Deformation Mechanism And Control Of Large-scale Foundation Pit With Cut-off Wall Inserting Rock Near The Yangtse River In Wuhan

The construction of the Yangtze River Economic Belt covers a large number of underground spaces including 11 provinces,such as cities,urban rail transit,tunnel and commercial complex along the Yangtze River.At the same time,the underground construction accidents and disasters along the Yangtze River are also increasing with the development scale.Wuhan is located in the middle reaches of the Yangtze River,and the first terraces are distributed on both sides of the Yangtze River.The confined water connected by the Yangtze River,silty clay and silty sand foundation exist widely,and the foundation pit engineering is very difficult.Construction is easy to induce environmental deformation and water permeability to cause collapse disaster accidents.For example,there are different seepage characteristics in different particle composition and interlayer ratio(silty clay and silty sand)structure,and the fluctuation of water level in the Yangtze River affects the deformation of the first terrace soil layer,and the deeply thick silty-fine sand is subjected to the confined water to trigger the seepage deformation of the foundation and so on.In addition,the micro-mechanism of seepage and consolidation deformation of silty clay near the river at present,the process of seepage deformation failure for silty foundation has not been fully revealed.Especially in this complex hydrogeological environment,this paper discussed the influence and control of the surrounding environment deformation caused by the foundation pit excavation with cut-off wall inserting rock near the river.In view of this,this paper adopts in-situ geological survey sampling,in-situ test,microstructure characterization,physical and mechanical test,centrifugal model test,macro-micro seepage analysis,a variety of numerical simulation calculation,theoretical mechanics analysis,statistics of measured data methods etc.,which obtained the plane deformation,spatial effect and influencing factors of foundation pit excavation under the condition of complex hydrogeology of the first terrace.The control measures and application analysis are put forward,which can provide reference for the design and construction of foundation pit near the river.The research results are as follows:(1)Based on the geological field investigation,regional geological data analysis and combined with the reconnaissance report,the engineering geology and hydrological condition of the first terrace have been ascertained,the characteristics of regional topography and geomorphology,geological structure,rock strata distribution,groundwater type,the characteristics of confined water movement and the soil structural,basic physical properties of regional soil,related constitutive relations and suitable parameter have also been obtained.The engineering geological structure model from the project site has been established,and the specific engineering problems existing in the long,deep and large pit near river are put forward.(2)The mechanism of seepage deformation of silty clay is studied from micro and macro level.Based on the action relationship between soil and water and pore connectivity,the laws of macro-micro deformation,the influence of pore connectivity on seepage field,the quantitative characteristics of pore structure are studied.Taking physical properties,micro-structure tests technology etc.(X-ray diffraction mineral analysis-XRD,clay specific surface area test-BET,mercury intrusion test-MIP,centrifugal low melting point metal intrusion-CLMI,scanning electron microscopy-SEM,image processing),obtaining the variation law of settlement deformation,water-earth pressure and displacement field of binary-structure soil layer under confined water,and the mineral composition,micro-structure information and morphology characteristics of undisturbed soil,compacted soil and silty clay with different sand content.On this basis,by the study of pore permeability with 2D LBM method,the relationship between pore connectivity and permeability of silty clay is revealed quantitatively and accurately,and the law of probability velocity and permeability standard deviation of silty clay with different connectivity is revealed.The parameters of image processing affecting the penetration of micro-structures are obtained.Clear pore profile images with nanometer resolution can be obtained through BSE imaging.The redistribution of pore size and the variation of porosity are revealed.Two stages micro-structure deforms are clearly observed: radial compaction and porosity deviation.With the Washburn's formula modified based on the relation of perimeter and area in pore,the measurement results of mercury intrusion porosity and permeability are reflected more accurately.The porosity and gap characteristics were analyzed by back scattered electron(BSE)images of three samples.