True Triaxial Tests On Coarse-Grained Soils And Study On Constitutive Model Validation

The stress and deformation of high earth rockfill dam is one of the important issues in the field of dam engineering. The rationality of the results of stress and deformation analysis of earth rock dam is mainly determined by the rationality of the constitutive model and the parameters. Although soil mechanics workers have done a lot of research work in the aspects, but because of the earth rockfill dam filler, especially the complexity of coarse-grained materials mechanical properties, the dam stress deformation analysis results are not very satisfactory. In recent years, in order to study the stress-strain relationship of the granular materials under complex stress conditions, the Changjiang River Scientific Research Institute developed a large real three axis instrument, provide the possibility for thorough research on the stress-strain relationship of coarse aggregate and construction of constitutive model of coarse aggregate. This thesis focuses on two kinds of coarse aggregate of Lianghekou, carried out systematic conventional triaxial and true triaxial test, make a preliminary analysis on the stress-strain relationship of the granular materials under complex stress conditions, expected to provide useful reference for the study of the stress and deformation of high earth rockfill dam.Through reading a great deal of literature, this thesis reviewed the development status of the triaxial instrument, and learned the process and the results of the research on the strength and deformation characteristics of coarse grained soil by the scholars at home and abroad. This thesis mainly introduces the test methods and instruments of the conventional triaxial test, the true triaxial test and the plane strain test.In this thesis, we make use of Lianghekou mixture material and slate material to carry out conventional triaxial test, true triaxial tests with a range of different confining pressure, different intermediate principal stress coefficient and plane strain tests. Integration analysis and discussion of the data obtained from the experiment, to study the strength and deformation characteristics of coarse grained soil under complex stress state; considered with Mohr-Coulomb criterion, Lade-Duncan criterion and Matsuoka-Nakai criterion, study the applicability of each failure criterion; based on the analysis of nonlinear shear expansion model proposed by Professor Cheng Zhanlin, Changjiang River Scientific Research Institute, derive and discusse model parameters, the process of constructing the model stiffness coefficient matrix is described systematically, based on comparison and verification of the experimental data curve, formula derivation calculation curve and Stiffness matrix inverse curve, used to test the applicability of the dilatancy model and the stiffness matrix for rockfill. The main results of this thesis are as follows:1. The strength characteristic of coarse grained soil is closely related to the middle principal stress coefficient. When the initial density and confining pressure are constant, the middle principal stress coefficient in different medium makes the strength of coarse grained soil different. On the whole, the strength of coarse grained soil has been increased gradually with the increasing of the middle principal stress.2. That the failure data points of coarse grained soil under different confining pressures compared with the damage trajectory on ? plane of Duncan strength criterion, SMP strength criterion and M-C strength criterion indicated: M-C strength criterion based on conventional triaxial test, the deviation is obvious when predicting the strength of soil mass, M-C strength criterion is used to be conservative in engineering, the Duncan-Lade failure criterion is the most close to the test results.3. It is obvious that the effect of intermediate principal stress on deformation characteristics of coarse grained soil under three dimensional stress conditions. When the confining pressure is constant, the slope of the curve of the relationship between the partial stress and the strain increases with the increase of the coefficient of middle principal stress; the absolute value of the slope of the principal strain curve increases with the increase of b value, that is to say the absolute value of the slope of the curve and the b value was positively correlated.4. Preliminary verified using the dilatancy model to describe the stress-strain relationship of coarse aggregate is reasonable. Based on the experimental data, the formula is derived and used to deduce the matrix of the model, the correctness of the model is verified.