Research On The Constitutive Model Of Unsaturated Loess

The loess is a kind of typical unsaturated soil, the constitutive relation of the loess has always been a focus in the research of engineering properties for loess. The L5 loess specimens were taken from the south plateau of Jingyang County in Shaanxi Province, and a systematic experimental study were carried out. Complete successions of the loess strata form S0 to L9 can be founded on the stratigraphic profile in Jingyang County, Shaanxi Province. Landslides within the loess strata have been well developed along the northern margin due to the agriculture irrigation. The L5 samples(Q2) for tests locate in main shear zone of the landslide here,is a typicalunsaturated structural loess. In the paper, the physical and mechanical paramaters of the L5 loess were obtained through the tests, and the constitutive model for unsaturated loess which could reflect the deformation characteristics of the loess was established, the main work and results are as follows:(1) The conventional triaxial CD tests were conducted on undisturbed samples and remoulded L5 samples respectively under five different moisture contents, i.e. 8%, 11%, 15%, 20% and 25%, and five different confining pressures, i.e. 100 kPa, 200 kPa, 300 kPa, 400 kPa and 500 kPa. It can be obtained that: the steady-state effective friction angle did not change with moisture content, which is basically a stable value; however, steady-state effective cohesive decreased significantly with the increase of the moisture content when the moisture content is lower than the plastic limit and it tended to a stable value when the moisture content is higher than plastic limit.(2) Based on the conventional triaxial CD tests and soil water characteristic curve obtained by the Fredlund SWCC pressure plate apparatus, the three different expressions of unsaturated strength theories put forward by Bishop, Fredlund and Lu Ning were analyzed. To determine the values of parameters χ and υb in Bishop’s and Fredlund’s unsaturated strength formulas, the soil water characteristic curve must be known in advance. However, the relationship between suction stress and the volumetric water content in Lu Ning’s strength theory can be obtained with conventional triaxial tests under different moisture contents and it can avoid the matric suction measurement which is very complicate and inconvenient, and is convenient for popularization and application in engineering practice.(3) According to the deformation and strength parameters of undisturbed and remoulded loess through the conventional triaxial CD test under different moisture contents and different confining pressure, an unsaturated ideal elastic-plastic model for L5 loess was obtained and the applicability of the model were verified; combined with the K0 consolidation tests for undisturbed and remoulded specimens under the different moisture contents, the expression of parameter μ in Duncan- Chang which ia an nonlinear elastic model was modified and nonlinear elastic models valuable for undisturbed and remolded unsaturated loess were obtained, and then the applicability of the models were verified;(4) The investigations on the oedometer tests of L5 remoulded and undisturbed specimens with different moisture contents and the same initial void ratio shown that the swelling line of samples under different moisture contents paralleled to each other in the a semi-logarithmic plot; similarlly, the slope of the normal consolidation lines were found to be constant; the magnitude of the slope of normal consolidation line λ was higher that of the swelling line κ; the higher the initial moisture content of was the smaller the void ratio the specimen would have with the same stress condition; the undisturbed soil could maintain a relatively larger void ratio than the remoulded specimen under the same skeleton stress.(5) Based on the modified Cambridge model, an unsaturated constructive model for remoulded loess was constructed by introducing the unsaturated state variable of remoulded loess ρs which represents a void difference between unsaturated remoulded loess and saturated remoulded loess under the same mean skeleton stress, and combining the overconsolidated state variable ρe. The parameter ρs can be determined via the consolidation curves of remoulded samples under unsaturated and saturated conditions. The proposed constructive model here can reflect the overconsolidation properties of unsaturated remoulded loess.(6) Furthermore, upon the constructive model of unsaturated remoulded loess, an unsaturated constructive model which can better reflect the structural property of undisturbed loess was proposed by introducing the structural state variable of remoulded loess ρst which represents a void difference between undisturbed loess and remoulded loess under the same mean skeleton stress and saturation.