非平衡荷载作用下桩（墙）撑式支护结构变形及受力机理研究

Retaining structure with interior bracing have a great adaptability in foundation pit engineering of various geological conditions, especially in soft soil. With the rapid development of underground construction and increasingly harsh conditions of deep excavation, foundation pits under asymmetric loads is becoming more common. Based on theoretical analysis, numerical analysis and model test, we study the deformation and stress mechanism of retaining structure with interior bracing under asymmetric loads. Considered that the theoretical research lags behind engineering practices and no standards can be referenced, the following research would be carried out: (1)the non-limit earth pressure pattern considering relative displacement between soils and retaining structure under asymmetric loads would be put forward; (2) by applying the non-limit earth pressure to the retaining structure, the deformation and stress mechanism of retaining structure with interior bracing under asymmetric loads would be uncovered; (3)based on the research result above, the designing method of retaining structure with interior bracing under asymmetric loads is proposed and designing software would be developed. It is anticipated that the findings of this project will guide the design of retaining structure with interior bracing under asymmetric loads in practical engineering, as well as perfect and normalize the standard in force named Technical Specification for Building Foundation excavation Engineering JGJ120-2012 and fill in gaps in theory.

桩（墙）撑式支护结构适用于各种地质条件下的基坑工程，尤其在软弱土中更是能发挥其优越性。基于我国城市地下建设快速发展及场地使用限制愈加严苛，非平衡荷载作用下的基坑愈发常见的现状，采用理论分析、数值模拟、试验方法相结合的研究手段，研究非平衡荷载下桩（墙）撑式支护结构的变形及受力机理。针对这方面的理论研究滞后于工程实践，没有明确的相关规范标准的现状，本项目开展以下三方面研究：（1）建立非平衡荷载作用下考虑支护结构侧向位移的非极限土压力模型；（2）基于支护结构所受的非极限土压力，揭示非平衡荷载下桩（墙）撑式支护结构受力变形机理；（3）在上述研究成果的基础上，建立非平衡荷载下桩（墙）撑式支护结构的设计方法并开发计算软件。研究成果可以在工程中指导非平衡荷载下桩（墙）撑式支护结构设计，对完善我国现行基坑设计规范《建筑基坑支护技术规程JGJ120-2012》与填补理论空白有重要的意义。

工程建设规模的日益增大以及建设用地的日益紧张，使得深基坑工程面临的荷载条件越发复杂，基坑围护结构受土方非对称开挖或两侧不平衡堆载的非平衡受荷情况时有发生，如何确保非平衡荷载作用下的基坑围护结构既安全又经济成为紧迫而重要的研究课题。本项目首先通过模型试验与数值模拟，系统揭示了非平衡荷载作用下内撑式围护结构的受力变形机理；基于库伦土压力理论，建立了可用于非平衡荷载的考虑支护结构变形及变位模式的非极限土压力理论；在此基础上，提出了基于变形控制的非平衡荷载基坑围护结构桩（墙）设计解析解；并进一步建立了基于变形控制的非平衡荷载基坑开挖引起的坑外地表沉降显式解。研究结果表明，非平衡荷载作用下，荷载大侧将会对荷载小侧形成围护结构顶部的“推回位移”，减小荷载小侧的桩（墙）长能够有效利用“推回位移”，围护结构上土压力的大小与其变形以及变位模式密切相关，土体极限位移不仅与土体类型、墙高和变位模式有关，在转动变位模式下还与转动变位参数相关；经典等值梁法由于采用了极限状态土压力理论，计算得到的支护结构插入比偏于不安全，且不安全程度在土体内摩擦角较小的情况下更为严重，在非平衡荷载情况下，荷载大侧对整体支护结构的受力起主导作用，荷载小侧支护结构的插入比将对基坑非平衡荷载更为敏感，与传统的仅按荷载大侧单边设计的方式相比，采用本项目提出的设计计算方法可减小荷载小侧插入比约0.1~1.0，可以在控制变形的同时有效节约工程成本。研究成果可以为非平衡荷载下桩（墙）撑式支护结构设计与施工，以及完善我国现行基坑设计规范《建筑基坑支护技术规程JGJ120-2012》提供理论基础，具有较强的科学意义与工程应用价值。