砂土地层盾构隧道稳定性三维离散元研究
Face Stability Analysis of Shield Tunnel in Sandy Ground Using 3D DEM

为探明砂土地层盾构隧道掌子面的稳定性，以Chambon和Corté开展的模型试验为基础，采用三维离散元方法研究了隧道埋深对隧道掌子面稳定性的影响规律，并从细观角度解释了开挖面失稳机理.离散元模型引入了三维柔性应力边界，将模型试验中空气或流体压力对掌子面的支撑效应抽象为作用在掌子面颗粒上的指定支护压力，逐步减少该压力，结合地层变形精确得到极限支护压力.通过删除进入隧道轮廓内的砂土颗粒模拟盾构开挖，以考虑该施工力学行为对掌子面稳定性的影响.研究结果表明：隧道埋深与隧道直径之比小于等于1.0时，掌子面极限支护压力随埋深增加而增加，此后趋于稳定，砂土地层中极限支护压力比随埋深增加而减少，地表沉降突增点对应的支护压力小于掌子面极限支护压力，失稳区直接发展到地表，工程中应同时关注地表沉降与仓内支护压力以保证开挖面稳定；隧道埋深与隧道直径之比大于等于2.0时拱顶上方形成了稳定的塌落拱，延伸高度分别约为0.7D（隧道直径）~1.3D与0.9D~2.3D.
：Based on the model test carried out by Chambon and Corte, the three-dimensional discrete element method (3D DEM) was used to study the face stability of shallow shield tunnels in sand, and the face failure mechanism was investigated from microscopic perspectives. A three-dimensional flexible stress boundary was implemented in the numerical model, and the support provided by air or fluid in the chamber for a tunnel face was simplified as specified normal pressure acting on face particles. Pressure was decreased gradually to 0 kPa, and ground deformation was closely recorded. Thus, the limit support pressure could be determined naturally. The tunnel excavation process was incorporated by deleting the particles that flowed into the tunnel, and its effect on tunnel stability was considered. Results show that when C (tunnel buried depth)/D (tunnel diameter) ≤ 1.0, the limit support pressure first increases with buried depth and then tends to be constant. The ratio of the limit support pressure to the initial support pressure decreases with buried depth. The support pressure at which ground settlement accelerates abruptly is smaller than the limit support pressure. The failure zone directly propagates up to the ground surface. In engineering practice, attention should be paid to the ground surface settlement and limit support pressure to keep the tunnel face safe. When C/D ≥ 2.0, a stable soil arch exists above the tunnel crown and extends upwards to approximately 0.7D-1.3D and 0.9D-2.3D