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

Coarse-grained soils are the main materials of earth rockfill dams, and there are many high earth rockfill dams to be built whose height reach 200 even 300 metres, therefore it is urgency to study on the strength and deformation characteristics of coarse-grained soils under three dimensional stress states, as well as the anisotropic characteristics of coarse-grained soils.True triaxial test is an important method to study the response of soils in three dimensional stress states. A lot of true triaxial tests of sand and clay have been performed by researchers inside and outside. However, for coarse-grained soils including gravel, restricted by apparatus etc, few such investigation is done. The two true triaxial apparatuses in Hohai University are used to perform true triaxial tests respectively on fine gravel and coarse-grained soils. And then on the basis of these tests, a new three-dimensional failure criterion and a two-yield-surface elastoplastic model are presented.The specific operating methods of true triaxial tests on fine gravel using the ZSY-1 Type True Triaxial Apparatus in Hohai University are discussed. Subsequently a few of true triaxial tests on fine gravel have been performed, in which bothσ3 and b are invariable. On the basis of the tests, the influences of intermediate principal stress on the stress-strain relationship, relations between the three principal strains and the strength parametersφand M are studied separately.The TSW-40 Type True Triaxial Apparatus in Hohai University which is developed recently has been examined. Several factors which may influence the test results are investigated, mainly including the following three points: the influence of the sample end restriction; the influence of the friction between the sample cap and the water-bag box; the influence of the metal-rubber complex and the water-bag box.Series of true triaxial tests on coarse-grained soils have been performed, includingσ3- and b-constant tests, p- and b-constant tests, plane strain tests, single-direction loading tests and p- and q-constant tests. Deformation and strength characteristics of coarse-grained soils in three dimensional stress states have been studied.Based on the results of true triaxial tests on fine gravel and coarse-grained soils, a new failure criterion is presented and verified. Theφb~b relationship of the failure criterion is deduced, and the shape of the failure locus on theπ-plane is investigated. A two-yield-surface nonassociative elastoplastic model is established, including elastic part, yield functions, hardening rules and plastic potential functions. The model incorporates seven parameters which can all be determined from simple experiments such as conventional triaxial compression tests. The predicted stress-strain curves show that the model is capable of successfully representing the experimental measurements. Additionally, the model has been incorporated in a three-dimensional FEM program for the solution of an earth rockfill dam problem.