by laboratory tests and augmented by field studies that both the concentrations of metallic components and the organics were well below the standards specified in two protocols in the United States, namely, toxicity characteristics leaching procedure regulatory limits and extraction procedure toxicity; this proved that recycled scrap tire is not a hazardous recycled material. There is a common concern regarding the increase in iron and manganese levels. However, iron level is only specified in the aesthetic drinking water standard (taste), rather than of health concern. Also, manganese is naturally present in ground water in many areas. Hence, it was concluded that there is little or no likelihood of significant leaching of tire chips for substances that are of specific public health concern. CONCLUSIONS This paper presented a new seismic isolation method using RSM, which is particularly suitable for developing countries. There are a number of distinctive advantages, including the ability of reducing both horizontal and vertical ground motions and the substantial consumption of the huge stockpile of scrap tires all over the world. A series of numerical simulations and parametric study have been carried out to demonstrate the effectiveness and the robustness of the proposed method. On average, it can reduce the horizontal and vertical ground accelerations by 60–70% and 80–90%, respectively. In the early development of such a new technology, it is of particular importance to identify and evaluate if there is any drawback or hidden problem. Five important issues regarding the concept and feasibility of the proposed method have been identified, namely (1) nonlinear site response, (2) soil resonance effects, (3) liquefaction, (4) ground settlement and (5) environmental effects. Background information has been provided on each issue. Further discussion and research are required.In addition, the proposed method can be generalized as a distributed seismic isolation system, which involves isolating the entire contact surface of the foundation structure. Seismic Responses of Geogrid Reinforced Wall with Tire Derived Aggregates (TDA) Backfill using Reduced-Scale Shake Table Test

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Tire shreds, also known astire derived aggregates (TDA), are pieces of processed andshredded waste tires that can be used as lightweight and quickfills for embankments, subgrades, bridge abutments, andretaining wall backfills. TDA of different sizes have beenwidely studied as alternative backfills in the past twenty yearsand vast literature references are available. In contrast to the relatively rich literature on the staticbehaviors of tire shreds, scarce experimental data are available on the seismic performances of mechanically stabilized walls and bridge abutments with tire shreds/chips as backfills. Tsang (2008) was one of few researchers who studied a rubber-soil mixture backfill under seismic conditions. In his shake table tests, it was found that site response of the backfill was nonlinear and helped absorb incident seismic waves.(Feng and Sutter (2000). Material There are two types of TDA that are used in the USA: type A with a maximum size of 7.5 cm and type B with a maximum size of 30.0 cm. In this research, the TDA was provided by a TDA vendor in California, USA. The size distribution is shown in Figure 1. It can be seen that the material’s maximum size is approximately 10 cm, and 76% (by mass) TDA are smaller than 7.5 cm. The TDA was judged to be close to type A.

Fig. 17

Young’s modulus of the TDA was also obtained using alarge-scale compression test. The dimensions of the TDA sample in the compression test was 112 cm long, 71 cm wide, and 50 cm tall and was confined in a wooden box. The stress- strain relationship, which was not included in this paper due to page limit, showed an apparent upswing trend as the compressive deformation continued. Within 10% strain, the curve appeared to be a linear line, and the Young’s modulus of the TDA is approximately 400 kN/m2. The bulk density of the TDA in the backfill was 721 kg/m3, which is at the lower end of the density range that is used in the engineering practice. Higher density was not able to be reach due to the compaction capability in the lab. In order to obtain the shear resistance of the TDA, large scale shear testing was conducted. The shear resistance of the TDA was found to be approximately c = 0, phi= 30. Experimental Setup A section of reduced-scale MSE wall was built in a 1.5 m x1.87 m x 1.8 m rigid steel box that was anchored on a 2.4 m x2.1 m one-dimensional shake table. The load capacity of the shake table is

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177.9 kN, the actuator provides 245 kN of hydraulic driving force, and the maximum travel distance of the table is 12.7 cm. The shake table is capable of replicating recorded historical earthquake motions that are within the table’s allowable displacement range. Figure 18 is a photo of the shake table and the box with a retaining wall built shows the completed model MSE wall with TDA backfill (Matthew Oman 2013). The model MSE wall’s configuration is shown in Figure 2.The wall was 1.5 m high, 1.2 m deep, and 1.5 m long. Fivewrap-around layers of reinforced TDA were used. Uniaxialgeogrid was used for both reinforcement and containment ofthe TDA.

Figure 18. Configuration and instrumentation of MSE wall beneath the first layer of the TDA, a 10 cm sand layer wascompacted to simulate the friction of the base soil. It is noted that in each of the wrap-around layer, the top geogrid sheet is only half of the length of the bottom geogrid sheet for that layer, since the top wrap-around sheet was not intended to serve as a reinforcement layer. The TDA were compacted using a 15 kg hand hammer with a long handle and 30 cm x30 cm steel base to reach the target density of 721 kg/m3. A concrete slab was placed at the top of the wall and anchored to the top layer with ten steel rebar, so that the slab did not move freely during the shaking. The concrete slab simulated a surcharge of3.4 kN/m2. Transparent Plexiglas sheets were used at the interface between the TDA wall and the sides of the box to minimize the friction between the TDA and the boundaries. Figure 2 also depicts the instrumentations used in the modeltest. Three linear potentiometers were used to measure the horizontal deflections of the wall face at the bottom, middle, and top layers. The potentiometers were fixed to an inertial frame outside of the shake table, and an inelastic wireconnected each potentiometer to the geogrid at the threedesignated levels. The fourth potentiometer was connected to the shake table in order to measure the actual seismic motions generated by the actuator. The potentiometers were spring- loaded, but the spring force was significantly smaller than the seismic force and therefore did not affect the responses of the walls. The vertical settlements of the MSE wall during the shaking were measured by LVDT transducers that were anchored on the shake table above the concrete slab. The transient vertical

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effective stresses in the backfill were measured using dynamic soil pressure cells, which were placed flat at the bottom of layers 1, 3, 4, and 5. Wire-free accelerometers were embedded in each of the five layers and were close to the wall face in order to measure the acceleration responses of the backfill. One accelerometer was attached to the shake table and one to the box to measure their acceleration responses as well. A delayed start timer was set in each accelerometer, and the data recording (100 data per second) predetermined time when the shake table test wasscheduled to run. In order to simulate the natural retained soil on the back ofand beneath the MSE wall, spring-loaded boards were installed at the back-side and the bottom of the box. The springs were so mic stiffness of dense sand,following the approach suggested by Gazetas (1991). Conclusion and limitation of this research: This paper presents a preliminary experimental research on theseismic responses of a reduced-scale geogrid-reinforced retaining wall with TDA backfill under the simulated Loma Prieta earthquake excitations. The research used a shake table to produce the scaled earthquake motions. Overall the wall with TDA backfill performed well with no apparent damage. The maximum horizontal deflection of the wall face occurred at the top of the wall and was 7 cm, or 4.7% of the wall height. Due to the difficulty in achieving higher density, the TDA had a small settlement (approximately 2 cm) in the first 10-15seconds, or 1.3% of the wall height. experimental study has several limitations. (1) Thegeogrid’s tensile strength was not scaled, this could result in an over-reinforced wall. (2) The reinforcement was based on static design. Seismic design using the methodologies presented by National Concrete Masonry Association (NCMA 2010) may change the internal configuration of the MSE wall and consequently the seismic behaviour. (3) The scaling law used in the model test should be improved to consider the scaling of the TDA-geogrid composite material properties. (4) External (global) stability, such as deep-seated rotational failure that can be caused by earthquakes, cannot be simulated in this test due to the shallow soil depth. Because of these limitations, extrapolation of the model results to the field is premature at this stage. This research work is continued to address the limitations in (1), (2), and (3). Furthermore, numerical model using Plaxis is being developed to simulate the laboratory conditions (including the boundary conditions, material properties, and seismic excitations). Using the same conditions, the numerical modelcan be calibrated using the model test results; then the numerical model can be used to predict the seismic performance of this type of retaining walls in the field. Liquefaction strength of fly ash reinforced with randomly distributed fibers A study on the improvement of liquefaction strength of fly ash by reinforcing with randomly distributed geosyntheticfiber/mesh elements is reported. A series of stress controlled cyclic triaxial tests were carried out on fly ash samples reinforced with randomlydistributed fiber and mesh elements. The liquefaction resistance of reinforced fly ash is defined in-terms of pore pressure ratio.

