宏细观尺度下碎石混黏土地层中深开挖与邻近既有盾构隧道相互影响机理研究

The clay-gravel-cobble mixed stratum exhibits strong discrete material behavior at relatively high rock contents. On the contrary, it shows apparent continuum material features at relatively high clay contents. As a result, the responses of the retaining structures, the stratum and the adjacent existing shield tunnel linings to excavation along with the associated interaction mechanism are distinctly different from those in fine particle soils and sand-gravel-pebble mixtures. This challenges the current state of arts and practice in the field of deep excavations. On the basis of the previous investigation results, the proposer intends to investigate the interaction mechanisms between the excavations and the adjacent shield tunnel linings in the clay-gravel-cobble mixed strata and then develope a model for estimation of the excavation behaviors as well as the responses of the ground and the adjacent tunnel linings to excavation. At the initial stage, the investigated items include: (1) developing the discrete element methodology which can simulate the diversity and randomness of natural rock particles, (2) exploring the key rock contents and the key particle size distributions which governs the soil shear strength, (3) proposing the constitutive model of clay-gravel-cobble mixtures which can considers the soil skeletons, and (4) improving the current centrifuge model test technology and equipments. In combination of discrete element analysis, finite element analysis and centrifuge model testing, the behaviors of retaining structures as well as the responses of the ground and the adjacent tunnel linings along with their inherent relations will be comprehensively examined. Thereafter, the relevant theoretical prediction model will be developed. The relevant investigation results will faciliate our understanding of the performances of excavations and the existing tunnels in the proximity and provide insightful guidance for design and construction. As a consequence, the risks associated with excavations adjacent to the existing shield tunnels can be minimized.

天然碎石混黏土中含石率较高时离散介质特征明显，但当黏粒含量较高时局部地层会表现出一定程度的连续介质特性。在此类地层中，当深开挖施工临近既有盾构隧道时，地层响应特征、隧道力学性能演化规律及地层-结构的相互影响机理有其复杂性，给现有设计与施工带来全新挑战。因此，本项目首先针对以下四个问题展开研究：（1）建立可以真实反映碎石颗粒形状、棱角度、级配和含石率差异性与随机性强等特征的细观算法；（2）分析影响碎石混黏土受力稳定性的关键含石率及颗粒级配区间；（3）构建能反映复杂应力路径下颗粒骨架作用的土体宏观本构模型；（4）改进现有离心试验装置使其可以真实还原现场开挖施工全过程。然后，基于以上成果，在宏细观尺度下对不同开挖尺寸和隧道位置工况条件下的开挖围护结构、地层和隧道的变形与受力演化规律及其内在关联机制进行研究，建立相关理论预测模型，为今后碎石混黏土地层中的地下近接施工控制提供理论依据。

碎石土粒径分布范围宽，结构离散，颗粒之间主要通过点接触传力。因此，地层反应灵敏，荷重易变形且自稳能力较差，受施工扰动后容易产生颗粒流动。因此在此类地层中进行基坑开挖、盾构穿越或地下近接施工具有较大风险，其诱发的地表塌陷具有突发性、滞后性和随机性的特点，给工程预测和风险管控带来极大困难。因此在把握不同基坑开挖条件及不同隧道位置工况下的基坑围护结构、地层与隧道的受力变形演化特征及三者之间的内在关联机制的基础上建立相关预测模型，可以为碎石土地区城市地下近接施工控制提供理论依据与参考。由于碎石土地层离散介质特征显著，且其力学行为受碎石颗粒形状、棱角、级配及含石率等关键因素影响，而现有研究手段无法合理考虑这些关键因素的影响。因此，课题组首先基于几何学、图形学及随机概率统计学发展了可以真实模拟天然碎石颗粒随机形状及尺寸的细观算法，通过计算机语言编程，在电脑软件中实现了对碎石地层的模拟。结合开发的离散元细观方法模拟和室内土工试验，确定了影响碎石土地层稳定性的关键颗粒级配区间，在此基础上并结合前人研究成果构建了可反映复杂应力状态下的颗粒骨架作用的土体宏观力学模型。此外，还专门设计了碎石土地层中基坑开挖对临近既有盾构隧道影响的模型装置。结合室内相似模型试验及所发展的离散元细观模拟方法，对复杂工况下的基坑开挖对临近既有盾构隧道影响展开了系统研究，所取得研究成果可以碎石土地区地下近接施工控制提供理论指导。所提出的碎石颗粒细观算法已经发表在美国土木工程师学会土木工程计算期刊上并已成功申请国家软件著作权；所设计的试验装置已经申请国家发明专利2项；研究成果发表在多本国内外权威专业学术期刊上（其中12篇论文发表在国际SCI学术期刊上、3篇论文发表在中文核心或EI检索期刊上）；项目开展期间获得上海市科技进步一等奖（排名第八）、教育部自然科学奖二等奖（排名第三），培养硕士生7名、博士生2名。