Experimental Study On The Phase Transfornation State Of Saturated Sand

So far, dilatancy and anisotropy are the fundamental characteristics of sands, which make it different from other materials significantly. As the object of study in this thesis, the salient difference in behavior of sands from that of ordinary solids can be found in the volume change under shearing and its dependency on stress. For example, loose sand will contract continuously upon application of a deviatoric stress. On the other hand, dense sand may only contract initially, and thereafter, expand due to a tendency for the particles to slip against and override each other for deformation to occur.The phase transformation state of sand is in the non drainage experiment, which means the change of pore water pressure is from increased to decrease.The phase transformation state of sand is in the drainage experiment, which refers to the change of soil volume deformation from compression to the beginning of expansion.In this paper, the influence of initial density and confining pressure on the phase transition state is studied by three axis shear test of saturated sand.With the help of the saturated sand under drained triaxial shear test, verify the constitutive model which is based of phase transfornation state, and it is a good description of soil shear expansion rule and the strain hardening and strain softening behavior.In this paper, the main research results are as follows.(1) In view of the saturated sand in Beijing, by carrying out the same initial void ratio, three axial compression tests with different initial confining pressure no drainage as well as the initial confining pressure is the same, the initial void ratio is different from the three axial compression tests of drainage and non drainage, and the relation between the phase transfornation stress ratio and the average effective stress is obtained. The variation of various state parameters (such as phase transfornation stress ratio and peak stress ratio) with the initial void ratio and initial confining pressure, revealed that the deformation characteristics of saturated sandy soil is infected by the initial void ratio and the initial confining pressure. Test results show that:1) In the case of drainage and non drainage, the ratio of the phase transformation is linear with the logarithm of the mean effective stress; 2) In the case of the drained and undrained, The initial void ratio is the same, the value of phase transformation stress increases with the increasing of the initial confining pressure, the peak stress ratio decreases with the increasing of the initial confining pressure, and eventually the two tended to be close to the critical state stress ratio; 3) Under the condition of drainage and non drainage, the phase transformation stress ratio increases with the increase of the initial void ratio when the initial confining pressure is constant, The value of the peak stress ratio decreases with the increase of the initial void ratio, and both of them are close to the critical state stress ratio.(2) Based on the results of experimental research, the constitutive model of sandy soil based on phase transformation is verified. The phase transition state line is used as the reference line, and combined with the critical state, The state parameters β are introduced to describe the deformation behavior of the soil. On this basis, The constitutive model of sandy soil based on the phase transformation in the three axis stress space is deduced. Using the test results, the calibration method and the specific value of each parameter in the model are determined. At last, by comparing between test results and model prediction results, the applicability of the constitutive model is verified in the prediction of the law of soil shear expansion and the characteristics of strain hardening and strain softening.