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The effects of parameters such as fiber content, fiber aspect ratio, confining pressure, cyclic stress ratio, on liquefaction resistance of fly ash have been studied. Test results indicate that the addition of fiber/mesh elements increases the liquefaction strength of fly ash significantly and arrests the initiation of liquefaction even in samples of loose initial condition and consolidated with the low confining pressure. The use of fly ash as alternative material to natural soilsfor construction of fills and for filling low lying areas couldsolve the problem of disposal of fly ash to a great extent. Toth et al. studied the use of fly ash as a structural fill and found that the physical behaviour of fly ash is similar to that of silt and the structural fill made with fly ash could perform better than the fill constructed with natural materials. Leonards and Balleysuccessfully used the untreated pulverized coal ash, with more cementing qualities as a material for structural fill to support the foundation of a new precipitator for a power generating station in Indianapolis, USA. Sridharan et al. investigated the geotechnical characterization of various ash ponds in India and reported that pond ashes, in general possess low unit weight, good frictional properties, low compressibility and low permeability and they are well suited for their use as a structuralfill (Matthew Oman 2013). Liquefaction behaviour of sands has been extensivelystudied and is currently a phenomenon that can bereasonably predictable. Perlea et al. reported that many silt and clay deposits with low plasticity index such as tailing materials have also been found vulnerable toliquefaction. Since fly ash predominantly consists of non-plastic silt size particles of relatively low permeability than sand, it is prone to liquefaction during earthquakes. Therefore, it is essential to improve liquefaction strength of fly ash by an appropriate ground improvement technique. Gandhi and Dey studied the improvement of fly ash by blasting techniques and found that fly ash is densified to great extent in deeper depths. However, for shallow depths,the blasting technique may not be suitable. Soil reinforcement technique with randomly distributedfibers is used in a variety of applications like, retainingstructures, embankments, subgrade stabilization etc. Random fiber reinforcement is a variant of admixture stabilization in which, discrete fibers are added and mixed with the soil in the same manner as cement, lime or other. Many studies have been conducted relating to the behaviour of soil reinforced with randomly distributed fibers under static loading conditions. Various types of randomly distributed elements, such as polymeric mesh elements , metallic fibers, synthetic fibers and discontinuous multi- oriented polypropylene elements had been used to reinforce soils. It was shown that the addition of randomly distributed elements to soils contributes to the increase instrength and stiffness. (Robinson and Sharon 2000) However, the studies on behaviour of soils reinforced withrandomly distributed elements under cyclic loading are verylimited in the literature. Vercueil et al.(2009) found the liquefaction resistance of saturated sand reinforced withcircular sheet geosynthetics and concluded that thereinforcement increases the liquefaction resistance significantly due to reduction in the interstitial pressure distri- bution. In this paper, the study on effectiveness of randomly distributed geosyntheticfiber and mesh elements inimproving liquefaction strength of fly ash is reported. Fly ash The fly ash collected from North Madras Thermal PowerStation, Chennai, India is used for testing. The fly ashconsists predominantly (about 82%) of silt size particles. Laboratory experiments were carried out on the fly ash to find out the index properties of fly ash and the resultingproperties of the fly ash are given in Table 1.

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Reinforcement Two forms of reinforcement namely; fiber and meshwere used for reinforcing fly ash samples. The fiber and mesh elements are made from commercially available non- woven polypropylene geogrid sheets. The sheets were cut into the specified length and width to get the required aspect ratio (AR). The physical properties of geogrid used are shown in Table 2. A typical view of fiber and mesh elementsused in the present study is shown in Fig.18.

Fig. 19. Details of reinforcement used.

Liquefaction strength of unreinforced fly ash In general, from the laboratory cyclic tests, the liquefaction state of the soil is identified either by considering thepoint when the pore pressure ratio (u/s3) becomes unity, i.e. when the pore pressure (u ) equals the confining pressure (s3) or when the peak to peak value of dynamic axial strain exceeds a certain limit, say 10% . In the present study, the liquefaction is defined as the state when porepressure ratio becomes unity. The liquefaction resistance ofan element of reinforced fly ash depends on how close the initial state of fly ash is to the state corresponding to liquefaction failure and it is expressed in terms of pore pressure ratio.Typical variation of applied cyclic deviatric stress and the corresponding variation of pore pressure ratio and induced strain with number of stress cycles for unreinforced fly ash with relative density of 70% are shown in Fig. 19. (Matthew Oman 2013). It is observed that during first few cycles, pore pressure increases rapidly and thereafter (i.e. beyond stress cycles of about 25 numbers) the rate of increase in pore pressure per stress cycle is appreciably low until the state of liquefaction is reached. This behaviour is just opposite to the behaviour of sand where a gradual build-up of pore pressure is immediately followed by a rapid increase, near the condition of liquefaction. It is mainly due to the uniform particle size of fly ash, which permits rapid propagation of pore water pressure through the sample. Fig. 19 also shows the response of fly ash under post-cyclic monotonic loading. It is noticed from Fig. 19 that decrease of pore pressure with an increase of axial strain indicating dilative response under post-cyclic monotonic loading as reported by Vaidand Thomas for water deposited Fraser River sand. The typical plot of effect of relative density on the pore pressure ratio variation with number of stress

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cycles is shown in Fig.3. It can be observed that the pore pressure ratio causing liquefaction decreases withthe increase in the magnitude of relative density at aparticular confining pressure.

Fig. 20. Variation of (a) cyclic deviatric stress, (b) pore pressure ratio and (c), axial strain with number of stress cycles.

Fig. 21. Effect of relative density on liquefaction resistance of unreinforced fly ash 3c = 60 kN/m2 At all the relative densities, theresistance of fly ash to liquefaction increases with increase

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in confining stress. However, resistance to liquefaction appears to be nearly same for all confining stress levels when fly ash is subjected to very low magnitudes of cyclic load indicating that fly ash liquefies within a very closerange of stress cycles.(Edil and Bosscher 1992). Reinforcedfly ash samples with mesh content 2% and above are notgetting liquefied even at higher number of stress cycles. However, the least value of pore pressure ratio, i.e. higher liquefaction resistance is observed at mesh content of 2%. Experiments carried outon fiber reinforced fly ash samples reveal that higher liquefaction resistance is attained at fiber content of 2%. At higher mesh/fiber content, the homogeneity of the sample will not be same because of the segregation between fly ash particles and reinforcement. As a result, there will be local deformations and, failure will happen due to rise of excesspore water pressure leading to liquefaction or due toexcessive axial strain. The effect of AR on liquefaction resistance of fly ash reinforced with distributed mesh elements is shown . The fly ash samples reinforced by mesh elements with AR of 10 and 20 are not getting liquefied. However, mesh reinforced fly ash with AR of 10 resulted in lower pore pressure ratio, i.e. higher liquefaction resistance than for reinforced samples with AR 20. At higher AR the mesh elements may not remain straight and thus the effective length of mesh element available to mobilizeshear strength is getting reduced. This is similar to thebehaviour, for fiber-reinforced sand under static conditionsreported.(Lee 2010). We also compare the behaviour of fly ash samples bothreinforced and unreinforced for different ranges of confiningpressure. From the figure, it is observed that unreinforced fly ash at low confining pressures liquefies at less number of stress cycles. But the number of stress cycles causing liquefaction, NL increases with increasing in confining pressure. that higher rate of gain in liquefaction resistance of fly ash is indicated at lower confining pressure. Since fly ash is of low unit weight and the low effective confining pressure of 40 kPa corresponds to 7 – 8 m below ground level in practice, it can be concluded that providing mesh reinforcements can significantly increase liquefactionresistance of fly ash. The comparison of liquefaction behaviour of mesh-reinforced fly ash with fiber-reinforced fly ash at optimummesh/fiber content of 2% is shown. In this case the AR for mesh elements is 10 and for fiber elements is 20. Fig. 10 clearly indicates the superiority of mesh elements over fiber elements for improving liquefaction resistance of fly ash. It is due to the fact that the mesh inclusions provide better interlocking property of the fly ash material, and the same time it also provides easy dissipation of pore pressurealong the sample length. (Rao and Dutta 2006)

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Fig. 22.(Cabalar 2011)

Conclusion: From the detailed experimental study carried out onreinforced fly ash to investigate the liquefaction resistance,the following conclusions are drawn.There is sudden build up of pore pressure with cyclicstress applications initially and this behaviour is just oppositeto the behaviour of sand. This behaviour is due to the uniform particle size of fly ash, which permits rapid propagation of pore water pressure through the sample. There is sudden increase in axial strain within the first ten stress cycles for fly ash samples at all relative densities.It is concluded that at low confining pressures, randomly distributed geosynthetic fiber/mesh reinforce- ment provides higher rate of gain in liquefaction resistance of fly ash. Addition of randomly distributed mesh and fiber elements increases significantly the liquefaction resistance of fly ash at low relative densities. The maximum value of pore pressure ratio is about 50% less when compared to unreinforced samples. Randomly distributed mesh elements better arrest liquefaction when compared with randomly distributed fiber elements. It is because the mesh elements provide better interlocking property of the fly ash material and also provide easy dissipation of pore pressure along the sample length. Optimum percentage of fiber/mesh content is found to be 2% against liquefaction. The gain in liquefaction resistance of fly ash due to mesh/fiber reinforcements is more pronounced at lower confining pressures and hence reinforcing fly ash with mesh/fiber elements is a better choice among available ground improvement techniques to improve liquefactionresistance of fly ash(Thenmozhi and Stalin 2010). An in-house computer program has been developd using finite element method to model the time-domaindynamic response of a two-dimensional soil–foundation–structure system

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Fig. 23(Thenmozhi and Stalin 2010)

The superstructure is modeled by an assembly of two-dimensional frame elements. The element massmatrix is derived as a lumped-mass matrix. A linear model has been adopted for the dynamic analysis of the building structures. The governing dynamic equations are solved on the basis of Newmark method. For foundation (either raft footing or piling system) and subsoil materials, four-node quadrilateral plane-strain elements are used in the modeling. The element mass matrix is also derived as a lumped-mass matrix. The element damping matrix is constructed using Rayleigh method, in which frequency-independent material damping ratio is considered. As the first mode of vibration dominates the dynamic response of low-to-medium-rise buildings (i.e., the subject of this study), the damping effects should be minimized at the fundamental frequency of the entire soil–structure system. Special treatment has been made for nodes located at the soil–structure interface (annotated in Figure 2). A node in a two-dimensional frame element has three DOFs, whereas a node in a plane- strain element has two DOFs. In the model, displacement compatibility has been ensured at the interface nodes, where the rotational DOF is free, and the two transformational DOFs as in frameelement are coupled with those in the four node quadrilateral element. Meanwhile, full contactbetween soil (or RSM) and piles is assumed.

