resilient modulus of black cotton soil

Accepted Manuscript Resilient modulus of black cotton soil K.H Mamatha, S.V Dinesh PII: DOI: Reference: S1996-6814(16)30131-6 http://dx.doi.org/10.1016/j.ijprt.2017.01.008 IJPRT 72 To appear in: International Journal of Pavement Research and Technology Received Date: Revised Date: Accepted Date: 28 June 2016 25 January 2017 28 January 2017 Please cite this article as: K.H Mamatha, S.V Dinesh, Resilient modulus of black cotton soil, International Journal of Pavement Research and Technology (2017), doi: http://dx.doi.org/10.1016/j.ijprt.2017.01.008 This is a PDF file of an unedited manuscript that has been accepted for publication As a service to our customers we are providing this early version of the manuscript The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain RESILIENT MODULUS OF BLACK COTTON SOIL Mamatha K H1 and Dinesh S V2 Research Scholar, Department of Civil Engineering, Siddaganga Institute of Technology, Tumkur, Karnataka, India, 572103 Professor, Department of Civil Engineering, Siddaganga Institute of Technology, Tumkur, Karnataka, India, 572103 Tel: +91-9449852695; dineshsv2004@gmail.com Corresponding Author RESILIENT MODULUS OF BLACK COTTON SOIL Mamatha K H1 and Dinesh S V2 Introduction Flexible pavement design is based on dimensioning multi-layered system where layer thicknesses vary depending on the subgrade load carrying capacity An excessive plastic and repeated elastic deformation in a pavement leads to cracking of the structure The resilient properties of the pavement components influence the short term deformations of the pavement structure Resilient modulus is a measure of the elastic modulus of the material at a given stress state Therefore, resilient modulus of subgrade soil is one of the key material property that is required for the mechanistic-empirical (M-E) design/analysis of multi-layered flexible pavement system The use of the resilient modulus for pavement design is recommended by the AASHTO [13] to replace bearing capacity parameters such as CBR and SSV The 1993 AASHTO Guide for Design of Pavement Structures [2] describes four different approaches i.e., laboratory testing, back calculation using non-destructive testing, estimation of MR from correlations with other properties and estimation of MR from original design and construction data for the determination of design resilient modulus value The factors that influence the resilient modulus of subgrade soils include physical condition of the soil (i.e., moisture content and unit weight), stress level and soil type Many studies have been conducted to investigate these effects on the resilient modulus The resilient modulus of soils is not a constant stiffness property but depends on stress state, which includes the deviator and confining stress, soil type and its structure [4], soil gradation, compaction method, specimen size and testing procedure [5] The effect of some of these factors on the resilient modulus of subgrade soils is significant Research studies showed that the resilient modulus of subgrade soil decreases with an increase of the moisture content or the degree of saturation [6-10] Unsaturated cohesive soils showed that the resilient modulus decreases with the increase in moisture content and pore pressure build-up [9] The resilient modulus increases with an increase in the dry unit weight of the soil [8, 11-13] However, this effect is small compared to the effect of moisture content and stress level on resilient modulus [14] In general, the increase in the deviator stress results in decrease of the resilient modulus of cohesive soils due to the softening effect [14] Several models [15 – 32] have been proposed for the prediction of resilient modulus of soils based on soil physical characteristics and stress state But, these models are region specific and there is need to verify these models for prediction of MR of local soils There are limited studies on the prediction of MR values of stabilized soils [33, 34] These studies have reported the prediction model for resilient modulus in terms of unconfined compressive strength (q u) [33] and back calculated falling weight Deflectometer (FWD) modulus in terms of qu [34] The correlation developed by Thompson [33] is recommended to determine the design