Settlement Monitoring And FLAC3D Simulation Analysis Of Soft Soil Subgrade Treated By CFG Pile

With the acceleration of Chinese economic development and urbanization,the construction of high-speed railways and expressways is still developing at a fast pace.Some roads often cross soft soil areas due to the local human environment,social economic development,geography and environmental factors.Cement-fly ash gravel pile(CFG pile)is a commonly used method in the current soft road foundation treatment due to its simple technology,low cost and fast cycle.Based on the research project of soft foundation treatment and settlement observation of Wuxue Section of Mayang(Macheng to Yangxin)Expressway in Hubei Province undertaken by the mentor,the soft foundation treatment technology of CFG pile and the settlement mechanism after soft foundation treatment are studied.(1)The engineering geological and hydrogeological conditions of Wuxue Section of Mayang Expressway are analyzed,and five soft foundation treatment technologies including preloading,replacement,lime pile,gravel pile and CFG pile are compared and analyzed.CFG pile is finally determined to be used for soft foundation treatment in this section.The reinforcement mechanism of CFG pile composite foundation is analyzed by using continuous medium theory.It is verified that the settlement of CFG pile composite foundation can be divided into instantaneous settlement,main consolidation settlement and secondary consolidation settlement.(2)The cushion thickness is calculated by using the cushion stress diffusion theory.Assuming that the stress diffusion curve of the CFG pile head is a quadratic curve,the stress diffusion thickness of pile head and the length of pile penetrating into cushion under the condition of critical cushion thickness are solved.The validity of the calculation method is verified by calculating the cushion thickness of typical cross section of Wuxue section of Mayang Expressway.(3)The settlement observation during the whole process of subgrade filling in the CFG pile soft foundation treatment area is carried out,after the collation and analysis of the observation data,it is found that the settlement rate and settlement of the subgrade filling period after CFG pile treatment are far less than the authority control standard.The settlement of the composite foundation is divided into the initial slow growth stage,the middle rapid growth stage and the later slow growth to stable stage.(4)FLAC3D software is used to simulate the settlement of soft foundation combined with the settlement observation data of each stage during the filling period.The calculation from the aspects of vertical settlement of subgrade,horizontal displacement and vertical settlement of pile is carried out.The simulation value is compared with the field observation value to verify the rationality of the simulation method and the value of simulation parameters.The mechanical properties of soft soil layer and CFG pile toe bearing layer which affect the settlement of composite foundation are analyzed to provide guidance for subsequent filling construction and ensure the quality of filling engineering.