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Fig. 24(Thenmozhi and Stalin 2010)

In order to simulate the non-reflective effects of the infinite soil transmitting half-space, the model ofviscous boundaries has been assumed as the boundary (transmitting base) of the computationaldomain. Six performance indicators have been chosen for comparing and evaluating the effectiveness of the GSI–RSM system. As most severe damages are caused by strong ground shaking produced by near- field earthquakes that are rich in high-frequency seismic wave components, horizontal acceleration response time histories have been collected at the mid-point of the roof of the building (referred to as the roof horizontal acceleration) and at the mid-point at the base of the footing (or the pile cap; referred to as the footing horizontal acceleration). The mid-point of the roof has been chosen because it typically represents the maximum horizontal acceleration response of the structure. The second location has been chosen because it is commonly considered as the location where earthquake input ground motion is applied in an ordinary structural analysis. Owing to the fact that soft-story mechanism is the major cause of collapse of many buildings in earthquakes, first-floor inter-story drift has been chosen as the third parameter. The peak and root-mean-square (abbreviated as RMS) values of the three parameters have been computed and shown by Hazarika(2008) hence, altogether, six parameters havebeen selected as the performance indicators. CONCLUSIONS This paper presented a potential GSI system by placing RSM around foundations (raft footing or pile cap) of low-to-medium-rise buildings for reducing seismic demand and exerting a function similar to that of a cushion. The use of scrap tires as the rubber material can provide an alternative way of consuming huge stockpiles of scrap tires from all over the world. Moreover, the possibly low-cost feature of this proposed method could greatly benefit impoverished regions where resources and technology are not adequate for earthquake mitigation with well-developed, yet expensive, techniques..(Cabalar 2011)

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(Thenmozhi and Stalin 2010) Figure 25: The correlation between the average percentage (%) reduction in the root-mean-square values of the three response parameters and the ratio of the fundamental natural periods between the model with rubber–soil mixtures and that with pure sand (i.e., period lengthening ratio). A brief review of the latest research progress made on the development of the GSI–RSM system has been presented, which is followed by an overview of the variety of GSI systems that have ever been proposed. An in-house computer program has been developed for the dynamic analysis of the soil–foundation–structure system involved in the GSI–RSM system. It is a time-domain, two-dimensional finite element program that can efficiently model the dynamic response of soil–foundation–structure system. In the program, superstructure is modelled by an assembly of two-dimensional frame elements, and foundation and subsoil are modelled by four-node quadrilateral plane-strain elements. The effectiveness of the proposed GSI–RSM system has been shown by a preliminary parametric study using three recorded earthquake ground motions. Six performance indicators, including the peak and RMS values of horizontal acceleration at the roof and the foundation as well as the first-floor inter-story drift, have been evaluated. On average, 40–60% response reduction could be achieved, and the results have been found to be the most sensitive to variations in the thickness of the RSM layer. Finally, the correlation between the period lengthening ratio and the reduction effectiveness has been briefly explored. Tire Shreds Used In Construction Introduction Based on its material properties and resultant characteristics some of the applications as a geotechnical material have been discussed. Lightweight fill Lightweight fills are used to reduce stress on the underlying soil in order to reduce consolidation settlements or to increase global stability of constructions by reducing load. The low bulk density of tyre shreds, compared to soil materials, makes the material suitable as lightweight fill material. The high porosity and drainage capability limits the presence of water in the fill and the low maximum water content in individual tyre shreds preserves the low bulk density over time. (Lee 2010).

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In large fills the fire risks should be considered. ASTM (1998) recommends maximum fill height of tyre shred fills to 3 m. In a large noise barrier fill, NPRA (2004), 1 m thick vertical clay layers every 70-80 m have been used as fire barriers in order to reduce the risk of horizontal fire spreading in the fills. Thermal insulation Frost penetration combined with accessibility of water causes frost heave in especially fine grained soils. Thawing and corresponding bearing capacity loss due to low draining capacity in the partly frozen soils is also common. Thermal insulation materials are used to reduce frost penetration. The low thermal conductivity in tyre shreds makes the material suitable for thermal insulation material. Combined with the high permeability the material could decrease the frost heave by acting as capillarity breaking layer and increase bearing capacity at thawing by draining of excessive water.(Feng and Sutter (2000). The low specific heat capacity of the tyre shreds in combination with the low water content, result in low freezing resistance. Thus the layer of tyre shred itself is at freezing condition, i.e. < 0oC, even at low freezing index, but effectively insulates the layer below. The insulation effect is acting on the underlying soil and reduces the heat transfer. A frost susceptible soil is fine grained, it is recommended to use geo textiles to reduce the migration of the fine grained soil into the tyre shred layer. This will preserve the thermal insulation properties and drainage capability of the material. In road constructions the resulting strain in the pavement is a limitation factor. The tyre shreds does not a high bearing capacity in frozen conditions like geological materials have. Using tyre shreds as thermal insulation layer will result in lower bearing capacity of the road during the frozen period compared to when granular soils are used. Tyre shred does not undergo the stiffness increase when pore water freezes to ice. Another effect which was notified was that the deflection of the road surface prevented an ice cover to be formed which was the case on the rest of the road. The high permeability of the tyre material will mitigate the bearing capacity loss due to excess water during thaw. This effect is not quantified. In order to have a road structure stiff enough the (ASTM 1998) recommendation is at least 900 mm of superstructure material above the tyre shred layer. Thermal insulation has also been studied by Humphrey and Eaton (1995), Lawrence et al. (1999) among others. Drainage layer In landfill construction, drainage layers are used in the bottom construction and in the top cover to protect the sealing layers to have water pressures being built up. The bottom drainage layer is a part of the leach-ate collection system used for transportation of leach-ate for treatment or release. Normally a gas drainage system is installed in landfills. The gas drainage system collects landfill gas. The gas has a high greenhouse effect potential due the high content of methane and it also increases the risk for landfill fires. In figure 6.3 the different drainage systems in a landfill are illustrated. The high permeability makes tyre shreds interesting to use as drainage material. In addition, the durability, resistance against chemicals, low bulk density and thermal insulation properties can be utilised in addition to the drainage capability. There are several studies of utilising tyre shreds as drainage material. Main focus has been on the use of tyre shreds as bottom drainage layer, since it includes most of the design issues. The suitability of tyre shreds used as bottom drainage layer has been investigated in several studies,

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Reddy and Saichek (1998a), Warith et al. (1997). Tyre shreds have high permeability even at high vertical stress. At a vertical stress of 1 GPa and 65 % compression the permeability was approximately 10-4 m/s. Reddy et al. (2005) shows that tyre shreds resists clogging even at high intrusion of fine soil material. The resistance against leach ate degradation has been tested on different lactates, e.g. acidic, and has proven to be persistent. Despite the high compressibility of tyre shreds the permeability is still high. The compressibility must be considered to maintain enough thickness of the drainage layer in a long term perspective. To preserve the permeability, geotextile material should be used to protect the tyre shreds from clogging. Stability and mobilised shear strength in the tyre shred layer and in the tyre shred/geotextile interface in a long term perspective must be considered. In studies, e.g. Cosgrove (1995), it has been concluded that a shear plane in between the tyre shred/geotextile interface is the limiting factor. Cosgrove (1995) recommends maximum slope angles between 10-16o for smooth surfaced geotextiles and textured surfaced geotextiles 21-28o in order to achieve a safety factor of to 1.3 and 1.5 respectively. Tyre shreds cannot be placed directly on geosynthetic liners for the risk of puncturing. A protective layer of 100-200 mm of e.g. sand is recommended by several authors, e.g. Duffy (1996), and Reddy and Saichek (1998a). An important design factor of the top cover constructions in cold regions is frost penetration. Freezing may affect the sealing layer negatively. By combining the utilisation of tyre shreds as drainage material with thermal insulation a lighter top cover may be used which is beneficial for the top sealing layer. It will thus reduce the total settlements. This shown in the figure where a construction using tyre shreds as thermal insulation layer is compared with a conventional construction.(Lee 2010). Backfill material Tyre shreds reduce earth pressure on constructions when used as backfill material, Tweedie et al. (1998) and Humphrey et al. (1997b). The use as backfill material is both technical and economical motivated, Cosgrove (1996). In addition to low earth pressure the material will serve as drainage layer and as thermal insulation. Seismic Stability Distributed seismic isolation system A new method of utilizing scrap tires for earthquake protection involving mixing scrap tires with soil sediments and placing the mixtures around civil engineering structures, for absorbing vibration energy and exerting a function similar to that of a cushion was developed. The validity of the method has been demonstrated by numerical simulations in order to show its effectiveness and robustness. Two new types of geotechnical seismic isolation systems analogous to the conventional structural seismic isolation systems using spherical sliding bearings and laminated rubber bearings which decouple the building or structure from ground motions by interposing elements or materials of low stiffness in between were introduced. A distributed seismic isolation system, which involves isolating the entire contact surface of the foundation structure in contrast with conventional systems which are based on isolation of certain discrete supporting points was effectively analysed through FEM.(ASTM 1998)

