Study On Mechanical Properties Of Unsaturated Loess Based On Suction-controlled True Triaxial Tests

In most cases,matric suction and net stress are used as two independent stress state variables to describe the stress-strain-strength characteristics of unsaturated loess.In the past,suction-controlled tests are usually carried out on consolidation apparatus,direct shear apparatus and triaxial apparatus,which promote the simulation of the key characteristics of unsaturated soil mechanical properties.However,these test methods can only allow stress loading along static,one-dimensional,axisymmetric confined stress path and deformation mode.In fact,in the actual engineering,the force on the soil is relatively complex,the force in three directions on the soil is often asymmetric,and the existence of gas phase in unsaturated soil makes the properties of unsaturated soil in the actual engineering become more complex.The loess slope,supporting structure,pavement foundation beam and shallow buried foundation above the groundwater level may be affected by the three-dimensional stress gradient caused by the change of net stress state variable.In order to reflect the complex stress state of unsaturated loess more truly in actual engineering,and more truly simulate the influence of matric suction on the mechanical properties of unsaturated loess,it is necessary to carry out true triaxial test of unsaturated loess under controlling suction condition.Based on the new suction-controlled true triaxial apparatus,a series of suction-controlled true triaxial shear tests and suction-controlled Isotropic compression tests were carried out on the unsaturated remolded loess.The influence of matric suction and intermediate principal stress conditions on the mechanical properties of unsaturated loess was combined to study.The relationship between the related strength parameters of unsaturated remolded loess and the matric suction and intermediate principal stress parameters was studied.The failure trajectories of unsaturated remolded loess in the p-q plane and π plane were described.The BBM model was modified by considering the nonlinear relationship between the parameters of the model and the matric suction,and the variation of the intermediate principal stress parameters was considered in the modified model.The following research results are obtained in this paper:(1)Based on the original true triaxial apparatus,a new kind of suction-controlled true triaxial apparatus with rigid-flexible-flexible mixed boundary is developed.The apparatus is equipped with a high intake value of clay plate and has a separate independent pore-gas and pore-water control and measurement system,which can enable control over a wider suction range and stress path range.The yield characteristics of remolded loess under isotropic compression with suction-controlled condition were studied based on the new apparatus.(2)The mechanical properties of unsaturated remolded loess under multi-axial stress paths with different matric suction and different intermediate principal stress parameters were studied by using the new suction-controlled true triaxial apparatus under different matric suction and different intermediate principal stress parameters,thus,the matric suction and intermediate principal stress were combined to study.The stress-strain curves of unsaturated remolded loess with different matric suction and different intermediate principal stresses were studied.Thus,the three-dimensional stress gradient induced by the change of net stress state variable in actual engineering is simulated.(3)The variation law of the correlation strength parameters of unsaturated loess with matric suction and intermediate principal stress parameters in the failure state was studied.A function g(b,h)was established to describe the relationship between the strength parameters of unsaturated remolded loess and the intermediate principal stress parameters approximately,so as to use the axisymmetric state to predict the strength parameters of other middle principal stress states.(4)The influence of matric suction on the strength failure line on the π plane and the strength failure surface in the principal stress space of the remolded loess was described.Matric suction has significant influence on the size,shape and position of the strength failure line on the π plane and the strength failure surface in the principal stress space of the unsaturated remolded loess.By comparing the predicted failure lines of different strength criteria on theπ-plane with the test points,the experimental results of unsaturated remolded loess under suction-controlled condition are in good agreement with those of the predicted failure lines of Lade-Duncan strength criterion.(5)The BBM model of unsaturated soil based on the critical state is analyzed,and the stepwise iterative calculation method at constant-b and constant-p(b=0)condition is given.Considering the nonlinear relationship between the relevant model parameters and matric suction in BBM model of unsaturated soil,the power function and exponential function were used to modify the model in two ways,and the modified model was verified.The function g(b,h)was used to consider the influence of the intermediate principal stress parameters in the modified BBM model,and the model was verified.