既有地铁隧道对基坑开挖的影响

The Impact of Existing Subway Tunnels on Excavation of Foundation Pits

为研究既有地铁隧道对基坑开挖的影响,本文以实际工程项目为例,通过有限元软件PLAXIS3D,对既有地铁隧道周边进行基坑开挖和坑外地下水下降时施工状态进行模拟,建模时对基坑排桩进行抗弯刚度等效,考虑地下水对土体材料的影响,考虑到隧道管片间的连接作用,沿其轴向和横向刚度进行折减,同时选取符合实际的基坑模型边界约束条件,对基坑水平和竖向的位移变化规律进行揭示。结果表明:分别考虑从施工基坑支护桩到降水开挖至坑底中的由深到浅的四个工况下,基坑周围土体的竖直方向位移最大,表现为基坑底部土体隆起,可达到32.2mm。基坑支护结构的最大位移可达到18.21mm,发生在水平方向上。施工降水开挖至坑底中,坑外地下水位下降1.5m时,基坑支护结构的最大位移可达到21.47mm,发生在水平方向上。可见,在既有地铁隧道周围进行深基坑施工时,应重点监控基坑支护体系的变形和坑外地下水位下降造成的影响,一般会对基坑支护体系水平和周围土体竖直方向产生较大的位移,建议将地下水位下降值设置为1.5m,应在施工中进行重点监控,防止超出位移限制,确保安全施工。

In order to study the impact of existing subway tunnels on foundation pit excavation,this article takes an actual engineering project as an example and uses the finite element software PLAXIS 3D to simulate the construction status of foundation pit excavation around the existing subway tunnel and the lowering of groundwater outside the pit.The bending stiffness of the foundation pit piles is equivalent,and the soil material ofthe model is simplified under the action of groundwater,taking into account the connection between tunnel segments,Reduce the stiffness to 75%along its axis and 25%horizontally.At the same time,select boundary constraints that are in line with the actual foundation pit model to reveal the horizontal and vertical displacement changes of the foundation pit.The results show that the horizontal and vertical displacement of the foundation pit under four working conditions of excavation,as well as the horizontal and vertical displacement when the underground water level outside the pit drops by 1.0m and 1.5m,when the foundation pit is excavated to the base,are all within the controllable range of the foundation pit displacement required by the specifications.The research results can provide reference for the safe construction of existing subway tunnel foundation pit excavation.