(3)Analysis of the mechanism of the seepage deformation and failure in silty sand foundation.The permeability characteristics of the one-dimensional soil column with different sand grain composition and the seepage deformation and failure in silty sand foundation are studied from the following four aspects: the permeability characteristics of the homogeneous soil which has different sand grain composition,the permeability characteristics of the interbedded soil with different structure,the seepage deformation in silty sand foundation,the mechanical mechanism of collapse in overburden soils.By analyzing the permeability coefficient,escape hydraulic gradient,critical hydraulic gradient,failure hydraulic gradient and the process of seepage deformation,which reveal the influence of the grain composition of sand and the structure of interbedded soil(sand with silty clay interbed)on macro permeability characteristics.The varying-head permeability physical model tests with different fracture width are designed and made to realize the permeability development stage,deformation characteristics and the failure mode of silty sand foundation with different fracture width and varying-head permeability,which include the dynamic process of seepage field,the dynamic process of pore water pressure,the relationship of drainage rate and upriver water level,the amount of exhausting mud and sand,etc.The numerical simulation calculation by FLAC is used to analyze the variation characteristics of seepage field and stress field under confined water head.The trend of velocity vector and the effective stress are used to predict the failure mode of the wedge permeability deformation generated on the permeability test.According to the deformation and failure mode,the aching mechanical model is introduced to explain the collapse mechanical mechanism of the overburden soils.Through relationship between the minimum overburden thickness and the parameters,the necessary condition for the formation of "arch" is revealed.(4)The selection of the horizontal support structure for pit with cut-off wall curtain was analyzed,in order to optimal deformation control from the following four aspects which includes the implementation scheme of the deep pit structure,the analysis of the enclosure structure of cut-off wall,the excavation simulated by 3D numerical model and the horizontal support structure optimization analysis,structural type selection of the pit is studied.Through the comparative analysis of normal construction method,the top-down method,and partly top-down method,top-down method is determined as mainly the construction scheme.The waterproof effect and structure problem of the diaphragm wall are solved by the law of seepage deformation under inserting rock curtain,and the features of force and deformation analysis of the diaphragm wall.Taking the side of the deepest section at the long pit as the research subject,It respectively analyses different foundation pit with three kinds of horizontal support structures simulated by FLAC3 D refined numerical model.Through the surface settlement deformation,the displacement of the diaphragm wall,the deformation of the deep soil,the horizontal support force and the displacement of the column,which reveals the deformation characteristics of foundation pit with different bracing structure and its influence on surrounding soil,and the deformation feature of large and long foundation pit near the river are comprehensively reflected.(5)The plane deformation and space effect of foundation pit was studied by numerical simulation and centrifugal physical model test.From 2-dimensional plane strain,3-dimensional spatial effect and factors influencing of spatial effect for foundation pit,were taken into account to research the near river deep and large foundation pit deformation feature under different construction measures.Centrifugal model tests of two-dimensional foundation pit with internal support,no support,multi-level vertical excavation and multi-level slope were respectively carried out to obtain the time-history curves of land subsidence and deformation diaphragm wall,soil pressure and pore pressure.The deformation behaviors of foundation pit in different measures were revealed by settlement,earth pressure,soil settlement,bending moment of diaphragm wall,foundation displacement field.Taking advantage of 3D Centrifugal Physical Model and numerical simulation analysis get the diaphragm wall displacement,the land subsidence outside pit etc.,the space effect rule was revealed by the changing of plane strain ratio(PSR),which was influenced by the following factors: the varying length,depth,area in foundation pit and changing confined water.The variation feature of plane strain ratio affected in the pit were obtained by different influence factors,which provided the theoretic elements for earth excavation and support structure construction measures of the long,deep and large foundation pit near river.(6)Combined with the engineering analogy,case statistics and numerical simulation results,the control scheme and optimization measures are put forward for the deformation of the pit near the river.According to the earthwork excavation measures,ABAQUS was used numerical simulation to predict the impact of excavation on the surrounding environment,and ANSYS was used to predict the integral deformation of horizontal bracing structure.Combined with the spatial effect characteristics of long,deep and large foundation pit mentioned above chapters,the construction of earthwork and structure were carried out in staging,subsection and zoning on the plane.Analysis of the influence of the first floor excavation on the surrounding buildings(structures)by PLAXIS2 D numerical simulation.Finally,it is proved that the selection scheme and optimization measures proposed in this paper are feasible to control the deformation of foundation pit and improve the construction efficiency,and the recommended values of relevant construction technological parameters are reasonable.