Mechanism and control of progressive collapse caused by failure of local anchors in multiple-level anchored pile excavation
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摘要: 多道锚杆支护体系具有节约施工空间、变形控制效果好等优点，广泛应用于深基坑支护中。然而局部锚杆失效引发的基坑连续破坏事故时有发生。针对此问题，采用有限差分法研究了多道锚杆支护体系中局部锚杆失效引发的土压力与支护结构内力变化及结构连续失效规律，初步揭示了局部锚杆失效后的荷载传递路径及其引发基坑连续破坏的机理。多道锚杆支护体系中，首道锚杆局部失效对邻近未失效锚杆影响大，但会引起支护桩弯矩下降，最下道锚杆局部失效则相反，但仅一道锚杆局部失效影响相对较小，均小于基坑深度相同的单道锚杆支护体系，然而多道锚杆整列同时局部失效影响显著大于单道锚杆支护体系，更易引发连续破坏，因此应采用隔道加强等设计或施工措施将局部锚杆失效控制在一道内。局部若干根锚杆失效导致邻近受影响最大的锚杆依次失效时，锚杆失效数量较多后，初始锚杆失效位置对连续破坏发展传递路径不再产生影响，锚杆连续破坏接近于整列失效，且呈倒梯形水平向扩展。多道锚杆支护体系中发生整列锚杆失效时，冠梁或腰梁极易发生破坏，造成支护桩弯曲破坏，加快基坑连续垮塌过程，因此应对冠（腰）梁进行局部锚杆失效工况下的设计，以提高基坑防连续破坏整体安全性能。Abstract: Collapse of tied-back excavations occurs frequently. In this study, the finite difference method is used to simulate failure of anchors. In a multi-level anchored pile excavation, the failure of the first-level anchors has a great impact on the adjacent anchors and causes the bending moments of piles to decrease, while the failure of the lowest-level anchors has the opposite effects. The impact of failure of column anchors in a multi-level anchored-pile system is significantly greater than that of a single-level anchored-pile system, which easily leads to progressive collapse. Therefore, the construction or design methods, such as interval-level anchor strengthening, should be adopted to limit the failure at a certain level. When the failure of anchors causes the most affected adjacent anchors to fail progressively and even many anchors to fail, the failure position of the initial anchors does not affect the transmission path development of failure of anchors. Accordingly, the progressive failure of anchors is close to the failure of column anchors, and shows an inverted trapezoidal horizontal expansion. The failure of column anchors in a multi-level anchored pile system easily leads to damage to the beam. Then it will cause pile failure and accelerate the process of excavation collapse. Therefore, the capping (waler) beams should be designed under the failure of column anchors to improve the overall safety performance of the retaining system.
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图 4 锚杆失效时荷载（锚杆轴力）传递系数变化
Figure 4. Load transfer coefficients (axial forces) under failure conditions 1~3 of anchors
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图 5 工况1，2，3锚杆失效时作用在#1桩上土压力与弯矩变化
Figure 5. Earth pressures and bending moments on pile No. 1 under failure conditions 1~3 of anchors
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图 6 锚杆失效时支护桩荷载（弯矩）传递系数
Figure 6. Load transfer coefficients (bending moments) of piles under failure conditions 1~3 of anchors
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图 7 不同数量锚杆失效引起冠梁和腰梁弯矩变化
Figure 7. Bending moments on capping and waler beams under failure of different amounts of anchors
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图 8 考虑冠梁及腰梁破坏时支护桩弯矩变化
Figure 8. Bending moments on piles considering failure of capping and waler beams
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图 9 不同数量锚杆失效引起冠梁和腰梁剪力变化
Figure 9. Shear forces on capping and waler beams under failure of different amounts of anchors
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图 11 锚杆失效时荷载（轴力）传递系数
Figure 11. Load transfer coefficients (axial forces) under failure conditions 4~5 of anchors
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图 12 锚杆失效顺序及荷载（轴力）传递系数
Figure 12. Failure sequence of anchors and load transfer coefficients (axial forces) under failure conditions of 6~8
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图 13 锚杆失效过程中荷载（弯矩）传递系数
Figure 13. Load transfer coefficients (bending moments) of piles under failure conditions 4~5 of anchors
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图 14 工况6，7，8锚杆失效时作用在#1桩上土压力与弯矩变化
Figure 14. Earth pressures and bending moments on pile No. 1 under failure conditions 6~8 of anchors
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图 15 锚杆失效时支护桩荷载（弯矩）传递系数
Figure 15. Load transfer coefficients (bending moments) of piles under failure conditions 6~8 of anchors
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