Feasibility of Lime Stabilized Expansive Soil as a Subbase Material for Flexible Pavements

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1 Feasibility of Lime Stabilized Expansive Soil as a Subbase Material for Flexible Pavements S. Patel 1, L. M. Reddy 2 And P. M. Chaudhary 3 Abstract A large portion of India roughly equal to 0.8 million sq. km, which is about 20% of the total land area, is covered with expansive soil popularly known as Black Cotton (BC) soil. This soil is considered problematic for road construction due to the presence of montmorillonite in its mineralogy which is capable of large volume changes from the dry to the saturated state. In this paper, an attempt has been made to stabilize the problematic expansive Black Cotton soil with lime for the effective utilization in the subbase course of flexible pavements. A number of cylindrical test specimens were prepared from BC soil with different lime (0-12%) contents and cured for 0, 7, 14 and 28 days. The geotechnical properties of different trial mixes, namely, unconfined compressive strength and resilient modulus were determined. The effects of lime content, curing period and curing temperature on unconfined compressive strength and resilient modulus were investigated. Different empirical models are proposed to estimate the resilient modulus of soil-lime mixes and model constants are determined. BC soil stabilized with a minimum lime content of 6% satisfies the criteria recommended by Indian Road Congress for utilization in subbase layer of flexible pavements. Keywords Repeated Load Triaxial Test, Resilient Modulus, Unconfined Compressive Strength. I I. INTRODUCTION NDIA, being called as sub-continent has various geographical, geological and climatic conditions. In developing countries the socio-economic development depends upon the connectivity between the different parts of the country. India is enriched with different types of soils in which more part of the country is covered with black cotton soil which is good for agricultural purpose and in contrary, problematic for construction purpose. Lime stabilization of black cotton soils waved back to several centuries in India and is a conventional method in so many countries. Since the pavement structure is subjected to traffic loading it is necessary to consider the cyclic loading effect in the design of the pavements. AASHTO [2] has recommended the use of resilient modulus in the design of pavements. Resilient modulus (M r ) is defined as the ratio of deviator stress to recoverable strain. Many organizations in the world S. Patel, Assistant Professor, Applied Mechanics Department, S. V. National Institute of Technology, Surat, India , Ph , spatel@amd.svnit.ac.in L. M. Reddy, Former Master Student, Applied Mechanics Department, S. V. National Institute of Technology, Surat, manoharreddy04@gmail.com P. M. CHAUDHARY, Superintending Engineer, Road & Building Division, Surat, pravin6765@yahoo.co.in. do not have the necessary facilities to conduct the resilient modulus test. Therefore, state agencies and pavement designers use available empirical models to estimate resilient modulus of the pavement materials. Various engineering properties of lime stabilized soil have been reported in the literature [3], [4], [7], [9] and [10]. However, a limited research has been reported in the literature on the behavior of stabilized subgrade under cyclic loading. Singh et al. [12] reported an increase in resilient modulus value with the addition of lime and class C fly ash to CL soil cured for 28 days. Solanki et al. [13] and [14] observed an increase in resilient modulus of subgrade soils for each of three different additives, such as hydrated lime, class C fly ash and cement kiln dust. Reduction in resilient modulus beyond certain amount of lime content was observed in the literature [14]. Ranjan et al. [11] observed an increase in resilient modulus on addition of lime and cement to three different types of subgrade soils. With the increase in deviator stress, resilient modulus of untreated soil decreased and resilient modulus of treated soil increased [11]. The objective of the present study is to determine the effect of lime content, curing period and curing temperature on unconfined compressive strength and resilient modulus of different soil-lime mixes and to compare different empirical models for predicting the resilient modulus of soil-lime mixes. The scope of the work includes conducting a series of unconfined compressive strength test and resilient modulus test on cylindrical specimens and carrying out regression analysis on different empirical models. II. EXPERIMENTAL PROGRAM The experimental program consists of determination of unconfined compressive strength