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    • International Journal of Civil Engineering and CIVIL ENGINEERING AND6308 INTERNATIONAL JOURNAL OF Technology (IJCIET), ISSN 0976 – (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME TECHNOLOGY (IJCIET) ISSN 0976 – 6308 (Print) ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December, pp. 61-66 © IAEME: www.iaeme.com/ijciet.asp Journal Impact Factor (2013): 5.3277 (Calculated by GISI) www.jifactor.com IJCIET ©IAEME CORRELATION BETWEEN CALIFORNIA BEARING RATIO AND SHEAR STRENGTH ON ARTIFICIALLY PREPARED SOILS WITH VARYING PLASTICITY INDEX Dr. K.V.KRISHNA REDDY* *Professor, Civil Engineering Department, MVSR Engineering College, Hyderabad, A.P. ABSTRACT The present study is aimed at evaluating a correlation between the soaked California Bearing ratio (CBRSoaked) and Undrained Shear Strength (Su) for artificially prepared soils with varying Plasticity Index. The naturally existing soil normally referred to as Red Moorum is considered for testing. The soil fraction passing 425 microns is replaced by red powder, clay powder and sand powder derived essentially from fines of red moorum, clay of medium plasticity and sandy soil in various percentages to obtain samples with varying Plasticity Index. The non linear correlation obtained was validated with experimental results. Key Words: California bearing ratio, Undrained Shear Strength, Plasticity Index 1. INTRODUCTION California bearing ratio (CBR) of the subgrade soils is an important factor in design of flexible pavements. CBR is a measure of resistance of material at controlled moisture content and density. In areas of moderate to heavy rainfall, 4 day soaking of the sample is considered to determine the CBR value (CBRSoaked). The CBR of the soil is evaluated as a ratio of the force per unit area required to penetrate a soil mass with a standard circular piston to that required for the corresponding penetration of a standard crushed stone and is essentially an empirical value influenced by the soaking time, soaking method, and interpolation of the CBR curves. The un-drained shear strength of the soil is evaluated from cohesion(C) and angle of internal friction (Phi) and is essentially an engineering property. The ‘C’ and ‘phi’ of the soil are dependent on the type and quantity of fines and granular soil fraction in any soil, which in turn affect the performance of the soil when used as a subgrade and subbase material. Most soils used for road construction in India contain varying types and percentages of fines resulting in varied Plasticity Index values. In the present study an attempt is made to correlate the soaked CBR value with the undrained shear strength of artificially prepared soils with varying plasticity Index. 61
    • International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME 2. LITERATURE REVIEW Danistan Joseph, et. Al. have reported extensive work done on correlation between CBR and index properties of CH CL and SC soils and have concluded with correlations which can be used on the specific soil groups. Patel, et. Al. developed correlations between Dynamic Cone Penetration results with Plate Bearing Test, Unconfined Compression Strength and California Bearing Ratio results for soils occurring in Gujarat. Soewignjo Agus Nugroho, et. Al. reported correlations between Index Properties and California Bearing Ratio Test of Pekanbaru Soils. Literature review suggests that there are a number of correlations developed between the CBR and different soil parameters, however most of them are soil, gradation and location specific and cannot be confidently used to evaluate the unknown parameter as the type of soil and gradation varies. In this context a need is felt to obtain a correlation between undrained shear strength and soaked CBR values for use on soils with varying fines. 