Theoretical And Numerical Analysis Of The Settlement And Consolidation Of CFG Pile Composite Foundation

In recent years, the composite foundation technology is being widely used in thetreatment of subsoil, and the composite foundation theory has become an importantresearch subject in the geotechnical engineering field. As one of these techniques, theCFG pile (i.e., Cement-Fly-Ash Gravel pile) composite foundation has quite a fewadvantages such as simple construction, reliable quality and low cost. When appliedto treat the soft subsoil, this kind of composite foundation can increase the bearingcapacity and reduce the settlement obviously. Furthermore, rubbish material (i.e.,fly-ash) can be turned into treasure with significant economic and social benefitachieved as well. However, at present relevant theoretical research about the CFG pilecomposite foundation is still far behind its engineering practice. Especially, fewliteratures or reports are available about the study working on the partially penetratedCFG pile composite foundation (composed by the reinforced region along the pileshaft and underlying unreinforced area down the pile tip) used in the treatment ofthick soft subsoil. Therefore, a further study was made in this dissertation to discussthe consolidation and deformation behavior of the partially penetrated CFG pilecomposite foundation, of which the following work was included:Firstly, based on the available research achievements and developing situation ofthe composite foundation and the CFG pile composite foundation, behavior of theCFG pile composite foundation, including the bearing and deformation characteristics,action mechanism, cushion setting rules, design and calculation methods of thebearing capacity, was deeply analyzed respectively.Then, the partially penetrated CFG pile composite foundation was simplified as adouble-layered foundation. Based on the one-dimensional consolidation theory, andconsidering the permeability of the pile shaft or not, consolidation equations of thepartially penetrated CFG pile composite foundation were established, respectively. Byintroducing the corresponding boundary conditions, formulas to calculate theconsolidation degree of the composite foundation under various loading process werederived, in which the consolidation degree were defined by two different parameters,i.e., the settlement and the pore water pressure. On this basis, a calculation programby MATLAB was worked out. A further comparative analysis was then carried out tofind out the influencing rules by some key factors (such as the pile length, pile shaft modulus, perturbation, pile shaft penetration ratio and area replacement ratio) on theconsolidation process. A conclusion was also verified that the consolidation degreedefined by the settlement is different from that by the pore water pressure in thepartially penetrated composite foundation.Based on the obtained calculation formulas and definition of consolidationdegree in partially penetrated CFG pile composite foundation, the settlementcalculation formula considering the time effect was derived, of which the feasibilitywas verified by the result of contrastive analysis of a computational case.Finally, a three-dimensional numerical analysis model of the partially penetratedCFG pile composite foundation was established by FLAC3D. Then, by changing theparameters in the model, influencing curves by some key factors (including the loadvalue, modulus of the subsoil between the piles, modulus of the underlying soil layer,and the pile length) on the settlement of partially penetrated CFG pile compositefoundation were obtained, which may be referred by the engineering practice.