Analysis On Vertical Bearing Behaviour Of In-service Piles Under The Condition Of Excavation Beneath Existing Building

Excavation under existing building to supplement or rebuild a new basement is an effective mean to expand underground space that caters for the increasing need of parking space in urban areas.As piles are served as the load transfer structures,excavation beneath existing basement will inevitably change the bearing behaviours of in-service piles.Additional displacement of existing in-service piles will then be caused,and is likely to lead to incline,even collapse of the whole building if some effective measures on improvement of the pile bearing capacity are not conducted.Therefore,there is a need to analyze the influence of excavation on the bearing characteristics of existing in-service piles,and to investigate the time effect of bearing capacity of piles for the full consideration of their bearing potential.To address these problems,a variety of methods were involved such as analysis of reported pile-test results,model test,numerical simulation and theoretical analysis.This study revealed the rule for variation of bearing capacity and bearing stiffness of pile foundation with time,and proposed an analysis method to clarify the time-dependent behaviour of existing in-service piles.Meanwhile,load transfer models of skin friction and end resistance of the in-service piles under the condition of excavation were also established in this study and an analysis method of existing pile response considering the effect of excavation was finally presented.The main work and conclusions are summarized as follow:(1)A comprehensive database with numbers of pile load test results included was established through literature search,and the ageing trend of bearing capacity and bearing stiffness of pile foundation embedded in different types of soils was carefully discussed and empirically quantified.Then,the normalized hyperbolic-type load-settlement model was adopted to simulate the relationship between the applied load and pile head settlement.Model parameters were clearly defined based on database assessment and theoretical analysis,and a method for analyzing the time-dependent load-settlement behaviour of single pile before the excavation was proposed.Moreover,an empirical formula for group stiffness coefficient of pile group was established by analyzing the field test results of pile groups.Using the concept of equivalent pier,the proposed load-settlement model for single pile was then extended to pile groups;(2)Model test study on the behaviour of single and group in-service pile under the condition of excavation was carried out.Through monitoring the axial force distribution of pile shaft,shaft and base capacity,and pile head settlement at different excavation depth under different applied load,the influence of load level and excavation depth on the bearing characteristics,including strength and stiffness,of pile foundation was analyzed;(3)Parametric study on the the influencing factors of pile-pile interaction within the group in-service pile under the condition of excavation was performed by finite element analysis.Numerical models with different excavation radius,excavator depth and pile spacing was established using the finite-element package ABAQUS and the value of interaction factors for different scenarios was back analyzed.The effect of excavation radius,excavator depth and pile spacing on the interaction factors was finally revealed;(4)According to the model test results,the hyperbolic model was adopted to simulate the relationship between pile-soil relative displacement and skin friction developed along the pile-soil interface,whereas the bilinear model was used to describe the unit end resistance-displacement response developed at the pile tip soil.The value of pile-soil relative displacement in load-transfer model was modified by analyzing the soil rebound caused by excavation,and the over consolidation ratio is introduced to calculate the skin friction of pile after the excavation since the soil at the bottom of the pit is in the state of over consolidation due to the stress relief.After that,based on the load transfer method,an iterative program for calculating the load-settlement behaviour of single pile after the excavation was proposed Furthermore,using the interaction factor method(IFM)and the interaction factor obtained from numerical analysis,the analysis method for single pile was extended to the pile group.