软土地铁隧道沿线既有桩筏基础长期性状演化规律与安全评估

Metro transit is widely used in the soft soil area of China due to the great need of commuting in the metropolis. The interaction between the tunnels and the existing piled-raft foundations has attracted much attention since the construction of tunnels, usually in shield-tunneling way, can lead to a serious environmental impact on the adjacent buildings. Therefore, most of the investigations focus on the behaviors of piled raft foundation during the tunneling. However, the leakage of linings, coupling with the vibrating effects from trains, indicates the long-term soil movement around the tunnels inevitably. Such phenomenon can cause the excessive settlement and differential settlement of the building near the tunnels and alter the mechanism of load transfer from the raft to piles, and a decrease of long-term safety of the building can be expected. In order to tackle this issue, the theoretical analysis, FEA and the model tests are employed in this study to investigate the long-term behaviors of the piled-raft foundation adjacent to the tunnels, and the key parameters of long-term safety for the buildings are consequently analyzed. Firstly, a hybrid solution is set up by embracing the soil movements-piles interaction developed with time and by combining the theoretical solution for the pile group and the finite element solution for the raft. The long-term behaviors of piled-raft foundation adjacent to the tunnel are analyzed. Secondly, the model tests are carried out and a parametric study is implemented for the long-term safety of building. The development with time of each parameter is analyzed and the key parameters are studied according to the criterion of building safety in different stages. Based on these analyses, this project aims at reducing the environmental impact from the tunnels, and consequently, the deformation of the piled raft is under control to maintain the long-term safety of the buildings.

地铁交通在我国软土地区得到了广泛的应用，隧道穿越邻近既有桩筏基础造成的环境影响分析大多局限于隧道开挖阶段，对于地铁运营后衬砌渗水、列车荷载等因素带来地面沉降及差异沉降过大、桩筏内力重分配等环境影响不够重视，桩筏基础的长期安全稳定性下降。针对该问题，本项目通过理论分析、数值模拟、模型试验等手段研究邻近地铁隧道的既有桩筏基础的长期性状，主要有两方面内容：（1）引入土体位移场时间效应，考虑隧道-既有桩筏基础相互作用，建立软土地铁隧道沿线既有桩筏基础的弹塑性混合模型，结合模型试验和数值分析，研究桩筏基础长期受力变形性状；（2）基于桩筏基础长期性状演化规律，分析隧道使用年限内邻近桩筏基础安全评估的关键控制参数。通过上述研究，本项目预期成果有助于实现隧道环境影响可降、桩筏基础变形可控、邻近建筑安全可期的目标，具有重要的学术研究意义和工程应用价值。

软土地铁穿越桩筏基础需考虑隧道开挖初始沉降和地铁运营长期沉降的环境影响，研究隧道-土体位移场-建筑物相互作用面临两个关键问题：（1）隧道开挖地层扰动引起桩筏基础内力重分布以及（2）复杂荷载作用下隧道周围岩土体受力变形性能长期演化。首先，开展隧道掘进过程中桩基三维受力变形特性、筏板调整桩土荷载分担、群桩遮拦效应等关键承载机理研究；分析桩基-隧道开挖面相对位置以及筏板调节桩顶变形的影响，提出了近隧道侧的筏板结构与其底部土体差异沉降而存在“脱空”的现象，分析筏板约束桩顶水平位移而引起桩顶次生弯矩的发展规律；基于隧道穿越引起建筑上部结构不均匀沉降的“跷跷板”效应，以楼板沉降为控制因素，开展桩筏基础静压锚杆桩加固的优化设计方法研究。其次，提出了列车循环荷载作用下隧道周围饱和土层状圆柱简化分析模型，研究开挖扰动区与原状土并存的工况中，饱和土在循环荷载作用下孔压发展规律；根据隧道掘进导致岩土体破裂分析了双重孔隙介质变形-孔压消散长期规律，研究了带裂隙岩体的应力集中发生区域以及孔压消散双重扰动效应；针对温度场作用下岩土体的长期变形特性，提出热-水-力耦合的多孔介质模型二阶段理论解答，首次得到积分变换域内环向、径向应力应变解答，能够较为全面地分析局部热荷载作用下岩土体应力集中现象；引入热渗效应，分析岩土体在热力梯度下作用下水分迁移特性，研究热渗效应对岩土体变形、内力、孔压演化规律的影响。最后，基于邻近隧道开挖的建筑实测数据，优化隧道开挖施工方案，提出建筑加固方法，降低隧道开挖环境影响；基于Timoshenko梁和Pasternak地基模型分析了隧道运营期间邻近堆载对隧道影响的简化分析方法，综合分析邻近堆载引起隧道结构弯曲和衬砌差异沉降，提出降低此类工程事故的有效措施。研究成果可为软土地铁隧道穿越桩筏基础的长期演化规律和结构安全评估提供重要科学依据和具体分析方法。