3. RESEARCH METHODOLOGY 3.1 Soil considered Naturally occurring red moorum soil is considered for testing. Though the percentage finer than 75 microns is considered as fines, to have an advantage of varying plasticity, the percent passing 425 microns is considered to vary the fines. Hereafter the fraction passing 425 microns for the red moorum is referred to as red powder. Two other soil powders passing 425 microns of medium plastic clay soil and sand are considered and are here after referred to as black powder and sand powder respectively. Table 1 indicates the Grain size distribution of the soils considered for testing. In this study soil samples with varying plasticity index are prepared by replacing the 425 micron passing fraction of the natural soil with varying percentages of red powder, clay powder and sand powder and also the coarse grained and fine grained fractions have been changed to evaluate a correlation. Table 1. Grain size distribution of soils considered for testing Red soil Black Powder Sand powder Size % Retained % Retained % Retained 38.8 4.75mm 21 2mm 35.6 425 µ 2.8 17.5 83.0 75 µ 1.8 82.5 17 Pan 3.2 Sample preparation with varying Fines As explained in the research methodology two sets of soil samples are prepared each with subsets to get samples of varied plasticity index. The first set with all its subsets has the red moorum soil retained on 425 microns at 70% and the soil fraction passing through 425 microns is varied (70% coarse and 30% fines, fines being varied). The second set has 9 subsets, with the portion of red moorum retained on 425 microns kept at 30% and the portion passing 425 microns is varied (30% coarse and 70% fines, fines being varied). Table 2 shows the varying fines content for both set I and set II samples. 62
    • International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME Table 2. Table showing Soil sample preparation by varying fines Sand Powder Red Powder Black Powder 40% 60% 30% 70% 10% 90% 100% 75% 25% 50% 50% 25% 75% 100% 4.0 TESTING, RESULTS AND ANALYSIS 4.1 Testing The Atterberg Limits of all the soil subsets are computed. Proctor standard compaction test was conducted on all the soil subsets to determine the optimum moisture content and maximum dry density. California Bearing ratio (CBR) test is conducted at the corresponding optimum moisture content and maximum dry density for all the soil samples under 4 day soaked condition. Cohesion (C) and angle of internal friction (Phi) were determined by direct shear test. The allowable compressive strength in the subgrade was taken as 1.5kg/sqcm (Yoder) to evaluate the undrained shear strength. 4.2 Results Figure1 shows the CBR curves plotted between load Vs penetration for the set I soil samples with 70% coarse and 30% fines, fines being varied as per Table 2. Figure 2 shows the CBR curves plotted for the set II soil samples with 30% coarse and 70% fines, fines being varied as indicated in Table 2. Figure 3 depicts the plot between Normal stress and Shear stress form the direct shear test for set 1 soil samples and Figure 4 shows the results of direct shear test for set II soil samples. The test results obtained and derived from the graphs for all the tests conducted are depicted in Tables 3 and 4 for set I and set II soil samples respectively. load in KG 40% Sand P owder and 60% Red P owder 30% Sand P owder and 70% Red P owder 600 10% Sand P owder and 90% Red P owder 100% Red Powder 550 50% Red P owder and 50% Black P owder 25% Red P owder and 75% Black P owder 500 100% Black P owder 450 400 350 300 250 200 150 100 50 Penetration in mm 1 2 3 4 5 6 Figure 1. CBR curves for Set I soil samples with 70% coarse and 30% Fines 63
    • International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME Load in KG 40% Sand Powder and 60% Red Powder 10% Sand Powder and 70% Red Powder 100% Red Powder 75% Red Powder and 25% Black Powder 300 25% Red Powder and 75% Black Powder 250 200 150 100 50 Penetration in mm 1 2 3 4 5 Figure 2. CBR curves for Set II soil samples with 30% coarse and 70% Fines Shear Stress (kg/sqcm) 40% Sand Powder and 60% Red Powder 30% Sand Powder and 70% Red Powder 0.45 10% Sand Powder and 90% Red Powder 100% Red Powder 50% Red Powder and 50% Black Powder 0.4 25% Red Powder and 75% Black Powder 100% Black Powder 0.35 0.3 0.25 0.2 0.15 0.1 0.05 Normnal Stress (kg/sqcm) 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 Figure 3. Mohr Envelopes for Set I soil samples with 70% coarse and 30% Fines Shear stress (kg/sqcm) 40% Sand powder & 60% Red powder 10% Sand powder & 90% Red powder 100% Red powder 75% Red powder & 25% Black powder f(x)=0.60645161*x+0.090043011; R²=0.9955 0.4 0.3 0.2 0.1 Normal Stress (kg/sqcm) 0.1 0.2 0.3 0.4 Figure 4. Mohr Envelopes for Set II soil samples with 30% coarse and 70% Fines 64
