efficacy of geosynthetics in stabilization of … · the application of geocell in road...

EFFICACY OF GEOSYNTHETICS INSTABILIZATION OF SUBGRADE

SOIL

Sangeetha DM1, Naveen Kumar DT2, Vishnu KB3,Sethuja B4, Sachu SM5, Jeevan KV6

Department of Civil Engineering,1,3−6Shreedevi Institute of Technology, Kenjar, Mangalore,

2Sri Venkateshwara College of Engineering, BangaloreIndia.

[email protected]

June 26, 2018

Abstract

Urbanization plays a vital role in generating economicgrowth and prosperity. The increase in urbanization hasled to scarcity of lands. As a result roads and buildingshave to be constructed over soils with low bearing capac-ity. The performance of road depends upon the propertyof subgrade. This paper focuses stabilizing locally availablesoil with various types of geosynthetics viz. Geomembraneand Geonets. CBR tests were conducted on soil that iscompacted to optimum moisture content and maximum drydensity. The variations in the CBR value on geosyntheticsfor subgrade stabilization considering both performance andeconomy could be suggested through this project.

Keywords: California Bearing Ratio (CBR), OptimumMoisture Content (OMC), Maximum Dry Density (MDD),Geomembrane, Geonet.

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International Journal of Pure and Applied MathematicsVolume 120 No. 6 2018, 6779-6797ISSN: 1314-3395 (on-line version)url: http://www.acadpubl.eu/hub/Special Issue http://www.acadpubl.eu/hub/

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1 INTRODUCTION

FOR the last four decades geosynthetics have been widely used ingeotechnical and environmental engineering. Over the years de-signers and contractors solved the problems of using conventionalconstruction materials which are restricted or considerably moreexpensive by these products. Geosynthetics is defined as a planarproduct manufactured from a polymeric material that is used withsoil, rock, or other geotechnical-related material. There are signifi-cant number of geosynthetic types and geosynthetic applications inconstruction world. Thousands of years before inclusions of differ-ent sorts mixed with soil have been used. They were used in Romanconstruction to stabilize road ways and their edges. These early at-tempts were made of natural fibres, fabrics or vegetation mixedwith soil to improve road quality. A fundamental problem with us-ing natural materials (wood, cotton, etc.) in buried environment isthe biodegradation that occur from microorganisms in the soil. Toovercome the limitations civil engineers searched for alternate solu-tions, one such alternative which has emerged as a popular material/ technique in recent years is geotextiles and related products nowcommonly described as geosynthetics. The first productions andpublications about goesynthetics started with the use of geotextilesas filters and reinforcement in the 1960s. In 1977 the first congresson geosynthetic were held in Paris and International Geosynthet-ics Society (IGS) were established in 1982 [1]. A series of designelements leading to the required geosynthetics strength, modulus,anchorage length, and surface friction along with design charts werediscussed for the safe and economical solution for soft soil stabiliza-tion projects [2]. Majority of cases, synthetics are produced fromman-made fibres; however, they can also be from natural fibres.In fact, geosynthetics are synthesized for use in civil engineeringprojects to facilitate construction, ensure better performance ofthe structures and reduce maintenance in the long run. They havewild applications in almost all geotechnical and hydraulic engineer-ing projects. Typically, such projects include airport and highwaypavements, railways, canals, dams, embankments, retaining walls,etc. Common types of geosynthetics used are geotextile, geomem-brane, geogrid, geonet, geocell, geofoam, geosynthetitic clay linersand geocomposites. Geotextiles, geogrids and geocells are mainly

