Experimental Study On Small Strain Stiffness And Constitutive Model Of Shanghai Soft Soils

In recent years,a large number of underground space development and construction have been carried out all over China,and the scale is getting larger and larger.It is more and more important to predict and control the deformation of underground engineering such as foundation pit and tunnel.A large number of measured data show that the strain of soil around the underground project is in a state of small strain and the characteristics of small strain of soil play an increasingly important role in the analysis of underground project.Shanghai shallow soft clay has the characteristics of high water content,high compressibility and low bearing capacity,etc.Although the mechanical properties of soil have been studied for a long time,the mechanical properties of soft clay in the state of small strain have not been systematically studied.It is not only of great scientific value but also of practical engineering significance to study the strength and deformation law of Shanghai shallow soft clay under the condition of small strain.In this paper,the mechanical properties of Shanghai shallow soft clay under small strain were studied through a series of laboratory test systems,and the strength and deformation laws of Shanghai soft clay in the whole strain range from small strain to large strain were analyzed and summarized.The test results were simulated by using three small strain constitutive models,and the constitutive parameters suitable for Shanghai soft clay were obtained.The main research contents include the following aspects:1.Carried out a series of laboratory tests(standard consolidation test,triaxial consolidation non-drainage shear test,load-unloading-loading triaxial test),to study the compression and shear characteristics of Shanghai(2)?(6)layer soft clay in a large strain range.Standard consolidation test results show that the(3)layer of silty silty clay,the(5)-1 layer of silty clay and the(6)layer of silty clay consolidation curve has no obvious inflection point;The(4)layer of silty clay phase has a strong structure,consolidation curve shows three.Triaxial test results show that: the(2)layer of silty clay,the(5)-2 layer of silty clay and the(6)layer of silty clay show obvious dilate,the soil layer is in the state of overconsolidation.The loading and unloading test results show that the stress-strain curve drops sharply at the initial stage of unloading,and the rate of decline slows at the later stage.In the process of reloading,the initial stress-strain curve increases sharply,the growth rate slows down in the later stage,and the growth rate slows down further after the unloading point is exceeded.2.The improved small strain triaxial instrument was used to conduct triaxial compression and tensile tests on Shanghai the(2)?(6)layer soft clay with small strain.The strength and deformation law of each layer clay in the state of small strain to large strain were obtained,as well as the initial shear modulus and the attenuation law of modulus with the increase of strain of each layer clay.The experimental results show that the shear modulus of different soil layers exhibits a nonlinear attenuation law with the increase of strain.Under the condition of very small strain,the shear modulus does not change with the change of strain,but is a constant value,that is,the initial shear modulus;Under the condition of small strain,the shear modulus decreases rapidly with the increase of strain.Under the condition of large strain,the shear modulus decreases slowly with the increase of shear strain.After normalization of the modulus,it is found that the attenuation law of the modulus of different soil layers is not related to the initial shear modulus,but to the stress state and overconsolidation of the soil.3.This paper introduces three kinds of Small strain constitutive models,including HS-Small model,Hardin & Black model and non-linear elastic constitutive model(Ramberg-Osgood model and Hardin Drnevich model),including the basic theory of the model and the method of parameter evaluation.Among them,there are 13 constitutive parameters of HS-Small model,which are obtained through simulation to provide reference for the numerical analysis of foundation pits in Shanghai.Hardin & Black model is used to calculate the initial shear stiffness.Through the simulation of the initial shear stiffness obtained by small strain test,the calculation method of the initial shear stiffness of soil mass in Shanghai area is determined.Hardin-drnevich model represents the change rule of shear stiffness with strain.By simulating the law curve of shear stiffness with strain attenuation under the condition of small strain to large strain of Shanghai soft clay,the theoretical relationship between stiffness and strain is obtained.