Volume change behavior of clayey soil–fly ashmixtures

B. A. Mir*1 and A. Sridharan2

At present, more than 150 million tonnes of fly ash is generated annually in India, posing serious

health and environmental problems. To control these problems, the most commonly used method is

addition of fly ash as a stabilizing agent in combination with soils. For the bulk utilization of fly ashes in

geotechnical applications such as embankments/dykes, as back fill material, as base material and in

water retaining structures either alone or with soil, the volume change behavior of soil–fly ash mixture

forms an important consideration. Only few data are available concerning the volume change

behavior of clayey soil–fly ash mixtures and require further investigations. In the present study, high-

calcium (ASTM Class C-Neyveli fly ash (NFA)) and low-calcium (ASTM Class F-Badarpur fly ash

(BFA)) fly ashes in different proportions by weight (10, 20, 40, 60, and 80%) were added to a highly

expansive soil [known as Black Cotton (BC) soil from India] to evaluate the effect of fly ashes on the

volume change behavior of clayey soils–fly ash mixtures. Compacted clay-fly ash samples were

cured for 7 and 28 days and subjected to consolidation test under different pressures ranging from

50 kPa to 800 kPa. In this study, the void ratio, the compression index, swelling potential, coefficient

of consolidation, permeability, and preconsolidation pressure of clayey soil–fly ash specimens were

investigated. The test results indicate a significant decrease in compressibility characteristics and

swell potential of the treated soils. It was seen that 20% high-calcium fly ash content is the optimum

quantity to improve the compressibility characteristics of clayey (BC) soil cured for 7 days against

60% for immediate tests. It is also observed that 10% of NFA is the optimum amount required to

minimize the swell potential compared to 40% of BFA. Thus, the main objective of the study was to

study the effect of fly ashes on the volume change behavior of fly ash treated clayey soil and bulk

utilization of industrial waste by-product without adversely affecting the environment.

Keywords: Compressibility, Clayey soils, Fly ash, Swell potential, Self-pozzolanic, Flocculation

An embankment, structural backfill, and other compactedfills should possess low compressibility to minimize settle-ments or differential settlements between structures andadjacent approaches. Consolidation occurs more rapidly incompacted fly ash than in silty clay/clay because fly ash hasa higher void ratio and greater permeability than soil. Forfly ashes with age-hardening properties, including most‘‘high lime’’ fly ashes from lignite or sub-bituminous coals,age hardening can reduce the time rate of consolidation, aswell as the magnitude of the compressibility. Knowledge ofcompressibility and permeability are essential in a number

of engineering problems concerning settlement, seepage,and stability of the structures. Excessive heave, settlement,low shear strength, and internal erosion of some soils causedamage to many civil engineering structures such as spreadfootings founded on clayey soils; roads, highways, andairport runways constructed on expansive subgrade; andearth dams constructed with dispersive soils. Swelling ofexpansive soils causes more damage to structures, particu-larly light buildings and pavements, than any other naturalhazards, including earthquakes and floods (Jones andHoltz, 1973).

In practice, lime has been used as an effective additive toimprove the engineering properties of soils and preventdamage to structures. Lime treatment in cohesive soilsgenerally reduces swelling and improves soil plasticity,workability, and bearing capacity (EI-Rawi and Awad,1981; Basma and Tuncer, 1991; Abduljauwad, 1993; Bell,

1Department of Civil Engineering, National Institute of Technology, Srinagar-190 006, Kashmir, Jammu and Kashmir, India2Indian National Science Academy, India

*Corresponding author, email: p7mir@nitsri.net

� 2014 W. S. Maney & Son LtdReceived 16 June 2013; accepted 25 July 2013

72 DOI 10.1179/1939787913Y.0000000004International Journal ofGeotechnical Engineering 2014 VOL 8 NO 1

1996; Narasimha and Rajasekaran, 1996). Pure limestabilization is very effective but can be expensive in largeprojects. With this in mind, and to improve the behaviorof clayey soils, attempts have been made to utilize low-costlocal materials including waste products. One of the mostcommon applications which include the bulk utilizationof fly ash is the construction of compacted fills andembankments, and a few embankments have beenconstructed in India using pond ash (Vittal, 2001). Flyash has also been used as a backfill material, a base coursematerial, and an embankment material due to its lowspecific gravity resulting in low unit weight but highfrictional value and free draining nature, apart from beingused as a stabilizing agent (Kumar, 1996; Prakash andSridharan, 2009). The Electric Power Research Institute’s(EPRI) manual (Glogowski et al., 1992) reported that 33embankments and 31 area fills in North America that wereconstructed with fly ash.