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used for soil reinforcement. Geotextiles are continuous sheets ofwoven, non woven, knitted or yarns. These sheets have a fabricappearance and are flexible and permeable. Geogrids have an ar-ray of apertures that are uniformly distributed between their lon-gitudinal and transverse elements. Geocells are three-dimentionalnetworks constructed from strips of polymeric sheets and are rel-atively thick. In some cases 0.5m to 1m wide strips of polyolefingeogrids have been linked with vertical polymeric rods forms deepgeocell layers called geomattresses. Geomembranes, geosyntheticclay liners, geonets, geocomposites and geopipes are widely usedin environmental protection projects. Geomembranes are manu-factured from one or more synthetic materials and are continousflexible sheets. They are relatively impermeable and used as linersof fluid or gas contaminant and as vapour barriers. Geosyntheticclay liners are prefabricated bentonite geomembrane or single layerof geotextile. Geonets are formed by two sets of coarse, parallel,extruded polymeric strands intersecting at a constant acute angle.Geocomposites are made by the combination of two or more geosyn-thetic type viz., geotextile-geonet, geotextile-geogrid. Geopipes areperforated pipes used for drainage of liquids or gas. The devel-opments in the technology of the polymer and engineering plasticindustries are greatly incorporated in geosynthetic products as it isa manufactured material. The research results led the developmentof new and more powerful designs and construction methods usinggeosynthetics. The improved materials and design methods supportengineers to face challenges and to construct structures in condi-tions which are considered unfavourable in the past. More than twodecades creative builders were using logs or tree limbs to stabilizeroads. Then by using different types of locally available materialssuch as rice husk ash (RHA), sugar cane bagass ash (SCBA) andcowdung ash subgrade soil can be stabilized and it was found thatthe natural soil was found as intermediate plastic clay which re-duces the dry density and increases the optimum moisture contentafter stabilization [3]. With the advent of polymers in the middleof the 20th century much more stabile materials become availableGeosynthetics have been used to reinforce the base layer of flexiblepavement systems for the past 30 years. The mechanisms as wellas key advances in each one of the multiple applications includethe migration of reflective cracking in asphalt overlays, separation,

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stabilising of road bases, road soft subgrades and lateral drainage[4]. Experiments were conducted to know the basic types of geosyn-thetic materials used in transport constructions and also found thatbuilding machines are vital for the preparation of earth body be-fore the application of geosynthetics [5]. Results of analysis aboutperformance of geosynthetics stabilized paved roads built over softsoil under cyclic plate loads shows that geosynthetics functions asbase reinforcement alone, the value of resilient modulus of the basecourse layer can be increased by about one quarter and the thick-ness of base layer can be reduced by one third for pavement [6].The United states department of agriculture (USDA) Forest Ser-vice has been using geosynthetics on its low volume roads for thepast 40 years in applications of separation, reinforcement, drainageand others. The use of geosynthetics on low volume forest roads arecost saving, yet utilization is variable and inconsistent [7]. Finiteelement modelling of unpaved road reinforced with geosyntheticsprovide a numerical investigation using a bi-dimensional Finite El-ement Method (FEM) using ABAQUS software to analyse the im-provement of reinforced unpaved road under repeated wheel trafficload conditions in terms of aggregate base course thickness saving.This is a technique of soil mechanical stabilization, using geosyn-thetics extensively used in the construction of unpaved roads witha low volume of traffic [8].

2 TYPES AND APPLICATIONS OF

GEOSYNTHETICS

Different types of geosynthetics are available which are used fordifferent applications with respect to their functionality are shownin figure 1.

There are many developments in mechanically stabilized earth(MSE) walls and slopes in basal stabilization. In 1993 to constructa road over karsts terrain, textile geogrid was employed using ul-tra high strength polymer. The reinforcement mechanism is shownin Figure 2. Another development of construction technique thatpermits bridge abutment to construct where the sill beam rests di-rectly on geosynthetic reinforced soil (GRS), while the GRS doesnot require stiffening facing [9]. To build steep slope on column

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Figure 1: Flow chart of types of geosynthetics

and piles, over geosynthetic encased column and in embankment acombination of textile geogrid reinforcement with the geosyntheticsystems are used. Reinforced construction shows increase in bear-ing capacity and decrease in displacements while comparing thereinforced and unreinforced case at same load [10].

