Study On The Unsaturated Strength Characteristics And Dynamic Behavior Of Intact Expansive Soil

The static and dynamic behavior of the expansive soil obtained from Jingmen, Hubei province were investigated through pressure plate,unsaturated triaxial and cyclic triaxial testing. The main results are summarized in the following:Firstly, the pressure plate test was used to measure the soil-water characteristic curve(SWCC) of expansive soil using the axis-translation technique, it was found that a good contact between the soil specimen and the ceramic disc is crucial in this test. The major factors that influence the test results are discussed and some suggestions are presented for the pressure plate testing.Secondly, a good control of suction during unsaturated triaxial testing can be achieved by adjusting water content according to SWCC. The friction angle associated with net normal stress of intact expansive soil varies with suction(water content). It seems that the contribution of matrix suction to the shear strength depends on the cell pressure. The shear strength derived from SWCC using the procedures summarized in this thesis are compared with test results, and the possible limitations associated with the procedures are discussed. It is shown that the variation of the apparent cohesionτus with suction can be described by power function.Thirdly, an empirical relation between dynamic modulus and dynamic strain or cell pressure are presented for unsaturated expansive soils, it was found that this relationship is similar to Janbu's empirical equation. The dynamic modulus of intact expansive soil increases as the consolidation ratio and cyclic frequency increase. It seems that the damping ratio of the intact expansive soil decreases as the water content decreases, and that the value range of damping ratio generally ranges from 0.03 to 0.15.Fourthly, the dynamic shear strength of intact expansive soil increases as water content decreases, which depends on the cyclic frequency. As cyclic frequency increases, dynamic cohesion increases, while the dynamic friction angle decreases.Finally, the results of static and dynamic tests are compared. In general, the dynamic modulus is larger than the static modulus, but the difference decreases as the cyclic process continues, especially for saturated soil. Compared with static shear strength, the dynamic shear strength can be larger or smaller, depending on two factors: loading velocity, which causes the shear strength increases, and disturbing effect due to cyclic load. The relative importance of these two factors depends on the water content of soil. The contribution of matrix suction to the dynamic shear strength is larger than that to the static shear strength.