Many researchers (e.g. Chen, 1975; Locat et al., 1990;Nicholson et al., 1994; Sivapullaiah et al., 1996; Du et al.,1999; Cokca, 2001; Nalbantoglu and Gucbilmez, 2002;Nalbantoglu, 2004; Phanikumar and Sharma, 2004; Kate,2005; Phanikumar and Sharma, 2007; Zha et al., 2008;Phanikumar, 2009) have reported successful stabilizationof expansive soils with admixtures such as lime and fly ash,which controls the potential of soils for a change involume, and improves the strength of soils. Saha and Pal(2012) have studied experimentally the compressibilitybehavior of soil and fly ash used in successive layers andreported that fly ash may be an effective stabilizingmaterial to reduce the volume change during both primaryand secondary consolidation.

Many other researchers (e.g. Pandian and Balasubra-monian, 1999; Kaniraj and Gayathri, 2004) studiedpermeability and consolidation characteristics of com-pacted fly ash and reported that the coefficients ofpermeability and compressibility of the compacted flyash were comparable to those of non-plastic silts. Ghazaliet al. (1991) have also observed a reduction in thehydraulic conductivity of chemically treated kaolin claydue to the decreased rate of consolidation.

The engineering properties of clayey soils are signifi-cantly altered by the addition of fly ash (Mir, 2001). Thecompacted dry unit weight of fly ash is usually in the rangeof 13?6–14?6 kN m23, which is well below that of mostconventional fill materials. Therefore, fly ash can beconsidered as a suitable and economical material forground improvement, where long-term settlements due toself-weight are also of concern (Indraratna et al., 1991).The quality of fly ash is a function of several factors. Theconstituents most likely to affect the engineering andphysical properties of fly ash are free lime and unburntcarbon. Either a high content of unburnt carbon or anegligible amount of free lime or both generally char-acterizes non-pozzolanic fly ash (low-calcium fly ash).Whereas, the pozzolanic fly ash (Class C or high-calciumfly ash) with self-hardening properties is most advanta-geous in ground improvement. The improvements in theengineering properties of clayey soils as fly ash is addedcan be explained by two distinct processes: (i) short-termreaction, consisting of cation exchange and flocculation as

a result of the reaction between clay and free limeimparted from fly ash, and (ii) long-term reaction,involving time and temperature dependent pozzolanicactivity, in which new cementious compounds-calciumsilicate hydrates (CSH) and calcium aluminate hydrates(CAH) responsible for long-term strength in soils areproduced. Fly ash can provide an adequate array ofdivalent and trivalent cations (Ca2z, Al3z, Fe3z, etc.)under ionized conditions that promote flocculation ofdispersed clay particles. Thus, clayey soils can bepotentially stabilized effectively by cation exchange usingfly ash. The proper use of fly ash can reduce the relativelyhigh cost of pure cement or pure lime stabilization,especially as fly ash is a waste material discarded inrelatively large quantities; at the same time solving theproblems posed by the disposal of fly ash. Therefore, forthe bulk utilization of fly ashes in geotechnical applica-tions, the volume change behavior of soil–fly ash mixesforms an important consideration.

Since only limited data are available concerning thevolume change behavior of fly ash treated clayey soil,more investigations are desirable. In the present study,high-calcium (ASTM Class C-Neyveli fly ash (NFA)) andlow-calcium (ASTM Class F-Badarpur fly ash (BFA)) flyashes in different proportions by weight (10, 20, 40, 60,and 80%) were added to a highly expansive soil [wellknown as Black Cotton (BC) soil from India] to evaluatethe effect of fly ashes on the volume change behavior ofclayey soil – fly ash mixtures. In this study, thecompression index, swelling potential, coefficient ofconsolidation, and permeability of clayey soil–fly ashmixtures were investigated. Thus, the main objectives ofthis study are:a. To utilize bulk quantity of fly ash as a stabilizing agent

avoiding the tremendous environmental problemscaused by large scale dumping of fly ash.

b. To control volume change behavior of clayey soils andto reduce the cost of stabilization of these soils byutilization of fly ash.

c. To find the effectiveness of the fly ash in reducing theswell potential of clayey soil and the possibility ofincrease in permeability and coefficient of consolida-tion.

d. To evaluate the role of fly ash as the primarystabilizing agent on the volume change behavior ofclayey soil.

