Research On Creep Test For Soft Soil Under The Condition Of Different Stress Paths

Nowadays,our country is undergoing the rapid urbanization development,the big cities are mainly distributed by the coastland or the regions near the rivers and lakes,where the soft soil is the main foundation subsoil.The characteristics of the soil is lower shear strength and permeability,bigger void ratio,higher water content,compressibility and sensitivity,high organic matter.And the soft soil shows the obvious creep properties under the different loadings,which is one of the main problems of engineering construction.The soft soil usually in the surface or shallow,within the scope of the excavation depth and impact,so the foundation pit engineering has significant time-space effect.The construction of the deep foundation pit engineering was a macro process with unloading for the soil.The soil was divided into several stress areas,and for the different position,the deformation patterns were also different.Therefore,the research of mechanical properties of soft soil under the different condition is very important to the exploiture of subterranean space in the big cities.In this paper,firstly a great deal of triaxial creep tests of soft soil was conducted using the original state of silt in Fuzhou region.On the basis of the test results,the 3D creep constitutive model was deduced,which was suitable for vary of stress paths on the soft soil foundation.Finally,with the assistance of external subroutine interface in ABAQUS,the model was applied to numerical simulation in practical engineering.The main contents are listed as follows:Firstly,the suitable method to the triaxial creep test in different discharge load was chosen through the comparison of the methods of creep tests,so as to simulate the state of stress changes and the soil deformation pattern with the time in the process of the foundation pit excavation.The results showed that the deformation of silt soft soil had obvious nonlinear rheological properties,especially under the condition of axial loading and lateral unloading,the nonlinear characteristics was more apparent.And with the ratio of loading or unloading increased,the total deformation increased significantly.Secondly,on the basis of the isochronous curve which was from the triaxial tests,the Nishihara's model which considered the creep in different stress path was chosen and extended to 3D by the ways of analogy and parameter conversion.Then the three-dimensional Nishihara's creep model was established,which can be used in vary of stress path under the condition of different discharge load.For the aim of obtaining each parameter value of the 3D Nishihara's creep model,combining with the data of triaxial creep tests and the Origin curve fitting tool,the creep test curve was fitted and get the parameters to put into the inversion equation.Thirdly,the 3D model was previously applied to numerical analysis by using the subroutine interface of ABAQUS,and the correctness and applicability was verified through the simulation of the triaxial creep tests,which contain axial loading and double-axial unloading.Finally,for the two engineering examples(layered filling of embankment and excavation of deep foundation),the 3D models were established for numerical simulation,and the necessity of considering creep effect was illustrated through the result comparison.