基坑柔性支护技术的理论及设计方法研究

High standard for the excavation deformation control is necessary with more and more deep foundation pit in the city and the complex pipe network underground. For the excavations with soft rock, although the traditional pile and anchor retaining structure is able to control the excavation deformation, boring the pile is very difficult in the rock. Moreover, it has the disadvantage of long duration and high cost. For soil excavation, although traditional soil nailing and composite soil nailing are more economical and convenient, but excavations with soil nailing have big deformation and soil nailing and anchor are not collaborative in excavations with composite soil nailing. Prestressed anchor flexible retaining system is aiming for the excavation of soft work, Micro-prestressed soil nailing retaining technique and low-prestressed composite soil nailing retaining technique for soil excavation, we called excavation flexible retaining technique. However, the theoretical analysis of the excavation flexible retaining technique is incomplete, and the appropriate design method and matched technical code are absent to guide the design and construction. Frequent engineering accidents resulted in bad social influence. Based on these backgrounds, this project intends to analyse the theory and designing method of excavation flexible retaining technique, by means of numerical analysis, centrifuge modeling and theoretical research, focuses on the mechanical behavior of the excavation, stability, distribution of earth pressure, anchor load-bearing structure and shotcrete. It is expected that this project can propose complete design and calculation methods to control the excavation deformation and avoid the accidents finally.

目前城市中深基坑越来越多，加上城市地下管网设施复杂，这对基坑的变形控制要求也越来越高。对于软岩基坑，尽管传统的桩锚支护等结构能较好地控制基坑的变形，但是在岩石中成桩非常困难，而且工期较长，造价较高。对于土质基坑，传统的土钉支护和复合土钉支护较经济便捷，但土钉支护基坑的变形大，复合土钉支护中土钉和锚杆受力不协同。针对软岩基坑提出了预应力锚杆柔性支护技术，针对土层基坑提出了微预应力土钉支护技术和低预应力复合土钉支护技术，暂统称为基坑柔性支护技术。但是，目前国内外对基坑柔性支护技术的理论研究相对滞后，仍缺乏相应的设计计算方法及配套的技术规范来指导该法的设计与施工。基于此，本项目拟系统地研究基坑柔性支护技术的三种技术形式的理论及设计方法，通过数值模拟、离心模型试验和理论分析，重点研究基坑的力学行为、稳定性、土压力分布、锚下承载结构及喷射混凝土面层的受力行为，并提出一套完整的设计计算方法。

针对传统的土钉支护和复合土钉支护存在一些缺点，课题组提出了预应力锚杆柔性支护技术，微预应力土钉支护技术和低预应力复合土钉支护技术，拉压复合型锚杆技术和劲性桩预应力锚杆联合支护技术，暂统称为基坑柔性支护技术。.课题组以离心模型试验为主要研究方法，结合解析计算模型和数值模型，对预应力锚杆柔性支护及微预应力土钉支护和低预应力复合土钉支护技术进行了分析研究。系统地研发了一套离心模型试验装置和技术。根据弹性理论、土楔理论与摩尔—库伦破坏准则提出了一个锚杆预应力扩散圆锥模型，应用预应力扩散模型，通过静力平衡及摩尔-库伦破坏准则推导得到了作用于喷射混凝土面层的水平土压力解析解。利用新型平面应变仪对粘土进行了固结不排水和排水平面应变压缩试验，基于侧向应变和竖向应变的关系曲线，确定了应变局部化的起始点和完全形成点。研究结果表明：剪切带内、外点和试样总体的应力-应变关系变化趋势一致，但剪切带内点的应变明显大于带外点的应变，试样的破坏过程是渐进性的。.对研发的新型拉压复合型锚杆的承载性能开展简化理论分析、模型试验及现场原位试验研究，推导得出了拉压复合型锚杆的抗拔承载力计算公式，及其与拉力型锚杆抗拔承载力之比（承载比），并对比研究了不同锚杆的极限抗拔承载力及其锚固性能。对劲性桩预应力锚杆联合支护方法的作用机理进行了理论分析，探讨了劲性桩预应力锚杆柔性支护的设计计算方法，结合工程算例，通过三维连续介质有限元方法，对支护结构的受力特性、基坑变形规律及稳定性进行了分析，并与预应力锚杆柔性支护和桩锚支护做出了比较。劲性桩预应力锚杆联合支护与普通预应力锚杆柔性支护相比，能够显著减小基坑的水平位移、地表沉降及坑后土体的塑性区范围；支护体系的潜在滑移面位置向基坑深层转移，支护结构的整体稳定性提高。