Materials and methods

Clayey soilIn the present investigation, clayey soil (BC soil) wascollected from Davengere District of Karnataka State.Black cotton soil is the Indian name given to the expansivesoil deposits in the country accounting for almost one-fifthof the surfacial deposits. Black cotton soils, which areclays of high shrinkage and swelling characteristics, causesextensive damage to civil engineering structures.

Fly ashesIn the present investigation, two fly ashes namely, BFA(Class F – from Badarpur thermal power station (UP),

Mir and Sridharan Volume change behavior of clayey soil–fly ash mixtures

International Journal of Geotechnical Engineering 2014 VOL 8 NO 1 73

and NFA (Class C – from Neyveli thermal power stationTamil Nadu) have been chosen for evaluating the effectof fly ash on volume change behavior of clayey soil andits bulk stabilization for its effective use. These two flyashes were chosen for this study as they represent theextreme cases based on calcium content among manyIndian fly ashes. Class F fly ash [low CaO withSiO2zAl2O3zFe2O3.70% – (ASTM C618 89)] is nor-mally produced from burning anthratic or bituminouscoal. It has pozzolanic properties, but little or nocementious properties. Class C fly ash [high CaO withSiO2zAl2O3zFe2O3.50% – (ASTM C61889)] is nor-mally produced from burning lignite or sub-bituminouscoal and in addition to having pozzolanic properties,possesses autogenous cementitious properties. Class C flyashes have more a glassy structure (calcium aluminate)and minor constituents of crystalline compounds, whichare highly reactive. Therefore, Class C fly ashes are morereactive than Class F fly ashes.

LimeIn this study, commercially available hydrated lime wasused as an additive to BFA to make it at par with NFA interms of lime content.

All the tests were carried out as per the relevant standards.The physical and chemical properties of materials used arelisted in Table 1 and compaction characteristics of clayeysoil–fly ash mixtures are given in Table 2.

Experimental workIn this study, the additives content is defined by the ratio ofthe weight of the additive to the dry weight of the naturalclayey soil (BC Soil) expressed as a percentage. The soil andfly ash samples were mixed in the dry state and the varioussoil–fly ash mixes used for conducting the compaction testsare given in Table 3. In the present paper, the soil was oven-dried and passed through a 425-micron sieve before being

used in this investigation. The major emphasis of theexperimental work was placed on the volume changebehavior of clayey soil treated with two fly ashes (low-calcium – BFA and high-calcium – NFA). The physical andcompaction behavior of clay soil–fly ash mixtures have beenreported by Mir and Sridharan (2013). For bulk utilizationof fly ashes in geotechnical applications, an understandingof volume change behavior of clayey soil–fly ash mixtures isessential.

Testing procedures

One-dimensional consolidation tests:‘‘Immediate’’ test seriesOven-dried soil samples were prepared as per standardprocedure (ASTM D421). For immediate test series, thesoil–fly ash samples were prepared by compacting at0?95cdmax (i.e., 95% of maximum Standard Proctor dryunit weight) and corresponding water content on dry sideof optimum and immediately tested in a fixed ringconsolidometer using brass rings of 60 mm diameter and20 mm height as per ASTM D2435. The height ofspecimen after compaction is 15 mm (using 5 mm brassspacer). The burette, the connecting tube, and the base ofthe consolidation cell were de-aired by allowing water toflow from the burette. The specimen was then assembled inthe consolidation cell. The chamber around the consolida-tion ring was filled with de-aired water. A seating pressureof 6?25 kPa was applied on the specimen and maintainedfor 24 hours. During this period, the specimen wasallowed to saturate by the capillary action of water fromthe surrounding chamber and head of water of the burette.The swelling of the specimens under nominal surchargecan be obtained by allowing the specimen to swell freelyunder nominal load of 6?25 kPa to reach its maximumpossible limit. After equilibrium was attained as indicatedby nearly constant readings in a vertical dial readings

