超深基坑承压水突涌破坏演化机理及控制

Risk of base instability due to hydraulic uplift is getting more critical as excavation goes deeper. To prevent hydraulic uplift failure, it is essential to understand how does hydraulic uplift trigger cracking in clayey excavation base, and how does the crack subsequently extend due to hydraulic fracturing. These two key processes are related to fundamental mechanisms of crack initiation as well as crack propagation in saturated clay. To address these two key issues, new experimental apparatus will be developed for soil element tests, to understand fundamental mechanisms of and develop criteria for crack initiation and crack propagation in clay. The newly developed criteria will then be incorporated into Extended Finite Element Analysis (XFEM) by programming a user-defined subroutine. Subsequently, the newly developed numerical tool will be used to simulate base instability of excavation in soft clay triggered by hydraulic uplift. Centrifuge model tests will also be performed to justify validity of the computed results. By doing so, it is hoping to (1) propose criteria for crack initiation and crack prorogation in saturated soft clay; (2) reveal failure mechanisms of base instability triggered by hydraulic uplift, and quantify effects of basal uplift on retaining structure, ground movement and development of excess pore water pressure; (3) propose engineering solutions to prevent hydraulic uplift failure. Not only is this research aiming at advancing scientific design of excavation in soft clay subjected to hydraulic uplift, but also it would provide theoretical frameworks and numerical modelling tool for all sort of geotechnical problems concerning cracking of saturated clay.

随着软土基坑开挖深度的不断增加，深层承压水对坑底软土层的顶托作用变得愈加明显，基坑突涌破坏时有发生。预防突涌破坏的前提是预测坑底软土层突水通道的形成条件和扩展路径。与这两点相对应的科学问题是饱和粘土裂缝萌生的判别准则，及裂隙粘土在水力劈裂作用下裂缝扩展的机理。围绕这两个科学问题，本项目创新设计单元试验装置，研究饱和软土裂纹萌生、扩展的机理，并建立相应的判别准则，据此二次开发扩展有限元（XFEM）仿真算法。利用新开发的数值工具，模拟深基坑高承压水作用下水力劈裂发生、演化的全过程；并开展离心模型试验，验证典型工况的计算结果。以上工作旨在：1）建立饱和软土裂纹萌生、扩展的判别准则；2）揭示坑底突涌破坏机理及演化过程中支护结构内力、坑外土体变形的响应规律，并提出警戒阀值；3）提出防突涌的控制方法。研究成果不仅将为承压水基坑的设计提供科学依据，还为其他存在裂缝开展的软土工程问题提供研究基础。

本项目研究主要围绕着超深基坑承压水突涌破坏演化机理及控制开展，建立滨海软土水力劈裂作用下失效准则，揭示相关的灾变机制和关键控制因素，提出抗突涌工程对策，为我国滨海地区广泛分布的含承压水的软土地层中的地下空间开发利用提供技术支撑。..项目执行过程中开展的主要研究内容有：(1)建立了基于线弹性断裂力学的饱和软弱土裂纹萌生、扩展的判别准则, 定量评价了砂质海床“与路径相关”的小应变刚度特性；(2)获得了承压水基坑的墙体、坑后土体三维变形的规律,并建立了定量评价三维变形的方法；(3)揭示了宽基坑和窄基坑在承压水顶托作用下的灾变机理，建立了不同尺寸、土层强度条件下，软土基坑抗突涌的临界坑底覆土厚度计算方法；提出并验证了分条、分步开挖，坑底及时水泥板压盖的基底抗突涌工法，(4)揭示了隧道在桩基附近不同位置开挖时，诱发的桩身荷载传递机制以及群桩内各桩的荷载重分布机制。..在项目资助期间，发表期刊论文24 篇（13 篇标注，含SCI 论文12 篇），其中发表在《Canadian Geotechnical Journal》期刊上关于承压水突涌灾变机制的论文，得到剑桥大学教授、英国皇家工程院院士Malcolm Bolton教授的正面引用。出版英文学术专著1 部（Springer 出版，标注），授权发明专利3项，申请发明专利5 项（已公开），获浙江省科技进步二等奖1 项（负责人排名第7，已公示）。..项目负责人获国际岩土力学和岩土工程学会（ISSMGE）的四年一届的“杰出岩土工程青年奖”（ISSMGE Outstanding Young Geotechnical Engineer Award），并被邀请担任国际土力学与岩土工程协会TC214（软弱地基中桩基工程）技术委员会委员。并应邀作国际学术会议大会主题报告1次（两人合作），特邀报告1次，组织国际会议2次。联合指导博士生3名（已毕业3 名），硕士2 名（已毕业1名）。