and resilient modulus of different soil-lime mixes. A. Materials Black cotton soil was collected from SVNIT campus, Surat, India and lime was procured from the local market. The different geotechnical properties of soil as obtained according to Indian Standards were as follows: sand content = 17.5%, silt content = 46.5%, clay content = 36%, specific gravity = 2.7, liquid limit = 60.3%, plastic limit = 27.2%, plasticity index = 33.1%, free swell index = 66.7%, optimum moisture content (OMC) = 20% and maximum dry density (MDD) = 1.75 g/cc. The soil was classified as high plastic clay (CH) as per Indian Standard. B. Unconfined Compressive Strength Test Unconfined compressive strength test was carried out on the 52

2 soil mixed with different percentages of limes (3%, 6%, 9%, and 12%). First, soil and lime were mixed in dry state manually and then water corresponding to the OMC was added and mixed thoroughly. Next cylindrical specimens of 50 mm diameter and mm high were compacted in three layers by static press to achieve the MDD of the mix obtained from modified Proctor compaction test. Samples were sealed in polythene bags and kept for curing. Specimens were cured in three different series for UCS test. In first series, specimens were cured at a temperature of 30ºC for 0, 7, 14, 28 days. In second series specimens were cured at temperatures of 40ºC and 50ºC for 7 days. In last series specimens were cured for 3 days at 30ºC and then soaked in water for 4 days before UCS test. UCS test was carried out by using a conventional compression testing machine at a strain rate of 0.6 mm/min Day 7 Days 14 Days 28 Days 0.5 Lime(%) Fig. 1 Variation of UCS with lime content for 30ºC curing temperature. C. Repeated Load Triaxial Test Repeated load triaxial test (RLTT) was carried out on different soil-lime mixes. Cylindrical specimens of 50mm diameter and mm high were prepared in three different series same as that of UCS test. The test was carried out in accordance with AASTHO T 307 [1] method. First, the specimens were subjected to 3000 loading cycles during the conditioning phase and then tested for the determination of resilient modulus at fifteen different stress levels. III. RESULTS AND DISCUSSIONS A. Unconfined Compressive Strength 1) Effect of lime content Figure 1 shows the variation of UCS with lime content. There is an increase in UCS values with increase in lime content up to certain extent and decrease thereafter. The lime content at which the compressive strength was maximum varies from 6% for 7 day curing to 9% for 28 day curing period. The strength gain in BC soil-lime mix is mainly due to the cementitious reaction which immediately begins by addition of lime in clay. The calcium ions of lime react with the silica and alumina present in the soil and form CaO SiO 2 H 2 O and CaO Al 2 O 3 H 2 O known as C-S-H and C-A-H gel. These products act as a glue to bind the soil particles together resulting in a stabilized mass. With an increase in lime content, the quantity of gel formation increases, thus increasing the compressive strength. However, for lime content more than the optimum value, the lime particles remain unutilized and simply serve as weak filler in the mix resulting in the reduction of the strength. Similar variations of UCS for lime treated soil have been reported in literature [8]. 2) Effect of curing period Figure 2 shows the effect of curing period on UCS of different soil-lime mixes. UCS value increases continuously with curing period for all mixes; however the rate of gain of strength is high during the initial 7 days but slows down thereafter. As the pozzolanic reaction is a slow process the quantity of gel formation increases with an increase in curing period, thus resulting in high compressive strength Curing period (days) Fig. 2 Variation of UCS with curing period for 30ºC curing temperature 3) Effect of curing temperature and soaking Figure 3 shows the effect of curing temperature and soaking on UCS values for different soil-lime mixes. With the increase in curing temperature UCS value increases for all lime contents. Higher curing temperature accelerates the cementitious reaction between soil and lime, thus increasing the strength. The soaked UCS values of the soil-lime mixes were found to be lower than the unsoaked UCS values. UCS values of different soil lime mixes at 7 day curing period before and after the cyclic tests were compared in Fig. 3. For third series of UCS tests, 7-day (3 days curing and 4 days soaking in water) UCS values of BC soil with 3%, 6%, 9% and 12% lime were 360, 1480, 1440 and 1310 kpa, respectively. IRC: 51 [6] recommends a minimum 7-day UCS value of 700 kpa for soil-lime mix to satisfy the strength and durability criteria for use in subbase course of flexible pavements. BC soil stabilized with a minimum lime content of 6% satisfies these criteria. UCS values obtained after cyclic test was higher than that obtained before cyclic test which may be due to the rearrangement and densification of soil particles during the application of cyclic loads. Similar trends were also observed for 14 and 28 days curing period. The variations of UCS values with curing temperature obtained in this study are consistent with the results reported in the literature [8]. 53

