Hear Strength Behaviour of Natural Expansive Soils

Rashedul H. Chowdhury, Shahid Azam

In the proceedings of: GeoRegina 2014: 67th Canadian Geotechnical Conference

Session: Expansive Soils

ABSTRACT: Saturated and unsaturated shear strength of natural expansive soils from southern Saskatchewan was investigated in this study. The shear strength parameters were determined using direct shear test for saturated specimen (c' = 16 kPa and f' = 24°) and unsaturated specimen (c' = 16 kPa, f' = 24° and fb = 9°). Residual shear strength was determined to be c' = 5 kPa and f' = 16°, respectively. Further analysis was conducted to predict the unsaturated of shear strength using different empirical equations that use the Soil-Water Characteristic Curve to examine the best suitable approach for the investigated expansive soil. RÉSUMÉ La résistance au cisaillement de sols gonflants naturels saturés et insaturés du sud de la Saskatchewan a été étudiée dans cette recherche. Les paramètres de résistance au cisaillement ont été déterminés en utilisant le test de cisaillement direct pour les spécimens saturés ( c'= 16 kPa et ' = 24 °) et les spécimens non saturés ( c'= 16 kPa , f' = 24 ° et fb = 9°). La résistance au cisaillement résiduel a été déterminée comme ayant c '= 5 kPa et f' = 17. Une analyse plus poussée a été effectuée pour prédire la résistance au cisaillement en utilisant différentes équations empiriques qui utilisent la courbe caractéristique sol - eau pour examiner la meilleure approche pour le sol expansif étudié. 1 INTRODUCTION Shear strength of expansive soils affects the bearing capacity of foundations and the stability of slopes. Their behavior is strongly related to the change in suction. Seasonal variations in suction or water content make the soil unsaturated hence unsaturated soil mechanics plays a crucial role. The shear strength equation for an unsaturated soil is given as follows (Fredlund et al., 1978): []bwaanuuucffsttan)('tan)('-+-+ = [1] where, = shear strength of an unsaturated soil, c'= effective cohesion of the soil, f' = effective angle of friction of the soil, (sn-ua) = net normal stress, (ua-uw) = soil suction, and fb = friction angle due to suction. Shear strength tests on unsaturated soils are costly and time-consuming. Simple and indirect methods have been developed to obtain the shear strength of unsaturated soils. Several researchers proposed different approaches to predict shear strength of unsaturated soils where most of them are based on the saturated shear strength parameters used in conjunction with the soil water characteristic curve (SWCC). Vanapalli et al. (1996) and Fredlund et al. (1996) have proposed a more general, nonlinear function using fitting parameter () and normalized water content (w/s). Oberg and Sallfors (1997) replaced Bhishop's parameter (c) by the degree of saturation (S) for predicting the shear strength of sands and silts. Khallili and Khabbaz (1998) extended Bishop's equation by introducing air entry value of the soil and a constant (based on 13 soils data published in the literature). Bao et al. (1998) suggested another equation taking into account the linear variation of the SWCC in the transition zone. Rassam and Cook (2002) used two estimated fitting parameters (a, b) based on air entry value and residual suction. Xu (2004) proposed a fractal model for the soil pore surface where he included two parameters, the surface fractal dimension and the air entry value. Tekinsoy et al. (2004) proposed a simpler method using a single air entry value from the SWCC to predict unsaturated shear strength. Apart from unsaturated shear strength, saturated shear strength (lowest peak strength) and residual shear strength (attained at large shear displacements) are useful for slope stability analysis of fissured clay soils and the progressive failures due to the strength reduction with time. Several undertakings have been done on natural expansive clays like Nanyang expansive soil by Miao et al (2002), Ningming grey-white clay by Yang et al. (2009), Hubei clay by Ye et al. (2010), Al-Qatif expansive clay by Elkady and Abbas (2012). Locally, the only reported work on Regina clay was conducted by Widger and Fredlund (1979) where they did not consider fb. The main objective of this research was to determine shear strength parameters ((c', f', fb) for a typical expansive soil. The geotechnical index properties were determined for preliminary soil assessment. The stress-strain relationship for saturated and unsaturated samples was measured using a conventional shear box. This was followed by the determination of shear strength parameters with the help of suction measuring device. Finally, unsaturated shear strength was predicted using the SWCC and the saturated shear strength parameters.

RÉSUMÉ: STRENGTH BEHAVIOR OF NATURAL EXPANSIVE SOILS Rashedul H. Chowdhury & Shahid Azam

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Rashedul H. Chowdhury; Shahid Azam (2014) Hear Strength Behaviour of Natural Expansive Soils in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper150,author = Rashedul H. Chowdhury; Shahid Azam,title = Hear Strength Behaviour of Natural Expansive Soils ,year = 2014}