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Underwater Caisson A series of large-scale underwater shaking table tests when performed on a gravity type model led to effective protection technique using tire chips and scrap tire derived recycled products. These tests concluded that, if the pore-water pressure build-up is used as a basis for defining the onset of liquefaction, the question of liquefaction does not arise at all at these locations for backfill with the tire chips cushion. When subject to an acceleration of 1.5 times that of the Kobe Earthquake, settlement in the conventional sandy backfill was of the order 3.162m whereas the structure reinforced with cushion did not undergo appreciable differential settlement. The reduction in the earth pressure at common peak ground acceleration was 75, 25, and 66% at the top, middle, and the bottom of the caisson, respectively. This implied that the seismic performance of the caisson improves with the use of the sandwiched cushion.(Hazarika et al.2008) Resistance to Liquefaction Since porosity of rubber shreds is much higher compared to soil, they inhibit capillarity. This prevents sudden rise in excess pore water pressure therby reducing chances of liquefaction during earthquakes considerably. Researchers have also qualitatively established that introduction of tire rubber shreds or granulated particles counteract development of excess pore water pressure under seismic loads hence can prevent liquefaction. Limitations in use Despite all the desirable characteristics of tire rubber , there are limitations which restrict its use in geotechnical applications. These have been summarised below. Creep Deformations The technical limitations of use of tyre shreds are related to the high compressibility and deformation properties. The high compressibility and creep deformation limits the use in foundation engineering design. (Thenmozhi and Stalin 2010) Large Deformations before failure Differential and long term settlements can cause concrete structures built upon tyre shreds to damage. Shear stress causes large deformations before failure in tyre shreds compared to that in granular soils. Relatively Thick Super Structure In road constructions tyre shreds requires a relatively thick superstructure to achieve required bearing capacity. Also due to presence of steel cords there is a chance of rupturing of any geo membranes used. However when used as a rubber soil mixture this disadvantage is mitigated considerably.(Anderson and Stokoe 1978). Self-Ignition Probability Tyre shreds have an ignition temperature of about 350 ºC. Spontaneous fires have been registered in large and thick tyre shred fills. The reason for self ignition is not completely understood, but heat generation by oxidation of free steel cord by microorganisms, presence of organic soils in

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combination with the low heat transportation out of the fills are suspected to be main factors. In the ASTM standard D 62070-98, ASTM (1998), the use is limited to 3 m thick fills of tyre shreds. It is however pointed out that no fills below 4 m thickness has been observed to self ignite. Fire assessments for tyre shred processing recommends temporary fills to be maximum 4 m, Hansson (2003). Environmentally Hazardous Compounds Since tyre shreds contains potential hazardous compounds, e.g. PAH and anti-degradants, some prevention acts is appropriate. Even if only low concentration of target compounds, except for iron and zinc is found in the leach-ate the use of tyre shred should be aimed to limit the potential leaching. From an environmental point of view tyre shreds should be placed above the ground or surface water table combined with good drainage conditions. From a leaching point of view neutral pH is ideally considering both element and organic compound leaching.(Edil TB, 2002) Leaching properties Among the metals it is primarily iron and zinc that is of concern due to the high concentrations found in the leaching studies. Iron hydroxides could be an aesthetic problem if precipitated outside a construction and will affect the release of other charged ions which may be accumulated absorbed on iron hydroxides or released if the hydroxides are dissolved. For zinc to be toxic high concentrations are needed. If the recipient is sensitive to additional zinc sources the use of tyre shreds from large constructions to small recipients should be considered. In most cases the zinc release are acceptable in terms of ecological effect levels in a potential recipient. Aerobic conditions in the recipient should be sufficient to biodegrade the obtained phenol concentrations in the leach ate.(Anderson and Stokoe 1978). Thermal insulation properties The increase in thermal conductivity caused by moisture (wetted tyre shreds under free draining conditions) was at average 6 %. Frozen samples have about 10 % higher thermal conductivity compared to not frozen. The water content, stress and tyre shred size have a minor influence on the thermal conductivity. The low influence of water content is due to the high draining capacity. Compaction properties Compaction increases the density, stiffness and reduces the compressibility of tyre shreds. It has been concluded that the increased stress, and thus the compression and density, increases the shear strength. Thus compaction also has an increasing effect on the shear strength. Most of the effect on bulk density is achieved at low compaction energy (SP). High compaction work (MP) results in slightly higher density compared to low compaction work (SP).

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Fig. 26 Effect on density of compaction energy level on tyre shred for Loose Fill (LF), Standard Proctor (SP) and Modified Proctor (MP). Non-vibratory compaction methods, such as Proctor compaction, are more appropriate for compacting tyre shreds than vibratory though the difference in achieved density increase is small, Ahmed and Lovell (1993). Water content has low effect on the compaction result in Proctor tests and is from an engineering point of view insignificant, . The effect of tyre shred size on bulk density in compaction tests is lower compared to used compaction energy, . The optimum size of the tested tyre shreds for obtaining maximum density is 75 mm. To sum up major factors affecting the compaction properties are tyre shred size and compaction energy. Minor factor is water content. There are no significant difference in increase of bulk density of a tyre shred fill by static (Proctor) or vibratory compaction.(ASTM 1998) Static liquefaction behaviour of saturated fiber-reinforced sand The problem of static liquefaction of sand is nowadays a classical soil mechanics subject. Using a ring-shear apparatus, we explore the possibility of fiber reinforcement as a new method to improve the liquefaction resistance of sand. In order to understand the effect of the fiber content and sand density on the static liquefaction behaviour of fiber-reinforced sand, a series of undrained ring-shear tests were carried out on saturated samples with different fiber content and sand density, and the test results and mechanisms of fiber reinforcement were then analyzed. The results indicate that the undrained shear behaviour of fiber-reinforced loose samples is not greatly influenced by the presence of fiber, but for medium dense and dense samples, the presence of fiber clearly affects their undrained behaviour. Untreated specimens showed a continuous decrease in shear resistance after failure, while the specimens treated with fiber showed fluctuations even after shear failure, and these fluctuations become stronger with increasing fiber content. The peak shear strength increases with the fiber content, especially in dense specimens. After shearing, all the fiber-reinforced and untreated dense samples maintained structural stability, while the unreinforced loose samples showed a completely collapse of structure. The presence of fibers may thus limit or even prevent the occurrence of lateral spreading that is oftenobserved in unreinforced sand.

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Unreinforced and fiber-reinforced samples were prepared using a moist tamping technique. This technique is commonly used in laboratory studies of fiber-reinforced sand and allows the control of sample density while preventing the segregation of fibers..(Cabalar 2011)

The maximum dry density of samplesdecreases with increasing fiber content Pf, and samples with a higher fiber content require more compaction for a given drydensity. In order to maintain uniform dry densities, lower drydensity/higher void ratios were used in this test. A maximum value of fiber content is required to keep dry sand density and sample volume unchanged for a given void ratio, and this value increases with increasing void ratio. Three different initial void ratios after consolidation and four different percentages of fiber- 0.2, 0.4, 0.6, and 0.8% e were chosen for this investigation. A moisture content of 10% was used for the mixing process of all the test specimens. The details of the void ratios at the end of vertical consolidation, dry sand density, density grade, and fiber contents of mixturein the preparation of all samples, the required water was first added into the dry sand, and then the proposed content of fibers was mixed in small increments by hand to obtain a uniform mixture. It is important to ensure that all fibers are mixed thoroughly. After that, the mixtures were divided into four equal parts, and each part was put into the shear box and compacted. Samples for this study with a height of 8 cm were prepared in four layers of equal height to achieve the proposed densities. The sample wasdirectly formed in the testing apparatus.