It is quite known that the compound behavior of geosyntheticsand soil are not completely understood. Recent research using largescale tri axial and biaxial combined with modern method of visu-alization of the displacement of soil particles tries to fill this gap[11]. In order to prevent the waste generated by the industrializa-tion it is found that silt soil can be mixed with waste materials andadmixtures to improve strength. Additionaly geogrid mesh also in-troduced in to soil for the upgradation of strength compared withthe parent soil [12]. Geogrids have been employed to resist reflec-tive cracking in asphalt for many years. By innovative studies it isclaimed that bitumen provides better bond to other polymers, pre-venting crack propogation. Geogrids enhances structural capacityand reduces potential distress. It can effectively reduce horizon-tal deformation of the aggregate base course layer with 203mm to457mm thickness in the traffic direction. For thinner layer of ag-gregate, the optimal location of geogrid in the unbound aggregate-subgrade interface is the upper third of the layer for thicker baselayer [13]. It can improve the low CBR pavement section perfor-mance as shown in the Table 1.

Geocells are used in aggregate stabilizing and it provides highvolume drainage and working platform support. In airports de-icig

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Figure 2: Geogrid reinforcement mechanisms. (a) lateral confine-ment, (b) Increasing bearing capacity and (c) tension membraneeffect

compound, the geocell confines the aggregate by improving the loadcapacity of aggregate and subgrade and it contains high volume offluids that will drain fluid in a control manner from structures in

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order to study the rutting behavior of geocell reinforced base layeroverlaying weak sand subgrade, a series of separated model loadtests were performed. The considerable improvement of differentnumber of cycles and plate settlements were observed on quantifi-cation of traffic benefit ratios (TBR), cumulative plastic deforma-tions (CPD), and rut depth reduction for geocell reinforced basecourses, comparatively it shows better improvement [15]. It is evi-dent that geocell has a proming sustainable ground reinforcementtechnique. Due to the use of geocell in infrastructure projects, thereis wide scope of further research to study better about the material.Geocells used for pavement applications are subjected to repeatedloads. Single and multiple geocell reinforcement granular base withthree infill materials like Kansar river sand, quarrey waste, and ag-gregate were tested under repeated loadings. The results showedthat it reduced the permanent deformation and increased the per-centage elastic deformation of granular bases [16]. The applicationof geocell in road construction is shown in figure 3.

Figure 3: Geocell geotextile for road construction [17]

Geonet is a mesh structures with overlapped threads (dia 3mmto 15 mm) and permanent angle 60-90 degree. The main appli-cations of geonets are constructing new road surfaces, reinforcingjoints of rigid pavements patch work, preventing cracking of roads[18] shown in figure 4(b). It is also used for reinforcing soft claysubgrade [19].

Geocomposites are made up of two or more geosynthetics toachieve better characteristic of each material and cost effectiveare shown in figure 4(a). The non perforated polyethylene ge-oweb has the greatest performance among all the geocomposites.

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Figure 4: (a) SITEDRAIN DS-180 Geocomposite and (b) place-ment of geonet on the road.

In late 1970, the US Army corps of engineering at the waterwaysexperiment stations (WES) used plastic tubes in a 300mm threedimensional mattress prototype with CBR 1-0 and cellular mat-tress was infilled with sand. By providing repeated loads was ob-served that wheel ruts under cumulative axle loads equivalent tothe 500mm thick unreinforced sand bases [20]. Geosynthetic clayliners have advantages such as rapid installation, less skilled labour,low cost and very low hydraulic conductivity to water. By properlyinstalling GCL, large differential settlement can withstand and bet-ter self healing characteristic can be achieved. Hydrated GCL is aneffective gas barrier and it reduces the over burden stress on com-pressible substructure [21]. In 1987, patent was filed in Germanyconcerning about shear resistance modes of manufacture. Needledfibre was used to stabilize and strengthen the products in this sys-tem. GCL applications expanded as seals in substructure of earthenembankment, hydraulic structure and a host of additional appli-cations. GCL are used in pavement construction to prevent theuncertainty of sinkhole formation underneath the pavement [22].

