泥水盾构开挖面动态泥膜渗透特性及失稳机理研究

During the slurry shield advancing in the sandy soil, the high-permeable filter cake on the tunnel face will lead to the tunnel face failure, the sinkhole, the buried shield, even the plan change. Due to the rotation of the cutter wheel, the permeability characteristics of filter cake change dynamically, which are difficult to measure. However, current researchers presume that the filter cake is completely permeable or impermeable. In the research, the mechanism of dynamic filter cake will be revealed based on the conventional cake filtration theory. The mathematic model for the dynamic filter cake will be built. The change rule of the permeability characteristics of filter cake and key factors will be investigated. The approach for evaluating the permeability characteristics of filter cake will be proposed. The influence of the content of bentonite and polymer additive on the slurry will be studied. The constitutive model of the filter cake that describe the relationship between the slurry pressure and hydraulic conductivity will be established. Its parameters will be obtained from the result of fluid loss test. The numerical model, including the permeable filter cake, the shield and the soil will be built to calculate the transient seepage field. The coupling relationship between the permeability characteristics of filter cake and the seepage field will be investigated. The rule of the slurry pressure transfer with the consideration of permeable filter cake will be studied. The mechanisms of the tunnel face failure will be revealed by the analysis of tunnel face stability. The equation to obtain the limit slurry pressure will be presented based on the limit equilibrium method. The results will help to reduce disaster and economic losses by the tunnel face failure and will be a theoretical guidance to ensure the safety of the slurry shield advancing.

泥水盾构在砂性土地层掘进时，开挖面上泥膜透水性过强会导致开挖面失稳，地面塌方，盾构掩埋，甚至隧道改线。掘进过程刀盘不断旋转切削土体，导致泥膜渗透特性动态变化，难以确定。但是，目前学者假设泥膜为完全透水和不透水两种情况。本项目基于传统滤饼过滤理论建立数学模型，揭示开挖面上动态泥膜形成机理，分析泥膜渗透特性时空变化规律和关键影响因素，建立动态泥膜渗透特性评价方法；分析膨润土和聚合物添加剂含量对泥浆基本特性的影响，通过压滤试验建立描述泥浆压力与泥膜渗透特性关系的本构模型；建立考虑泥膜渗透特性的数值模型模拟盾构掘进时地层的瞬态渗流场，研究泥膜渗透特性与渗流场的耦合关系及考虑泥膜渗透性的泥浆压力转换规律，开展开挖面稳定性分析揭示泥水盾构开挖面失稳机理，基于极限平衡法提出极限泥浆压力的计算方法。将有助于减少开挖面失稳造成的灾害和经济损失，为盾构隧道安全施工提供理论支持。

泥水盾构开挖面泥膜将泥浆压力转化为支护压力，转化程度与泥膜的渗透特性有关。若泥膜渗透系数过大，易导致开挖面失稳事故。为了明确泥膜形成机理，保证施工安全，本文对泥水盾构动态泥膜渗透特性以及开挖面稳定性进行了系统的研究，主要工作及研究成果如下：.（1）针对15种泥浆进行了不同压力差下的滤失试验，研究泥膜的渗透特性。得到了15种泥浆形成的泥膜的孔隙比-渗透系数-压力的相互关系，发现CMC-Na对泥浆的改性效果最好。提出泥膜渗透系数与平均泥膜厚度的计算公式。.（2）提出了预测泥水盾构开挖面前泥浆渗透距离的计算模型，具有精度高了，使简便的特点。考虑泥膜时渗透最大距离约为盾构直径的1.05%。.（3）开展数值模拟研究开挖面前地层的孔压情况。结果表明，隧道开挖面前2D、下1D、上0.5D内土体受超静孔压影响较大。针对数值模拟结果进行参数化分析，研究泥膜前后孔压损失与孔压总损失之比（即泥膜压降占比）的影响因素。泥膜压降占比随着开挖时间和有效泥浆压力的增大而增大；随着泥膜渗透系数和埋深的增大而减小；水位变化对泥膜压降占比的影响很小。.（5）基于数值模拟结果，采用极限平衡法得到了极限有效泥浆压力的计算方法。发现本文提出的泥浆压力计算方法与工程符合，相较于目前工程中的泥浆压力的取值方法更加安全。发现隧道埋深和水位对极限有效泥浆压力的影响较小，而地层的内摩擦角、黏聚力和泥膜渗透系数对极限有效泥浆压力的影响较大。提出了泥浆压力的简化计算方法，能够快速计算保证开挖面稳定的泥浆压力。