Research On Safety Insurance Of Subsurface Excavation Metro Tunnel In Deep Saturated Soft Loess

Xi'an,as the first city in northwestern China to run a subway,has a large number of loess and saturated soft loess distributed in the region.The special engineering characteristics of saturated soft loess and the influence of groundwater will bring tremendous difficulties to the underground construction.Curtain grouting is an effective method to deal with groundwater and reinforce the saturated soft loess layer.Limited by the complex engineering characteristics of saturated soft loess and the concealment of grouting construction,the theory of water plugging and reinforcement for grouting in saturated soft loess layer is far behind the engineering practice,and which often ignores the impact of water pressure.This paper relies on the mining tunnel construction between Taiyi Road Station and Xingqing Road Station on the No.5 line of Xi'an Subway.The technical difficulties encountered in the construction of the saturated soft loess layer are taken as the starting point and studied by theoretical calculations and numerical simulations.Based on the distribution of water pressure under grouting conditions,through theoretical analysis and numerical simulation,the distribution of water pressure under grouting conditions was first calculated,on this basis,the deformation law of surrounding rock and supporting force characteristics after tunnel excavation effected by thickness of grouting and water table were studied.Finally,a reasonable construction strengthening condition suitable for saturated soft loess layer was determined.This article mainly carried out the following research:(1)Based on the engineering characteristics of the saturated soft loess and the engineering survey,the main cause of the construction risk in the saturated soft loess layer is the consolidative settlement caused by the loss of water in saturated soft loess.(2)Through comparison and selection,the rationality of pre-grouting method for tunnel excavation in saturated soft loess layer is clarified,and the distribution of water pressure of lining after grouting is calculated through theoretical formulas,the reduction effect of different thickness of grouting on lining water pressure is analyzed.(3)Through numerical simulation,the effects of different grouting thickness and different water table on the ground settlement,deformation of surrounding rock and stress status of lining structure after tunnel excavation were analyzed.The optimum thickness of the curtain grouting method and the optimal precipitation level were determined based on the analysis results.(4)The site monitoring data was used to analyze the settlement of the surface and surrounding buildings after grouting reinforcement,and compared with the precipitation data caused by the previous precipitation,it was considered that the excess settlement caused by the original surface and surrounding buildings was relieved after the grouting plan was adopted.The grouting reinforcement scheme has a significant effect on the settlement control of the surface and surrounding buildings and it has effectively solved the technical problems encountered during construction.The research results of this article can provide a theoretical basis for the construction of the supporting project,and reference experience for similar projects in the future.It is of great significance.