resilient modulus for lime stabilized subgrade [3, 35] But, it is reported that the MR values predicted from the correlations developed by Thompson [33] and Little et al [34] demonstrated the lack of clear relationship between MR and q u [36] and these relations have a severe limitation as they not take into account the stress state The literature demonstrates that the clayey soils can be effectively altered with lime stabilization Lime stabilization reduces plasticity, swell potential and improves strength and stiffness of the soil [37-45] Cation exchange and flocculation/agglomeration reactions takes place relatively rapidly and produces quick changes in plasticity, workability and engineering properties [39] The cementation is mostly by pozzolonic reaction and can significantly improve the long term performance of the lime stabilized soils [46, 47] Black cotton soils are formed by the weathering of Deccan lava in the major parts of India The black cotton soils are inorganic clays characterized by high plasticity, higher fraction of fines, low strength, high compressibility and are expansive in nature These soils show very high swell – shrink behaviour due to moisture variations which makes them unsuitable for foundations, subgrades etc There are many failures of road bases, foundations, canal slopes founded on such expansive soils BC soils are formed over large geographical areas and replacement of such soils locally will not be cost effective Therefore, such soils have to be stabilized and MR values are to be determined to develop empirical correlations for the prediction of MR values for the pavement design This paper reports the results of resilient modulus of black cotton soil at relative compaction levels of 95% and 100% under both standard and modified proctor conditions An attempt is made to explore the effectiveness of lime for stabilization for improving the resilient modulus of expansive black cotton soil subgrade, verify the suitability of existing models for prediction of MR and development of a new model for the prediction of MR of lime stabilized BC soil Materials and Methods 2.1 Materials Black cotton soil which is widely available in several parts of Karnataka state, India is considered for the present investigation Black cotton soil was collected from Bagalkot, Karnataka, India and tested for its engineering properties All the tests were carried out as per relevant Indian standard guidelines and Table shows the engineering properties of the selected soil The soil consists of 10% sand, 36% silt and 54% clay and figure shows the grain size distribution curve The soil is classified as A-7-C as per HRB classification system and A-7-6 AASHTO classification system [48] and highly compressible clay with the group symbol CH as per IS classification system and unified soil classification system [49] The liquid limit and plasticity index of the soil are 71% and 48% respectively The BC soil has a free swell of 34% and the soil is classified as low swelling clayey soil [50] The soaked CBR is less than 2% under modified proctor conditions The unconfined strength is 89kPa under unsoaked condition and the soil showed collapse behaviour when it is subjected to soaking As per MoRT&H [51] guidelines the soil having liquid limit greater than 71% and plasticity index greater than 45% respectively is unsuitable for subgrade In addition, the MoRT&H [51] guidelines specifies minimum dry unit weight of 18kN/m3 for compacted subgrade IRC:SP:72-2007 [52] specifies the use of unit weight corresponding to standard proctor condition for low volume roads The selected soil fails to meet the MoRT&H criteria (i.e., liquid limit, plasticity index and unit weight requirements) to be used as subgrade for low volume roads (village roads) The selected soil shows reasonably good strength under unsoaked condition but under soaked conditions strength is low In view of the above, the BC soil considered for the present study is a problematic soil Therefore, it is necessary to improve the strength of soil by adopting any of the available strength improvement techniques In the present case, additive stabilization is considered and industrial lime is selected for stabilization Tables and show the physico-chemical properties of black cotton soil and industrial lime (quick