3 Fig. 3 Variation of UCS with lime content for different curing temperatures at 7 day curing period. B. Resilient Modulus 1) Effect of lime content Figure 4 shows the variation of resilient modulus with lime content for different curing periods at a deviator stress (σ d ) of 93.1 kpa and cell pressure (σ 3 ) of 34.5 kpa. There is an increase in resilient modulus value with increase in lime content up to certain extent (9% lime content) for all curing periods and decreases thereafter. For lime content more than the optimum values the lime particles remain unutilized and serve as weak filler resulting in the reduction of resilient modulus. Similar variations of the resilient modulus with lime content were reported in literature [5], [12], [13], and [14]. 2) Effect of curing period Variation of the resilient modulus with curing period for different soil-lime mixes at a deviator stress (σ d ) of 93.1 kpa and cell pressure (σ 3 ) of 34.5 kpa is shown in Fig 5. The resilient modulus of all the soil-lime mixes increases continuously with curing period. However, the rate of strength gain is high during the initial 7 day curing and slows down thereafter. The amount of gel formation in the pozzolanic reaction increases with increase in curing period which binds the soil particles more efficiently resulting in higher stiffness C soaked 0 Day 7 Days 14 Days 28 Days Before RLTT After RLTT 80 Fig. 4 Variation of resilient modulus with lime content for different curing period Curing period (days) Fig. 5 Variation of resilient modulus with curing period for different lime content C soaked σ d = 93.1 kpa σ 3 = 34.5 kpa 80 Fig. 6 Variation of resilient modulus with lime content for different curing temperature. 3) Effect of temperature Figure 6 shows the variation of resilient modulus with lime content for different curing temperatures and soaking condition. There is an increase in resilient modulus with increase in temperature due to the accelerated pozzolanic reaction at higher temperature which shows that the lime stabilization is more effective in tropical countries where temperature is high. Resilient modulus of the soaked specimens were found to be higher than that of the unsoaked specimens except for 3% lime content. This may be due to the development of pore water pressure during the cyclic test as the specimens were tested under undrained condition. The presence of pore water increases the resistance to the deformation resulting in higher stiffness. 4) Effect of major principal stress Variation of resilient modulus with major principal stress (σ 1 ) for different soil-lime mixes are shown in Fig. 7 and Fig. 8. There is an increase in resilient modulus with increase in stress value for all lime content and curing period. Similar trend was also observed for other lime contents and curing periods. 54

4 Resilient moduls (MPa) Days Major principal stress (kpa) Fig. 6 Variation of resilient modulus with major principal stress for different lime content. Resilient Modulus (MPa) 200 9% Lime 0 day 7 days 14 days 28 days Major principal stress (kpa) Fig. 7 Variation of resilient modulus with major principal stress for different curing period. Several constitutive stress based models for the estimation of M r are available in the literature. In this study the fitness of four stress based models for the determination of M r for soillime mixes are compared in Table 1. The residual sum of squares (rss) and the coefficient of determination (R 2 ) obtained from the non-linear regression analysis were used to compare the goodness of fit for the four models. The regression constants obtained were used for determining the predicted resilient modulus of the mixes. A graph was drawn between measured resilient modulus and predicted resilient modulus, and the corresponding R 2 and rss values were determined. Comparison between the four models shows that the three parameter model provided a best fit regression equation for the determination of resilient modulus of lime stabilized soil. TABLE I STRESS BASED MODELS WITH R 2 AND RSS VALUES Model R 2 rss =k = k = k P a is atmospheric pressure equals to kpa k 1, k 2, k 3, k 4, k 5, k 6, k 7, k 8 are model constants Model constants (k 1, k 