Approaches To The Triple-Shear Unified Elasto-plasticity Bounding Surface Model For Saturated Sand

Soils around the subway tunnels at great depth are subjected to the static loads during the construction and dynamic loads during the operation.In this paper,the constitutive model of saturated sand with different compactness is studied based on the bounding surface model and the triple-shear unified strength theory in accordance with the deformation characteristics of saturated sand under the action of dynamic and static loads in Nanchang metro project.The state parameter considering the phase transformation void ratio of dense sand are introduced in the modified Cambridge model to reflect the dilatancy and strain softening characteristics of the soil under loads.In order to solve the problem that the modified Cambridge model can not consider the effect of full stress state and cohesive force on the stress and strain characteristics of soil,the triple-shear unified yield criterion is combined with the above models.By means of the methods of the equicalent substitution and the coordinate translation,we get the yield surface equation which can reflect the effect of the intermediate principal stress,stress interval effect,failure difference between tension and compression and different cohesion on the soil's stress and strain.The triple-shear unified bounding surface constitutive model of saturated sand is established on the basis of the bounding surface theory.The sand under the import and export floor of a station of Nanchang subway is taken as the test soil sample for experiments which to verify the correctness of the model.In order to consider the effect of cohesive force on soils,t he mud content of sand is set with 0%,5% and 10% by adding fine clay particles.The conventional three axis tests are carried out with the surrounding pressure of 100 kPa,200kPa and 400 kPa.The isotropic consolidation compression tests are carried out by the step by step loading method,and the maximum pressure is 1600 kPa.Single variable undrained uniaxial cyclic loading tests are carried out with the mud content of 0%,5%,and 10%,the surrounding pressure of 100 kPa,200kPa and 400 kPa,the frequency of 0.5Hz,1Hz and 2Hz and the amplitude of 30 kPa,40kpa,50 kPa etc.The parameters of the model are determined according to the above correlation tests.The new model can reflect the stress-strain characteristics of saturated sand under the conventional tri-axial stress state and the undrained cyclic tri-axial stress state by comparing the test data and the numerical data.Finally,the relevant numerical calculation program is compiled by Fortran 90 according to the stress-strain relationship of the new model,and numerical experiments are carried out in the true tri-axial stress state.Under the monotonic loads,the influence of the influences coefficient of intermediate principal stress on soils are studied,and under cyclic loads,the effects of intermediate principal stress,minimum principal stress,the soil mud content and the amplitude are researched.