Experimental results:

In order to discuss the results of the ring-shear tests on thesaturated fiber-reinforced sand, the samples have been divided into three groups (loose, medium dense and dense states), according to their undrained shear behaviour. (Robinson and Sharon 2000) Loose samples The normal stress, pore pressure and shear resistance against shear displacement for loose specimens with different fiber contents are presented and the effective stress paths are presented. the trends of change in pore pressure and shear resistance are fairly similar for unrein- forced and reinforced samples with fiber contents of 0.2%, 0.4% and 0.6%. In the initial shearing period, the shear resistance showed a sharp increase to peak shear strength and then underwent a quick reduction, subsequently decreasing slowly until it reached a steady state. The pore pressure built up quickly and then increased grad- ually until it reached a steady state accompanying further shearing. Compared with the above results, the curves for 0.8% fiber-rein- forced samples show a different trend after the peak value, i.e., the pore pressure and shear resistance fluctuate. the effective stress paths of the unreinforced and reinforced specimens show similar trends. Soon after the start of shearing, with increasing shear stress, the stress path moved leftwards until it reached a final steady state point (Lee 2010). The unreinforced sample clearly shows a completely collapsed structure ,while the reinforced sample still maintains structural stability even after removal of the upper ring. It seems that the presence of fibers can limit or even prevent the lateral spreading of the soil which is one of the consequences of liquefaction. This phenomenon in triaxial undrained tests on saturated sand was alsopresented.(ASTM 1998)

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Discussion The distributed fibers might act as a spatial three-dimensional network to interlock soil grains, helping the grains to form a unitary coherent matrix and restricting the displacement. Several researchers reported that the fiber surface roughness strongly affected the fiber sliding resistance As the fiber was mixed or samples were compacted, the hard sand particles impacted and abraded the fiber surface, resulting in plastic deformation and even removal of part of the surface layer. It is speculated that pits and grooves that formed on the fiber surface constituted an interlock and improved the interactions between the fiber surface and the sand matrix. These interfacial mechanical interactions between the fiber and sand are greatly dependent on the sand dry density and fiber content. Loose sand corresponds to a higher void ratio and larger pore diameters. It is speculated that when the sample wassaturated, some of fiber was separated from the sand particles. After the sample shearing, the fiber can move easily and has no effect on the static liquefaction behaviour of the saturated speci- mens. Furthermore, the fiber occupies only a part of the volume of sample pores in low density samples, so the low fiber content hardly changes the shearing behaviour of the saturated sand. But the presence of fiber still improves the structural stability and prevents the lateral spreading of sand due to static liquefaction.

An increase of sand density gives rise to a more effective inter- facial contact area between the fiber and the sand matrix. In the process of preparing samples, more compaction should be applied to obtain a high density, and this will result in a larger contact forceand interlock between adjacent sand particles and a greater plasticdeformation and roughness of the fiber surfaces. Meanwhile, the interlock effect increases with the content of fiber. So the peak shear strength of samples increased with the fiber content. During ring shearing, the interfacial friction strongly depends on the resistance of sand particles to rearrangement and rotation. Nor- mally, if sand particles are less likely to be rearranged during shearing or are more interlocked, this leads to a higher interfacial resistance to shear and if the resistance offered by mechanical interlocking between the particles and fiber surface is larger than that between adjacent sand particles, it will result in sand particle rotation. It is speculated that the fiber is re-oriented gradually due to the sand rearrange- ment and rotation in the shearing process, and this fiber orientation in a shear zone might lead to the volume expanding, the pore pressure decreasing and the shear resistance increasing. These orientations increase with the fiber content. So the fluctuation in shear resistance occurred more frequently when the fiber contentincreased in medium and dense samples in this study.(Thenmozhi and Stalin 2010)

Conclusions

A series of ring-shear tests were performed to study the staticliquefaction behaviour of sand reinforced with short polypropylenefiber. The effects of the fiber content and sand density on the staticliquefaction behaviour of the fiber-reinforced sand were investigated.The main conclusions from the present study can besummarized as follows:

(1) The ring-shear test provides an efficient analysis tool forevaluating the static liquefaction behaviour of fiber-reinforcedsand. The ring-shear apparatus used in this study can measurethe entire process of undrained shear even after sample failureand check the shear behaviour at large shear displacements.

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(2) The undrained shear behaviour of fiber-reinforced loose sand is not significantly influenced by the presence of fiber, but the unreinforced samples after shearing clearly show a completely collapsed structure, while the reinforced samples still maintain structural stability even after the removal of the upper ring. It seems that the presence of fibers can limit or even prevent the occurrence of the lateral spreading of sand as normallyobserved for unreinforced sand.

(3) The presence of fibers clearly affects the undrained behaviour of medium dense and dense samples. The results show that the test on sand showed a continued decrease in shear resistance after failure, while those treated with fiber showed fluctuations even after shear failure. This fluctuation becomes stronger with increasing fiber content. All the medium dense and dense reinforced samples maintained structural stability after shearing, while the unreinforced medium dense sample showed a partly collapsed structure and the dense sample showed structural stability. It seem that densification and fiber reinforcement both can limit or even prevent the occurrence of lateral spreading of the soil due to static liquefaction.(Edil TB, 2002) (4) The failure shear strength and peak shear strength increases with the fiber content, especially for medium dense and dense samples. And they also increase with increasing dry sand density for the same fiber content. The presence of fiber has negligible effects on the residual shear strength of specimenswith different densities.

(5) The results of this investigation indicated that fiberreinforcement is useful for improving the static liquefaction resistance of sand, and the sand density and fiber content must be considered in practical applications. Further study will beperformed to examine the cyclic shear behaviour.

Experimental investigation:

Materials tested A relatively uniformly graded sand is used in this study. The sand is classified as SW by Unified Soil Classification System (USCS). The particle size distribution of the sand is shown in Fig. 2. Engineering properties of the soil are listed in Table 1. For better performance tire shreds are cut from waste tires with approximately the same size and thickness. Tires are cut with a special cutter by hand into rectangular shape and different sizes (see Fig. 3). As much as 170 tires are used for the tests. Soil is mixed with different tire shred contents and size. The shred contents in this study are 10%, 20%, 30%, 40% and 50% by volume. The shreds widths are 2 and 3 cm with aspect ratio of 2, 3, 4 and 5. Physical properties of the tire shreds are presented in Table 2..(Cabalar 2011)

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Fig. 2. Grain size distribution curve for sand.[HATAF AND RAHIMI 2005]

Fig. 27. Typical tire shreds used in this study.[HATAF AND RAHIMI 2005]

Fig. 28 Table 1[HATAF AND RAHIMI 2005] Engineering properties of soil used in the study

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Table 2[HATAF AND RAHIMI 2005] Physical properties of tire shreds

2. Footing and testing apparatus Model footing was made of steel with a cylindrical shape 15-cm diameter and 7 cm thickness. A cylindrical tank with 1.0 m diameter and 1.0 m height was built. Testing tank was made from steel plate, 4 mm of thickness, to accommodate the sand. The steel cylindrical tank was designed big enough to avoid boundary effect on bearing capacity. A static loading system was used by using a simple lever arm system. Load and settlements were measured using the load cell and dial gauges. 3. Test procedure Sand and tire shreds were measured properly with respect to the desired tire shreds contents in the test program (10%, 20%, 30%, 40% and 50% volume of shreds compared to total volume). Then, the soil and tire shreds were mixed carefully using a spade. The tank was, then, filled up to specified thickness (20 cm) with the mixed material. Each layer was tamped and compacted with a specific wooden plate, dropping from a certain height (certain energy for each layer) before the next layer was poured. Small cans were used to identify sand density in different places in the tank. Relative density of sand ranged from 35% to 45% with respect to shred content. Relative density decreased with increasing shred content because shreds absorbed compaction energy. Footing was loaded statically until failure reached. The settlement of the footing was measured for each load. The bearing capacity was obtained using tangent method. In this method, two tangents were plottedalong the initial portion and latter portion of theload–settlement curve and the load corresponding to the intersection of these two lines was taken as ultimate bearing capacity of the footing.(Anderson and Stokoe 1978). 4. Testing program The sum of 34 tests were carried out on circular footing on reinforced soil with different tire shred contentsand aspect ratios, as shown in Table 3. The results of all tests were compared to the results of tests carried out on unreinforced soil to indicate the effect of reinforcing on bearing capacity of the soil.(Edil and Bosscher 1992). 5. Test results In general addition of tire shreds to sand increases the bearing capacity and limit reductions in post peak resistance. Effect of sand reinforcement on bearing capacity (as the ratio of reinforced soil bearing capacity to unreinforced soil bearing capacity BCR) is shown in Table 3. Figs. 4–10 show load–settlement curves for sand reinforced with tire shreds obtained from performed tests. Fig. 11shows BCR values versus aspect ratios of shreds for different width and shred contents. It seems that for both selected widths (2 and 3 cm) optimum aspect ratio is about 4. For a constant shred contentminimum BCR was observed for 2 · 4 cm shreds. This might be because of the short length of shreds whentensile force becomes greater than pull out resistanceforce of the reinforcements. (ASTM 1998)

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So it can be concluded thatfor better performance a minimum length of shred must be provided. It can also be seen that for a certain width increase in length, greater than optimum length, decreases the BCR. This may be due to the fact that increasing the length will decrease area ratio (i.e., AR/A), which is an important parameter. The 3 · 12 cm shreds have maximum BCR for all shred contents. Maximum and minimum BCR reached were 3.9 and 1.17 for 40%tire shreds of 3 · 12 cm and 10% tire shreds of2 · 4 cm, respectively.The effect of shred contents on BCR is shown inFig. 12. As this figure depicts increasing shred contentincreases BCR. However, it seems that there is an optimumshred content (about 40%) after that BCR will notincrease further if shred content is increased. The behaviour of soil mixed with tire shreds in high tire shreds content tends to be more influenced by the tire shreds material and intensity of the composite material rather than the soil characteristics Table 3 [HATAF AND RAHIMI 2005]

Bearing capacity ratio (BCR) for reinforced sand

[FIG 4-12 IN HATAF AND RAHIMI 2005]

Fig. 29. Load–settlement curves for 2 · 8 cm shreds with different shred contents.

Fig. 30. Load–settlement curves for 2 · 6 cm shreds with different shred contents.