Geofoam can be used as a light weight fill material in sub baseon subgrade and simplify the design and construction activities [23].But in 1998 it was reported that the road construction problemsusing geofoams, fires raised during construction in Euro road EC inVestby, Norway, pavement failure due to block shifting under traffic,street reconstruction project in Rotterdam, the Netherlands. Alsogeofoam damaged due to insect infestation, icing of pavement sur-face, failure of insulated pavement system, water absorption prob-

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lem and so on [24].Geomembranes are used for containment of liquid or vapour

barrier or both. Okhalhoma DOT used geomembrane for undu-lation problem due to expansion of clay under road surface. Theclay tends to swell due to increase in moisture content and causenon-homogenous expansion and contraction. This results to detri-mental effect on pavement and minimize moisture changes in thepavement. It also provides many facilities like strength properties,lighter weight, portability, cost savings and so on [25].

Figure 5: More than 630,000 sq. ft. of geomembrane was installedover expansive clay in Ardmore, Oklahoma to stabilize Interstate35 highway [25].

3 ADVANTAGES AND LIMITATIONS

The ease of installation of geosynthetic is significant in comparisonto thick soil layers requiring large earth moving equipment. In caseof geogrid reinforcement, it reduces the permanent settlement ofrailway tracks particularly on soft soils [26], this geogrid system ismore flexible so they can be used as earthquake resistant, this canbe installed in any weather conditions [27].

Geocell can be used to improve the performance of unsurfacedrural pavements by reducing the rutting, inclusion of geocell asreinforcement increases the bearing capacity of foundation over soil,use of geocell will reduce the risk of the materials sliding down thesteep slope [28].

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The construction of coastal structures such as breakwaters ordikes in shallow water by geosynthetic mat method will providegood lateral stability. While using geofoam, it is 1% to 2% thedensity of soil with equal strength, i.e, when density is low it willachieve high strength, break down of particles is less, so it will notspread to surrounding soils, geofoams can be installed by simplehand tool thus it requires limited number of labours for the con-struction. Geofoam eliminates the high operation cost of heavymachines.

Geotextile-geomembrane composites increases the resistence topuncture, friction related to sliding as well as tensile strength in thereinforcement area, they can also be used as a drainage medium.While comparing to compacted clay liner, geosynthetic clay linerhas some advantages mainly in its hydraulic conductivity perfor-mance which is less than compacted clay liners.

There are several ways of geosynthetic functionality, well knownmechanisms are functions of separation, filtrating, protecting andslope reinforcing. There are several publications that explainingthe influence of geosynthetic in engineering aspects, and these mayhave some limitations.

Although the reinforcing functionality of geosynthetics is de-scribed and used in connection with slopes reinforcement, Thereare many practical examples of geosynthetic usage as a reinforcingelement of pavement, subbase or subgrade layers at the same timethis application is put to laboratory, full scale or real constructiontests. The results and outputs of the tests are highly incompa-rable because of the non uniform test methodologies and some ofthem being purpose-built by geosynthetic producers. The followingparameters should be taken into account for each experiment: ma-terial properties (soil, aggregate, geosynthetic, etc.), test condition(soil bearing capacity, compaction rate, etc.), geometrical parame-ters (dimensions, etc.). Unfortunately, many published papers aremissing the mentioned parameters this is a serious cause for theincomparability of the mentioned parameters [29].

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4 CASE STUDY IN LATERITE SOIL

In the present work Geomembrane and Geonets are introduced toincrease the stability and used as a strengthening material for thestabilization of the subgrade soil. Present investigation is carriedout using available laterite soil in the Mangalore region.

A Laterite soilLaterite soil usually develop in tropical and other regions with

similar hot and humid climatic conditions. The laterite soil is col-lected from borrow pits at a depth of 2m from Yekkar, Mangalore.It is important to understand the detailed properties of soil for thedesign of pavement, therefore various basic tests were conducted todetermine the properties of the soil. The results are as tabulatedin Table 2.

B Geosynthetic materialsGeosynthetics materials for the project are collected from Geod-

hukan, Velandipalayam, Coimbatore. The properties of geosyn-thetic are given in Table 3.