Table 1 Physical and chemical properties of materials used

Physical properties Chemical properties

Property CS* BFA** NFA*** Composition (by wt-%) CS BFA NFA

Particle size SiO2 49?2 57?5 36?5Clay size (%) 63 03 05 Al2O3 24 33 41Silt size (%) 27 87 85 Fe2O3 5?8 4?8 4?5Fine sand (%) 10 10 10 TiO2 0?7 1?4 1?4Coeff. of uniformity, Cu — 6?3 1?4 CaO 0?4 0?5 9?00Coeff. of curvature, Cc — 1?1 0?9 MgO 0?4 0?2 3?8Specific gravity 2?71 2?18 2?64 MnO 0?2 bd# ,0?1Atterberg limits K2O 0?12 0?4 0?1Liquid Limit (%) 84 50 40 Na2O 0?1 0?2 0?4Plastic Limit (%) 25 NP$ NP$ LOI

ˆ(900uC) 18?1 1?5 3?5

Plasticity Index 59 — — Clay mineral Mont — —Shrinkage Limit (%) 8 36 38 Free Lime — — 3?2Free swell ratio (%) 65 0?75 1?2 *: CS – Clayey soil

**: BFA – Badarpur fly ash***: NFA – Neyveli fly ash$ NP – Non-plastic# bd – below detection

ˆLOI – loss on ignition

Swell pressure (kPa) 280 — —Std. Proctor Maximum dryunit weight (c5rg), kN m23

14?4 10?6 12?6

OMC (%) 28?6 38 33 OMC: Optimum moisture content

Mir and Sridharan Volume change behavior of clayey soil–fly ash mixtures

74 International Journal of Geotechnical Engineering 2014 VOL 8 NO 1

(1 divn50?002 mm), a pressure increment ratio of one wasused for subsequent load applications. Each pressureincrement was maintained normally for about 24 hoursand readings were recorded before changing the nextpressure increment (up to 800 kPa). At the end ofconsolidation, i.e., at an elapsed time of 24 hours, thepermeability test was conducted under the existingeffective pressure. The consolidation and permeabilitytests were then repeated in the same manner and carriedout at different vertical pressures varying from 12?5 to800 kPa, the readings of the dial compression (1divn.50?002 mm) were recorded for each loading combi-nation with time and the test results are presented as ‘‘e–log p curve’’. The load-settlement curves for each loadincrement are used for the determination of t90, which inturn is used for the determination of coefficient ofconsolidation Cv. The coefficients of consolidation Cv

values are also calculated by using rectangular hyperbolamethod (Sridharan and Rao, 1981). After carrying out thepermeability test at 800 kPa, the vertical pressure wasdecreased in steps, each time to one-fourth of its previousvalue, and at each pressure the vertical deformation dial

gage readings were noted for 24 hours. No permeabilitytests were conducted during unloading of the specimen.

Curing series – 7 and 28 daysFor the 7 and 28 days curing test series, samples wereprepared as described above for each series and were curedin a dessiccator at 100% relative humidity. The sampleswere removed from the dessiccator at the end of therequired curing periods and tested in a fixed ringconsolidometer. The effect of increasing the fly ash contenton the coefficient of consolidation, compression index,hydraulic conductivity, and preconsolidation pressurewere also investigated.

Results and discussions

Compressibility characteristicsThe compressibility characteristics, namely compressionindex, which is used to determine the magnitude ofsettlement and the coefficient of consolidation, Cv, whichis used to calculate the rate of settlement are determinedby a standard procedures.

Table 2 Compaction characteristics of clayey soil (CS)–fly ash (FA) mixtures

Clayey soil–Badarpur fly ash mixtures Clayey soil–Neyveli fly ash mixtures

CS soilzBFA (%)

Maximum dry unit weightand optimum moisture content

CS soilzNFA (%)

Maximum dry unit weight andoptimum moisture content

cdmax kN m23 w (%) cdmax kN m23 w (%)

100% BC soil 14?4 28?3 100% BC soil 14?4 28?920 BFA* 13?9 30?0 10 NFA* 14?1 29?540 BFA 13?6 31?1 20 NFA 13?9 29?760 BFA 12?7 33?0 40 NFA 13?7 29?980 BFA 11?8 35?4 60 NFA 13?5 30?5100 BFA 10?6 38?2 80 NFA 13?1 31?1100% BFA** 10?57 34?8 100 NFA 12?6 32?0

*20BFA520% Badarpur fly ash (BFA-by weight)z80% CS soil and so on;**8?5% of lime (CaO) was added to BFA to make it at par with Neyveli fly ash (NFA) in terms of lime.

