Study On The Unloading Creep Characteristics And Constitutive Model Of Marine Deposit Soft Clay Under Excess Static Pore Water Pressure

Marine sedimentary soft soil is widely distributed in coastal areas such as the Pearl River Delta and the Yangtze River Delta.Which is the most population concentrated and economically developed area in China.With the rapid development of urban construction in the above areas,a large number of soft soil underground space has been developed,such as: Subway shield tunnel,soft soil deep foundation pits and so on.The engineering practice shows that the deformation,instability,and damage of the surrounding rock of soft soil do not occur immediately during the excavation or after the excavation,but they experience a period of history,that is,the unloading creep damage of soft soil occurs.In addition,there will usually be high groundwater in the coastal soft soil area,the construction vibration will cause the excess static pore water pressure which is difficult to dissipate.It will weaken the structure of soft soil particles,further aggravate the occurrence of soft soil unloading creep,and eventually cause the rheological disaster of soft soil.Therefore,it is of great scientific significance and engineering application value to study the unloading creep characteristics and failure law of soft soil under the action of excess static pore water pressure to ensure the construction safety and reasonable construction of underground space development of water-rich soft soil region.Based on the experimental study of the unloading creep characteristics of soft soil,this paper takes Shenzhen marine sedimentary soft soil as the research object,carries out the undrained triaxial unloading shear test and unloading creep test under 4 kinds of initial excess static pore water pressures,analyzes the test results and establishes the unloading creep constitutive model.The specific research contents and results are as follows:(1)The unloading shear test of soft soil under different initial excess static pore water pressures was carried out by using TSZ-2S automatic stress-strain control triaxial instrument,and the loading stress path test was added to compare with the unloading stress path.Based on the test results,the unloading shear mechanical properties of soft soil under various experimental conditions were studied.the influence of stress path and excess pore water pressure on the shear mechanical properties of soft soil was analyzed,and the variation law of the initial unloading modulus of soft soil with the initial excess static pore water pressure was obtained by fitting the hyperbolic function of the stress-strain curve,which has certain guiding significance for controlling the excess pore water pressure in practical engineering.(2)According to the unloading shear mechanics test results of soft soil,the unloading creep test under the same initial test conditions was carried out.It was found that the ultimate failure load of unloading creep was obviously lower than the unloading shear mechanical strength,and the creep deformation and pore water pressure variation of soft soil unloading creep under different initial excess static pore water pressures was obtained.The results show that the higher the initial excess static pore water pressure,the more obvious the creep deformation of soft soil,the lower the limit failure load of creep.Therefore,the excess static pore water pressure should be controlled as far as possible in practical engineering.(3)Further analysis of the experimental results show that the unloading creep of soft soil is nonlinear.The higher the initial excess pore water pressure,the more obvious the creep nonlinearity,and the initial excess static pore water pressure also has a significant weakening effect on the yield stress of soft soil unloading creep.(4)Based on the soft soil unloading creep test results,the elastic modulus in the Kelvin model satisfies the nonlinear functional relationship with the initial excess pore water pressure and time as variables,and the improved nonlinear viscoelastic Merchant model and nonlinear viscoplastic Merchant model are obtained,a nonlinear viscoelastic plastic unloading creep model of soft soil under the action of different initial ultra-static pore water pressure was established by connected the two models.The BFGS algorithm in 1stOpt was choosed to identify the parameters in the model,the identification effect is ideal and the error is small.