C Results and discussionsLaboratory tests were performed to determine the properties

like specific gravity, grain size analysis of soil sample were deter-

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mined as per IS: 2720 [30]; Modified Proctor compaction were car-ried out to determine the optimum moisture content and maximumdry density of the sample as per IS: 2720 (Part 8)-1983 [31]. Thereafter, a series of CBR tests were conducted on laterite soil usinggeonet and geomembrane placed at various heights. Each geosyn-thetic material were placed at three different heights and the heightat which maximum CBR value obtained is found out in unsoakedcondition. The soaked CBR tests were then conducted by rein-forcing the soil with geosynthetics at optimum height. The variousheights considered in the present investigation are h/4, h/2 and3h/4.1) CBR Test on laterite soil reinforced with Geonet

CBR test was conducted on soil reinforced with geonet at threedifferent heights (v.z., h/4, h/2 and 3h/4). The load-penetrationcurve is plotted for all the three heights and is shown in figure 6.The maximum CBR value for unsoaked condition is observed whenthe geonet was placed at one-fourth of the height of the specimen.

Figure 6: Load-penetration graph for soil reinforced with geonet inunsoaked condition.

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2) CBR Test on laterite soil reinforced with GeomembraneCBR test was conducted on soil reinforced with geomembrane

at three different heights (v.z., h/4, h/2 and 3h/4). The load-penetration curve is plotted for all the three heights and is shownin figure 7. The maximum CBR value is observed when the ge-omembrane was placed at one-fourth of the height of the specimen.

Figure 7: Load-penetration graph of soil reinforced with geomem-brane in unsoaked condition.

3) CBR Test on laterite soil reinforced with Geonet and Geomem-brane at a depth of h/4 in soaked condition.

Load versus penetration curves were plotted for the CBR valuesobtained for geomembrane and geonet placed at a optimal depth ofone-fourth of the height of the specimen in soaked condition and isshown in fig. 8. The maximum CBR value is observed in geomem-brane, as compared to geonet. Hence, it can be concluded as thebest material among both.

4.) Design of pavementFlexible pavement design by CBR method is used to determine

the total thickness of pavement. Generally there are two methodsto design the pavement from CBR (California bearing ratio) valuenamely, CBR method recommended by California State of highwaysand CBR method recommended by IRC. For the present study IRCcharts are used to design the pavement and the thickness of soil inpavement is found to be:

• 460mm in unreinforced soil.• 230mm in soil reinforced with geomembrane.

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Figure 8: Load-penetration graph of geomembrane and geonet insoaked condition

• 280mm in soil reinforced with geonet

5) Cost AnalysisTotal cost of the pavement can be analysed by considering the

factors like unit cost of soil, unit cost of the required material, andthickness of the pavement reinforced with material. Thus the costof soil required for unit area of pavement can be determined and thesum of both soil and material cost gives the total cost of pavement.Figure 9 shows the cost involved in the construction of pavementwith/without geosynthetics. It can be observed that the cost ofconstruction is high for the pavement reinforced with geosyntheticswhen compared to unreinforced soil. The percentage increase incost when compared to unreinforced soil is found to increase by180% using geomembrane and 213% using geonet.

Figure 9: Cost Analysis with/without geosynthetics

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5 CONCLUSION

Nowadays, the available land with good bearing capacity is notsufficient enough to meet the increasing population demand. Thisleads to the construction of different structures on weak soil withpoor bearing capacity. In those situations stabilization of soil playsa vital role in achieving the efficiency and performance of the struc-ture. Geosynthetics are one such material, which are widely usedin soil stabilization.

From the present study, it is observed that both the reinforce-ments are effective in increasing the strength of soil. The CBRvalue of the reinforced laterite soil was obtained as 1.23% after re-inforcing it with Geonet it was found to increase by 1.4% and withgeomembrane it was 4.70%. Also, the optimum height of placementwas found at a depth of H/4 from the top of the specimen in both.Hence it is concluded that the maximum strength of the soil wasobtained when the Geomembrane was placed at a height of H/4from the top of the specimen. And also it gives minimum thicknessof pavement.

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