Table 3 Experimental program

Clayey soil–Badarpur fly ash mixes Clayey soil–Neyveli fly ash mixes

RemarksClayey soil(%) (G52?71)

Badarpur fly ash,BFA (%) (G52?18) Gmix*

Clayey soil(%) (G52?71)

Neyveli fly ash,NFA (%) (G52?64) Gmix

100 0 2?71 100 0 2?71 # The additive content is definedby the ratio of the dry weight ofthe additive to the dry weight ofthe natural clayey soil expressedas a percentage.$8?5% of Lime (Ca (OH)2) byweight was added to BFA to makeit at par with NFA in terms of limecontent, (the lime content differencebetween the two fly ashes).

80 20# 2?58 90 10 2?7060 40 2?47 80 20 2?7040 60 2?37 60 40 2?6820 80 2?27 40 60 2?670 100 2?18 20 80 2?650 100$ 2?18 0 100 2?64

*For example, for clayey soil (G52?71)–Badarpur fly ash (G52?18) ratio of 80:20 for total mass, M5100 g (80 g of soilz20 g of fly ash). Thespecific gravity of this soil–fly ash mixture is calculated as: Gmix5M/(VszVf) (soil particle density of mix); Vs5Vol. of soil sample580/2?71(cc) and Vf5Vol. of fly ash520/2?18 (cc); and Gs5rs/rw, rs5Gs (rw51), V5M/rs5M/Gs, rs5M/V5soil particle density of mix, therefore,Gmix5M/(VszVf); V5(VszVf)5 total volume of mixed sample.Likewise, the specific gravity of other samples of soil–fly ash mixtures is calculated in the same manner.

Mir and Sridharan Volume change behavior of clayey soil–fly ash mixtures

International Journal of Geotechnical Engineering 2014 VOL 8 NO 1 75

Void ratio and compression index

Figure 1a and b shows the effective pressure (p) – voidratio (e) curve without any curing, commonly referred toas the ‘‘e–log p curve’’. The compressibility parameter,

namely, compression index, Cc, which is the slope of the

linear portion of ‘‘e–log p curve’’ indicates the amount of

compression undergone by the soil or soil–fly ash mixture.

Fly ash can reduce compressibility quite effectively. As the

1 a e–log p plot for clayey soil–Badarpur fly ash (BFA) mixes for immediate test series, b e–log p plot for clayey soil–

Neyveli fly ash (NFA) mixes for immediate test series, c e–log p plot for clayey soil–NFA mixes for 7 days test series, d e–

log p plot for clayey soil–NFA mixes for 28 days test series

Mir and Sridharan Volume change behavior of clayey soil–fly ash mixtures

76 International Journal of Geotechnical Engineering 2014 VOL 8 NO 1

percentage of fly ash increases, soil–fly ash mixture canresist the compression loading much better and conse-quently shows lesser compressibility.

Figure 1c and d shows the compressibility curves forcured samples for 1 week and 28 days. It is seen that curedsamples resist the external load very effectively. The

load–compression curves are much flatter. Fly ash alonegives much lesser compression.

Figure 2a–d shows the variation of compression indexwith pressure for different curing periods. Compressionindex was calculated for every pressure increment in thefollowing manner.

2 a Variation of Compression Index, Cc with pressure for Clayey soil (CS)–Badarpur fly ash (BFA) mixes for immediate test

series, b variation of Compression Index, Cc with pressure for CS–Neyveli fly ash (NFA) mixes for immediate test series, c

variation of Compression Index, Cc with pressure for CS–NFA mixes for 7 days test series, and d variation of

Compression Index, Cc with pressure for CS–NFA mixes for 28 days test series

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International Journal of Geotechnical Engineering 2014 VOL 8 NO 1 77

For each pressure increment the change in void ratiowas calculated. Then the compression index is given by‘{Change in the void ratio divided by (log p22log p1)}.This is nothing other than (de/d (log p)).

It is observed that compressibility increases withincrease in effective consolidation pressure and decreasesas the fly ash content increases. It can also be seen thatwith increase in curing time, the compressibility decreases.This is due to the cementation bonds that are formed(during the curing period) between free lime and reactivesilica and thereby improving the compressibility charac-teristics of the clayey soil.

