黏土地层中深基坑开挖引起邻近既有隧道长期变形及超孔压消散机理研究

It is not uncommon to encounter large deformation, crack and even damage in existing tunnels due to nearby basement excavation. Because of its low permeability, excess pore water pressure in clay due to stress relief and shearing cannot be dissipated rapidly. Long-term tunnel response is expected as a dissipation of excess pore water pressure. To ensure the safety and serviceability of existing tunnels, performance of existing tunnels during and after basement excavation should be reviewed. However, previous studies mainly focused on tunnel response during basement excavation, and most of centrifuge tests were conducted in dry sand. In this project, centrifuge test and numerical simulation will be adopted to investigate the influence of a multi-propped deep excavation on an adjacent existing tunnel in clay. The influence of soil stress history on the long-term tunnel response and dissipation of excess pore water pressure will be explored. The mechanisms of tunnel response due to basement excavation in normally and overconsolidated clays will be compared and examined. An advanced soil model which can capture small-strain soil stiffness together with simple constitutive models are used to carry out numerical back-analyses of the centrifuge tests to improve the understanding of stress transfer and redistribution mechanisms. By comparing numerical results from different soil models, significance of considering small-strain soil stiffness is highlighted. Based on a systematic numerical parametric study, calculation charts will be developed for direct estimation of long-term tunnel response due to basement excavation in clay.

基坑开挖导致既有隧道产生过大变形、裂缝甚至于破坏的案例时有报道。由于黏土渗透系数小，基坑开挖引起的应力释放和剪切作用产生的超孔隙水压力不能及时消散。超孔隙水压力消散导致隧道产生附加变形。因此，隧道在基坑施工期的短期安全性到施工完成后的长期安全性是岩土工程迫切需要认识的课题。然而，现有研究主要针对基坑施工期隧道的短期变形，离心模型试验大都在干砂中开展。本项目拟通过离心模型试验和数值模拟研究黏土中多支撑深基坑开挖对邻近既有隧道的影响。研究不同土体应力历史即超固结比下基坑开挖引起的隧道短期、长期变形及超孔隙水压力消散机理，揭示从正常固结黏土到强超固结黏土中基坑开挖引起隧道渐变的变形机理。采用简单土体模型和反映土体小应变刚度特性的模型开展有限元仿真模拟，揭示考虑土体刚度随应变折减的重要性和隧道周围土体刚度变化、应力传递及重分布规律。开展系统的有限元参数分析，建立预测基坑开挖引起隧道长期变形的方法。

黏土地层中，深基坑施工期隧道的短期安全性至工后的长期安全性是岩土工程迫切需要认识的课题。现有研究主要针对基坑施工期的隧道短期变形，基坑施工后的隧道长期变形研究甚少；隧道—基坑相互作用问题的平面应变假定忽视了基坑和隧道三维变形效应。通过开展系统的三维土工离心机试验和有限元数值模拟，深入研究了深基坑开挖引起的既有隧道长期变形和三维变形特性。首先，通过开展土工离心机试验，首次揭示了不同土体应力历史下基坑开挖引起的隧道长期三维变形特性和超孔隙水压力消散机理，建立了从正常固结黏土到强超固结黏土中基坑开挖引起隧道渐变的变形机理。其次，开展了考虑土体小应变刚度特性的三维数值分析，精确预测了基坑开挖引起的土体和隧道变形，克服了传统的弹塑性本构模型无法准确预测土体变形的不足。针对方形和圆形基坑的不同应力释放机理，分别揭示了既有隧道的三维变形和应力传递规律。再次，对工程中常见的基坑和隧道尺寸、基坑—隧道相对位置、土体性质等参数开展系统的三维有限元分析，建立了考虑三维效应的基坑开挖引起隧道变形的预测方法，改进了以往平面应变假定导致隧道变形被严重高估的不足，大幅提高了隧道变形的预测精度。通过与基坑—隧道相互作用的平面应变假定计算结果对比发现：圆形基坑开挖直径小于8倍开挖深度、矩形基坑开挖长度小于9倍开挖深度时，需要考虑基坑三维效应对隧道变形的影响，否则隧道变形被严重高估。反之，基坑—隧道相互作用可简化为平面应变问题。最后，对基坑和隧道的传统加固措施进行了有效性分析，揭示了不同加固措施的作用机理。项目研究成果可用于指导城市地下空间开发中的基坑—隧道相互作用问题，具有较大的工程应用价值。