Study On Unsaturated Mechanical Characteristics Of Granite Residual Soil In Shenzhen Area And Its Application Of Constitutive Model

The granite in coastal areas of my country is strongly weathered by climate and topography to form residual soil.In the study of its hydraulic,mechanical,deformation characteristics and constitutive model,not only the complex engineering properties of residual soil itself should be considered,but also its unsaturated property should not be ignored.Due to the needs of national strategic deployment and major decisions,such as the construction of the"Guangdong-Hong Kong-Macao"Greater Bay Area,there will definitely emerge a large number of new construction,new construction and new renovation projects.Most of the residual soils in it are in an unsaturated state,and the mechanical properties of the soil will change significantly in the process of changing from a saturated state to an unsaturated state.The increase of saturation will not only deteriorate the strength of the soil due to the decrease of the effective stress of the soil,but also change the properties of the soil,such as the expansion or contraction of the yield surface.Unsaturated granite residual soil is a very complex soil material,so it is necessary to conduct systematical research on its properties and develop the unsaturated constitutive model suitable for granite residual soil.At present,most studies on foundation pit instability under convection-solid coupling are based on the effective stress principle,and few studies consider the change of soil material properties caused by saturation change.Therefore,it is important to construct a constitutive model of unsaturated soil material and apply it to practical foundation pit projects.Taking the granite residual soil of Shenzhen,one of the construction cities of guangdong-Hong Kong-Macao Greater Bay Area proposed in the national"14th Five-year Plan"as the research object,relevant experiments are carried out to study the hydraulic power,mechanics and deformation characteristics of the local granite residual soil.Based on the modified Cambridge model,the unsaturated secondary modified Cambridge model suitable for granite residual soil is constructed.Under the framework of fluid-solid coupling calculation,the secondary development of ABAQUS realizes the modeling and analysis of a foundation pit in Shenzhen,and provides engineering practice guidance for the Guangdong-Hong Kong-Macao Greater Bay Area and other regions in China.The main research results and conclusions are as follows:(1)The soil-water characteristics of soil samples were tested by GEO-Experts stress-related soil-water characteristic curve pressure plate instrument.This paper analyzes the variation laws of soil-water characteristic curves under different vertical stresses,particle size distribution and dry-wet cycles.Among them,the vertical stress has the most significant influence on the soil-water characteristic curve.The pore and saturation of soil samples with vertical stress are reduced,and the soil-water characteristic curve always remains above the SWCC curve without vertical stress and undisturbed sample is more sensitive to vertical stress than remolded sample.The effect of particle size on soil-water characteristic curve is weak,and the increase of coarse particles will reduce the air intake value of soil-water characteristic curve.The hysteresis effect of soil samples is obvious in the drying-wetting cycle.The area of the hysteresis loop decreases significantly with the increase of the number of drying-wetting cycles and the vertical stress,and the hysteresis loop tends to disappear after three drying-wetting cycles.By comparing Van Genuchten model,Fredlund&Xing model and Gardner model,it is found that Van Genuchten model has the most outstanding fitting effect,and the model parameters are used to predict the permeability and strength coefficient fitting model of unsaturated granite residual soil,which provides parameter support for the verification of shear strength in Chapter 4 and the subsequent numerical model of foundation pit in Chapter 6.(2)Based on GDS unsaturated triaxial apparatus,unsaturated triaxial shear tests are carried out on the remolded soil samples.This paper analyzes the effects of different particle size distribution,net consolidation pressure and matric suction on the strength and deformation characteristics of soil samples.The stress-strain curves of unsaturated remolded soils are mostly strain-hardening under the combined influence of matric suction and net confining pressure,and strain softening occurs only at high matric suction and low net consolidation pressure.The strength of the remodeled soil increases with the increase of the matrix suction and net confining pressure,and the volume variable increases with the increase of the matrix suction and net confining pressure.Shear dilatation occurs only under the conditions of low matrix suction and low net confining pressure.With the increase of coarse particle content,the water holding capacity of remolded soil decreases,and the water content of samples decreases and the strength increases under the same matric suction.Compared with matrix suction and net consolidation pressure,particle size has little effect on the stress-strain and strength deformation of unsaturated remodeled soil.The shear strength parameters obtained from the test can not only verify the prediction results of soil-water characteristic curve in Chapter 3,but also provide experimental support for the constitutive model proposed in Chapter 5.(3)On the basis of the modified Cambridge model,an unsaturated secondary modified Cambridge model is proposed,which takes into account the tensile yield strength P_tof the soil and the influence coefficient?of strain strengthening.The former moves the yield surface to the left as a whole,and the latter expands the yield surface to the right.At the same time,the Jacobian matrix of the unsaturated secondary modified Cambridge model is deduced to improve the convergence of numerical modeling.Under the solution framework of seepage-force coupling,the unsaturated secondary modified Cambridge model UMAT subroutine is developed and embedded in ABAQUS.And the USDFLD subroutine is used to realize the control of soil saturation on the yield surface of the unsaturated secondary modified Cambridge model in the solution process.(4)The modified Mole Coulomb model and the unsaturated extended modified Cambridge model are used to establish the numerical model of the granite residual soil foundation pit in Shenzhen area.The optimal embedded depth analysis of the saturated soil foundation pit is verified by the prepared large-scale indoor physical model test.By comparing the calculation results of the saturated soil foundation pit and the unsaturated soil foundation pit at different embedding depths,this study takes the pile top displacement,the pile body displacement and the surrounding soil settlement as the basis for the instability and failure of the foundation pit,and finally obtained the optimal embedding depth of saturated soil foundation pit and unsaturated soil foundation pit.The research results have a guiding role for the foundation pit engineering of the supporting structure in the diaphragm wall in Shenzhen,and can provide engineering reference for other areas such as the Greater Bay Area.