Study On Consolidation Theory Of Composite Ground Improved By Composite Piles

Formed by two materials with different properties,composite pile integrates both accelerated drainage and enhanced bearing capacity.This new technology has been widely used in the constructions of highway and housing projects for ground improvement.To reasonably predict the consolidation progress of composite ground with composite piles,relevant studies have been carried out.However,the current results are relatively few,and there is a certain gap between the predicted and measured values.In this thesis,theoretical methods combined with engineering cases are used to investigate the consolidation of composite ground improved by concrete-core gravelshell piles,and the following contents are involved:(1)The mathematical model on the consolidation of composite ground improved by composite piles is proposed with the consideration of constant well resistance.Firstly,based on the annular equivalence theory,the composite pile is transformed into an annular unit.Then,pile cross-sectional shape effect,the construction disturbance effect and the distribution of additional stress varied with time and depth are considered.Based on the equal strain assumption and Darcy’s law,a mathematical model about the ratio of cross-sectional area of the stiffer core to gravel shell is proposed to investigate the consolidation of composite ground improved by composite piles.After that,solutions for degree of consolidation are achieved under arbitrary loads and special loads,and a parametric sensitivity analysis is carried out.The result shows that the degree of consolidation is inversely proportional to the hollow ratio αo;as αp increases,the rate of consolidation increases and then decreases;the degree of consolidation is proportional to Ep/Eg,kg/kv,and σiT/σiB;the optimum kg/kv is 5000,and the optimum αp is 0.5;the hollow ratio of the square composite pile is the smallest,and the consolidation is the fastest under a given perimeter.(2)Considering the effect of clogging within the sand shell,the mathematical model on the consolidation of composite ground improved by composite piles is established.The fine soil particles flow into the gravel shell following the pore water,and then a clogging area is formed during consolidation,resulting in smaller drainage area and slower consolidation.Firstly,the permeability coefficient is assumed to decrease exponentially with time,and a mathematical model for consolidaion is established considering the clogging effect.Then,based on the separation of variables method and Fourier function,the analytical solutions for the consolidation of composite ground under single-stage and muti-stage instantaneous load are derived.Finally,the effect of parameters on consolidation properties are analyzed.It shows that the consolidation rate is proportional to αB while it is inversely proportional to the initial permeability coefficient of the sand and gravel shell kg0/kh;when kg0/kh>3 000,clogging has no effect on consolidation;the effect of clogging on radial consolidation mainly acts in the later stage and makes consolidation cease finally;when radial consolidation stops,the vertical seepage within soil allows ground continue to consolidate until completion;as H increases,the effect of kv/kh on the consolidation rate changes from acting mostly in the early stage to the later stage.(3)Considering the effect of time-and depth-dependent well resistance on sand shell,the analytical model on the consolidation of composite ground improved by composite piles is established.The permeability of the gravel shell decreases with timevarying clogging and increasing lateral pressure,so the well resistance varying with time and depth needs to be considered.Thereby,the well resistance is assumed to decrease linearly with depth and exponentially with time,and an analytical model for the consolidation of soft ground improved by composite piles is developed based on the equal strain assumption.Then,the separation variable method and the correlation properties of Bessel function are applied to solve the explicit results for it under instantaneous loading.Finally,the effects of the parameters on the consolidation properties are analyzed:The rate of consolidation is proportional to B1 and inversely proportional to B2 and a3,with a3 responding more significantly than B2.(4)Combined with practical engineering cases,the solutions are compared and verified.The theoretical results of various consolidation models with and without considering the well resistance effect and clogging effect are compared using same parameters.Then,combined with engineering examples,the consolidation model of soft ground with composite piles just considering clogging effect is applied to predict consolidation in the laboratory.Compared with the measured data,it shows that the results show good consistency.Finally,the consolidation model is degenerated to that for composite ground with gravel piles for predicting settlement in embankment projects.Compared with the measured settlement values in situ,its reasonableness and applicability are verified.