Analysis Of Coupled Mechanical Characteristics Of Creep And Unloading For Deep Foundation Pit In Soft Soil Of Subway Station

Deep marine sedimentary soft soil strata are widely distributed in the Pearl River Delta area.Due to the special mechanical behavior of the muddy soft soil in this area,the mechanical properties of the silt soft soil are significantly influenced by stress path and stress history.As a result,there is a significant spatio-temporal effect in the process of excavation of deep foundation pit in this area.At present,the creep-unloading mechanical characteristics of soft soil have not been fully considered in the design and construction of deep foundation pit,which leads to the great difference between the actual deformation and the calculated value of the soft soil deep foundation pit engineering in this area.The deep foundation pit project and its surrounding environment have great hidden danger of safety.In order to simulate the creep-unloading force of the typical silty soft soil in the Pearl River Delta region objectively and truly.In this paper,the creep and unloading mechanical properties of typical silty soft clay in the Pearl River Delta region are analyzed and studied by means of laboratory test,theoretical analysis and field test.Taking the deep foundation pit engineering of a subway station in the Pearl River Delta as the research background,the space-time coupling characteristics of the soft soil deep foundation pit engineering are systematically analyzed,and the research conclusions of reference value to the practical engineering are obtained.The specific research contents and conclusions are as follows.The main results are as follows:(1)Based on the laboratory triaxial creep tester of unsaturated soil,the undrained creep tests of typical silty soft soils in the Pearl River Delta region under different confining pressures were carried out,and the creep mechanical properties of the silty soft clay were obtained.A semi-theoretical and semi-empirical viscoelastic-plastic creep constitutive model,which can comprehensively reflect the nonlinear creep characteristics is established.(2)Using the GDS triaxial test instrument of unsaturated soil stress path,threedifferent unloading stress paths of RTE,TE and RTC were studied in the active and passive regions of foundation pit.The mechanical characteristics of unloading under different stress paths are obtained.The normalized equations under different stress paths are obtained by using the normalization method,and the constitutive models of unloading stress paths in the active and passive regions are derived.(3)Based on the secondary development function of ABAQUS finite element analysis software,the finite element analysis subprogram of soft soil unloading stress path constitutive model is programmed by Fortran language.(4)Based on the deep foundation pit engineering of a subway station in Pearl River Delta,a three-dimensional finite element model of creep-unloading coupling analysis for deep foundation pit excavation is established,and the space-time effect characteristics of soft soil deep foundation pit engineering are studied.Through the field monitoring data,the correctness of the analysis results is verified,and some research conclusions with practical engineering reference value are drawn.In this paper,the creep and unloading characteristics of typical soft clay in Pearl River Delta region are studied scientifically,and the influence of creep and unloading characteristics of soft clay on foundation pit excavation is analyzed,which provides theoretical and engineering application value for the design and construction of foundation pit in similar areas.