2 and k 3 ) of the three parameter model obtained for different soil-lime mixes were given in Table 2. The value of k 1 increases with an increase in lime content upto 9% and decreases thereafter for all curing periods and curing temperatures. However, no consistent trends have been observed for values of k 2 and k 3. Figure 9 shows the graph between predicted M r using three parameter model and actual M r of all the soil-lime mixes for fifteen different stress levels. Predicted Resilient Modulus (MPa) Actual Resilient Modulus (MPa) Fig. 8 Predicted M r versus actual M r for three parameter model. TABLE II MODEL CONSTANTS OF THREE PARAMETER MODEL FOR DIFFERENT LIME CONTENT AND CURING PERIOD Curing Period k 1 k 2 k 3 M r p 0 day 7 days 14 days 28 days 7 days at 7 days at 7 * days M r a M r p is predicted resilient modulus in MPa and M r a is actual resilient modulus in MPa at σ 3 = 34.5 kpa and σ d = 93.1 kpa; *3 day curing at 30 0 C and 4 day soaking in water. 55

5 IV. CONCLUSIONS Engineering properties such as unconfined compressive strength and resilient modulus of soil-lime mixes were investigated and the following conclusions were drawn: UCS values increases with increase in lime content up to certain extent and decreases thereafter. UCS value was maximum at 6% and 9% lime content for 7 days and 28 days curing period, respectively. The rate of gain of strength is high during the initial seven days but slows down thereafter. Higher the curing temperature, higher is the UCS value obtained. It was observed that soaking of the specimens in water reduces its compressive strength for 7 day curing period. BC soil stabilized with a minimum lime content of 6% satisfied the strength criteria for use in subbase course as recommended by IRC 51. Resilient modulus of the soil-lime mixes increases with the increase in lime content upto 9% and decreases thereafter for all curing periods and curing temperatures. Mr value increases continuously with increase in curing period and curing temperatures. Resilient modulus of the soaked specimens were observed to be higher than that of the unsoaked specimens for 6, 9 and 12% lime contents The performance of four stress based models were compared and observed that the three parameter model outperformed the other models with high coefficient of determination values providing good fit model constants for the determination of resilient modulus. class C fly ash, Geoflorida 2010, ASCE, pp [13] P. Solanki, M. M. Zaman, and D. Jeff, Resilient modulus of clay subgrade stabilized with lime, class C fly ash, cement kiln dust for pavement design, J. of Transportation Research Board, 2010, pp [14] P. Solanki, N. Khoury, and M. M. Zaman, Engineering properties and moisture susceptibility of silty clay stabilized with lime, class C fly ash, cement kiln dust, J. of Materials in civil Engg., ASCE, Vol.12, Dec. 2009, pp REFERENCES [1] AASHTO T 307, Determining the resilient modulus of soils and aggregate materials, [2] AASTHO, Guide for design of pavement structures, [3] S. Chakkrit, J. P. Anand, C. Vivek, S. Sireesk, and R. H. Laureano, Combined lime and cement treatment of expansive soils with low to medium soluble sulfate levels, Geocongress 2008, ASCE, pp [4] S. Z. George, D. A. Ponnaih, and J. A. Little, Effect of temperature on soil-lime stabilization, J. of Construction and Building Materials, Vol. 6 No. 4, 1992, pp [5] Z. Hossain, M. Zaman, C. Doiron, and P. Solanki, Evaluation of mechanistic empirical design guide input parameters for resilient modulus of stabilized subgrade soils, ICSDEC 2012, pp [6] Indian Road Congress 51, Guidelines for the use of soil-lime mixes in road construction, [7] A. Muhamed, and D. Wanatowski, Effect of lime stabilisation on the strength and micro structure of clay, IOSR J. of Mechanical and Civil Engg., Vol. 6, June 2013, pp [8] A. Nasrizar, K. Ilamparuthi, and M. Muttharam, Quantitative models for strength of lime treated expansive soils, Geocongress 2012, ASCE, pp [9] K. C. Pranay, K. V. Sai, J. P. Anand, and H. Laureano, Evaluation of strength, resilient moduli, swell and shrinkage characteristics of four chemically treated sulfate soils from north Texas, GSP 136 Innovations in Grouting and soil improvement, ASCE [10] K. D. Radha, P. D. Vincent, D. Kim, and R. Chen, Assessing the quality of soils modified with lime kiln dust measuring electrical conductivity with time domain reflectometry, J. of Transportation Research Board, 2006, pp [11] K. R. Ranjan, R. Pinit, V. Shashank, and J. P. Anand, Resilient moduli behavior of lime cement treated subgrade soils, Geocongress 2012, ASCE, pp [12] D. Singh, G. Rouzbeh, G. L. Joakim, and M. Zaman, Laboratory performance evaluation of stabilized sulfate containing with lime and 56