Settlement Calculation And Numerical Analysis Of Work Behavior Of Piles Group With Variable Stiffness

Settlement calculation of piles group and optimum design of piled raft foundation are always hot topics in geotechnical engineering.The reinforcing and restraining effect of piles embedded in soils is objective,however,this effect has not been or effectively been considered in current theory and practice.And in traditional interaction factor method,only the elastic settlement of piles group could be calculated,and the interaction factors are significantly greater than the real results.Numerical analysis method has been greatly developed,but its epgineering application was still restricted due to the complexity of modeling,geotechnical parameters and constitutive relations of soils.Aiming at those problems,a series of researches were done,including deducing settlement formulas considering reinforcing and restraining effect,and studying the work behavior and optimezed design of piled raft by numerical analysis method.The research outcomes are of theoretical and practical significance.The main researches in this dissertation are as follows.Firstly,based on the shear displacement method,a matrix equation of flexibility coefficient or interaction coefficient was built to calculate the settlements of piles group with constant stiffness(equal pile length,diameter and spacing),in which the reinforcing and restraining effect was taken into account,i.e.the reduction effect caused by the existence of the other adjacent piles.In the process of the deducing,the multilayer of soils and the interaction of settlement between piles'bases were taken into consideration,and a new relationship between pile-end resistance and pile-end settlement was proposed based on Mindlin displacement solution.Secondly,interaction coefficient between any two piles was redefined,and then a simplified formula with regard to the newly defined interaction coefficient was presented,in which reinforcing and restraining effect was also considered.Thirdly,on the basis of the analysis on piles group with constant stiffness and the new definition of interaction coefficient,the settlement caculations of the pile groups with variable stiffness(different pile lengthes,diameters,moduluses and spacings)considering reinforcing and restraining effect were studied.And the research focused on the derivation for piles with different lengthes(long-short piles),including influences of long pile on short one as well as short pile on long one.Fourthly,using three dimensional numerical simulation program FLAC3D to fit the load-settlement curve measured in the static load test of a single pile,the elastic parameters of soil and pile-soil interface parameters were determined through the elastic and elasto-plastic analyses.Then the influences of various parameters on the interaction coefficient were analyzed by numerical simulation of two-pile models,and a polynomial curve was used to fit the results.Based on the results of numerical analysis,a method of using single pile load test to calculate pile groups'elastic and elasto-plastic settlement in the interaction factor method was proposed,so that this method was extended to compute the nonlinear settlement of piles group.Lastly,a series of field model tests were analysed to verified the feasibility of FLAC3D,and then full scale models of five different piled raft foundations were built,including natural groundsill,uniformly distributed piles,varied piles spacing,varied piles length,and varied piles diameter,and the work behavior were systematically studied and compared.The results from model test and numerical analysis could be references for the optimum design of piled raft foundation.