Study On The Interaction Mechanism And Stability Of Multi-bench Retaining Structures Of Excavations

Retaining system with horizontal struts is often used in soft soil excavations.But horizontal strut system is costly and needs a long construction period;the space is limited;and it will be a problem to deal with the abandoned struts.The multi-bench retaining system can be used in deep excavations for the purpose of cancel horizontal struts.Three multi-bench retained excavation projects were introduced and the efficiency of this system was proved by comparing the deflections of the retaining structures in different retaining systems.Large-scale model tests were designed and conducted to study a two-bench retaining system.The test results showed that three failure mechanisms,were observed with different distances between two rows of retaining piles?bench width?,which were the overall overturning failure,mutual-effect failure and separate failure.When the overall overturning failure occurred,the soil between two rows of retaining piles?bench zone soil?did not reach the critical state.There was no obvious shear band or slip surface in the bench zone soil.When the mutual-effect failure occurred,the slip surfaces can be observed in the soil in the retained side and the bench zone.The overturing failure was observed but the failures of two retaining structures were not simultaneous.When the separate failure occurred,one of the two retaining structures overturned,while the other one was not failure.The slip surfaces caused by the failure of the overturned retaining structure were formed in the bench zone soil.Corresponding finite difference simulations were conducted to back-analyze the model tests and for further analysis.The test and numerical results showed that the distance between two rows of retaining piles and their embedded lengths can significantly influence the relative movement between them.The bench zone soil can be divided into“relaxation zone”and“compaction zone”based on the relative horizontal displacements of two rows of piles at corresponding height.When the first retaining piles were embedded deeper than the second ones,the relaxation and compaction zone located at the upper and downward part of the bench zone,respectively,and the first retaining piles would undertake a larger bending moment than the second ones.When the second retaining piles were embedded deeper,the location of the relaxation and compaction zone would exchange,and the second retaining piles would undertake a larger bending moment.As the bench width increases,the deformations and bending moments in the retaining piles decrease.Shear strength reduction method was used to the further analysis of the failure mechanism of the mlti-bench retained excavation.The result of shear strength reduction calculation showed that the failure modes are related to the soil strength and the geometric forms of the retaining system.The parameter“R_fos”was introduced to reflect the influence of the soil strength,which is the ratio of the factor of safety of the two level retaining structures.The critical separate failure width,B_d and the critical overall overturning failure width,B_i decreased with increasing R_fos.A simplified analysis method can be used to calculate the safety factor when R_fosos is small.B_s and B_i will be constant when R_fosos is large enough.The influence of the the embedment ratio of the first and second retaining structures were related to the velue of R_fos.Based on the study of the serviceability and the failure mechanism of the multi-bench retaining system,a new design method was introduced by the introduction of the concept of“mobilizable soil strength.”Comparisons with the numerical results show the reasonability of the proposed method.