Study On Dynamic Response Of Composite Foundation Subgrade Of High-Speed Railway Crossing Ground Fissure Zones

Ground fissures is a slowly changing geological hazard and widely distributed in China.Wherever it goes,surface buildings(structures),roads,bridges,and underground pipelines and other infrastructures will be affected and undergo deformation and cracking.With the deepening of China’s "Western Development" and the implementation of national strategies such as the "the Belt and Road" initiative,The large-scale high-speed railway construction is facing serious ground fissure disaster risks and challenges in China.Meanwhile,as a weak and fractured zone,even if ground fissure is not active,under the long-term train vibration loadhigh-speed railway subgrade crossing ground fissure can have differential settlement effects,endangering the safe operation of high-speed railway projects.Therefore,the research on the dynamic response of high-speed railway composite foundation subgrade crossing ground fissure zone has important theoretical and practical significance.This thesis takes the Daxi Passenger Dedicated Line(high-speed railway)as the engineering background,selects the high-speed railway foundation section spanning the ground fissure site of Dongguan Substation as the research object,conducts large-scale physical model tests on the dynamic response of embankment-ground fissure-CFG pile(cement fly ash gravel pile)composite foundation subgrade,reveals the dynamic response law of composite foundation subgrade under train vibration load,and combines finite element dynamic numerical simulation analysis,a comparative analysis and verification supplement were conducted on the dynamic response characteristics of natural foundation and composite foundation subgrade when the train crosses the ground fissure zone at different intersection angles.The main research results are as follows:(1)Based on a physical model test with geometric scale 1:20 of composite foundation subgrade of high-speed railway,the dynamic response laws of the subgrade,interior of the foundation,and CFG piles in a ground fissure site under the action of train moving vibration loads were revealed.It was found that the overall dynamic response of the subgrade in the form of composite foundation is similar to that of natural foundation.CFG pile composite foundation has a significant effect on reducing the dynamic response of the subgrade and controlling the deformation and settlement of the subgrade,effectively weakening the dynamic response caused by the gradual increase of train speed on the subgrade.The dynamic strain response of CFG pile body gradually decays with the increase of depth.(2)With the dynamic finite element numerical software,a three-dimensional dynamic finite element model was established to simulate and reveal the differential characteristics of the dynamic response of the subgrade in the natural foundation and composite foundation of the orthogonal crossing ground fissure zone.It was found that the dynamic response,influence range,and response difference between the hanging wall and footwall of the composite foundation are smaller than those of the natural foundation.CFG pile composite foundation can effectively reduce the dynamic response impact of high-speed trains on the subgrade.(3)Based on dynamic finite element numerical simulation,considering the influence of the intersection angle between the line and the ground fissure zone,the dynamic response laws and differential characteristics of natural and composite foundation subgrade under the condition of intersection angle(30 ° and 60 °)were simulated and analyzed.The results indicate that the smaller the intersection angle between the ground fissure and the high-speed railway line,the greater the difference in dynamic response,influence range,and response between the hanging wall and footwall of the subgrade.CFG pile composite foundation can effectively suppress the settlement deformation of the subgrade caused by train vibration loads.