Study On The Mechanism Of Progressive Collapse Caused By Partial Anchor Failure In Pile-anchor Retained Excavations

The progressive collapse of the excavations caused by the failure of partial-anchor often occurs.In this paper,a number of large-scale anchor-failure experiments were carried out by self-designed devices.The height of anchor erection,excavation depth and anchor failure rate were studied for the impact on earth pressure,anchor axial force,cap beams and piles.In addition,this paper also explores the load transfer laws of partial-anchor failure.At the same time,the results of the experiments were compared and verified by PLAXIS 3D numerical analysis models.On this basis,the effects of excavation depth and soil parameters on the progressive collapse mechanism are systematically researched.The main contents are as follows:(1)During the failure process of anchor,the earth pressure behind the piles in the failure area is reduced,and the earth pressure behind the pile away from the failure area is increased.The reason is that the support piles in the failure area are deformed into the excavation area,causing stress unloading and forming a stress arch.At the same time,the failure of the anchor will exert a loading effect on the adjacent anchor,and the axial force transmission coefficient of the anchor increases as the number of failure anchor increases.(2)The experiments of different heights of single anchor system was carried out,with corresponding numerical simulations.It is found that the bending moment of the supporting piles in failure area is reduced when the anchor is placed on the cap beam or the waist beam,but the bending moment is reduced more when the anchor is arranged on the waist beam.At the same time,when the number of failure anchor is the same,the axial force transmission coefficient is larger at the position of the waist beam.(3)The experiments of different excavation depth conditions was carried out,with corresponding numerical simulations.It is found that when the excavation depth increases,the bending moment transmission coefficient of the pile outside the failure area increases.Therefore,when the excavation depth is deeper,the bending moment transmission coefficient of supporting piles outside the failure area may be greater than the axial force transmission coefficient of the anchor,so that the piles will be destroyed before the anchor.When the excavation depth is lower and the number of failure anchor is large,the support piles under the excavation surface gradually subjected to tensile force on the side which contacted the outer side of excavation area,and the bending moment is gradually increased.Therefore,when the depth of excavation is low,the number of failure anchor is large,the support piles in the failure area may got collapsed before the anchor.(4)The experiments of different failure rates of anchors was carried out.It is found that when the number of failure anchor is the same,compared with the instantaneous failure of the anchor,the drop of the earth pressure behind the pile is much smaller than that when the anchor is slowly failed.At the same time,in the case of slow failure,the bending moment of piles in the failure area is larger,but the bending moment transmission coefficient outside the failure zone is relatively close.In addition,the axial force transfer coefficient is greater when the failure of anchor is lower.(5)Multi-anchor experiments was carried out with relevant numerical simulations.It was found that when the number of failure anchor is small,the bending moment of support piles in the failure area is reduced.But when the number of failure anchor increases,the bending moment of supporting piles increases and exceeds the bending moment when the excavation is completed.Therefore,when the number of failure anchor in a certain area of multi-anchor structure is large,the bending moment of piles may be too large and the progressive collapes occurs.