软土超大面积基坑梯级悬臂支护稳定与破坏机理研究

With the development of the super-large excavation in soft ground, the conventional retaining structure with strut has shown some prominent disadvantages, such as high cost, long construction period, unconvience of construction, and difficulties of strut removal. The sustainable develepment requires more reasonable retaining method. The applicant proposes several types of benched cantilever retaining structure without strut. It has the advantages such as fast bulk exavation and convient construction of underground structure. The benched cantilever retaining structure has been successfully applied in several super-large excavations in soft ground. However, for this new type of retaining system, there has been no systematical research on the mechanism of the interaction between soil and each stage of cantilever, the failure mechanism of excavation, the method to evaluate the excavation stability, and the corresponding design method. Besides, some case histroies have benn reported that the dominolike failure occurred in the excavartion using benched retaining structure. Consequently, in this research the interaction between soil and each stage of retaining structure is focused. Based on the centrifuge model test and field test, the stability mechanism of the excavation was analyzed using the DLO which could obtain the failure mechanism precisely and efficiently. The failure process of the excavation is investigated through the MPM which could consider the large deformation. The deformation mechanisms of the benched cantilever retaining structure during excavation, approaching failure and post failure are studied. Furthermore, the parametric study is conducted to explore the effect of excavation and retaining structure geometry, retaining structure strength and pattern, soil condition, surcharge and over excavation on the failure modes and process of excavation. Finally, based on the upper and lower bound limit analysis, this research proposes a method to evaluate the stability of the super-large excavation supported benched cantilever retaining structure in soft ground.

随着超大面积基坑的发展，常规内支撑支护在造价高、工期长、施工不便、支撑拆除困难等方面的问题更为突出，发展更为合理的支护方法是可持续发展的需要。申请人提出了多种形式无水平支撑梯级悬臂支护方法并具有施工速度快、土方开挖与地下结构施工方便等优点，已在多个软土超大面积基坑中成功应用。但该新型支护形式的支护机制、破坏机理、稳定计算方法及相关设计理论的研究尚未系统开展，工程实践中已有梯级支护产生多米诺骨牌式破坏的案例报道。本课题拟采用可高效准确获取破坏模式的DLO法、可考虑大变形的MPM法及其它数值分析方法、离心机模型试验、工程现场监测等方法，以梯级支护形式中支护结构-土相互作用为核心，研究梯级支护在基坑开挖过程中的变形机理、趋于破坏的机理及破坏后的大变形性状，并进一步研究开挖和支护形式、土质条件变化、超载、超挖等因素导致梯级支护稳定破坏的机理，基于极限分析法建立适用于梯级悬臂支护的稳定计算方法。

随着超大面积基坑的发展，常规内支撑支护在造价高、工期长、施工不便、支撑拆除困难等方面的问题更为突出，发展更为合理的支护方法是可持续发展的需要。.项目提出了多种形式无水平支撑梯级悬臂支护方法并具有施工速度快、土方开挖与地下结构施工方便等优点，已在多个软土超大面积基坑中成功应用。但该新型支护形式的支护机制、破坏机理、稳定计算方法及相关设计理论的研究尚未系统开展，工程实践中已有梯级支护产生多米诺骨牌式破坏的案例报道。..针对大面积梯级支护基坑的稳定机理，项目按计划采用上限DLO法、可考虑大变形的MPM法、有限元强度折减法开展了大量的数值分析，结合离心机模型试验、工程现场监测等方法，以梯级支护形式中支护结构-土相互作用为核心，研究梯级支护在基坑开挖过程中的变形机理、趋于破坏的机理及破坏后的大变形性状，并进一步研究开挖和支护形式、土质条件变化等因素导致梯级支护稳定破坏的机理，基于极限分析法建立适用于梯级悬臂支护的稳定计算方法。..主要研究成果如下：第一，通过采用强度折减法结合DLO法考察了梯级悬臂支护基坑的稳定性机理，发现梯级支护基坑存在分离式、关联式和整体式三种破坏模式。第二，采用MPM法的对梯级悬臂支护基坑的破坏过程进行了模拟，结合有限元法到了支护变形与基坑安全系数的关系。第三，提出实用和适用的稳定计算方法。对于该研究目标，根据三种破坏模式，提出了相应的稳定计算方法；此外，基于MSD理念，提出了变形和受力简化计算方法。