基坑放坡开挖对下方既有地铁隧道影响计算及风险预测

Calculation for effect of foundation pit sloping excavation on underlying metro tunnels and risk prediction

为研究基坑放坡开挖对下方既有地铁隧道的影响以及预测隧道结构的风险,通过改进的计算方法得到放坡开挖基坑引起下方既有地铁隧道的竖向和横向附加荷载、位移、相对变形曲率共6个隧道结构安全的物理表现因子;将位移计算结果与前人理论计算结果、实测数据对比验证,并分析各土层物理力学参数对6个因子的敏感性;最后,基于正态分布概率模型对较敏感的土层物理力学参数随机取值,利用蒙特卡罗方法计算6个因子各级风险发生的概率和竖向、横向2类因子综合影响下隧道结构各级风险发生的总概率。研究结果表明:与原来仅限于矩形开挖基坑的计算方法相比,改进后的计算方法适用范围更广、实用性更强;在算例二分析中,隧道竖向位移和相对变形曲率超过控制值的概率分别为12%和68.7%,其余因子均为0,隧道竖向相对变形曲率是隧道结构处于不安全状态的最主要因子;若不采取预防措施,隧道结构将有高达73.27%的概率处于不安全状态,其中有68.7%的概率处于很不安全状态。

In order to study the effect of foundation pit slope excavation on the underlying metro tunnels and predict the risk of tunnel structure,six physical performance factors of tunnel structure safety,including the vertical and lateral additional load,displacement and relative deformation curvature of underlying metro tunnels caused by the foundation pit slope excavation,were obtained by the improved calculation method.The calculation results of displacement were verified through the comparison with the theoretical calculation results of predecessors and measured data,and the sensitivity of various physics and mechanics parameters of soil layers to six factors was analyzed.Finally,based on the normal distribution probability model,the values of the more sensitive physics and mechanics parameters of soil layers were randomly selected,and the Monte Carlo method was used to calculate the probability of risk occurrence at different levels of six factors and the total probability of risk occurrence at different levels of tunnel structure under the combined influence of vertical and lateral factors.The results showed that the improved calculation method had wider application range and was more practical than the original calculation method which was only limited to rectangular excavation of foundation pit.In the second practical example,the probability of the vertical displacement and the relative deformation curvature exceeding the control value was 12%and 68.7%respectively,and the other factors were all 0.The vertical relative deformation curvature of tunnel was the main factor for the unsafe state of tunnel structure.If without any preventive measures,the probability of tunnel structure in an unsafe state would be as high as 73.27%,of which 68.7%was in a very unsafe state.