lime) respectively The specific surface area of the selected soil is 300m2/gm and cation exchange capacity is 49.35milli equivalence per 100gm The higher specific surface area leads to higher reaction capacity of the soil during hydration and pozzolonic reaction and this justifies the selection of lime as additive for stabilization 2.2 Methods 2.2.1 Sample Preparation For preparing untreated sample, calculated quantity of oven dried soil was mixed with calculated volume of water and mixed thoroughly to get a homogeneous soil mass In the preparation of lime treated soil specimens, it was observed that when dry lime powder was added to the soil, it absorbed water present in the soil and there was a noticeable change in the consistency of the soil lime mixture This will interfere with the role of water content in soil stabilization In the field application, lime is added in the form of slurry in the jet grouting method By trial and error, it was found that by using water content equal to 100% by weight of lime, not much change was observed in the consistency of the specimen Therefore, additional water content equal to 100% by weight of lime was provided to prepare lime treated specimens The samples were prepared at standard and modified proctor conditions by static compaction The untreated samples were tested immediately after compaction and the lime treated samples were cured for 7, 14 and 28 days in a desiccator at 100 percent relative humidity at a temperature of 23°C [53] in a temperature controlled chamber so that reaction between soil particles and lime is continued In case of unsoaked condition, the samples were tested immediately after curing, whereas under soaked condition the samples were soaked for one day after curing For soaking, the samples were covered by a membrane with the filter paper and porous stones kept at top and bottom and then immersed in a water bath where the height is maintained below the top surface of compacted soil sample such that water enters through porous stones from bottom by capillary action The soaked samples were kept in air for drying for about one hour and then testing was carried out The unconfined compression tests were carried out on samples prepared at modified proctor condition The repeated load triaxial tests were carried out on samples prepared at standard and modified proctor conditions Also, the samples prepared at 95% of dry and wet sides of optimum density for both standard and modified proctor conditions were considered for the repeated load tests 2.2.2 Repeated Load Test Untreated and lime treated black cotton soil samples of 50mm diameter and 100mm [54] height were prepared at the desired moisture contents and dry unit weights for determining resilient modulus A lime content of less than 2% is not sufficient to improve the strength of the soil and therefore, for preparing treated samples, lime contents of 2, 2.25, 2.5, 2.75 and 3% were considered for the experimental study The dosage of lime content was fixed from the consideration of development of a minimum unconfined compressive strength value of 420kPa [55] for subgrade applications A series of repetitive load tests were conducted on both lime treated and untreated samples Untreated black cotton soil samples were tested under both unsoaked and soaked conditions and lime treated samples were tested under soaked condition A repeated axial cyclic stress of fixed magnitude with a load duration of 0.1 second, followed by a 0.9 second rest period was applied to cylindrical test specimen Load and rest period together constitutes one loading cycle (1 sec) which amounts to Hz frequency The stress pulse shape was haversine in nature The repeated load tests were performed at the confining pressure and deviator stress levels recommended by the AASHTO T-307-99 [56] The sample in the repeated load test was subjected to a combination of three confining pressures and five deviator stresses Each combination is applied in 100 cycles after preconditioning of 500 cycles The total resilient or recoverable axial deformation response of the specimens were measured and used to calculate the resilient modulus The last five cycles in each combination of confining pressure and deviator stress were considered to calculate resilient modulus and then the mean