ABSTRACT
Recently, the discussion on the effective stress and stress state variables of unsaturated soil becomes hot spot. Xing et al. (2001) proposed an effective stress equation which accounted for the anisotropic influence of matric suction on the mechanical behavior of unsaturated soil. Li (2003) also preformed a similar work. Xie et al. (2006) analyzed the microstructure and macrostructure nature of effective stress of unsaturated soil, and
1 INTRODUCTION
Soil is a multi-phase porous medium. The characterization of stress state of soil is an important theoretical issue, and it is the base of modeling mechanical behavior of soil. Terzaghi’s effective stress principle successfully characterizes the stress state of saturated soil. However, there is no agreement on the characterization of stress state of unsaturated soil. Many attempts have been made on this topic, in which the Bishop’s effective stress equation (Bishop 1959) and the Fredlund’s two stress state variables (Fredlund & Morgenstern 1977, Fredlund & Rahardjo 1993) are most wellknown, and can be expressed respectively:
′ −σ σ= δ χ+ δij j w ijij aa iu ( )− u (1)
σ σ δ′ij a ij a wu s u u− =ij (2)
where ′σ ij , ua and uw are the effective stress, pore gas pressure and pore water pressure, respectively, of unsaturated soil, χ is a parameter depending on degree of saturation and stress path and varying between 0 and 1, and σ ′ij and s are the net stress and the matric suction, respectively.
