INTERNATIONAL JOURNAL OF CIVIL ENGINEERING AND TECHNOLOGY (IJCIET) FRAMEWORK FOR ASSESSMENT OF SHEAR STRENGTH PARAMETERS OF RESIDUAL TROPICAL SOILS

Size: px

Start display at page:

Download "INTERNATIONAL JOURNAL OF CIVIL ENGINEERING AND TECHNOLOGY (IJCIET) FRAMEWORK FOR ASSESSMENT OF SHEAR STRENGTH PARAMETERS OF RESIDUAL TROPICAL SOILS"

Brianna O’Connor’
5 years ago
Views:

1 INTERNATIONAL JOURNAL OF CIVIL ENGINEERING AND TECHNOLOGY (IJCIET) International Journal of Civil Engineering and Technology (IJCIET), ISSN ISSN (Print) ISSN (Online) Volume 4, Issue 2, March - April (2013), pp IAEME: Journal Impact Factor (2013): (Calculated by GISI) IJCIET IAEME FRAMEWORK FOR ASSESSMENT OF SHEAR STRENGTH PARAMETERS OF RESIDUAL TROPICAL SOILS Nagendra Prasad.K 1, Sivaramulu Naidu.D 2, Harsha Vardhan Reddy. M 3, Chandra.B 4 1 Professor, Dept. of Civil Engineering, SV University, Tirupati, India, 2 Research Scholar, Dept. of Civil Engineering, SV University, Tirupati, India. 3 Former under-graduate student, Dept. of Civil Engineering, SV University, Tirupati, India. 4 Post-graduate student, Dept. of Civil Engineering, SV University, Tirupati, India. ABSTRACT Failure of soil may cause collapse of structures resulting in loss of lives and economic damage. Most geotechnical instability problems including failure of soil are associated with shear failure. Shear strength is one of the most important properties for design of engineering structures and also one of the most difficult to evaluate. In order to determine the shear strength parameters that govern shear strength, such as angle of internal friction and cohesion, typical laboratory tests such as the direct shear test and triaxial test are used. However, these laboratory tests have some shortcomings regarding sample collection such as lack of in-situ conditions and difficulties for obtaining undisturbed soil samples. In-situ testing methods are also used to determine the shear strength of soil such as the Vane Shear Test, the Standard Penetration Test and the Cone Penetration Test. However, these tests estimate the shear strength of the soil with appropriate empirical correlations that have a wide margin of error. Traditional testing methods to acquire the shear strength parameters are expensive, complicated, time consuming, and require extreme care during the process of collecting, storing, transporting and preserving samples. The objective of this paper is to develop a phenomenological model that could be used to predict the shear strength parameters from their index properties (liquid limit) and other engineering properties (specific gravity, void ratio, maximum dry density), which are relatively easy to determine. The validity of the method was proven by determining shear strength parameters for various types of soils and by comparing them with the results taken from a conventional testing method. This could be used to rapidly estimate cohesion and friction angle in situations where either the good quality samples or the equipment needed to conduct such tests are not available. 189

2 Keywords: shear strength parameters, bulk modulus, normal compression line, triaxial test, volumetric strain, maximum dry density. 1. INTRODUCTION The structural strength is primarily a function of shear strength of soil. Soil failure usually occurs in the form of shearing along internal surface within the soil. Shear strength is soils ability to resist sliding along internal surfaces within the soil mass. The strength of clayey soil is influenced by compaction energy, optimum moisture content, dry density, percentage of fines, degree of saturation, consistency limits, cohesion and frictional resistance between the particles. According to Mohr s theory, a soil mass will fail when the shearing stress on the failure plane, which is a definite function of the normal stress acting on that plane, is greater than the shear resistance of the soil i.e. S = f (σ n ). The shearing strength of a soil is represented by the following Mohr-Coulomb s equation, S = c + σ n tan ф Where, S = Shear stress at failure c = cohesion i.e. the resistance of soil particles to displacement due to intermolecular attraction and surface tension of the held water σ n = Normal stress ф = Angle of internal friction. The angle of internal friction depends upon dry density, particle size distribution, shape of particles, surface texture, and water content. It is directly proportional to the applied normal force acting between the particles. In clayey soils, partially saturated soils, and cemented soils, the individual soil particles are bonded together. This is another source of the shear strength of soil which is independent from the normal force, called cohesion. Cohesion depends upon size of clayey particles, type of clay minerals, valence bond between particles, water content, and proportion of the clay. In geotechnical design practice, two important considerations that need careful examination are whether construction will cause deformation of the soil and /or instability due to shear failure. An engineer has to ensure that the structure is safe against shear failure in the soil that supports it and does not undergo excessive settlement. Therefore knowledge about the stress-strain behaviour, deformation and shear strength of the soil is essential. These considerations are more complicated and challenging when dealing with clayey soil, which is known to be highly deformable and have low shear strength. It can be determined either in the field or in the laboratory, or both. The tests employed in the laboratory may include unconfined compression test, triaxial test, laboratory vane, direct shear box and direct simple shear test. In situ tests are normally conducted to test the validity of the laboratory tests and for design purposes. However, these laboratory tests have some shortcomings regarding sample conditions such as lack of in-situ conditions 190