    • International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME Subset 1 2 3 4 5 6 7 Subset 1 2 3 4 5 Table 3. Results of Testing done on subsets of Set I soil samples Red Black PI C Ph Su CBRSoaked Sand powder powder powder % Kg/sqcm Deg Kg/sqcm % 40% 60% 8.8 0.03 40 1.289 24.6 30% 70% 9.6 0.034 40 1.293 23.7 10% 90% 13.2 0.044 39 1.259 22.0 100% 16.7 0.05 39 1.265 22.5 50% 50% 23.0 0.09 36 1.180 19.9 25% 75% 23.5 0.10 35 1.150 19.6 100% 28.8 0.12 33 1.094 14.4 Table 4. Results of Testing done on subsets of Set II soil samples Sand Red Black PI C Ph Su CBRSoaked powder powder powder % Kg/sqcm Deg Kg/sqcm % 40% 60% 8.8 0.10 28 0.898 11.2 10% 90% 13.2 0.13 26.5 0.878 9.0 100% 16.7 0.15 26 0.882 9.6 75% 25% 22.3 0.18 24 0.848 8.2 25% 75% 23.5 0.232 20 0.778 5.8 4.3 Analysis Relation between CBR soaked and Su is obtained by plotting a graph as depicted in Figure5. A polynomial correlation is found to better correlate the two parameters. The correlation obtained is validated by the actual tests conducted on different soil samples. The validation details are presented in Table 5. Figure 5. Correlation between CBR soaked and Su 65
    • International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME Table 5. Validation of the correlation with test results SET Sand powder Red powder Black powder PI C Kg/ sqcm Phi Deg Su Kg/sqc m CBRSoaked Actual CBRSoaked Correlation % variation I 25 75 - 10.4 0.04 39.5 1.277 22.9 23.25 1.5% II - 50 50 23 0.23 24 0.897 9.7 9.82 1.2% 5.0 ACKNOWLEDGEMENT At the outset the author would thank the Head, CED, MVSR Engineering College and SE (R&B) for their valuable guidance and encouragement during experimentation. 6.0 CONCLUSION 1. The soil samples with varying plasticity Indices studied, has resulted in a polynomial relationship between the soaked CBR and undrained shear strength. 2. The correlation obtained is CBR Soaked = 11.08Qu2 + 11.27 Qu - 9.201 with R- Squared value of 0.983. The same is validated with test results and the variation was fond to be within 1.5%. 7.0 REFERENCES [1] Danistan Joseph, et Al. (2010), “Correlation Between California bearing Ratio and soil parameters” , Proceedings of CIGMAT-2010, Conference and Exhibition. [2] Dr. K.V.Krishna Reddy, “Stabilization of Medium Plastic Clays using Industrial Wastes”, International Journal of Civil Engineering & Technology (IJCIET), Volume 4, Issue 3, 2013, pp. 38 - 44, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316. [3] Dr. K.V.Krishna Reddy, “Benefit Analysis of Subgrade and Surface Improvements in Flexible Pavements”, International Journal of Civil Engineering & Technology (IJCIET), Volume 4, Issue 2, 2013, pp. 385 - 392, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316. [4] Dr. K.V.Krishna Reddy, “Influence of Subgrade Condition on Rutting in Flexible Pavementsan Experimental Investigation”, International Journal of Civil Engineering & Technology (IJCIET), Volume 4, Issue 3, 2013, pp. 30 - 37, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316. [5] Dr. K.V.Krishna Reddy and Mr.K.P.Reddy, “Maturity Period and Curing as Important Quality Control Parameters for Lime Stabilized Clay Subgrades”, International Journal of Civil Engineering & Technology (IJCIET), Volume 4, Issue 2, 2013, pp. 393 - 401, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316. [6] SoewignjoAgus NUGROHO, et. Al. (2012), “Correlation Between Index properties And California bearing Ratio Test Of Pekanbaru Soils With And Without Soaked” Canadian Journal on Environmental, Construction and Civil Engineering, Vol. 3, No. 1, pp. 7-17. [7] Yoder, E.J. and Witczak, M.W. (1975) Principles of Pavement Design, 2nd edition, John Wiley & Son, Inc, New York, USA. [8] Dr. K.V.Krishna Reddy, “Rutting Resistance of Filler Modified Bituminous Concrete Surfaces”, International Journal of Civil Engineering & Technology (IJCIET), Volume 4, Issue 2, 2013, pp. 250 - 257, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316. [9] Patel, S.R. and Desai, M.D. 2010. CBR Predicted by Index Properties for Alluvial Soils of South Gujarat, Dec. 16-18, 79-82, Proceedings of Indian Geotechnical Conference–2010, India. 66