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Fig. 31. Load–settlement curves for 3 · 12 cm shreds with different shred contents.

Fig. 32. Load–settlement curves for 3 · 9 cm shreds with different shred contents.

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Fig. 33. Load–settlement curves for 2 · 10 cm shreds with different shred contents.(Lee 2010).

Fig. 34. Load–settlement curves for 2 · 4 cm shreds with different shred contents.

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Fig. 35. Load–settlement curves for 3 · 6 cm shreds with different shred contents.

Fig. 36. Bearing capacity ratio (BCR) vs. aspect ratio of shreds.

Fig. 37. Bearing capacity ratio (BCR) vs. shred content.

Fig. 38

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Conclusion

A series of laboratory test have been carried out on the model of shallow footing resting on reinforced sand. Tire shreds were used as reinforcement elements. Two parameters were selected to identify their influence on bearing capacity of sand: shred content and shred aspect ratio. It was found that addition of 10% shreds by volume increases BCR from 1.17 to 1.83 (increasing bearing capacity from 17% to 83%), 20% tire shreds increases BCR from 1.6 to 2.2, 30% tire shreds increase BCR from 2.15 to 3, 40% tire shreds increases BCR from 3.2 to 3.9 and 50% tire shreds increases BCR from 2.95 to 3.9 with respect to shreds width and aspect ratio. Aspect ratio of 4 was found as the best aspect ratio for two widths used in this study (i.e., 2 and 3 cm). Shreds of 4 cm length and smaller work improperly as reinforcement because of the small length. Optimum shred content found in this study is 40%, further addition of shreds will not increase the BCR significantly.(Lee 2010). The optimum value of the rubber content:

The optimum value of the rubber content is obtained from testing program described in Table 6. The tests are done for different rubber contents, Rc different thicknesses of soil cap, hs/B and different thicknesses of rubber-reinforced soil, hrs/B. The corresponding bearing pressure with rubber content for different values of hrs/B = 0.25, 0.5, 1, and 1.5, while hs/B value is kept constant (hs/B = 0.25) at different values of settlement is depicted in Fig. 6. This figure may be classified into two groups; one for hrs/B <=1 (first group), and the other for hrs/B = 1.5 (second group). For the first group (Fig. 18a–c) the improvement in bearing capacity initially is increasing when rubber content increases from 0% to around 5%, but, thereafter, the bearing capacity decreases with rubber content, regardless of the footing settlement ratio, s/B and the thickness of rubber-reinforced layer, hrs/B. For example, in the case of hrs/B = 0.5 and hs/B = 0.25 (Fig. 6b), the bearing pressure obtained at settlement ratio of s/B = 2.5%, is about 50 kPa, 118 kPa, 154 kPa, and 87 kPa for 0%, 2.5%, 5%, and 7.5% of rubber content, respectively.(Edil TB, 2002) These values show that the bearing pressure increases about 136%, 208% and 74%, respectively for 2.5%, 5% and 7.5% of rubber content compared to that of the unreinforced bed. The results depict an optimum shred rubber content around 5% which delivers the maximum increase in the bearing capacity. The increase in performance improvement with rubber content of 5% could be due to the available competent reinforced layer beneath the footing. The decrease in bearing capacity after optimum content of rubber may be attributed to swapping the soil grains with soft material, like rubber, and also possible increasing the void ratio of mixture tends to the compressibility of mixture – consequently leading to increase in the footing settlement. It may be expected when the rubber content increases to more than 7.5%, the bearing pressure of footing leads to less than the bearing pressure of unreinforced bed. The excess of soft rubber particles separates soil particles and forms a soft rubber fabric and consequently decreases the bearing capacity of footing due to significant compressible foundation bed.(ASTM 1998)

For the second group (Fig. 39) where a thicker layer of rubber– soil mixture (hrs/B = 1.5) is employed in foundation bed, the general trend in variations of bearing pressure of footing with rubber content, is similar to those obtained for the first group (hrs/B <= 1). In this case, at the settlement ratio of s/B = 2.5%, the optimum shred rubber content is obtained around 2.5% which has only delivered a 56%

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enhancement in bearing pressure of footing while in the first group using hrs/B = 0.5 and 5% of rubber, 208% enhancement in bearing pressure of footing has been delivered at the same settlement ratio. It is, therefore, inferred that use of the thicker mixture (hrs/B = 1.5) could not be compared to that of the thinner mixture even where the optimum rubber content used in the rubber–soil mixture layer. Consequently, the second group is not as efficient as the first one to be considered in practical design. Fig. 39

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Fig. 40 Variation of bearing pressure with rubber content for constant hs/B value of 0.25 at different values of settlement, (a) hrs/B = 0.25, (b) hrs/B = 0.5, (c) hrs/B = 1, and (c) hrs/B = 1.5. The optimum value of the thickness of rubber-reinforced soil:

Fig. 19 depicts the variation in bearing pressure with the thickness of rubber-reinforced soil (hrs/B) for the experiments with the three different rubber contents of 2.5%, 5% and 7.5% and unreinforced soil bed at different footing settlement ratio, s/B. The rubber- reinforced layer was placed at a depth of 0.25 time of the footing width (hs/B = 0.25) from the base of the footing. From this figure, it has been found that with an increase in hrs/B ratio, the value of bearing pressure increases up to the value of hrs/B = 0.5, approximately, after which, with further increase in hrs/B ratio, the value of bearing pressure decreases at all settlements, irrespective of rubber content used in the mixture. As can be seen from Fig. 19b, at settlement ratio of 2.5% (s/B = 2.5%), the bearing pressure increases about 100%, 206%, and 62%, respectively for 0.25, 0.5, and 1 of rubber-reinforced layer thickness ratio (hrs/B) compared to that of the unreinforced bed. Overall, these results reveal that at all footing settlement level, regardless of rubber content value, the maximum improvement in the bearing pressure of footing have been obtained at optimum thickness of rubber reinforced soil layer (hrs/B = 0.5).Fig. 41

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Fig. 42 Variation of bearing pressure with thickness of rubber-reinforced soil, hrs/B for the soil cap ratio of 0.25 (hrs/B = 0.25) at different settlement ratio, s/B, (a) rubber content = 2.5%, (b) rubber content = 5%, and (c) rubber content = 7.5%. The optimum value of the thickness of soil cap In order to investigate clearly the beneficial effect of the soil cap over the rubber–soil mixture layer, the variation of bearing capacity with the soil cap thickness at different levels of footing settlement, are shown in Fig. 20. This figure shows the results of tests including 2.5%, 5%, and 7.5% of rubber in the rubber reinforced layer of 50 mm in thickness (hrs/B = 0.5). The rubber reinforced layer was placed at depths of 0, 0.25 and 0.5 times of the footing width (hs/B = 0, 0.25 and 0.5) from the base of the footing. This figure depicts that the bearing pressure increases as the hs/B ratio increases, up to approximately 0.25, but decreases as the hs/B ratio increases further, irrespective of the footing settlement level.[TAFRESH AND DAWSON 2010]

Fig. 43

Fig. 44 Variation of bearing pressure with thickness of soil cap, hs/B for optimum value of thickness of rubber–soil mixture (hrs/B = 0.5) at different values of settlement, (a) rubber content = 2.5%, (b) rubber content = 5%, and (c) rubber content = 7.5%.(Anderson and Stokoe 1978). Limitation and applicability The results presented herein provide significant encouragement for the application of randomly distributed shredded rubber as soil reinforcement, similar to conventional geosynthetic reinforcement to improve the strength and settlement behavior of foundation bed. But it should be

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noted that the present experimental results are based on the tests conducted on a small model of square footing and they are obtained for only one type of shredded rubber, one size of footing width, and one type of soil. Thus, full application should only be made after considering the above limitations. However, further study is needed to assess other important factors such as the importance of shred length, the economic aspects of using shredded waste tires as soil reinforcement compared to other reinforcement materials, the effectiveness of shredded waste tires as soil reinforcement in cohesive soil and expansive soil(Lee C etal 2012), and to see if results obtained in the laboratory are representative of field applications. Furthermore, although Milligan et al. 1986 and Adams and Collin 1997 in their studies on large- and small-scale tests on the behavior of granular layers with geogrid reinforcement showed that the general mechanisms and behavior observed in the small model tests could be reproduced at large-scale, nevertheless future tests could be conducted with larger scale footings at various conditions. For example, different footings (in size, shape and depth) and different characteristics and size of the shredded rubber could be studied to validate the present findings. However, in order to directly correlate the results of a prototype-scale footing to a model-scale test, the scale effects should be considered on geometrical dimensions of effective factors and the properties of rubber and soil used, if any. Although, the results of this study may be somewhat different to full-scale foundation behavior in the field, the general trend may be similar. Overall, qualitatively, this study provides insight into the basic mechanism that establishes the bearing pressure versus settlement response of the shredded rubber-reinforced soil bed and would be very useful and a fruitful avenue for future studies. On the whole, these results could be helpful in designing large scale model tests and their simulation through numerical models.(Edil TB, 2002) Summary and conclusions In this study, a series of laboratory tests under monotonic load has been carried out on square footings supported on the rubber reinforced and unreinforced soil beds. The test results have been used to assess and understand the potential benefits of reinforcing soil with rubber shreds and soil cap in terms of the increased bearing pressure of footing compared with footing on unreinforced beds. Based on the results obtained, the following conclusions are derived: (1) The results prove the usefulness in recycling of tires waste in geotechnical aspects of waste management. These lead to overall saving in competent soil material costs and re-use of tires waste. (2) The results strongly suggest the re-use of tire waste in the form of shredded rubber mixed with soil as reinforcing elements beneath the footing. From the results of tests, the bearing capacity of footing increases with increase in the rubber content, the thickness of rubber-reinforced soil layer and the soil cap thickness up to their optimum values, after which the bearing pressure decreases. (3) The optimum percentages of shredded waste tire rubber are measured around 5% of the total volume of soil-rubber mixture. This leads to the maximum improvement in bearing capacity of footing regardless of soil cap thickness and the thickness of reinforcement layer. (4) Tire shreds-soil mixture used as a reinforcement layer under footing base performs more effective when covered by a soil cap layer compared to tire shreds-soil mixtures without a soil cap