resilient modulus was determined and reported This yields 15 resilient modulus values for each sample for different stress state The tests were terminated when the total vertical permanent strain exceeds 5% [56] Results and Discussions 3.1 Lime Fixation Lime rapidly modifies the clay fraction of the material involving ion exchange and flocculation when sufficient stabilizer is available, continues with the development of hydrated calcium and alumina silicates and eventual cementation Cementation usually takes longer than modification and will continue provided clay, moisture, and a pH in excess of about 12.0 is available During this process, the clay mineral structure is broken down and forms colloidal gels of calcium aluminate and silicate hydrates which have cementing properties similar to those of portland cement The lime – soil proportion requirement of soil stabilization was carried out as per ASTM D 6276 – 99a [57] The lime dosage was varied from 1% to 10% in an increment of 0.5% It was observed that a lime content of 2% yielded a stable pH of 12.6 Thus, lime contents of 2% to 5% with an increment of 1% were considered for the determination of consistency limits, compaction characteristics and unconfined compressive strength of lime stabilized black cotton soil 3.2 Consistency Limits The effect of lime stabilization on consistency limits was evaluated for lime stabilized soil with lime content varying from to 5% Figure shows the variation of consistency limits with lime content It is observed that the addition of lime to the soil reduced liquid limit and plasticity index The plastic limit was found to increase with the lime content With a lime content of 2%, the liquid limit and plasticity index were reduced to 64% and 38% respectively from 71% and 48% making the soil suitable for subgrade application [51] With 5% of lime, the liquid limit reduced from 71% to 58% and plasticity index reduced from 48% to 28% The reduced plasticity index is attributed to flocculation and agglomeration that occurs with the addition of lime to the soil At this stage, the calcium ions from lime gets attracted to the surface of clay particle and displace water and other ions imparting improved workability and reduced plasticity index of soil 3.3 Compaction Characteristics The compaction characteristics of lime stabilized black cotton soil was investigated under both standard and modified proctor conditions The soil was mixed with lime paste (as detailed in section 2.2.1) and mixed uniformly to get a homogeneous mixture Water was then added to the soil – lime mixture and mixed thoroughly The uniform mix thus obtained was filled into the compaction mould followed by compaction in accordance with ASTM D 698 – 07 [58] and ASTM D 1557-09 [59] Figures and shows the compaction curves for the lime stabilized black cotton soil under standard and modified proctor conditions respectively It is observed that the addition of lower lime contents (2 to 3%) has no significant effect on the maximum dry unit weight and optimum moisture content As the lime content increases (>3%), the maximum dry unit weight was found to reduce and optimum moisture content increased slightly The addition of higher percentages of lime to the BC soil results in rapid cation exchange phenomenon which ultimately results in soil-lime interaction causing the soil particles to possess flocculated structure resulting in lower dry unit weight This is due to the resistance offered by the flocculated structure to the impact applied during compaction The flocculated structure of the soil requires additional amount of water to fill the voids resulting in increased water contents compared to untreated soil [40, 60, 61] With a lime content of 5%, the maximum dry unit weight is reduced from 14.6kN/m3 to 13.6kN/m3 and optimum moisture content increased from 24% to 28% under standard proctor condition Similarly, the maximum dry unit weight reduced from 16.8kN/m3 to 15.9kN/m3and the optimum moisture content increased from 19% to 22% under modified proctor condition 3.4 Unconfined Compressive Strength Test A series of unconfined compressive strength tests were carried out to study the strength behaviour and to obtain the optimum lime content corresponding to standard proctor condition to achieve a reasonable strength