3 and difficulties for obtaining undisturbed soil samples apart from difficulties associated with simulating drainage conditions appropriately. Insitu tests available include field vane, standard penetration test, cone penetration test, and piezocone and pressure meter. However, these tests estimate the shear strength of the soil with appropriate empirical correlations that have a wide margin of error. The present work aims at evaluating the shear strength parameters of soil at a state of maximum dry density taking into consideration its liquid limit and Proctors maximum dry density since soil is compacted to its maximum dry density in almost all earth structures. 2. BACKGROUND INFORMATION Investigation carried out by Burak (2008) has established correlation between index properties and shear strength parameters of normally consolidated clays by statistical and neural approaches. Amin (1997) made studies to predict and determine undrained shear strength, a very important parameter in design practice, for Klang clay, Malaysia. Shear strength is determined using field and laboratory vane shear and recompression method utilizing the direct simple shear apparatus. Analysis of the triaxial test results of Satija (1978) reveals some nonlinearity in the shear stress versus matric' suction failure envelope (Fredlund et al. 2000). Fredlund and Vanapalli (2000) in a recent study have provided comparisons between the measured and predicted values of unsaturated shear strength using the shear strength functions published in the literature. Comparisons were provided both for low suction range (i.e., 0 to 1,500 kpa) as well as large suction range (0 to 10,000 kpa or higher). Vanapalli et al. (2001) predicted the shear strength of an unsaturated soil with a semi-empirical shear strength function developed at the University of Saskatchewan both for low and as well as large suction ranges. Rajeev Jain et al. (2010) presented an artificial neural network technique to predict the shear strength parameters of medium compressibility soil, which influenced by basic properties of soil in unconsolidated undrained conditions. Kamil Kayabali (2011) investigated the shear strengths at plastic limit and liquid limit by reappraising a large body of shear strength and soil consistency data.. If the shear strength at plastic limit and liquid limit are set properly, the undrained shear strength of remolded soils at any water content between Plastic limit and liquid limit can be determined easily. Erfan Hosseini (2012) studied shear strength parameters by using grading test, Atterberg limits, compression, direct shear and consolidation. Soil State It is widely known that the stress and strain are inseparable for all materials under loading. The stress the particulate materials experience depends on the associated strain and vice versa. Accordingly, an attempt has been made to analyse the mobilisation of shear strength in relation to the volumetric strain, the sample experiences to exhibit maximum resistance. The volumetric strain is reckoned with reference to the possible loose state in order to arrive at the current state. It is the current state of soil that determines the shear strength of soils irrespective of the stress path the soil follows to reach the current state as demonstrated in the Figure 1. At (a) the soil is under a pressure of 1 kpa and at (b) the soil is at maximum dry density. 191

4 Figure 1: Depiction of Soil State Bulk Modulus Bulk modulus (K) of a substance measures the substance's resistance to uniform compression. It is defined as the ratio of the infinitesimal pressure increase to the resulting relative decrease of the volume. The bulk modulus K>0 can be formally defined by the equation, Where, P = Pressure V = Volume = Derivative of pressure with respect to volume. 3. EXPERIMENTAL INVESTIGATION 3.1 Introduction The study area lies to the extreme south of Andhra Pradesh state (India) approximately between 12 37' ' north latitudes and 78 3' ' east longitudes. The experimental methods of different laboratory investigations are carried out on the tropical residual soils of Tirupati region. 3.2 Details of the Experimental Investigation The present experimental investigation is carefully planned to understand the behavior of tropical residual soils. The experimental program involves determination of the following aspects. Basic properties Compaction properties Undrained triaxial compression test All the tests were conducted as per the relevant provisions stipulated in Bureau of Indian Standards. 192

Laboratory data of the samples 1 to 15 are used to analyze and predict the correlation among c, ф and bulk modulus (K) of various soil samples.