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layer. The optimum depth of soil layer beneath the footing (i.e. the thickness of soil cap layer over the mixture) is obtained approximately 0.25 times the footing width (hs/B = 0.25) which results the best performance in increasing the bearing capacity.(Edil TB, 2002) (5) The optimal thickness of the rubber-reinforced soil layer to achieve the maximum improvement in bearing capacity of footing is measured to be approximately 0.5 times of the width of the footing. More increase in the thickness of rubber– soil mixture than its optimum value increased the compressibility and the settlement of foundation bed, and consequently the reduction in reinforcing effect of rubber– soil mixture. It may reduce the performance of foundation bed compared to fully unreinforced bed.

(6) At all the footing settlement levels, bearing pressure of footing has substantially increases for shredded rubber-reinforced bed, when considering the optimum values of soil cap thickness, the thickness of rubber-reinforced soil layer and the rubber content compared to unreinforced bed. At the settlement level of 5%, maximum improvement in bearing capacity was observed as the value of bearing capacity of footing reaches around 2.68 times of the unreinforced bed.

USE OF TIRE CHIPS IN LANDFILL GAS EXTRACTION APPLICATIONS

In landfills, tire chips have been used as alternative daily cover, in leachate recirculation trenches, as selective fill above the leachate collection and removal system (LCRS) in new cells (i.e., the operations layer), and as a primary drainage component in the LCRS. In Florida, the use of tire chips as a substantial component of the LCRS has been demonstrated by the Hillsborough County (Florida) Solid Waste Management Department where tire chips are used in the drainage layer of new cells at the Southeast County Landfill. In the late 1990s, Waste Management Inc. of Florida used tire chips in augmenting an older bottom liner design to enhance leachate removal prior to initial filling. That system included supplemental tire chip trenches with a geotextile buffer layer installed between the tire chips and the geo-membrane liner.(Anderson and Stokoe 1978).

Hydraulic Conductivity:

As vertical loading increases and the tire chip layers compress, thereby reducing void space, the hydraulic conductivity of the tire chip backfill decreases. A detailed comparison of reported hydraulic conductivity for various tire chip sizes was prepared by Reddy and Marella (2001).

Various tests indicate that the hydraulic conductivity of tire chip sizes typical of landfill gas applications (i.e., greater than 2 inches) ranges from a minimum of 0.5 cm/sec to over 20 cm/sec. Texas DOT (2004) reports tire chip permeability greater than 10 cm/sec. The lower end of the reported range is representative of smaller chips and/or higher applied loads (i.e., over 5,000 psf, which corresponds to the pressure exerted by over 80 feet of waste on top of the chips).[ California Integrated Waste Management Board 1998 ] It has been reported that the pore space provided by the tirechips allows approximately 100 percent more gas transmission than traditional aggregate (Nebraska State Recycling Association) and promotes condensate or leachate drainage on the order of 10 times higher than well-graded soils

Flammability

Tires are flammable and exothermic reactions in tire chip fills have been documented. However, as reported by the Texas DOT, of the 70 installations of tire chip backfill for road construction projects

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documented by 2004, only three experienced exothermic, or heat producing, reactions. These projects in Washington and Oregon were both over 50 feet deep and contained a mix of soil and tire chips, which resulted in a fill material that was less compressible than tire chips alone.They also included tire chips that contained significant amounts of exposed steel and were contaminated with liquid petroleum. The authors are aware of no landfill fires that have been directly or indirectly affected by, or attributed to, the presence of tire chip backfill within a landfill.

Permeability

The hydraulic conductivity of 3-inch tire chips is comparable to that of typical drainage materials, and except under the most extreme compressive forces, tire chips provide the same, if not better, conductivity as compared to aggregate[Donovan et al 1996]. The authors have not witnessed any situations where the permeability of 3-inch nominal size tire chips used as backfill in landfill gas collectors or wells appeared to be less than that of stone.

Compaction and Settlement

In order to minimize post-construction settlement, tire chip backfill must be properly compacted in order to minimize excess void space. The Texas DOT (2004) reports that settlement of 3-inch tire chips is less than the settlement of 12-inch chips. If the tire chip backfill is not adequately compacted, over time and/or with the application of vertical loads in the future, the tire chips will settle into these voids and result in localized ground settlement. The presence of excess voids around the walls of the collection pipe also diminishes the pipe strength.

For horizontal collectors, compaction can be accomplished by using an excavator bucket to press down onto the installed tire chips or by driving over the trench with track-mounted equipment. Settlement caused by inadequate compaction typically is not evident once waste has been filled on top of the collectors.

However, with vertical extraction wells, localized settlement can be significant. In the case of vertical wells installed with tire chip backfill at a landfill in Delaware, settlement ranging from 5 to 10 feet was reported around well casings. While the cause of this settlement was not fully known, the contractor suspects that large tire shreds caused “bridging” and the formation of voids within the tire chip backfill. Once soil backfill was placed above the tire chip layer, these voids collapsed, and the ground surface subsided. Unless the tire chip backfill extends a substantial distance above the top of the slotted collection pipe, such settlement can result in soil infiltrating the well casing and reducing the effectiveness of the well.

Compressibility

The compressibility behaviour of sand-tyre mixtures is of important consideration especially in the design of geotechnical structures of which both stability and serviceability are of paramount concerns. Compressibility of scrap tyres is quite high compared to that of pure sand. Hence, the compressibility of sand-tyre mixtures might be governed primarily by the amount of tyre contents in the mixture. Ahmed (1993) performed compressibility tests on sand-tyre chip mixtures. The behaviour of sand-tyre chip mixtures under repeated loads was determined after 3-4 loading/unloading cycles. The influences of sample preparation, compaction efforts, and size of tyre chips were investigated. It was found that percentage of chips (on gravimetric basis) had significant

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effect on the compressibility of the mixtures; however, compaction efforts had minor effect on load deformation response of the mixtures. Edil and Bosscher (1994), Bosscher et al. (1997), and Humphrey et al. (1993) reported that tyre-soil mixtures exhibit significant initial plastic compression under load, which is related to the porosity of the mixtures.

Once the material was loaded to a level of reduced initial porosity, it behaved like an elastic material. The test results indicated plastic (unrecoverable) strain under the first cycle of load application followed by reduced plastic and elastic strains under subsequent cycles of loads. Subsequent cycles had similar load-displacement curves with a diminishing rebound compared to the first cycle. Additionally, Bosscher et al. (1997) found that smaller chips and larger shreds had similar responses. It is evident from the literature review above that a number of studies were carried out on shear and compressibility behaviours of sand-tyre chip/shred mixtures. However, only a few of these studies carried out detailed investigation on the behaviour of S-TC mixtures, which has equal potential to be used in civil engineering projects. It is noted that the authors and coinvestigators have recently proposed the use of soil-scrap tyre mixtures for low-cost seismic isolation (Tsang 2008; Tsang et al. 2009; Tsang et al. 2012) of civil infrastructure, especially suited for developing countries. The success of such low-cost seismic isolation method mainly depends on the engineering properties of the soil-tyre mixtures.

This paper presents results of experimental investigations on shear and compressibility behaviours of S-TC mixtures which have not yet been adequately researched in the literature. It is noted that development of a constitutive model of S-TC mixtures is considered beyond the scope of the paper and is part of future research investigations of the authors.

While proper backfilling techniques can minimize some of the settlement of tire chip layers, designers should consider compressibility when designing horizontal collectors and vertical extraction wells with tire chip backfill. For horizontal collectors, it is prudent to increase the thickness of the permeable backfill layer in accordance with the expected overlying pressures. Assuming a compression of 30 percent, if the desired trench thickness is 3 feet deep, the trench depth at the time of construction may need to be increased to 4 feet in order to account for settlement and compression.

For vertical extraction wells, a similar approach should be used such that the resultant settlement due to compression yields a tire chip backfill that extends above the top of the perforated or slotted well casing. While backfilling of horizontal trenches enables compaction with standard equipment, when backfilling a borehole, compaction is normally accomplished only via the weight of the tire chips and the fill dirt. The expected compression of the tire chip backfill can be estimated based on empirical and laboratory data presented in various literature referenced at the end of this paper.