of 420kPa which is the minimum strength requirement for subgrade as per NCDOT [55] Untreated and lime treated black cotton soil samples of 38mm diameter and 76mm height were prepared at the modified proctor condition as detailed in section 2.2.1 for determining unconfined compression strength The test specimens were prepared at the respective maximum dry unit weight and optimum moisture contents as obtained from figure The specimens were tested in accordance with ASTM D 2166 – 13 [62] and ASTM D 5102 – 09 [53] respectively for untreated and lime treated conditions The soil was treated with 2%, 3%, 4% and 5% lime and cured for 3, 7, 14 and 28 days All the prepared samples were tested under soaked condition and soaking was performed as detailed in section 2.2.1 Figure shows the variation of unconfined compressive strength with curing period and lime content It is evident that the addition of lime to black cotton soil shows improvement in strength The strength further improves significantly with curing period It is observed that the soil possesses a strength of 420kPa at a lime content of 3% when cured for days NCDOT [55] specifies that a lime treated soil having strength of 420kPa can be used as subgrade Though the lime stabilized soil possesses higher strength with increase in lime content and longer curing period, a lime content yielding the subgrade strength requirement of 420kPa with a nominal curing period of days has been considered for further investigation Higher lime contents yielding higher strength will be uneconomical During the pavement construction, it is necessary to open for traffic at the earliest A curing period of 28 days is too long and cannot be adopted practically Therefore, an optimum lime content of 2.5% and a nominal curing period of 7days was considered for durability studies and CBR test based on the minimum strength criteria in terms of unconfined compressive strength as per NCDOT [55] For resilient modulus determination, lime content varying from to 3% which provides an unconfined compressive strength in the range of 300kPa to 750kPa was considered Table Results of repeated load tests on black cotton soil treated with 2.75% lime 13.8 14 302 255 228 199 160 253 209 178 158 131 221 193 160 136 120 28 331 289 261 235 207 292 260 240 210 186 267 231 216 193 171 14 28 Sample Collapsed 240 226 201 169 121 200 176 155 126 100 156 136 118 101 87 Wet side of std proctor condition Sample Collapsed 27.6 28 Std Proctor condition Sample Collapsed 41.4 13.8 27.6 41.4 55.2 68.9 13.8 27.6 41.4 55.2 68.9 13.8 27.6 41.4 55.2 68.9 14 Sample Collapsed 10 11 12 13 14 15 Sample Collapsed Confining Deviator Pressure Stress (kPa) (kPa) Sample Collapsed Sl No Dry side of std proctor condition Dry side of modified proctor condition 14 28 183 266 301 170 234 259 147 207 231 122 185 206 81 144 177 157 232 262 135 185 230 107 152 210 86 124 180 61 115 156 132 187 238 108 163 201 82 133 186 69 113 163 48 92 141 Modified Proctor condition 291 251 228 195 148 221 192 162 140 117 200 164 134 122 101 14 371 298 267 223 192 270 226 202 188 175 246 212 176 168 152 28 419 367 340 314 288 369 345 321 293 269 348 311 300 270 249 Wet side of modified proctor condition 14 28 177 254 281 157 210 239 142 181 211 114 161 185 73 133 157 150 210 242 123 171 210 101 145 190 83 115 160 54 98 136 123 171 218 93 152 181 72 126 166 55 104 143 38 84 123 U – Unsoaked condition S – Soaked condition 27 Table 10 Results of repeated load tests on black cotton soil treated with 3% lime 13.8 14 329 277 248 216 173 275 227 193 171 142 241 210 174 147 130 28 360 314 284 255 224 318 283 261 228 202 291 251 235 210 185 14 28 Sample Collapsed 261 246 218 183 131 218 191 168 136 108 170 148 128 109 94 Wet side of std proctor condition Sample Collapsed 27.6 28 Std Proctor condition Sample Collapsed 41.4 13.8 27.6 41.4 55.2 68.9 13.8 27.6 41.4 55.2 68.9 13.8 27.6 41.4 55.2 68.9 14 Sample Collapsed 10 11 12 13 14 15 Sample Collapsed Confining Deviator Pressure Stress (kPa) (kPa) Sample Collapsed Sl No Dry side of std proctor condition Dry side of modified proctor condition 14 28 194 284 322 179 249 276 152 218 244 125 194 216 79 148 184 170 252 285 146 201 250 115 164 228 92 134 195 65 124 168 143 203 259 117 177 218 88 144 202 74 122 176 51 99 152 Modified Proctor condition 