Study On Direct Shear Creep Test Analysis And Constitutive Model Of Soft Soil

Soft soil in China is mainly distributed in the lake,the middle and lower reaches of the river and the Delta region of the coastal areas.The strong creep properties of soft soil have great influence on the post-construction settlement of the buildings in the actual project.With the rapid development of the urbanization of the coastal and riverside areas in the Pearl River Delta region,the development and utilization of a large number of soft soil projects have been promoted.At the same time,many building foundations that pass through or even build on the soft ground may cause uneven post-construction settlement with the long-term creep properties of soft soil,which will become a potential threat to the safety and stability of buildings.The suitable creep constitutive model can accurately and truly reflect the variation of stress,strain and time during the creep process,which can also better predict the development of soft soil deformation in practical engineering.Due to the different depositional environments of soil layers in different regions,the composition of soft soil in each region is more complex,so that the creep characteristics and constitutive model of soft soil are also different.Therefore,the method of combining direct shear creep test and rheology theory is adopted to study the original soft soil in Luogang,Guangzhou.In addition,based on the behavior and mechanism of macroscopic creep mechanics,the creep properties and creep constitutive models of soft soil are explored in depth.The main research work and conclusion of this paper are as follows:(1)The creep behavior of Luogang original soft soil was studied by the creep test of direct shear.The experimental results show that the shear strain of Luogang soft soil tends to change staircase with the increase of time under the action of grading and loading.The instantaneous shear deformation of Luogang soft soil will occur at the moment of loading at all levels,and the deformation is independent of time.Under the lower shear stress,the creep deformation of Luogang soft soil is smaller and the deformation reaches a stable state in a short time,which mainly shows the characteristics of decay creep.Under the medium and high shear stress,Luogang soft soil has both obvious attenuation creep characteristics and isokinetic creep process,and the higher the shear stress is,the longer the creep time is,the more stable and longer the creep deformation is.The soft soil of Luogang has significant creep properties,meanwhile,and its phenomenon of creep deformation increaseing with stress will be more obvious,and its creep time is longer.(2)Based on the relationship curver of strain-time in Luogang soft soil,the fitting effects of three typical linear rheological constitutive models on the test curves are compared and analyzed.It is found that the simulation results of the Merchant model are better only under the lower and higher shear stress levels.The fitting effect of Burgers model is not ideal at all shear stress levels.The five elements generalized Kelvin model can better describe the variation characteristics of each creep curve.Because the parameter of the fitting parameter changes with the shear stress in the linear rheological constitutive model,any constitutive equation under the parameters of linear rheological constitutive model can not completely simulate the creep curve process.It shows that the linear rheological constitutive model can not describe the creep characteristics of the soft soil more accurately and conveniently.(3)Based on the creep stress-strain curve of Luogang soft soil,the nonlinear characteristics of the creep process of Luogang soft soil are analyzed.When the shear stress is less than the yield stress,the creep behavior of Luogang soft soil is mainly viscoelastic creep properties.When the shear stress is greater than the yield stress,the creep process of the soft soil in Luogang begins to show the viscoplastic creep properties.The whole creep process of Luogang soft soil has significant nonlinear characteristics,and it is more obvious with the passage of time.(4)Based on the analysis results of the fitting effect and the relationship between creep compliance and time variation based on the linear rheological model,a nonlinear creep constitutive model for soft soil is established.Considering the nonlinearity of soil rheology,the creep deformation of Luogang soft soil is simplified to three parts: linear viscoelastic,linear viscoplastic and nonlinear viscoplastic.The five element generalized Kelvin model is selected to model the linear viscoelastic part.The linear viscoplastic part is described by a combined creep model,which consists of a friction slider parallel to a single spring and a parallel Maxwell model.However,the nonlinear viscoplastic part is simulated by an empirical model of power function.Combined with the above three models,a nonlinear creep constitutive model for soft soil of semi theoretical and semi empirical is established.The 9 fixed parameter values in the model are solved by Origin9,which shows that the model overcomes the variability of parameter values in the linear rheological constitutive model.At the same time,the model fits well with the results of all creep curves in direct shear tests.In addition,The model has achieved good results in predicting the creep deformation of natural soft soil in Nansha,Guangzhou,which validates the rationality and applicability of the nonlinear creep constitutive model to describe the creep deformation process of the soft soil in the Pearl River Delta region.