Also, due to cation exchange reaction, an increase in theflocculation and aggregation causes a chemically inducedpreconsolidation effect, which increases the verticaleffective yield stress and reduces the compressibilitycharacteristics. The calcium ion is accepted to be aflocculating agent in soils and some cation exchangereactions occur on the addition of additives, which causethe replacement of the exchangeable sodium, magnesium(Nalbantoglu and Tuncer, 2001), or other cations pre-viously held by the soil clay by calcium cations(Abduljauwad, 1993). This is believed to produce a soilwith a more flocculated fabric and result in a reduction inthe compressibility characteristics. The compression index,Cc values vary from 0?035 to 0?4 for BC soil and 0?04–0 1for BFA and 0?03–0?07 for NFA for the immediate testseries for the pressure range between 50 and 800 kPa,respectively. For 7 and 28 days curing (Fig. 2c and d), theCc values for NFA reduces further. The general trend isthat as the curing time increases, Cc values decrease, butwith increase in pressure, these values increase. It has beenobserved that, in general (for each stress level), compres-sion index decreases with the increasing fly ash content.For instance, at the maximum stress level of 800 kPa, thecompression index of the clayey soil decreases from 0?4 toabout 0?1 with NFA content of 80%. The reduction in thecompressibility characteristics is explained by the aggrega-tion formations of soils treated with fly ash and lime,which results in stronger lime particle aggregates and giveshigher resistance to compression. With increase in fly ashand curing time, the compression index decreases indicat-ing improvement in the compressibility of the compositesample due to the formation of cementation bonds. This isbecause of the self-hardening property of NFA due to thepresence of free lime and hence more effective comparedto BFA, which is non-pozzolanic in nature.

Effect of fly ash on swell potential

The ‘‘free swell’’ testing method was used to determine theswelling potential of the test specimens (ASTM D4546-90). The swelling potential of the specimens, based on thefree swell test data was determined under the condition ofno curing, 7 days curing, and 28 days curing. The effect offly ash on the swell potential/heave of the natural andtreated clayey soil is shown in Fig. 3, which indicates thatfly ashes are effective in reducing the swell potential of thetreated clayey soils. A decrease in the swell potentialvalues was obtained with an increase in the percentage offly ash. The specimen treated with 10% NFA and 40%BFA gives a swell potential of 0?1 and 0?25%; with curing

of 7 days and in 28 days, this value of swell potentialdrops to almost zero. The decrease in swelling potentialdue to curing can be attributed to the time-dependentpozzolanic and self-hardening properties (formation ofcementitious compounds) of fly ashes. Thus, it is seen that10% of NFA (Class C fly ash) is the optimum amountrequired to minimize the swell potential compared to 40%of BFA (ASTM Class F fly ash).

Coefficient of consolidation, Cv

Coefficient of consolidation, Cv, the parameter governingthe time rate of consolidation, has been determined atdifferent percentages of fly ash for different pressureranges (50–800 kPa). Of the various methods of determin-ing coefficient of consolidation, two common curve-fittingmethods are Taylor’s method and Casagrande’s method.Both of these curve-fitting methods, however, need somejudgment and the interpretation is not free from someerrors. Therefore, this paper presents another simplermethod to determine the coefficient of consolidation,which is known as rectangular hyperbola method(Sridharan and Rao, 1981; Sridharan et al., 1987). Thismethod can be used for all types of time-settlement curvesand the interpretation of the test results based on thisapproach is simpler than other classical curve-fittingmethods.

Rectangular hyperbola method

Sridharan and Rao (1981) and Sridharan et al. (1987) haveproposed rectangular hyperbola method, which is rela-tively simple and reliable. In this method a plot of timedivided by compression versus time (t/d v. t) is used(Fig. 4) in which the straight-line portion is obtained in

3 Variation of swell potential with fly ash for clayey soil–fly

ash mixes for different curing periods

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78 International Journal of Geotechnical Engineering 2014 VOL 8 NO 1

between 60–90% consolidation. The coefficient of con-solidation is obtained as

(Cv)~0:24|m|h2

av

c(m2s{1) (1)

where, m is the slope of straight-line portion of t/d v. t plot,c is the vertical intercept of straight line, and hav is theaverage length of drainage path.