At the Orange County (Florida) Solid Waste Management Facility, SCS Engineers (SCS) designed 10 vertical extraction wells with tire chip backfill extending 1 foot above the top of the slotted well casing. After four years in operation, field personnel report no settlement at the well casings, which indicates that the tire chips have not compressed significantly. A recently installed landfill gas system in Hillsborough County (Florida) was designed by SCS to include 10 vertical wells with a minimum of 5 feet of tire chip backfill extending above the top of the slotted well casing. Three months after installation, there are no signs of localized settlement that would indicate significant settlement of the tire chips.

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Economic Feasibilty

The economic feasibility of using tire chips in lieu of coarse aggregate as a construction material for landfill gas collectors and wells depends on the availability of both materials. In terms of economics, the cost of tire chips is driven by the market for the chips. Where there is a source of tire chips and/or a market for their use, costs can be comparable to rock, which is the case in Florida and Louisiana. Based on recent material quotes in Florida, tire chips are approximately $41 per cubic yard (not including delivery), compared to non calcareous rock costs of $46 per cubic yard for AASHTO No. 3 stone and over $60 per cubic yard for No. 4 stone. Note that these costs have been converted from a cost per ton to cost per cubic yard basis to account for the differences in density of the materials.(Humphrey 1995)

In states where rock is relatively inexpensive (e.g., Georgia), tire chip use is not common, processors are not plentiful, or the burial of tires in landfills is permitted (e.g., Alabama), tire chip costs are generally higher than for No. 3 or No. 4 rock. For example, typical rock prices in Georgia (not including delivery) range from $18 to $20 per cubic yard, which is approximately one half the $35 per cubic yard cost of tire chips. In Alabama, the cost of rock is less than $10 per cubic yard and tire chips are not available. In Louisiana, the cost of 4-inch tire chips is the same as for No. 3 rock

In circumstances in which a landfill owner processes tires on site, tire chip costs may be essentially zero and the economics strongly favor tire chips over rock. Similarly, at a site in Louisiana, the tire processor provided the tire chips for free. This example illustrates that landfill owners may find significant cost savings by investigating the availability of tire chips, especially if a tire processor is located in close proximity to the site, or is currently paying a tipping fee to dispose of tires.

Environmental Incentives

As part of an integrated solid waste management program, using tire chips as a backfill material can be seen as environmentally beneficial, which may be important to some landfill owners.

REGULATORY CONSIDERATIONS

There appears to be little reluctance from regulators in the Southeastern U.S. and California regarding the use of tire chips in landfill gas collection systems. In Florida, the Central, Southwest, and South districts of the Department of Environmental Protection (FDEP) have approved landfill gas collectors and wells that use tire chip backfill. In Mississippi, the Department of Environmental Quality (MDEQ) has published literature that encourages the use of tire chips in landfills.

Louisiana Department of Environmental Quality (LDEQ) recently approved a series of horizontal collectors at an active landfill in the southern part of the state. Staff from the Alabama Department of Environmental Management (ADEM) and Georgia Environmental Protection Division (GA EPD) have stated that using tire chips in landfill gas systems would be no different than standard construction methods from a permitting perspective.

The waste tire program funded by the State of Louisiana offers an economic incentive for using tire chips in environmental projects. Therefore, LDEQ requires the permitted to quantify the volume of chipped tires that will be used in the project. Once the project is completed, the State of Louisiana pays the tire processor $150 per ton of tire chips that are reused in beneficial use projects. Also, in

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its 1998 Guidance Manual on the Use of Tire Shreds as Gas Collection Material at MSW Landfills, the California Integrated Waste Management Board stated that “there are no restrictions of the use of tire shreds as LFG collection material for collection trenches within the waste mass or as vertical well backfill.”(Edil and Bosscher 1992).

CONCLUSIONS

Utilization of tire chips in landfill gas applications is becoming more common where there is an economic or environmental incentive to do so. In summary:

• Tire chips have hydraulic and gas conductivity values comparable to or higher than coarse aggregate, which makes them acceptable for use in landfill gas collection systems.

• Successful projects have use 3-inch tire chips with a gradation similar to AASHTO No. 3 stone.

• Compressibility of the tire chips must be considered in designing the backfill thickness.

• For vertical extraction wells, the tire chip backfill should extend a minimum of 5 feet above the top of the slotted pipe. A greater thickness is warranted for higher expected compression rates.

• Construction CQA inspectors should ensure proper backfilling and compaction methods are used to minimize “bridging” of the tire chips and avoid excessive future settlement.

• Tire chip backfill has been used successfully in horizontal collectors throughout Florida in landfills that generate a substantial amount of landfill gas. In some cases, these collectors are buried under more than 100 feet of waste and are still functioning properly.

• The use of tire chips for backfilling vertical wells has not been well documented.

FLY ASH AND TYRE CHIP USED TOGETHER

Recycled Materials in Embankments

One way recycled materials have been used successfully is in highway embankments. The three recycled materials used most often are fly ash (Type A or Class F), tire chips and wood chips. In the 1970s and 1980s fly ash was popular, but in the last five years tire chips have become more popular in embankment construction. Compared to fly ash, both tire chips and wood chips are a lighter-weight fill.

Study

The Center for Innovative Grouting Materials and Technology at The University of Houston conducted study 0-1351, "Recycled Materials in Embankments, Except Glass, for TxDOT, the Texas Commission on Environmental Quality (TCEQ) and the Federal Highway Administration (FHWA)" to research the use of recycled materials in highway embankments and develop specifications as needed.

Researchers performed limited laboratory tests to verify the engineering and leaching properties of random samples of the selected materials obtained from various parts of Texas. All of the recycled materials showed very low levels of contaminant leaching during the Toxicity Characteristics

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Leachate Procedure (TCLP) and TCEQ tests and were characterized as Class 3 non-hazardous waste materials.

The behavior of the materials with simulated Texas soils was evaluated. The behavior of recycled materials without soils was also studied. The behavior of the recycled materials is comparable to the behavior of sand with Texas soils. Hence recycled materials could be incorporated into earth embankments effectively.Placing the recycled material in the core of the embankment was the most popular configuration. Research also indicates that the use of recycled material can either increase or decrease the cost of embankment construction, with transportation costs being an important factor.

Through the collection and analysis of this information, researchers developed a specification for the use of recycled materials in embankments.The research team believes future research on using recycled materials in embankments should:

-Perform field studies to evaluate the success of various embankment configurations using Texas soils and recycled materials. Instrument the embankments to quantify short- and long-term settlement of side slopes and evaluate the leachate quality.

-Evaluate the long-term risk to the environment from using recycled materials in embankments.

-Develop a new compaction test to evaluate the densities of long tire chips with and without soils.

-Develop an assurance program to minimize the effects of the variability of recycled material properties.

-Establish appropriate safety and health practices for handling recycled materials in embankment construction.

The contents of this summary are reported in detail in The Center for Innovative Grouting Materials and Technology Research Report 0-1351, "Recycled Materials in Embankments, Except Glass," C. Vipulanandan, M. Basheer, and M. W. O'Neill, Preliminary Report Dated - January 1996. This summary does not necessarily reflect the official views of the FHWA, TCEQ or TxDOT.

CONCLUSION

A series of laboratory model tests has been carried out to investigate the using of shredded waste tires as reinforcement to increase the bearing capacity of soil. Shred content and shreds aspect ratio are the main parameters that affect the bearing capacity. Tire shreds with rectangular shape and widths of 2 and 3 cm with aspect ratios 2, 3, 4 and 5 are mixed with sand. Five shred contents of 10%, 20%, 30%, 40% and 50% by volume were selected. Addition of tire shreds to sand increases BCR (bearing capacity ratio) from 1.17 to 3.9 with respect to shred content and shreds aspect ratio. The maximum BCR is attained at shred content of 40% and dimensions of 3 · 12 cm. It is shown that increasing of shred content increases the BCR. However, an optimum value for shred content is observed after that increasing shreds led to decrease in BCR. For a given shred width, shred content and soil density it seems that aspect ratio of 4 gives maximum BCR.

Based on model footing test it is found that for the optimum value of rubber content of 5% at footing settlement level of 5%, the maximum improvement in bearing capacity of rubber- reinforced

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bed was obtained as 2.68 times of the unreinforced bed. This value of improvement was achieved using the optimum thickness of reinforced layer of 0.5 times of footing width and the optimum thickness of soil cap of 0.25 times of footing width.(Edil and Bosscher 1992).

Tyre pieces mixed with soil and aggregates separately in various proportions and tested for California bearing ratio to determine its optimum content. Aggregate crushing value, impact value and abrasion value decreased with increase in waste tyre content in the aggregates. Waste tyre pieces reinforced with soil showed improvement in CBR vaule with its addition upto 7.5% and there onwards decreased with further increase in tyre content in unsoaked condition. However, waste tyre pieces reinforced with soil does not show any improvement in the CBR value in soaked condition.(Rao and Dutta 2006)

It is found that load carrying capacity of the soil + RW + soil layer and soil + RW inter mix are found to be better than the other combinations such as soil + soil + RW or soil + RW + RW, both in reinforced and unreinforced case. The soil+RW+soil or soil+RW inter mix can be effectively used in order to reduce the plastic deformation that the subgrade clay undergoes and also to enhance the recoverable elastic strain. The placement of geogrid reinforcement at the interfaces of soil and RW layer improved not only the load carrying capacity but also very much the recoverable elastic strain.

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