浅埋盾构穿越渗透性地层时极限支护压力分析

基于半承压水模型综合考虑土压盾构穿越渗透性地层时覆土层及下卧层的渗透性，推导了盾构穿越层中沿掘进方向的水头分布的解析解，将其与现有的二维渗流场的解析解结合扩展为相应的三维近似解. 同时采用数值仿真得到稳态渗流条件下浅埋渗透性地层的主、被动破坏模式，建立了相应的柱体+弧形转角体模型，将前述三维渗流场引入该模型，通过力矩平衡法得到了相应两种极限状态下开挖面支护压力的计算公式，与既有结果进行对比，此计算方法更接近数值解. 研究发现:施工对开挖面前方渗流场的扰动基本局限在3倍洞径以内，主、被动极限支护压力的值随水头差的增大均线性增加，盾构直径和水头差是影响主动极限支护压力的主要因素，拱顶埋深与盾构直径是影响被动极限支护压力的主要因素，实际施工过程中，支护压力值应尽可能接近水土分算下的土体原始地层侧压力值，并在其附近（最好在其上方）小幅度波动，波动范围应以变形控制标准为依据. 

Analysis of Limit Support Pressure due to Shield Tunnelling with Shallow Overburden under Seepage

Based on the semi-contained water model, comprehensively considering the permeability of the shield-crossing soil layer and the overburden layer when shield passed through the permeable soil, the analytical solution of the head distribution along the tunnelling direction within the shield-crossing soil layer was derived, combining this, the analytical solution of the two-dimensional seepage field was extended to the corresponding three-dimensional approximate solution. Meanwhile, the active and passive failure modes of shallow-buried soil under steady-state seepage were obtained using numerical method, and the corresponding cylinder-arc-corner shaped model was established, after that, the calculation formulas of the two limit support pressures at tunnel face were obtained by introducing the above-mentioned three-dimensional seepage solution, and the calculated results were compared with the existing one, the current results were closer to the numerical solution. Besides, the study found that the disturbance of the seepage field in front of the tunnel face is limited to three times the tunnel diameter, the values of active and passive limit support pressure increased linearly with the increase of the head difference. The shield diameter and head difference were two main factors affecting the active limit pressure, and the overburden thickness and the shield diameter were two major factors affecting the passive limit pressure. During tunnelling, the support pressure should be as close as possible to the in-situ transverse earth pressure using approach calculating separately for soil and water, and should be slightly fluctuated in the vicinity (preferably above it). The fluctuation range should be determined according to the deformation control standard.