软土超深开挖性能演化与极限失效机理的离心模拟研究

Ultra-deep excavation has been shown to be the trend in underground construction projects in soft ground. Compared with normal excavation, ultra-deep excavation has features of high level of stress and loading, high safety risk, severe damage extent and influence in case of failure accident. However, the current researches are not enough relating to the deformation pattern, behavior evolution and failure mechanism during ultra-deep excavation process, nor are the high quality experiments. This research proposal will first establish a reliable and precise inflight system of ultra-deep excavation and propping simulation, and based on series of experiments, deformation pattern and kinematic mechanism of the ground and related embracing structure will be investigated in details. Equations of deformation pattern of the soft ground and the structure will be proposed under kinematic allowable principle. The interaction between soft ground and structures will also be studied in terms of soil pressure and the internal forces in the structures, and the trends of their transition will be observed carefully. Above all, ultimate cases will be set to study the failure mechanism of ultra-deep excavation and the failure pattern will be summarized. Based on the above studies, the analysis method incorporating mobilizable strength theory and energy conservation principle will be developed to deal with the whole process of ultra-deep excavation. The purpose of this study is to upgrade the theoretical understanding of the ultra-deep excavation, make deeper insight into the rules and laws relating to the deformation development and safety issue, and help to guide relevant design to increase safety and decrease the risks.

超深开挖是软土基坑工程发展的趋势。相比一般基坑，其应力及荷载水平高，风险大，事故的损失和影响更大。目前对超深开挖的变形模式、性能演化和失效机理研究针对性不够，理论深度欠缺，高精度试验验证少。本课题针对这一问题，在既有离心试验平台上开发形成可靠性好、精度高的动态超深开挖和支撑模拟试验系统。基于系列离心场模拟试验，首先对软土超深开挖的变形模式和运动机构进行精细观察，并在对不同几何尺寸和刚度比的工况考察的基础上，分类建模。其次对超深开挖中的深厚软土与结构共同作用进行考察，对土压力荷载和结构内力的特征及演化进行研究。同时对超深开挖设置极限荷载工况，研究超深开挖的失效机理，探讨运动机构演化与失效的关系。在此基础上，以动员强度理论和能量守恒原理为框架，形成超深开挖性能演化及失效的全过程分析方法。通过研究，提升软土超深开挖特性的理论认识，并可对设计形成指导，以增强安全性，降低风险。

(1)本项目的背景：随着我国经济社会的快速发展以及城市化进程的加速，城市各类基础设施建设出现高潮。而作为各类基础设施建设中必不可少的基坑工程也正在朝大深度方向发展，其深度、数量和密度均大幅超过以往的经验。尤其是软土地区，地层强度低、压缩性大，非线性显著，其稳定和变形方面的风险更为复杂。目前对软土超深开挖性能演化与失效的研究尚不深入，较少采用模型试验手段进行验证。在这一背景下，采用离心动态模拟的试验手段研究超深开挖的变形机理,具有重要的学术意义和实用价值。. （2）主要研究内容：主要研究内容如下：.　　 1）软土超深开挖体系的变形模式及演化特性的离心模拟研究. 2）软土超深开挖体系内力及荷载的特性及演化机理. 3）软土超深开挖极限失效机理及模式的离心模拟研究. 　 4）基于动员强度理论和能量守恒超深开挖全过程分析；. （3）重要结果及关键数据：通过本课题的研究，首先建立了一整套采用机械手的高离心加速度下不停机开挖及加支撑的软土基坑开挖动态模拟系统。通过一系列的试验及结果分析，对体系变形、受力等性能演化进行了探讨。超深开挖下地下墙变形逐渐增大并且最大值点下移；在基坑开挖的后期，挡墙的墙体变形并没有继续增大，而是处于相对稳定状态，仅仅最大值点下移。墙后土体沉降大致符合Peck公式，沉降最大值与挡墙水平变形最大值存在对应关系。土压力较好地吻合了挡墙的变形情况。被动区土压力变化情况是，接近开挖面的浅层处的土压力随着基坑的开挖逐渐减小，而深处的土压力随着基坑的开挖有时增大。浅层土压逐渐减小，说明是土体卸载的影响要大于挡墙变形挤压的效果，深处土压增大是由于基坑的挤压影响要大于基坑卸载影响，土压力的变化是由于坑内土体开挖卸载和挡墙变形挤压共同作用的结果。. （4）科学意义。本课题的研究对于超深基坑开挖时地层变形机理及结构体系受力分析均有较好的借鉴意义。