暴雨气候下砂砾石地层中城市深开挖工程灾变机制研究

With massive development of urban underground space in recent years, many deep excavations were or are being conducted in sand-gravel strata which feature obvious discrete material behaviors and high permeability. Meanwhile, as a direct consequence of global warming (climate change), there is a high incidence of intense rainfalls globally, which may cause catastrophic excavation failure in sand-gravel strata. Failure of deep excavation in sand-gravel strata, triggered by intense rainfall, is not only associated with loosening, separation, suspension and migration of soil particles at microscopic level, but also related to sudden change in characteristics of seepage-, stress- and displacement-field and their coupling interaction effects at macroscopic level. Thereby, the relevant failure mechanism is very complex and the current states of design, construction and environment protection are challenged. In light of these, the first priorities of this proposed study are to: (1) determine major rainfall models affecting excavation safety, (2) to identify key particle size distribution and key particle size governing soil permeability and also to develop a constitutive model reasonably accounting for strength-degradation behavior of sand-gravel materials under seepage erosion, (3) to develop a multiphase fluid-solid coupling discrete element modeling method accounting for solid-water-air interaction during seepage erosion, and (4) to realize man-made rainfall during centrifuge testing by modifying the current experimental equipments and improving testing skills. Then, attention is to be focused on investing mechanisms of initialization, evolution and formation of excavation failure incurred by heavy rainfall. On the basis of the comprehensive investigation results, theoretical model for predicting rainfall-induced excavation hazards is to be developed and corresponding countermeasures are to be proposed. The relevant results to be yielded from this proposed study will provide theoretical basis for future design and construction of urban deep excavation in sand-gravel strata.

随着我国城市地下空间的大规模开发与利用，许多深开挖工程位于离散介质特征明显、渗透性强的砂砾石地层中。在当前全球极端暴雨频发的气候条件下，此类工程易遭受渗漏、渗透破坏等气象工程地质灾害。其灾变机理复杂，不仅涉及细观层面岩土介质颗粒松动、脱离、悬浮与迁移，还涉及宏观层面渗流场、应力场和位移场突变特性及三者耦合影响，给现有设计、施工和环境保护带来全新挑战。因此，本项目首先针对以下问题展开研究：（1）确定影响开挖工程安全的主要雨型；（2）确定影响砂砾石渗透性能的关键颗粒级配与粒径并构建反映雨水入渗导致土体力学性能劣化的本构模型；（3）发展反映固-液-气相互影响的多相流-固耦合细观模拟技术；（4）改进现有离心装置使其可以真实模拟天然暴雨过程。然后基于以上成果，针对暴雨作用下开挖工程灾变的诱发、演变及破坏机制进行宏细观研究，建立灾变预测模型并提出防控措施，为今后砂砾石深开挖工程的设计、施工提供依据。

随着我国城市地下空间大规模开发与利用，越来越多的深开挖工程修建于离散介质特征明显、渗透性强的砂砾石地层中。近年来，受全球极端暴雨频发的气候变化影响，此类工程于土方施工阶段频繁遭受渗漏、渗透破坏等气象工程地质灾害。其灾变机理复杂，不仅涉及细观层面松散岩土介质颗粒松动、脱离、悬浮与迁移，还涉及宏观层面渗流场、应力场和位移场突变特性及三者耦合影响，给现有设计、施工和环境保护带来全新挑战。为深入研究相关科学问题，课题组首先建立了中国城市暴雨事件以及与之相关的城市地下工程灾害事故数据库，全面分析总结了我国城市最新暴雨气候的总体特征、代表性雨型模式以及致灾作用。鉴于砂砾石土地层离散介质特征显著，且具有复杂形状的砾石体在渗流场中的力学行为与迁移规律属于典型的三维问题，现有的基于平面二维简化方法的数值仿真手段无法合理模拟并预测渗透破坏。因此，课题组基于计算几何学、图形学及随机概率统计学发展了可以真实模拟颗粒材料随机形状及尺寸的细观算法，通过计算机编程开发软件实现了三维砂砾石土在离散元模型中的模拟。此外，还专门再设计了一种可精确模拟天然暴雨多样性的自动控制装置和多情景下砂砾石地层中深开挖工程渗透灾变的缩尺模型试验系统，对于多种暴雨雨型条件下深开挖工程渗透破坏的细观机理与灾害演化机制展开了系统研究，所取得研究成果可为砂石土地区气象工程地质灾害的防控提供理论指导。所提出的三维碎石颗粒材料随机形状生成算法已经发表在国际隧道与地下空间技术期刊上面；所设计的试验装置已经申请国家发明专利1项、实用新型专利2项；研究成果发表在多本国内外权威专业学术期刊上（包括13篇SCI论文、3篇中英文核心与EI论文）；项目开展期间获得教育部自然科学奖二等奖（排名第三）、爱思唯尔 2020、2021 中国高被引学者、美国土木工程师学会2021 年度杰出审稿人、获得建造设施性能期刊颁发的2021年度杰出期刊论文奖、培养硕士生9名、博士生1名。