5 3.3 Soils Tested The soils considered in the present investigation have been obtained from the surroundings of Tirupati region. The location of soil samples can be seen from Figures 2 and 3. The details of locations of sampling are shown in Table 1. Laboratory data of the samples 1 to 15 are used to analyze and predict the correlation among c, ф and bulk modulus (K) of various soil samples. Data of samples A, B and C obtained from the laboratory are examined to verify the accuracy of prediction in a phenomenological model. These soils are residual in nature, which are deposited at the place of formation. Figure 2: locations at Tirupati region in India map Figure 3: Detailed sample locations at Tirupati region 193

6 Table 1: Soil Locations S.No Location 1 Beerakuppam (Village) 2 Gongutapalli (Village) 3 RC Kandriga 4 Nagari 5 Avalkonda 6 Renigunta by-pass 7 Tiruchanur 8 Kottramangal(village) 9 Pillaripattu 10 Padmavathipuram 11 Nagari Station 12 Dhodlamitta (village) 13 Kandriga(village) 14 Daminedu 15 Padmavatipuram A B C K.T.Road Kothapalem layout Padipeta 3.4 Collection of s Soil samples considered represent wide spectrum of typical soils encountered in practice, ranging from predominantly clayey sand to clay with low to high compressibility. Soil samples have been collected by exercising necessary care to see that the natural constituents are represented and the same were transported to geotechnical engineering laboratory. The samples were air dried and stored in air tight containers for use in rest of the investigation. 194

7 3.5 Properties of Soils Index properties and the compaction properties for all the samples including A, B and C are presented in Tables 2 and 3. It may be seen that most of the soils represent clayey sand (SC) and few samples fall under clay with intermediate and high compressibility (CI, CH). The liquid limit values for the samples considered ranges from 31% to 67% and the plastic limit varies from 14% to 22%. The fine fraction ranges from 29% to 83% which is typical for the soils encountered in practice in this region. The cohesion values ranges from kpa to kpa and angle of internal friction ranges from o to o. 4 ANALYSIS OF TEST RESULTS The usual object of detailed experimental investigation will be to propose a mechanistic approach for understanding the behavior of materials tested in a coherent manner by properly analyzing the observed behavior. Accordingly a detailed analysis of test results is presented in the following section. 4.1 Triaxial test data Triaxial compression tests have been conducted on samples 1 through 15 and the test results are depicted from Figures 4 to 21. Mohr s circles are drawn for soil samples 1 to 8 as shown in Figures 22 to 29. Similar Mohr circles can be drawn for other soil samples also. The values of c and ф thus determined from the Mohr s circle approach are represented in Tables 2 and 3. The stress-strain response of the sample is noticed to be typical with greater deviatoric stress for greater confining pressures. The shear strains experienced by the samples seem to be related to the degree of compression to which the samples is subjected. Figure 4: Deviatoric stress verses strain for sample 1 Figure 5: Deviatoric stress verses strain for sample 2 195

8 Figure 6: Deviatoric stress verses strain for sample 3 Figure 7: Deviatoric stress verses strain for sample 4 Figure 8: Deviatoric stress verses strain for sample 5 Figure 9: Deviatoric stress verses strain for sample 6 196

9 Figure 10: Deviatoric stress verses strain for sample 7 Figure 11: Deviatoric stress verses strain for sample 8 Figure 12: Deviatoric stress verses strain for sample 9 Figure 13: Deviatoric stress verses strain for sample

10 Figure 14: Deviatoric stress verses strain for sample 11 Figure 15: Deviatoric stress verses strain for sample 12 Figure 16: Deviatoric stress verses strain for sample 13 Figure 17: Deviatoric stress verses strain for sample

11 Figure 18: Deviatoric stress verses strain for sample 15 Figure 19: Deviatoric stress verses strain for sample A Figure 20: Deviatoric stress verses strain for sample B Figure 21: Deviatoric stress verses strain for sample C 199

