Lecture 7 Shear strength of unsaturated soils.ppt

p q ? ? O ? c ? a ?f线 Kf线 O’ A R Stress invariants The independent properties of principal stresses are expressed in terms of constants called stress invariants The first stress invariant The second stress invariant The third stress invariant Stress point The stress point is defined using the coordinates (p,q,r) where p is the average of principal net major and minor stresses, q is the maximum shear stress and r is the matric suction. Stress point The coordinates of the stress point Stress path To indicate a change in the stress state, A curve drawn through the stress points for successive stress states (Lambe, 1967) The size of the Mohr circle depends on the relative magnitude between the principal stresses Stress path The stress path is established by connecting the stress points The stress path can be linear or curved depending on the loading pattern The matric suction might remain constant or not; The matric suction influences the position of the stress path Stress states occurring in the field during deposition, desaturation, and soil sampling can be described using the stress path method. The isotropic consolidation condition is represented by a stress point on the p-axis. During natural deposition of soil sediments the shear stress in the soil increases during K0-loading. Critical state for unsaturated soils P’ O q CSL Critical state for unsaturated soils Critical state for unsaturated soils Homework 1: An initially saturated soil specimen with an initial void ratio of 0.8 is placed on a pressure plate apparatus and subjected to an applied matric suction (ua-uw) of 20 kPa. At equilibrium condition, the void ratio is 10% less than the initial void ratio while the gravimetric water content is 15% less than the initial gravimetric water content. Calculate the change in volumetric water content Dqw due to the application of matric suction and degree of saturation S at equilibrium condition. Assuming that the solids density of the soil rs is 2.7 Mg/m3.