| With the development of metro construction in loess area,we will inevitably encounter bad geological phenomena of collapsible loess,etc.Loess subsidence has great harm to project in Collapsible loess area,especially for strictly controlling deformation metro project.Currently “Code for building construction in collapsible loess zone” has been implemented mainly for industrial and civil buildings,the mentioned related standard is not applicable to underground metro project,since not combining the characteristics of metro project.Therefore,investigation of the interaction mechanism has important theoretical and application value between collapsible loess and underground structure.The metro structure and formation deformation law and deformation-controlled standard is crucial for underground structures design and construction especially there is no uniform standard of approaching construction and foundation residual settlement allowable value for metro underground engineering in loess area,even so,the engineering investigation has important practical significance.In this paper,based on summarizing the experience and the existing research results on Engineering,the interaction mechanism between formation and underground structure and underground structure deformation-controlled standard in collapsible loess area are studied by means of data investigation,theoretical analysis,model test,numerical simulation and so on,the main work is as follows:(1)Analyzing the nature and factors of collapsible loess From the engineering point of view,established a more systematic and comprehensive collapsible mechanism model on the basis of experimental data to express the collapse property?collapse settlement?moistening drying and stress at the early stage of loess.(2)According to the influence of loess collapse and underground structure failure,described the catastrophe process and causes of structure deformation,cracks,seepage,then discussed the collapsible induced process from the perspective of the loess collapse deformation mechanism.(3)Recognizing that metro underground construction is an uninstall process of formation,generally do not need to check its foundation bearing capacity.There is still need to accumulate the calculated value of the metro underground structure foundation collapse settlement in collapsible loess stratum.According to the characteristics of metro underground structure,the foundation collapse settlement calculation method and the residual settlement controlling standard of metro underground structure in collapsible loess stratum are put forward.(4)The damage caused by the loss strength of the loess collapse which covered by conventional laboratory soil test and the actual construction is realized.On the basis of attaching importance to strength loss of saturated loess collapsible,proposed collapsible loess strength index at water saturated collapsible moment,is similar with the test unconsolidated undrain index of saturated soft clay.In this state,the internal friction angle is equal to zero and all shear strength is assumed by cohesive force and has nothing to do with the surrounding pressure.In this case soil lateral pressure coefficient is K ? 1.Through case studies,obtained the influence of lateral pressure coefficient K=1 on metro structures.(5)Based on the earth pressure theory of buried culvert and combined with theoretical analysis,this paper comes up with the conclusion that vertical earth pressure at the top of the tunnel is larger than that of the original vertical earth pressure,when the deformation of the soil around and above the metro tunnel occurs.An example is given to analyze the problem and corresponding law are obtained.(6)According to practical engineering,the stress situation of the metro interval tunnel in the collapsible loess stratum is analyzed in the moment of saturated collapse as well as the relationship between basement pressure of metro structure and foundation bearing capacity.By numerical simulations,the new tunnel through existing tunnel at different vertical spacing and different geotechnical parameters are studied,the response law of formation and existing tunnel is obtained.And deformation controlling standard is proposed to ensure the safe operation of the existing line.By simulating the pit near existing line construction at different levels of net distance,response law of the existing structure and formation is analyzed.(7)By means of geotechnical centrifuge model test,influence of different displacement of new deep foundation pit retaining wall on the metro station and tunnel in collapsible loess area has been studied.Then,the deformation controlling standard of new retaining structure is proposed to ensure the safe operation of the existing line.At the same time,the influence law of new tunnel under excavation at different distance and formation loss rate is studied.And deformation controlling standard is proposed to ensure the safe operation of the existing line.Furthermore,the deformation controlling standards proposed by numerical simulation and model test are compared and verified.The research results will help enrich our metro construction technology in the complex environment loess areas,and provide important reference for the subsequent construction of similar projects. |
PDF Full Text Request