Shear strength and dilative characteristics of an unsaturated compacted completely decomposed granite soil Md. Akhtar Hossain and Jian-Hua Yin Abstract: Shear strength and dilative characteristics of a re-compacted completely decomposed granite (CDG) soil are studied by performing a series of single-stage consolidated drained direct shear tests under different matric suctions and net normal stresses. The axis-translation technique is applied to control the pore-water and pore-air pressures. A soil-water re- tention curve (SWRC) is obtained for the CDG soil from the equilibrium water content corresponding to each applied matric suction value for zero net normal stress using a modified direct shear apparatus. Shear strength increases with matric suction and net normal stress, and the failure envelope is observed to be linear. The apparent angle of internal friction and cohesion intercept increase with matric suction. A greater dilation angle is found at higher suctions with lower net normal stresses, while lower or zero dilation angles are observed under higher net normal stresses with lower suctions, also at a saturated condition. Experimental shear strength data are compared with the analytical shear strength results obtained from a previ- ously modified model considering the SWRC, effective shear strength parameters, and analytical dilation angles. The experi- mental shear strength data are slightly higher than the analytical results under higher net normal stresses in a higher suction range. Key words: soil-water retention curve, shear strength, dilation angle, completely decomposed granite, matric suction, axis- translation technique. Re ´sume ´: La re ´sistance au cisaillement et les caracte ´ristiques de dilatation d’un sol de granite comple `tement de ´compose ´ (GCD) re-compacte ´ sont e ´tudie ´es a ` l’aide d’essais de cisaillement directs consolide ´s et draine ´s a ` une e ´tape, et ce sous dif- fe ´rentes succions matricielles et diffe ´rentes contraintes nettes. La technique de translation de l’axe est utilise ´e pour contro ˆ- ler les pressions interstitielles. La courbe de re ´tention d’eau (CRE) du sol GCD a e ´te ´ obtenue a ` partir de la teneur en eau a ` l’e ´quilibre correspondant a ` chacune des valeurs de succion applique ´es lors d’un essai avec l’appareil de cisaillement di- rect modifie ´, pour des contraintes normales nettes de ze ´ro. L’augmentation de la re ´sistance au cisaillement avec l’augmen- tation de la succion matricielle et de la contrainte normale nette est line ´aire, ainsi que l’enveloppe de rupture. L’angle de friction interne apparent et la cohe ´sion augmentent avec l’augmentation de la succion matricielle. L’angle de dilatation est plus grand a ` une succion e ´leve ´e et a ` une contrainte normale nette faible, tandis que des angles de dilatation faibles ou nuls sont observe ´s lorsque la contrainte normale nette est e ´leve ´e et la succion est faible, ainsi qu’en condition sature ´e. Les don- ne ´es expe ´rimentales de re ´sistance au cisaillement sont compare ´es aux re ´sultats analytiques obtenus a ` partir d’un mode `le pre ´alablement modifie ´ qui tient compte de la CRE, des parame `tres de la re ´sistance au cisaillement effective et des angles de dilatation analytiques. Les donne ´es expe ´rimentales de re ´sistance au cisaillement sont le ´ge `rement supe ´rieures aux re ´sul- tats analytiques obtenus sous des contraintes normales nettes plus e ´leve ´es dans un intervalle de succion plus grand. Mots-cle ´s : courbe de re ´tention d’eau, re ´sistance au cisaillement, angle de dilatation, granite comple `tement de ´compose ´, succion matricielle, technique de translation de l’axe. [Traduit par la Re ´daction] Introduction Since the 1950s, several investigations have been carried out to help understand the principles of unsaturated soil me- chanics (Aitchison and Donald 1956; Donald 1956; Aitchi- son 1960; Bishop and Blight 1963). It is very difficult, costly, and time-consuming to carry out tests to investigate and understand shear strength behavior of unsaturated soils in a general stress state. Hence, there have been numerous intensive studies conducted that investigate the shear strength behavior of unsaturated soils by using simplified test methods (Fredlund and Rahardjo 1993a; Vanapalli et al. 1996; Khalili and Khabbaz 1998; Rassam and Williams 1999; Vilar 2006). Research studies performed on unsaturated soils confirm the relationship between the soil-water retention curve (SWRC) and the unsaturated soil properties (Fredlund and Rahardjo 1993b). Empirical and analytical models have been developed for predicting the shear strength of unsatu- Received 4 June 2009. Accepted 26 February 2010. Published on the NRC Research Press Web site at cgj.nrc.ca on 30 September 2010. Md.A. Hossain and J.H. Yin. 1 Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China. 1 Corresponding author (e-mail: cejhyin@polyu.edu.hk). 1112 Can. Geotech. J. 47: 1112–1126 (2010) doi:10.1139/T10-015 Published by NRC Research Press