The shear strength evaluation of unsaturated soils by Triaxail testing

摘要

The alternative meaning of unsaturated soil is partially saturated soil, in which thedegree of saturation is less than unity. Unsaturated soils have three phases of solid (soil particles), air andwater. Therefore the applications of mechanical and hydraulic laws are inadequate in describing unsaturatedsoil behaviour. Unsaturated soil behaviour is associated of forces and energies with multi-phase interactions.Since the fundamentals of interfacial physics are added in the application laws to explain unsaturated soilbehaviour. In particular, Terzaghi’s effective stress law for saturated soil is not appropriate for unsaturated soilbecause of negative pore water pressure. The stress, which acts on unsaturated soil, not only acts through thesoil skeleton and the pore water, but also acts through the air phase. The stress state theory at failure ofunsaturated soil must consider the thermodynamic properties of pore water in terms of soil suction, materialvariables, state variables and the consequent interparticle forces. In 1978, Fredlund et al. formulated the shearstrength equation for an unsaturated soil. The assumption that the internal friction angle (Φ′ ) is constant withmatric suction and the relationship between matric suctions and shear strengths is linear. As the shearstrength of unsaturated soil is influenced by matric suction and the third shear parameter (Φ b ) needs to bedetermined (Fredlund et al 1978), the conventional triaxial equipment has to be modified. Thu et al. (2006)found that non-linearity of the Φ′ angle with respect to matric suction was in agreement with data reported byGan et al. (1988), Escario et al. (1989) and Vanapalli et al. (1996). Moreover Toll et al (2008) proposed that theangle of friction due to net stress (Φ a ) being greater than Φ′ as measured in saturated conditions.