土的剪胀性对隧道开挖地表沉降的影响机理及其计算方法研究

A reasonable prediction of the surface settlement induced by tunneling is very important for the security construction of subway and normal use of the existing structures. However, the surface settlement troughs that are calculated by current methods are usually ‘wider and shallower’ than the actual condition. To solve the above problem, this project starts from a fundamental characteristic of soil--dilatancy. Model test, theoretical analysis and numerical simulation are adopted together, to reveal the influence mechanism of dilatancy on the shape of surface settlement trough, as well as to propose a numerical calculation method of surface settlement considering dilatancy. The main content is as follows. (1) A large-scale model test of shield tunneling will be conducted to measure the surface settlement in the ground with different degree of dilatancy. With the help of discrete element simulation, the internal relationship between the transmission of the ground loss and the distribution of the contraction/dilatancy zone will be analyzed, so that we can explain how dilatancy affects the shape of surface settlement trough. (2) New equations of the phase transformation stress ratio and dilatancy will be established, and a practical elastoplastic constitutive model will be developed to describe the soil dilatancy under the stress path during tunneling. By introducing proper stress integration algorithm, user material subroutine of the constitutive model is then written to perform numerical calculation of the surface settlement. This project grasps the fundamental characteristic of soil firmly, and provides a new idea and solid foundation to predict the tunneling-induced surface settlement reasonably.

合理预测隧道开挖引起的地表沉降，对于保障地铁的安全建设和既有建构筑物的正常使用具有重要意义。然而，现有方法计算出的地表沉降槽，普遍比实际情况“宽而浅”。为了解决上述问题，本项目从剪胀性这一土的基本性质出发，综合运用模型试验、理论分析、数值模拟等方法，揭示剪胀性对地表沉降槽形态的影响机理，提出考虑剪胀性的地表沉降数值计算方法。主要内容包括：（1）开展大尺寸盾构隧道开挖模型试验，监测不同剪胀程度地层的地表沉降，并借助离散元模拟，分析地层损失的传递与剪缩剪胀区的分布之间的内在联系，解释剪胀性如何影响地表沉降槽的形态；（2）建立新的相转换应力比公式和剪胀方程，发展能够描述隧道开挖应力路径下土的剪胀规律的实用弹塑性本构模型，然后引入合适的应力积分算法，开发本构模型的用户材料子程序，对地表沉降进行数值计算。本项目紧紧抓住土的基本性质，为实现地表沉降的合理预测提供新思路，打下坚实基础。

合理预测隧道开挖引起的地表沉降，对于保障地上建筑的安全有着重要意义。然而，现有方法得到的地表沉降槽，普遍比实际情况“宽而浅”。为了解决这一问题，本项目从土的剪胀性出发，开展了模型试验、理论分析、数值模拟等方面的研究。.首先，在相对密度分别为35%、55%和80%的砂土地层中，开展了三组盾构隧道开挖模型试验，测量了地表沉降、地中沉降和隧道周边土压力。得到的规律如下：随着相对密度的增大，地表沉降槽的形状由正态分布曲线形变为三角形，宽度和深度均减小，且沉降的出现表现出越来越明显的滞后性；在地层损失从开挖截面向上传递至地表的过程中，松砂地层的地中沉降槽围成的面积逐渐增大，表明土体发生剪缩，而密砂发生剪胀，使得地中沉降槽围成的面积逐渐减小；拱顶处的大主应力从竖直方向突变到水平方向，平均正应力减小，而广义剪应力先增大、后减小、再增大，直至土体塌陷，上拱肩处由于土拱效应发生应力主轴旋转，拱腰和拱底处的应力路径分别接近于等p剪切路径和常规三轴压缩路径。.然后，利用离散元软件，模拟了典型隧道开挖应力路径下颗粒材料的变形，发现相转换应力比的大小与应力路径、土的密度状态有关，据此改进了统一硬化（UH）模型的相转换应力比公式，能够合理地描述隧道开挖应力路径下土的剪胀性。基于最近点投影法，开发了新模型的隐式应力积分算法，对统一硬化参量进行了改进，并在应力变换前对拉应力予以修正，从而消除了数值计算中的奇异问题，提高了算法的收敛性和运算效率。编写了用户材料子程序，嵌入到商用有限元软件中，发现随着土体剪胀程度的提高，地表沉降槽逐渐变窄，从而证实了考虑土的剪胀性能够在一定程度上改善地表沉降的预测效果。.本项目的研究成果为实现隧道开挖地表沉降的合理预测奠定了坚实基础，具有重要的理论和实际意义。

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