| With the development of underground rail transit,the scale of development and utilization of underground space is constantly increasing.In order to make rational use of underground space,the design form of overlapping tunnels is gradually adopted in tunnel design.During the construction of the overlapping tunnels,the soils in the overlapping sections are disturbed many times,the stress,strain,displacement are repeatedly changed,it is easy to cause ground settlement or even large-scale collapse,which will cause damage to pavement,underground pipelines and various buildings on the ground.In order to control the ground settlement in the overlapping area,the control measures for grouting reinforcement in the overlapping zone are generally adopted at present,and different reinforcement schemes are necessary for different geological condition.Therefore,before the construction of the overlapping section,it is necessary to predict the ground settlement caused by the overlap section construction,so as to timely adopt corresponding reinforcement schemes for the stratum according to the predicted settlement results.At the same time,simulate the proposed reinforcement schemes of the overlapping site and predict the ground settlement generated by excavation under various reinforcement schemes,so as to determine the most economical and reasonable reinforcement scheme.In order to study the ground settlement of the overlap caused by the overlapping construction of the left and right lines of the shield tunnel in Foshan area,this paper takes the overlap of the left and right lines of the Ludao Lake-Liantang section of Foshan Metro Line 2 as the main research object.Collecting and sorting out on-site monitoring data through onsite monitoring and summarize the law of surface settlement in the test section.And then use the finite element software to simulate the construction process of the overlapping section,and combined with the field measured data,so as to demonstrate the reliability of numerical analysis results.Finally,use the numerical model to simulate the ground settlement caused by excavation in overlapping section without stratum reinforcement and reinforcement measures.And determine a more economical and reasonable reinforcement scheme by comparing the reinforcement effects and benefits of each reinforcement scheme.The research results show that:(1)In the overlap construction of the upward and down lines of the tunnel,the upward tunnel constructed later will cause a large settlement after passing through the built down tunnel.The construction of the upward tunnel is the main factor affecting the surface settlement.With the decrease of the distance between the two-lane tunnels,the overlap effect of the formation deformation caused by the shield construction increases,and the surface settlement value caused by the excavation of the subsequent tunnels also increases.(2)When left and right lines of overlapping tunnel are constructed successively,the ground settlement is realized in four stages,that is,the first settlement-the first stable subsidence-the second stable subsidence-the second stable settlement,which is mainly related to the time interval between the left and right lines.(3)The finite element model can be used to effectively predict surface settlement if we select the constitutive model,model parameters and boundary conditions reasonably and combined with field measured data.(4)Before the construction of the overlapping tunnel,adopting a reasonable and effective grouting reinforcement scheme for the stratum,improving the cohesive force and shear resistance of the soil,can effectively reduce the surface settlement and control the final settlement value of the surface within the safe range.Within a certain reinforcement range,the effect of ground grouting reinforcement increases significantly with the increase of reinforcement range,and beyond a certain reinforcement range,the effect of ground reinforcement on controlling surface subsidence becomes weaker gradually. |
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