Matric Suction, Tension Suction and Their Equivalent in Four-Grain Packed Unsaturated Soils with Isolated Pore Water

摘要

In order to define correlation of shear strength with matric suction which further depends on water contents for unsaturated soils, it is recognized by examining four-grain packed system in this paper that in additional to the conventional matric suction distributed on contact area of water, there exists a normal tension force which is distributed along the saturation line on grain surface due to surface tension while the air-water interface overlaps on particle surface. The line-distributed tension can be transformed equivalently into a distributive tension stress which is defined as tension suction and will pose the similar effect on shear strength and deformation as that of matric suction. According to the actual contact states of air-water interface with the surface of soil particles, the unsaturated soil with the characteristics of continuous water with continuous air (CWCA) can be further classified into the overlapping and non-overlapping unsaturated soil. The tension suction appears only in the unsaturated soils of discontinuous water with continuous air (DWCA) and overlapping continuous water with continuous air (CWCA). The thermodynamics and geometric conditions are considered to establish a set of nonlinear equations for defining the air-water interface shape between two mono-sized spheres. These nonlinear equations are solved by iterative procedure for different combinations of contact angles and water contents to obtain theoretical matric and tension suctions in a four-grain packed system with isolated pore water. Then the equivalents of matric and tension suctions are made in order to take both intensity and action area of matric and tension suctions on grain surface into consideration. For the unsaturated soils of DWCA with different contact angles, it is concluded that the ratio of tension suction to matric suction increases from a small value to 2.5 or 4 with increasing saturation degree. It is implied that the tension suction cannot be overlooked. The matric suctions falls down sharply with increase of saturation degree whereas tension suction is almost kept constant and is larger than matric suction from a saturation degree around 8%. The resultant suction of both matric and tension suctions in the meaning of equivalence varies slightly with saturation degree and may be taken as a constant which can well agree with the experimental observations that shear strengths almost keep constants with increasing matric suctions as soils approach to completely dry.