The ground subsidence accident caused by the instability of shield tunnel face has been a major problem that puzzles urban subway construction in weak rock. Maintaining the stability of tunnel face is the key to ensure the safety of shield construction and reduce the environmental impact. In view of the shield with a diameter of 8600 mm in Chengdu metro line 18 through the sandy cobble stratum, it is more difficult to maintain the stability of the shield tunnel face than other strata. The characteristics of the sandy pebble stratum were taken into account firstly in this paper. The analytical equation was obtained by the limit equilibrium method. The optimization of the trapezoidal bottom has been improved based on the existing 3D trapezoidal wedge model. And the optimal solution of the ultimate support pressure was obtained. The influencing factors of support pressure are also analysed. When the tunnel diameter is fixed, the deeper the ground covering depth is, the greater the required limit support pressure becomes. But the ground covering depth has almost no effect on the ultimate support pressure even if the tunnel is deep. The limit support pressure is inversely proportional to the internal friction angle of the soil and is proportional to the diameter of the tunnel. Finally, theoretical limit support pressure can be also verified by the numerical simulation results and the field monitoring results. It provides a reference for the subsequent setting value of the soil warehouse pressure.

盾构隧道工作面失稳引起的地面塌陷事故一直是困扰弱势城市地铁建设的主要问题。保持隧道工作面的稳定性是确保盾构施工安全，减少环境影响的关键。鉴于成都地铁18号线穿过沙卵石地层的直径为8600毫米的盾构，与其他地层相比，保持盾构隧道面的稳定性更加困难。本文首先考虑了砂卵石地层的特征。通过极限平衡法获得解析方程。梯形底部的优化已基于现有的3D梯形楔形模型进行了改进。并获得了极限支撑压力的最优解。还分析了支撑压力的影响因素。当隧道直径固定时，地面覆盖深度越深，所需的极限支撑压力就越大。但是即使隧道很深，地面的覆盖深度对最终的支撑压力几乎没有影响。极限支撑压力与土壤的内摩擦角成反比，与隧道的直径成正比。最后，数值模拟结果和现场监测结果也可以验证理论极限支撑压力。它为土壤仓库压力的后续设定值提供参考。但是即使隧道很深，地面的覆盖深度对最终的支撑压力几乎没有影响。极限支撑压力与土壤的内摩擦角成反比，与隧道的直径成正比。最后，数值模拟结果和现场监测结果也可以验证理论极限支撑压力。它为土壤仓库压力的后续设定值提供参考。但是即使隧道很深，地面的覆盖深度对最终的支撑压力几乎没有影响。极限支撑压力与土壤的内摩擦角成反比，与隧道的直径成正比。最后，数值模拟结果和现场监测结果也可以验证理论极限支撑压力。它为土壤仓库压力的后续设定值提供参考。数值模拟结果和现场监测结果也可以验证理论极限支撑压力。它为土壤仓库压力的后续设定值提供参考。数值模拟结果和现场监测结果也可以验证理论极限支撑压力。它为土壤仓库压力的后续设定值提供参考。