Richwein and Meyer (2003) have reported that theinterpretation of laboratory tests in this method is free ofspeculative judgment and the method gives a unique valueof Cv. The main advantage for commercial laboratories isthat the method takes a testing time as short as or evenshorter than Taylor’s method.

The time required for the end of primary consolidationand beginning of the secondary consolidation is shortenedin fly ash treated soils. The primary consolidation getsover within a very short interval of time. The test resultspresented in this paper are based on rectangular hyperbolamethod. In Fig. 5a and b for immediate test series, Cv

shows a different trend with increasing pressure for bothfly ash mixes. The effect of additive on Cv is particularlynoticeable for NFA mixes. The variation of Cv withpressure is not very definite. However at higher percen-tages of fly ash, the coefficient of consolidation isquite high. At larger axial pressures, the effect of fly ashon the coefficient of consolidation becomes less signifi-cant. From Fig. 5c and d, it is observed that there is lesservariation beyond 7 days of curing period, which is ofvital importance for field engineers. Pandian andBalasubramonian (1999) determined the values of Cv for

two Indian fly ashes using the Casagrande method, Taylor’smethod, and the rectangular hyperbola method. There weresubstantial differences in the values of Cv determined by thethree methods, with the Taylor’s method giving highervalue than that given by the rectangular hyperbola method.The high rate of consolidation of coal ashes is advantageousduring its bulk utilization in embankments and in reclama-tion of fills, as the primary consolidation will be practicallyover by the end of construction period itself.

Permeability test resultsPermeability is an important parameter in designing theliners to contain leachate migration. The consolidometer–permeameter system (fixed ring) offers the best means forquantitatively assessing the coefficient of permeability ofclays/ashes under confined state. The samples are preparedas ASTM D421 and compacted at 0?95cdmax andcorresponding water content on the dry side of optimum.After placing the oedometer cell in position, the samplesare saturated with water under a surcharge of 6?25 kPa.Water is allowed to flow upwards through the samplesfrom the bottom. The time periods required for fullsaturation of samples was well within 24 hours. Aftersaturation and change of next increment of pressure, thepermeability test was conducted by the falling headmethod and the relation below gives the coefficient ofpermeability

k~2:303�aL

Atlog10

h1

h2

(2)

where, k5coefficient of permeability (m s21); a5crosssectional area of the burette (m2); A5cross sec. area of thesoil sample (m2); t5time for the head drop from h1 to h2

(sec); h15initial height of the fluid in the pipe (m); h25finalheight of the fluid in the pipe (m); and L5sample heightfor corresponding load increment (m).

There are substantial differences in the values ofcoefficient of permeability determined from consolidationdata with the Taylor’s method giving much higher valuethan that given by the rectangular hyperbola method. Thecoefficient of permeability by falling head method lies in-between the values given by these two methods. The largevariation between the measured values of k from the fallinghead method and the back-calculated values was perhapsdue to errors in the determination of Cv by the conventionalmethods. This has also been reported by Porbaha et al.(2000). Therefore, it is concluded that the coefficient ofpermeability be determined directly rather than backcalculated from consolidation test results to obviate theseinaccuracies. The test results in the form of variation ofpermeability with pressure for some soil–fly ash mixes fordifferent curing periods are plotted in Fig. 6a–d. The valuesof k vary from 1?3561027–6?761029 m s21 for clayey soil,5?1161026–4?1061026 m s21 for BFA and 2?461028–1?761028 m s21 for NFA for the pressure range of 50–800 kPa under immediate test series (Fig. 6a and b). Thevariation of permeability with pressure for different curingperiods is shown in Fig. 6c and d. The values of k for flyashes are typically in the range of the coefficient ofpermeability of non-plastic silts. Therefore, clay-likeadmixtures should be added to the fly ash to reduce its

4 Typical plot for the determination of Cv by rectangular

hyperbola method for clayey soil

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International Journal of Geotechnical Engineering 2014 VOL 8 NO 1 79

permeability. Test results reveal that at the same effectivevertical stress, the treated soil specimen has higherpermeability than the untreated clay. This implies that atthe same depth below the ground surface, the treated clay

will show a higher void ratio than the untreated soilspecimen. This has been verified from the e–log p relation oftreated and untreated clay. Similar work has also beenreported by Broderic and Daniel (1990) and Locat et al.