12 Figure 22: Mohrs circle approach to determine c and ф for sample 1 Figure 23: Mohrs circle approach to determine c and ф for sample 2 Figure 24: Mohrs circle approach to determine c and ф for sample 3 Figure 25: Mohrs circle approach to determine c and ф for sample 4 200

13 Figure 26: Mohrs circle approach to determine c and ф for sample 5 Figure 27: Mohrs circle approach to determine c and ф for sample 6 Figure 28: Mohrs circle approach to determine c and ф for sample 7 Figure 29: Mohrs circle approach to determine c and ф for sample 8 201

14 Table 2: Soil Properties Values Sl. No: Description Gravel (%) Sand (%) Silt+Clay (%) mm Size (%) Liquid Limit, W L (%) Plastic Limit, P L (%) Plasticity Index, P I (%) IS Classification CL SC CI CI SC CI SC CI CH 9 Free Swell Index (%) Degree of Expansion Low Low Low Low Low Medium Medium Low Mediu m 13 Optimum moisture content, (%) Maximum dry density, γ d (kn/m 3 ) Shear strength parameters 15 Cohesion, C in kpa Angle of internal friction, Φ in degrees

15 Table 3: Soil Properties Sl. No: Description Values A B C 1 Gravel (%) Sand (%) Silt+Clay (%) mm Size (%) Liquid Limit, W L (%) Plastic Limit, P L (%) Plasticity Index, P I (%) IS Classification SC SC CH CH SC CH SC CI SC 9 Free Swell Index (%) Degree of Expansion Medium Medium Medium Medium High High Mediu m Mediu m Mediu m 13 Optimum moisture content, (%) Maximum dry density, γ d (kn/m 3 ) Shear strength parameters 15 Cohesion, C in kpa Angle of internal friction, Φ in degrees Behaviour with respect to Normal Compression Line (NCL) An attempt has been made to examine the compression behavior with respect to Normal Compression Line (NCL) for which the equation given by Nagaraj et.al. (1994) as reproduced below has been adopted. e e L = ' logσ v (1) Where, e = Void ratio at a given pressure of σ v e L = Void ratio corresponding to liquid limit. 203

16 4.3 Determination of Void Ratio Void ratio corresponding to liquid limit will be minimum and can be determined as the product of specific gravity (G) and liquid limit (W L ). When the soil is compacted to its maximum dry density, void ratio decreases accordingly which can be determined from the equation, Where, = Maximum dry density G = Specific gravity of soil = Unit weight of water e = Void ratio at a given pressure of σ v σ vmax, pressure corresponding to maximum dry density is now determined from equation (1), by substituting e and e L values. The void ratio (e o ) in the loosest state under a pressure of 1kPa is determined from equation (1) for all the soil samples knowing their liquid limits. 4.4 Volumetric Strain The volumetric strain (ϵ v ) can now be determined from the equation, (2) Where, e o = void ratio under pressure of 1kPa e = void ratio at a state of maximum dry density 4.5 Bulk Modulus Bulk modulus (K) can be obtained as the ratio of the infinitesimal pressure increase to volumetric strain, dp for all the 15 soil samples (1-15) can be evaluated as the difference of the pressure between loosest state (corresponding to a normal stress of 1 kpa) and pre-compression stress (σ vmax, referred to a normal compression line of natural state of soil). 4.6 Bulk Modulus versus c and ф A graph of bulk modulus (K) versus c and bulk modulus (K) versus ф is plotted as depicted in Figures 30 and 31 respectively. Experimental results usually show small deviations and a best fit straight line from plotted data is normally drawn to establish a definite relation. A correlation of 97.60% and 96.10% are obtained for bulk modulus (K) versus cohesion (c) and bulk modulus (K) versus angle of internal friction (ф) respectively. The equation thus obtained for bulk modulus (K) versus c is as follows: And for bulk modulus (K) versus ф it is: c = 0.034K (5) ф = 0.007K (6) 204

