Research On The Influence Of Foundation Pit Excavation And Unloading On The Deformation Of The Underlying Shield Tunnel And Its Control Method

With the development and utilization of unban underground space on a large scale in our country,more and more foundation pits located above the existing subway tunnel.The construction and excavation of foundation pit would change the stress field and displacement field of the surrounding soil,which will bring additional internal force and deformation to the underlying subway tunnel.What’s more,it may affect the operation safety of subway system.How to make a reasonable prediction of the impact of excavation and unloading of foundation pit excavation on the deformation of tunneling and engineering control is an urgent task.Based on the measured statistics,theoretical analysis and numerical simulation method,combining to the excavation unloading to lie down under has built of shield tunnel deformation effect and control method are studied,the paper main research contents and conclusions are as follows.(1)In collect domestic 17 in soft soil area across the subway shield tunnel excavation on the basis of engineering examples,makes a statistic analysis on the characteristics of the deformation of the tunnel,the results show that after the completion of the foundation pit excavation,the vertical displacement of the tunnel is characterized by uplift deformation.In the 17 engineering examples,12 of the tunnel uplift is smaller than the alarm value 10mm,indicating that the maximum uplift of the tunnel under the effective control measures can be controlled.The measured value of tunnel horizontal displacement is small,and the horizontal displacement of the tunnel is not affected by the excavation of the foundation pit.The deformation of the tunnel is "vertical drawing and horizontal compression",and the transversal deformation of the tunnel is"vertical ellipse".(2)Through the engineering case of statistics and the analysis of the influence of excavation unloading mechanism,found lying under the excavation causes has been built is the measure of the uplift of shield tunnel are the main factors:the excavation depth h,excavation unloading ratio N,A,foundation pit excavation area,and shape coefficient a,etc.,therefore,was carried out by two-dimensional finite element numerical analysis,the results show that:with the excavation depth of foundation pit and the increase of unloading ratio of foundation pit,the vertical deformation of tunnel is increased linearly.As the width of the foundation pit increases,the tunnel vertical uplift becomes parabola.In the analysis of the commonly used control deformation measures,it is found that the vertical deformation of the tunnel can be reduced significantly by strengthening the soil,setting up the anti-pulling pile,and adopting the method of stacking the bottom of the tunnel.(3)In predecessors on the basis of theoretical analysis and actual measurement,this paper analyses the tunnel is established formulas for calculating maximum uplift of experience,and for the second phase correction method Kerr model,which can be used to predict maximum uplift Smax of shield tunnel in soft soil area and the influence of deformation of tunnel longitudinal uplift BR.(4)Based on a certain channel excavation project of hangzhou subway line 1 project,through the Plaxis 3 d three-dimensional finite element model is set up,the whole process of excavation of simulation,analysis below the earthwork excavation in the process of the deformation of the tunnel,and,with the measured results on the improvement of two-phase Kerr foundation model results were analyzed.Finally,based on the analysis of the various measures to control tunnel deformation on the impact and effectiveness of the deformation of tunnel,the results show that the improved two-phase method Kerr model theory analysis method the results basically accord with the actual situation,the theoretical calculation of the maximum uplift Smax and influence scope of BR and measured results are close to,has a good practicability;The effect of the anti-pull pile to control the deformation of the existing tunnel is the most obvious,and the deformation effect of the soil reinforcement and the load return control tunnel is reduced in turn.The deformation of tunnel is reduced obviously with different construction control measures.