Establishing And Application Of 3D Elastic Visco-plastic Constitutive Model Considering Unloading And Disturbance

Disturbed state concept theory(DSC) maintains that actual response state of materials under loading comes forth as a weighted average value of two conference response states, namely relatively intact state and fully adjusted state, with a weight factor named disturbance factor. Aimed at rheologic problems under unloading of soft soils, a three-dimensional rheology constitutive model considering unloading and disturbance were established based on disturbed state concept theory and results of laboratory test. Main work and results in the paper are showed as follows:1. Rheologic laboratory tests on stress paths were designed and carried out based on the analysis of stress results of the excavation by FEM. The main results of laboratory tests were listed as follows :(1)The linear relation between void ratio and log-time was found and the slopes of void ratio-logt curves were almost the same in one-dimensional unloading laboratory test; the exponential decay function was adopted for heave modulus-t curve under constant unloading ratio; heave modulus is much higher than compression modulus and the least ratio between them is about 2～7.(2)With the increase of unloading,instantaneous strain and three periods in creep strain,namely damp creep, equal velocity creep and acceleration creep, occured both in active zone and in passive zone. However, equal velocity creep can be found not to change acceleration creep in the test.(3)Pore pressure decreased under unloading, then increased to positive value after unloading whether in active zone or in passive zone. so the most dangerous time is not at the completing the construction but after the construction when pore pressure reaches the biggest2.Based on laboratory test results under unloading and the concept of equivalent time according to Yin Jian-hua, one-dimensional elastic visco-plastic constitutive equations including substep unloading and constinuous unloading were established. The elastic visco-plastic constitutive model under unloading and Yin-Graham model can be normalized as one dimensional elastic visco-plastic constitutive model.3.The generaliazed Kelvin model was adopted for relative intact state and extended to three-dimensional stress state. By taking modified Cam-bridge model as yielding function and the associated flow rules, three-dimensional elastic visco-plastic model was established based on equivalent time and adopted for fully adjusted state. Volumetric strain and shear strain were considered in the disturbance function according to Desai. Three-dimensional rheologic constitutive model considering unloading and disturbance was established based on disturbed state concept theory.4.Three-dimensional disturbed state model was applied to deep excavation near metro tunnels. Dynamic construction process of the pit was simulated with FLAC^3D. The results indicated that:(1)Excavating different zone in pit has a different impact on deformation of retaining wall. Rheology velocity obviously increased when excavating the zone near diaphragm wall. Therefore ,the pit should be excavated symmetrically with block excavation and restriction on the time cost of excavation. (2)The model can simulate the time-dependent longitudinal non-uniform settlement of metro tunnel...