17 Figure 30: Bulk modulus (K) versus Cohesion (c) Figure 31: Bulk modulus (K) versus Angle of internal friction (ф) 205

18 5 PREDICTION OF SHEAR STRENGTH PARAMETERS (C & Ф) The validity of the present investigation can be checked by determining shear strength parameters of the samples A, B and C. The shear strength parameters c and ф are determined from the conventional triaxial test to check the accuracy of predicted data. Using liquid limit, void ratio in loosest state under a pressure of 1 kpa (e o ), is determined by equation (1). Void ratio at maximum dry density (e), for samples A, B and C are determined from equation (2). Now volumetric strain and bulk modulus are determined from equations (3) and (4) respectively. From bulk modulus, the cohesion(c) values for each sample A, B and C are obtained using equation (5). Similarly the angle of internal friction (ф) for these samples is obtained from equation (6). 5.1 Accuracy in Prediction Data thus predicted is compared with the laboratory data obtained from conventional triaxial test. It is observed that the accuracy of prediction in the evaluation of both c and ф accounts to about 96%. 6. CONCLUDING REMARKS The objective of this study is to suggest a phenomenological model to correlate liquid limit, maximum dry density with shear strength parameters such as cohesion and angle of internal friction. 1) The values of cohesion (c) and angle of internal friction (ф) alters with the state of soil or simply, they represent the state of soil. 2) Void ratio decreases when the soil is compacted from loosest state to its maximum dry density. 3) Both cohesion (c) and angle of internal friction (ф) increases with increase in bulk modulus (K). 4) Relation between bulk modulus (K) and cohesion (c) is almost linear. 5) Also, the relation between bulk modulus (K) and angle of internal friction (ф) is almost linear. 6) Increase in cohesion (c) is more when compared to increase in angle of internal friction (ф) with increase in bulk modulus. 7) The present state of soil determines its shear strength irrespective of the path followed. 8) The compacted soil state lies on left hand side of the Normal Compression Line and hence the state is quite akin to over-consolidated state. 9) The volumetric strain to which the sample undergoes depends on the stress which in turn depends on the compaction energy imparted. 10) Accuracy of prediction in the evaluation of both cohesion (c) and angle of internal friction (ф) accounts to about 96%. 206

19 REFERENCES [1] Amin (1997), Prediction and Determination of Undrained Shear Strength of Soft Clay, Pertanika J. Sci. & Techno! 5(1): [2] Burak (2008), Shear strength estimation of plastic clays with statistical and neural approaches, Journal of Building and Environment, vol. 43. [3] Fredlund (1987), Nonlinearity of strength envelope for unsaturated soils, proceedings of the 6th international conference on expansive soils, New Delhi. [4] Fredlund, D.G. and Vanapalli (2000), Comparison of different procedures to predict unsaturated soil shear strength, ASTM Proceedings, Unsaturated Soils, Geo-Denver [5] Erfan Hosseini, Mohammad K. Alizadeh, and Amir Mesbah (2012), Evaluation of Shear Strength Parameters of Amended Loess Using Common Admixtures in Gorgan, Iran, International Journal of Science, Engineering and Technology. [6] Kamil Kayabali (2011), Assessment of Shear Strength at Consistency Limits - A Reappraisal, Vol. 16, EJGE Journal. [7] Nagaraj,T.S. & Srinivasa Murthy B.R. and Vatsala, A. (1994), Analysis and Prediction of Soil Behavior, Wiley Eastern Journal, New Delhi. [8] Rajeev Jain and Pradeep Kumar Jain (2010), Computational approach to predict soil shear strength, International Journal of Engineering Science and Technology. [9] Satija B. S. (1978), Shear behavior of partly saturated soils, Ph.D. thesis, Indian Institute of Technology, Delhi, India. [10] Vanapalli (2001), Predicting the shear strength of an unsaturated soil, The Canadian Geotechnical Society Journal. [11] Ercan Serif Kaya, Takuro Katayama and Toshitaka Yamao, Seismic Characteristics Of The Folded Cantilever Shear Structure, International Journal of Civil Engineering & Technology (IJCIET), Volume 4, Issue 2, 2013, pp , ISSN Print: , ISSN Online: [11] M. Alhassan and I. L. Boiko, Effect of Vertical Cross-Sectional Shape of Foundation and Soil Reinforcement on Settlement and Bearing Capacity of Soils, International Journal of Civil Engineering & Technology (IJCIET), Volume 4, Issue 2, 2013, pp , ISSN Print: , ISSN Online:

8.1. What is meant by the shear strength of soils? Solution 8.1 Shear strength of a soil is its internal resistance to shearing stresses.

8.1. What is meant by the shear strength of soils? Solution 8.1 Shear strength of a soil is its internal resistance to shearing stresses. 8.2. Some soils show a peak shear strength. Why and what type(s)