Study Of The Mechanism Of Progressive Collapse Along The Length Direction Of The Long Strip Excavation Retained By Cantilever Contiguous Piles

In recent years,the scale of excavation have become bigger,deeper and longer than before.Progressive collapse of deep excavation in the longitudinal direction that induced by partial failure could extent to tens of meters or even more than one hundred meters,The research of the mechanism of progressive collapse induced by partial failure is still limited.The analysis method of progressive collapse are still based on two-dimensional theoretical or elastic foundation beam method,these methods neither can analysis how the partial failure evolves to large scale progressive collapse nor can provide theoretical basis for design theory of progressive collapse.This paper conducted the studies of deep excavation progressive collapse as follows.(1)This paper have explore means of how to simulate lagre deformation questions in excavation by model test.Large-scale physical model test platform,various model pile and the strain monitoring system have been build for model test.(2)Model test and finite difference method is used to analyze the partial excavation in long strip pit,the results show that partial over-excavation could lead to significant increments of the internal forces of the piles in the range of excavation and the adjacent piles due to soil arching effect.When the normal excavation depths are the same,the maximum moment of pile and the displacement of pile top will increase with the range of partial excavation until the trend tends to be stable when the partial excavation is larger than certain value.(3)Model test and finite difference method is used to analyze the partial failure in long strip pit,the results show that partial collapse will cause sudden increasing of adjacent earth pressure and structural internal forces beacause of soil arching effect.After the initial-failure piles begin to failed,the soil behind the initial-failure piles flows into the excavation,the displacement between the soil bodys induced the soil arching effect,the moment of the adjacent pile increased beacause the adjacent pile have been the foot of the soil arching.When the the ultimate bearing capacity of the adjacent pile is lower,progressive collapse along the length direction will happen.As we see in test 4,after 4 initiating-failure pile failed 4 following-failure pile rapidly failed.(4)The concept of a load transfer coefficient was proposed,the load transfer coefficient of a adjacent pile were proportional to the distance between the adjacent pile and the initial-failure pile.When the normal excavation depths are the same,the load transfer coefficient were proportional to the amount of initial-failure pile;When the amount of initial-failure pile are the same,the lower the normal excavation depths are will induced to a higher load transfer coefficient.(5)After the initial-failure piles failed,the adjacent pile failed one by one,soil around the collapse will slide into the excavation,which produces an unloading effect in the adjacent piles.The unloading effect lags behind the loading effect of soil arching,and it is the main reason for the natural termination of the progressive collapse.