Mechanism of overturning progressive collapse of excavations retained by double-row piles induced by over-excavation
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摘要: 基坑事故屡有发生,其中不乏由于局部超挖引起的倾覆型连续破坏事故,然而此类连续破坏的机理和评价指标尚缺乏研究。为此,以某双排桩基坑倾覆垮塌事故为依托,采用有限差分法,研究了局部超挖对双排桩内力、变形及稳定性的影响,并提出抗倾覆稳定状态值（即桩在施工过程中任一时刻所受的抵抗倾覆力矩与倾覆力矩的比值）作为实时判断支护桩倾覆稳定状态的指标。结果表明,局部超挖发生后,倾覆破坏主要通过主动区土拱效应及冠梁的荷载传递作用向超挖区外传递,支护结构稳定状态值瞬间减小,其最小值决定了支护结构是否发生倾覆型连续破坏及其扩展范围。当稳定状态值不考虑冠梁荷载传递作用时,倾覆破坏的范围在超挖区外将被严重低估。对于倾覆型连续破坏,冠梁不能降低连续破坏发生风险,反而会加重超挖区以外的倾覆程度,扩大倾覆垮塌范围,与在弯曲型连续破坏中的作用不同。施工过程中的不利工况,如超载及振动荷载扰动,会加剧倾覆破坏的程度,扩大倾覆型连续破坏的范围。Abstract: Collapse accidents of excavations occur occasionally, including some overturning progressive collapses (OPC) induced by local over-excavation. However, the mechanism and evaluation index of OPC lack studies. Based on a case study on OPC of an excavation retained by double-row piles, the effects of local over-excavation on the internal force, displacement and stability of the cantilever retaining system are investigated, and an evaluation index is proposed, i.e., the anti-overturning status value (ASV), which is the ratio of anti-overturning moment to overturning moment of a pile at a certain moment, to evaluate the status of the pile. The results show that after a local over-excavation, OPC in the zone of over-excavation extends to the adjacent piles owing to the arching effect and capping beam, and ASV of the piles decreases suddenly. The minimum value of ASV during the over-excavation process determines the possibility and the range of OPC. When the retaining system does not include the capping beam, the range of the OPC outside the over-excavation zone will be underestimated. The capping beam cannot decrease the risk of the OPC, because it will enlarge its range, which is different from that in the bending-failure type progressive collapse of cantilever piles. The harmful conditions during the construction process (e.g., overload and disturbance of the vibrating loads) may intensify the level and range of the OPC.
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Keywords:
- deep excavation /
- local over-excavation /
- overturning collapse /
- progressive collapse /
- FDM
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图 1 基坑平面、双排桩剖面及土体参数[14]
Figure 1. Plan view of excavation, section of double-row pile retaining structure and soil properties
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图 4 桩顶位移与弯矩增大系数随桩号变化曲线
Figure 4. Curves of displacement at pile top and load transfer coefficient
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图 7 #1、#8、#10桩身抗倾覆稳定状态值及力矩时程曲线
Figure 7. Time history curves of anti-overturning status value and moments of pile No. #1, #8, #10
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图 8 超挖稳定后桩身稳定状态值及力矩随桩号变化曲线
Figure 8. Anti-overturning status values and moment curves of piles after over-excavation
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图 11 桩身稳定状态值及达到最小值所需时间
Figure 11. Minimum anti-overturning status values and time of reaching minimum value
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图 13 土压力云图与主应力方向分布
Figure 13. Contour plot of earth pressure and direction of principal stress tensor
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