Study On Ground Surface Subsidence Law And Control Of Large Section Shallow Tunnel In Sandy Soil Layer

Most urban subway tunnels are shallow in depth,large in section,complicated in geological conditions,and dense in ground construction and transportation facilities.When the surface settlement caused by tunnel construction is too large,it will seriously threaten the safety of construction workers,the safety of ground buildings,and underground pipelines.Normal use.Shijiazhuang Metro Line 1 Liucun Station ~ Torch Square Station section(fire-retaining section)is a single-hole double-line large-section shallow-buried tunnel,with the main stratum crossing the ground layer being fine sand and medium coarse sand,and the main stratum of the arch is loess.The silty clay belongs to the sand composite stratum,and the surrounding rock conditions are poor.At the same time,it has to cross two underground passages.The minimum distance from the tunnel floor to the tunnel is 1.5m,and the interval is constructed by double-wall guide pit method.Can better control the deformation of surrounding rock.In this paper,the fire-retaining interval is taken as the research object.By means of theoretical analysis and numerical simulation,the tunnel failure modes of the tunnels excavated in the double-side tunneling method are systematically studied and analyzed,and the supporting parameters and construction are fully considered.The influence of parameters on the failure mode of the tunnel is summarized.The law of ground settlement in the target study area is summarized,and the corresponding settlement control measures are proposed for the tunnel.In addition,a large number of on-site measured data processing and analysis work was carried out,and the numerical simulation results were compared and verified,which confirmed the rationality of the numerical simulation scheme and the accuracy of the simulation analysis results.The specific research work is as follows:1.Through a large number of analysis and analysis of domestic and foreign literatures,summarize the causes of surface subsidence induced by shallow buried tunnel construction,and analyze the influence mechanism of different factors on the surface settlement of the target study area.Based on the Peck formula,the regression analysis of the lateral settlement data of the two monitoring sections of the fire-retaining section is carried out,and the width coefficient of the surface settlement trough and the loss rate of the formation applicable to the Shijiazhuang subway are obtained.Based on the section measurement curve and numerical simulation curve,the vertical settlement law of the surface is summarized.2.Selecting the area with shallow depth of construction,high construction risk and detailed monitoring data as the target research area,using MADIS/GTS /NX finite element software to construct a three-dimensional numerical model with a width of 90 m,a height of 45 m and a longitudinal length of 40 m.Contains 50,660 units and 54,495 nodes.According to the actual excavation and support sequence,the three-dimensional dynamic simulation of tunnel double-wall guide pit construction is carried out.The effects of excavation on surface settlement and deformation are analyzed in detail from the aspects of displacement,stress,plastic zone size and distribution.3.In order to reduce the influence of boundary effect,the lateral settlement simulation results of the tunnel with the longitudinal excavation direction of Y=20m are taken to analyze the displacement of the surface and tunnel section.By observing the lateral settlement cloud map of the earth's surface,the shape and influence range of the lateral settlement of the surface are obtained.By simulating the variation law of lateral settlement above the section after excavation of each guide hole,the influence of excavation of each guide hole on surface settlement is compared and analyzed.The representative section measurement points are selected to simulate the vertical settlement law of the surface,and the influence range of tunnel settlement is obtained.In addition,the study of the embankment settlement and the displacement simulation of the side wall is carried out,and the dynamic development law of the displacement of the surrounding rock is revealed,which is quite different from the displacement mode of the surrounding rock of the general tunnel.4.Through the simulation calculation,the stratigraphic stress cloud map and the plastic zone cloud map of each guide hole during dynamic excavation are obtained.By analyzing the formation stress cloud map,the stress distribution in different directions during excavation is obtained.Through the plastic zone simulation analysis,the deformation of the face of the tunnel during tunnel excavation and the development of the plastic zone at the junction of the middle partition and the vault and the inverted arch are obtained.In addition,in the simulation process,the reinforcement effect of the small small conduit is considered equivalently,and the reinforcement effect is judged by combining the plastic zone cloud map.5.Data processing and analysis work was carried out in combination with a large number of surface measured data.It is concluded that the surface settlement of the left side of the tunnel has been slowly increasing after the completion of excavation of the two sides of the tunnel,and the surface settlement has been delayed after the surface grouting reinforcement treatment.The shotcreting reinforcement zone formed at the same time can also reduce the influence of the intermediate pilot hole excavation on the ground surface.By comparing the measured data and numerical simulation results of horizontal and vertical settlement,the rationality of the numerical simulation scheme and the accuracy of the simulation analysis results are verified.6.Based on the control mechanism of support measures on surface subsidence,measures for surface subsidence control are proposed from the aspects of advanced small pipe reinforcement and initial support.By simulating and comparing the surface settlement under the three conditions of grouting,ungrinding and grouting strength,it is revealed that in the weak surrounding rock,especially in the sandy stratum,the pre-grouting is used to strengthen the stratum to reduce the formation.The effect of surface subsidence.In addition,the initial support of the simulation comparison adds surface settlement under one-step and two-step conditions of delayed excavation,and analyzes the influence of support implementation timing on reducing surface settlement.