5 a–b Variation of Cv with pressure for clayey soil–fly ash mixtures for immediate test series, c–d variation of Cv with pres-

sure for clayey soil–fly ash mixtures for different curing periods (by RHM)

Mir and Sridharan Volume change behavior of clayey soil–fly ash mixtures

80 International Journal of Geotechnical Engineering 2014 VOL 8 NO 1

(1996). At lower consolidation pressure, BFA exhibitshigher permeability values. At higher consolidation pres-sures, the order of permeability is almost same for both thefly ashes. This is because the reduction in the pore spaceavailable for flow for BFA is more compared to NFAwith increasing consolidation pressures. The appreciable

decrease in permeability with increase in pressure in thecase of BFA is due to the higher decrease in void ratioexperienced by it (compared to NFA). The cementationcaused by free lime in NFA resists volume decrease andhence NFA experiences a comparatively lower decrease invoid ratio than BFA.

6 a–b Variation of permeability with pressure for clayey soil–fly ash mixes for immediate test series, c–d variation of perme-

ability with pressure for clayey soil–fly ash mixes for 7 and 28 days test series

Mir and Sridharan Volume change behavior of clayey soil–fly ash mixtures

International Journal of Geotechnical Engineering 2014 VOL 8 NO 1 81

ConclusionFrom the study of volume change behavior of soil–fly ashmixes, it can be concluded that:

1. Compression index appreciably decreases with theaddition of fly ash indicating improvement in compressi-bility of the composite sample due to the formation ofcementitious bonds. It is seen that 20% high-calcium flyash content is the optimum quantity to improve thecompressibility characteristics of clayey soil cured for7 days against 60% for immediate tests.

2. Addition of fly ash to clayey soils significantlyreduces their swelling due to reduction of plastic fines ofclay by non-plastic fines of fly ash. Swell potentialdecreases significantly as percentage of fly ash increases.The use of NFA (class C) even in small percentageproduces significant changes (i.e., 10% class C fly ash ismore effective for reducing swelling characteristics com-pared to 40% of class F fly ash).

3. Neyveli fly ash (Class C) has higher void ratio thanBFA (Class F) for a given effective consolidation pressure.However, for a given void ratio, the permeability of NFAis less compared to that for BFA. This is because of theless effective void space available for the flow of water.The reduction in the effective void space is due to thepresence of free lime, which causes cementation in NFA.

4. Coefficient of consolidation of fly ash treated claysincreases with increase in percent fly ash. However, atlarger axial pressures, the effect of percentage of fly ash onthe coefficient of consolidation becomes less significant.The high rate of consolidation of fly ashes is favorable forits use as embankment and reclamation fills and otherapplications. Rectangular hyperbola method is suitablefor finding the coefficient of consolidation.

5. With increase in percent fly ash, void ratio andpermeability of the composite samples increase. Thisindicates that the addition of fly ash to fine-grained soilsmakes it granular leading to higher coefficient of perme-ability. The plasticity of fine-grained soils is reduced andworkability increased.

6. The secondary cementitious products appear to bedeposited on or near the surfaces of the clay clusters,which gives rise to a reduction in entrance pore diameterwith increase in particle size leading to reduced perme-ability over time as indicated in the permeability of 7 daysand 28 days of curing.

7. The coefficient of permeability is to be determineddirectly from the falling head permeability test and notback calculated from consolidation test results. Likewise,Cv can be calculated from the equation:

8. Cv5k/(mvcw), where ‘‘k’’ is measured value frompermeability tests.

9. It is also observed that the addition of 8?5% lime doesnot improve the behavior of BFA (class F fly ash) muchbecause of non-availability of reactive silica.

10. From this study it is also concluded that not onlycan problematic soils be easily stabilized by bulk utiliza-tion of fly ash but fly ash can also be stabilized by addingclay-like admixtures to reduce its permeability on needbasis.

11. Recycling/utilization of fly ash has the advantage ofusing an industrial waste by-product without adverselyaffecting the environment or potential land use. Inaddition, fly ash proves to be an effective admixture forimproving the soil engineering behavior considerably.

Acknowledgement

The investigation reported in this paper forms a part of theresearch at the Indian Institute of Science by the firstauthor. The support and assistance given by the Instituteis gratefully acknowledged.

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