Bearing Characteristcs Of Pile And Soil Materials Subjected To Pile-supported Excavation For Underground Storey Supplement

Further excavation beneath existing buildings offe rs a new solution for supplementing underground space in developed urban region.This can effectively alleviate the urban space congestion and parking problems.Recent years,a few successful real cases have been reported.Further excavation involve s many complex problems such as supported excavation and the bearing behavior of existing in-service foundation piles.However,present research mainly focus es on new excavation and zero-load foundation pile.This leads to the lack of theoretic base for related practice and restricts further application.This thesis aimes to simulate the practice of pile-supported excavation for supplementing underground storey.A large-scale test chamber was established to simulate the working performance of the internal and external layered pile-supported structure and the interaction between the existing piles and the supporting piles,together with the theoretical analysis of excavation-induced soil rebound.Several key parameters were involved to study the behavior of retaining piles and foundation piles.The involved parameters included the excavation depth,the row spacing,the retaining pile length and the pile-top load.The major contributions and findings are summarized as follows:(1)Under the condition of double-row piles retaining excavation,considerable bending moment is generated in the front-row retaining piles during further excavation and;its magnitude varies inversely with the embedment of the back-row retaining piles but positively with the row spacing.Presence of the existing piles gives advantage to the new piles;as thus,their absence results in an abrupt increase in bending moment and a downward movement of zero bending moment along the new piles.The plane position of the individual retaining pile puts significant effect o n the pile's bending moment;however,the discrepancy can be adjusted to be smaller by strengthening the lateral stiffness at the pile top.(2)The new consolidation method considered the coupling effect between the over-consolidation ratio and the coefficient of lateral static pressure during excavation unloading.The initial residual stress in each soil layer was derived so as to modify the over-predicted unloading stress.Then the effective overburden stress was assumed to be equal to the calculated residual stress.The resilient modulus of soil was determined in accordance to the residual stress method.Comparative analyses involving several case histories show that the proposed method yields satisfactory agreement with the measurements and the over-consolidation ratio can reflect the variation of residual stress.(3)As to the interaction effect between the single-row supporting pile and the existing foundation pile,results show that the foundation piles with zero load affect the retaining piles largely in terms of deformation rather than internal force;the in-service foundation piles put significant effect on the distribution of back earth pressure and the failure mode of the retaining piles,and the effect is related positively to the loading level applied on the foundation piles;the so-called "sheltering" and "counter-pressed" effects induced by the existing engineering piles are also affected by the spatial effect of excavation,and the effect decreases with the increase of the distance between the foundation piles and the retaining piles.