Study On Subway Tunnel's Thickness-span Ratio And Stratum Deformation Regularity In Sandy Stratum

The disturb of the rock which caused by the construction of civil subway is unavoidable which may lead to the stress distribution of the surrounding soil and the removal of stratum. Overlying soil thickness and excavation span of civil subway especially may have effect on the stratum removal, which may have serious effect on the structure, pile foundation and underground pipelines etc. Thickness-span ratio means the ratio of overlying soil thickness to excavation span. So ascertaining reasonable thickness-span ratio is the prerequisite for the safety and economical efficiency of the civil shallow buried underground excavated tunnels.Under the background of subway project in sandy stratum, the author summarized the deformation mechanism and regularity of stratum caused by the shallow buried underground excavated tunnels in sandy stratum. In this article, stratum control theory and technique are introduced in order to provide a foundation for the research of the influence that the thickness-span ratio have on the stratum deformation. In the same time, after the analysis of different width coefficient of the ground settlement tank, the author chart the relation curve (H/D) between ratio of the maximum ground settlement to the vault crown settlement thickness-span ratio and used the FLAC3D numerical simulation to verify the result. Using the finite element software, the author simulate the typical subway tunnel in sandy stratum. Based on plenty of calculation and analysis, the author got the thickness-span ratio related regularity of ground settlement spread, ground settlement, stress redistribution and plastic distribution after excavation.The author consider that the diameter of the tunnel can be ignored and the O'Reilly& New formula can be used to forecast the ground settlement and deformation when the thickness-span(H/D) is larger than 3; whereas the thickness-span ratio is smaller than 1.5, each ground settlement which obtained using the experimental formula will larger than the vault crown settlement, which does not accord with the theory. In other words, the applicable conditions of experimental formula is that the thickness-span of the tunnel is larger than 1.5. In the construction stage, the arch structure can only come into being in the place where is 1.2D(D is the equivalent diameter) height above the tunnel, and the settlement curve may meet the turn point in the place where is 1.3D above the tunnel. From the sight of stability of surrounding rock and surrounding environment control, the reasonable thickness-span ratio is 2.0.