Discussion and analysis of accident reasons of deep foundation pits
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摘要: 介绍了深基坑工程的特点和破坏形式，探讨了深基坑事故发生的内在因素。针对北京某地铁基坑事故，分析了设计、施工、监测等各方存在的问题。主要原因是钢腰梁与支护桩之间未按设计要求设置抗剪蹬，导致钢腰梁与支护桩之间的连接不能有效承受斜撑轴力的切向分力，钢腰梁与基坑侧壁间产生摩擦滑动，支撑结构丧失承载能力。斜撑与牛腿间未按设计要求焊接，支撑体系在重力方向上的受力机制发生了本质的改变，导致在钢腰梁与基坑侧壁间无有效的水平方向防滑约束条件下，钢腰梁发生倾覆失稳，钢斜撑一端自由下落。深基坑工程应加强施工质量的过程管理与控制，相关各方应加强相互沟通，施工单位应掌握主要设计意图和设计中的关键环节。应综合分析监测数据异常的深层次原因，提出有针对性的应急措施。Abstract: The characteristics and modes of failure of deep foundation pits are introduced. The internal factors of accident reason are discussed. For the accident of subway foundation ditch, the existing problems in the design, construction and monitoring are analyzed. The main reasons of the accident are that the shear-resistance rungs are not installed according to the design requirements between steel wailings and soldier piles, resulting in that the joints between steel wailings and soldier piles cannot support effectively the tangential component force of axial force of diagonal bracings, and the sliding friction between steel wailings and side wall of ditch is aroused, so the structural supports lose bearing capacity. The diagonal bracings and corbels are not welded according to the design requirements. The mechanism of load-carrying of support system in the direction of gravity changes essentially, as a result, without effective anti-sliding constraints, the steel wailings exhibit overturning and unstability, and one end of the steel diagonal bracings falls freely. Deep foundation pits should enhance the process management and control of construction quality, related parties should enhance intercommunication, and construction units should grasp the main design intention and the key design link. The unusual underlying reasons of monitoring data should be comprehensively analyzed, and specific emergency measures should be proposed.Key words: deep foundation pit; damage of foundation pit; engineering accident; monitoring
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