The Analysis And Research On Related Problems For Shallow Tunnels Construction Method In Urban Subway

In recent years, the construction of subway has developed so fast. Meanwhile, shallow tunneling method is also widely used in subway construction. But there are many problems to be resolved, such as how to ascertain the cycle drilling length, the influence of the step-length and core-soil length on construction in bench cut method, the problem of multi-cavern effect in subway station construction and so on. These problems are crucial to the construction safety, construction periods and construction costs in tunnel. In allusion to these problems, this paper mainly researches the problems by the way of theoretical analysis and numerical simulation analysis. The study work is summarized as follows:(1) Theoretical analysis:The stress status and development trend before and after the excavation and support of surrounding rock is studied from the view of mechanics.(2) Research on cycle drilling length:Reasoning formula of cycle drilling length and formula of maximum shear strain is inferred from the view of theory. The cycle drilling length in clay soil and sandy soil is ascertained from the aspect of comparison and analysis about displacement field, shear strain, plastic area by simulating 8 kinds of conditions.(3) Numerical simulation analysis of bench cut method:The optimal core-soil length and cycle drilling length is ascertained from the aspect of numerical simulation analysis about longitudinal displacement, crown settlement, land subsidence, maximum principal stress and minor principal stress, by simulating 12 kinds of conditions.(4) Analysis on multi-cavern effect caused by several pilot tunnels:The mutual influence of the construction process of several pilot tunnels is researched by different excavation sequence and then the best construction scheme is determined. Research result has important skill guide functions and scientific references to the relevant design and construction of subway in bench cut method, and it can also be referred in the design, construction and research of similar subway station construction in PBA method in future. Based upon the study content, some main results are got in this paper:(1) Cycle drilling length is ascertained in soil surrounding roek by the formuala: t=2*fkp*c/γ.By numerical simulation analysis, the cycle drilling length can be also ascertained from the aspect of maximum shear strain.(2) Stratum deformation is controlled effectively in construction with the bench cut method by the step-length of 0.5D-1D and core-soil length of two-thirds of the step-length.(3) The problems of large stratum deformation are solved effectively, which are caused by multi-cavern effect, by increasing the distance of adjacent pilot tunnels.