The Research On Calculation Method Of Settlement Of Geogrid-encased Stone Columns Based On Thick-walled Cylinder Model

Geosynthetic-encased stone columns(GESC)composite foundation is a composite foundation encased by geogrid and other geosynthetics near the top of gravel pile in traditional stone column composite foundation.It not only inherits the advantages of gravel pile including accelerating the consolidation of natural soft foundation,improving bearing capacity and controlling settlement,but also can minimize the bulging of gravel pile and further improve the bearing capacity of composite foundation.However,due to the existence of different materials including geosynthetics,gravel and soft soil,the calculation and analysis of bearing capacity and settlement of GESC are complicated.At present,there is still plenty of work need to be done in the theoretical approach of calculating deformation of pile-geogrid-soil simultaneously.In addition,the research about the diameter of the unit cell,which is widely used in analyzing and engineering calculation of gravel pile and GESC,are rarely seen.Therefore,through numerical simulation analysis and theoretical derivation,the diameter of unit cell and settlement calculation of GESC were discussed in this thesis.Firstly,after introducing the development,mechanism and failure mode of gravel pile composite foundation,the mechanism and failure mode of GESC were summarized and compared with the traditional gravel pile.The approach of research was proposed for investigating unit cell and settlement calculation method of the GESC.Secondly,the unit cell of GESC was studied in detail through finite element numerical model.According to the calculation results,the influence of geogrid's strength,reinforcement length and gravel pile strength on the radial and circumferential stress state of pile as well as soil were analyzed separately.Then,three functions describing diameter of unit cell were linear fitted.The functions showed that the commonly used approach of calculating diameter of unit cell is overestimating it with respect to the bearing capacity and deformation capacity of GESC.Thirdly,the unit cell concept was employed in the analysis while assuming GESC together with soil was linear elastic materials and evenly deformed.Stone column was constrained by geogrid and earth ring at the same time while the earth ring can be considered as a ring with earth pressure at rest provided by soil outside and bulging pressure caused by the stone column installed inside.Hook's law was then introduced to gain the stress-strain formula of the unit cell.Then A different method of settlement calculation of GESC composite foundation was established.The rationality of the results of this method was verified by an engineering example and had compared with the existing method.The result demonstrated that the method agreed well with the engineering example.Finally,the finite element numerical modeling was carried out by previous engineering examples and the results of vertical stress of piles and soils were analyzed and compared with the theoretical calculating results.Then,the influence of loading level as well as the pile-soil stiffness ratio on pile-soil stress ratio and the influence of replacement rate on the total settlement of composite foundation were analyzed utilizing the theoretical results.