310 267 240 204 152 235 203 169 144 118 217 178 145 131 108 14 398 318 283 234 200 288 240 212 197 181 267 230 191 182 164 28 450 393 363 334 305 396 370 342 311 284 379 338 326 293 270 Wet side of modified proctor condition 14 28 192 276 305 170 228 259 153 196 229 124 175 201 78 143 169 163 228 263 133 185 228 109 157 206 89 124 173 57 105 147 133 186 237 101 165 197 77 136 180 59 112 155 40 90 133 U – Unsoaked condition S – Soaked condition 28 Table 11 Regression co-efficients k1 K2 K3 K4 K5 K6 K7 12.935 0.473 6.98 0.428 0.809 0.508 0.373 29 100 90 80 Silt Gravel Sand 60 50 40 30 20 10 0.001 0.01 0.1 10 100 Paricle Size (mm) Fig Grain size distribution of black cotton soil 80 70 Consistency Limits (%) % Finer 70 C l a y 60 50 40 30 20 LL PI PL 10 0 Lime Content (%) Fig Variation of consistency limits with lime content 30 Maximum Dry Unit Weight (kN/m3) 17 0% 2% 16 3% 4% 15 5% 14 13 12 10 15 20 25 30 35 Water Content (%) Fig Compaction curves of lime stabilized black cotton soil under standard proctor condition Maximum Dry Unit Weight (kN/m3) 17 0% 16 2% 3% 15 4% 5% 14 13 12 10 15 20 25 30 35 Water Content (%) Fig Compaction curves of lime stabilized black cotton soil under modified proctor condition 31 Unconfined Compressive Strength (kPa) 2000 2% 3% 4% 1500 5% 1000 500 0 10 15 20 25 30 Curing Period (Days) Fig Variation of unconfined compressive strength of black cotton soil with curing period and lime content under soaked condition Volumetric Strain (%) 14 Volumetric strain during wetting 12 Volumetric strain during drying 10 Differential strain (%) 0 10 No of Cycles Fig Variation of volumetric strain on wetting, drying and differential strain with no of cycles of alternate wetting and drying for lime treated soil sample 32 Fig Collapsed samples after soaking (standard proctor condition) Fig Sample subjected to large deformation during testing 33 Resilient Modulus (MPa) 250 200 150 100 AASHTO T 307 50 Carmichael and Stuart Amber and Quintus 0 20 40 Deviator Stress (kPa) 60 80 Fig Comparison of laboratory MR with the predicted MR 500 R2 = 0.875 Predicted MR 400 300 200 100 0 100 200 300 400 500 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Characteristics of Soil using Modified Effort 40 60 Suhail A Khattab, Ibrahim M Al-Kiki and Abderrahmane H Al-Zubaydi (2011) Effects of Fibres on Some Engineering Properties of Cement and Lime Stabilized Soils, Engineering and Technology Journal, 29(5), pp 886-905 61 Naveena, P C., Dinesh, S V., Gowtham, G and Umesh, T S (2016) Prediction of Strength Development in Black Cotton Soil Stabilised with Chemical Additives, Indian Geotechnical Journal, DOI: 10.1007/s40098-016-0209-3 (In Press) 62 ASTM D 2166 – 13 Standard Test Method for Unconfined Compressive Strength of Cohesive Soil 63 ASTM D 559 - 03 Standard Test Methods for Wetting and Drying Compacted Soil-Cement Mixtures 64 Seed, H., Chan, C and Lee, C (1962) Resilient Modulus of Subgrade Soils and Their Relation to Fatigue Failures in Asphalt Pavements, Proceedings of International Conference on the Structural Design of Asphalt Pavements, University of Michigan, Michigan, pp 611-636 65 Pezo, R and Hudson, W (1994) Prediction Models of Resilient Modulus for Non granular Materials, Geotechnical Testing Journal, 1(3), pp 349 -355 66 Thompson, M R and Robnett, Q L (1979) Resilient Properties of Subgrade Soils, Transportation Engineering Journal, 105(1), pp 71-89 67 Maher, A., Bennert, T., Gucunski, N and Papp, W J (2000) Resilient Modulus of New Jersey Subgrade Soils, Final Report, Report No FWHA-NJ-2000-01, New Jersey Department of Transportation, New Jersey 68 IRC:37-2012 Guidelines for the design of Flexible Pavements, Indian Roads Congress, New Delhi 41 ... improving the resilient modulus of expansive black cotton soil subgrade, verify the suitability of existing models for prediction of MR and development of a new model for the prediction of MR of lime... mentioned by various codes of practices for selection of design MR value 3.4 Prediction of MR of Unstabilized Soil The resilient modulus of untreated black cotton soil under soaked condition... Prediction Model for Resilient Modulus of Lime Stabilized Soil The available prediction equations were found to be not suitable for the prediction of resilient modulus of the black cotton soil except