Researches On The Progressive Collapse Mechanism And Control Strategy In The Braced And Tied-back Retaining Systems Of Deep Excavations

Excavation engineering has strong uncertainty,complexity and contingency.At the same time,the retaining structure of excavation is a temporary structure with relatively low safety reserve,which leads to high risk of deep excavations and frequent collapse accidents.The existing excavation accidents at home and abroad show that the excavation collapse often starts with the failure of partial components,and then continues to expand,eventually resulting in a large-scale of collapse,that is,the excavation collapse is a process of progressive failure.However,at present,systematic and in-depth study has not been carried out on the progressive failure mechanism of the excavation and the theory and method of progressive failure control.In this study,aiming at the braced and tied-back retaining systems commonly used in engineering,large-scale physical model tests and numerical simulation are carried out to analyze the mechanism of progressive failure caused by partial failure,and the corresponding progressive failure evaluation indexes and control strategies were proposed.The main contents are as follows:The physical model tests and numerical modelling of braced retaining excavation shows that when the partial failure or large deformation occurs in retaining piles,due to the soil arching effect in the active zone and the stress redistribution of the structure,the internal force of the retaining piles adjacent to the initial failure zone can be significantly increased.This effect can be expressed by the load transfer coefficient(that is,the multiple times of the rise in the component's internal force due to a partial failure).Under a certain component safety factor,the higher the load transfer coefficient,the greater the risk of progressive failure caused by a partial failure.When partial failure of the struts occurs,the loads released by partial failure of the struts will be intensively transferred to most adjacent struts rather than uniformly transferred to a large number of the adjacent non-failure struts,which will easily cause the large additional internal forces of these struts and trigger the progressive failure.In deep excavations,the lateral stiffness of the structure and the unloading degree caused by the partial failure have significant influence on the load transfer and the progressive failure development.The struts can enhance the lateral stiffness of the retaining system and restrict the load transfer as well.Therefore,compared with the partial failure of retaining piles in the cantilever pile system,the load transfer coefficient caused by pile partial failure in the braced retaining system is larger,but the influenced range is smaller.In the braced retaining system,when the number of struts is constant,different strut elevation set along the excavation depth can affect the lateral stiffness of retaining pile.When the lateral stiffness of retaining system is larger,the load transfer coefficient caused by the retaining pile failure is also larger.When the excavation depth becomes deeper,the magnitude of unloading caused by the failure of the retaining pile and load transfer coefficient will also become larger.For the anchored pile retaining system,this study focuses on the mechanism of progressive failure caused by partial failure of anchor in single-channel and multi-channel anchor retaining system.For the single-channel anchored pile retaining system analyzed in this paper,partial failure of single anchor will lead to obvious increase of axial force of adjacent 3-4 anchors on each side,and lead to increase of maximum shear force and bending moment of capping beam.The capping beam is easy to be damaged according to structural reinforcement.As the number of initial partial failure anchors increased,maximum load(axial force)transfer coefficient(I_t)increased gradually and tended to constant value,and the constraint effect of the capping beam on stress and deformation of the retaining piles decreases within the range of failure.The deformation and stress mode of pile gradually transformed braced type to cantilever type(the capping beam provides support for the piles within the scope of partial failure zone),and the maximum bending moment decreased first,then increases to constant value.Meanwhile,load(bending moment)transfer coefficient(I_m)is generally bigger than I_t.It can be seen that when the failed anchors are less,the progressive failure is firstly transmitted along the anchor and trigger more anchors failure.When the failed anchor is more,the progressive failure will develop to the damage of retaining pile,which is close to the impact of strut failure in the bracing support system.In practice,slow failure occurs in the bolts gradually,and the anchors with slow failure have a smaller load transfer coefficient(I_t)compared with anchors with transient failure.Compared with the internal braced retaining system,the stiffness of anchor is generally less than that of steel strut,so the load(axial force)transfer coefficient(I_t)caused by the same failure range is small.In the progressive failure of excavation along the length direction,the relative magnitude of load transfer coefficient and safety coefficient of the structure determine whether the adjacent retaining system will cause secondary progressive failure and the development range of progressive failure due to partial failure,which is an important target of progressive failure research.On this basis,the design principles of the blocking element method for preventing progressive failure and its use in typical excavation support systems(cantilever,internal bracing and tied-back)are proposed.The finite difference method is used for simulation and verification.