Study On Dynamic Response Of Foundation Pit Retaining Structure Under Train Vibration Load

With the rapid development of China’s infrastructure construction,many large and medium-sized cities are now planning subway lines have constituted a network,becoming the main component of rail transportation in large and medium-sized cities.The vibration and its propagation when the subway is moving will make many existing buildings and projects under construction suffer the influence of dynamic loads.Especially for under-construction projects,it is one of the major concerns in the engineering field to study the effect of underground train vibration loads on them and to take appropriate reinforcement measures for weak areas.This study focuses on actual engineering cases to investigate the dynamic response law of foundation pit support structure under the action of train vibration load,The following are the main contents.First,data collection and modeling are carried out for a new secondary school campus project in Luohu District,Shenzhen.The nearest distance to the north side of the project is about 14 m from Line 9(in operation),and the nearest distance to the shield interval of Line 6(not in operation)is about 74.0 m.The north side of the support structure is affected by the vibration effect of the subway for a long time,and the project is used as the research background.Based on the hydrogeological conditions investigated in the field,the parameters of the soil layer in the under-construction area and the shape size and excavation depth of the foundation support structure are obtained.Second,for the finite domain fluctuation response problem,the setting method of functional gradient viscoelastic artificial boundary layer is proposed.The numerical simulation of the fluctuation problem is carried out based on COMSOL software.The numerical simulation of the fluctuation problem is verified by two-dimensional structure and three-dimensional structure.The results show that the artificial boundary layer is more effective in reducing the influence of the artificial boundary on the actual fluctuation than other traditional methods in time domain analysis.Therefore,we further propose a composite boundary based on a functional gradient viscoelastic artificial boundary layer and a low-reflection boundary for the three-dimensional structure.Numerical arithmetic examples show that the artificial boundary can effectively achieve the requirements of low reflection,no distortion and rapid decay,and is used in the subsequent research of this paper.Further,in this paper,based on the previous research on the train vibration model,MATLAB is used to program the acceleration into the load as the train vibration load force.And the maximum frequency of the input acceleration data after processing is used to determine the grid size and time step.Final,in this paper,based on the previous research on the train vibration model,MATLAB is used to program the acceleration into the load as the train vibration load force.And the maximum frequency of the input acceleration data after processing is used to determine the grid size and time step.Finally,COMSOL software was used to model the train vibration load applied to the support structure at different excavation depths,and the dynamic response of the support structure at different excavation depths subjected to train vibration load was analyzed by numerical simulation.The simulation results show that as the excavation depth increases,the displacement and stress peaks of the support structure subjected to train vibration loads become larger.The distribution of tensile pressure in the piles of the support structure and the distribution of Mises equivalent force in the support structure were analyzed,and the top of the piles,bottom of the piles and crown beam were more stressed;the stress state of the rear piles was smoother when they were subjected to train vibration load;the top of the front piles,bottom of the piles and crown beam of the support structure were the weak positions when they were subjected to vibration;the dynamic response of the support structure on the far tunnel side and the dynamic response on the near tunnel side were compared.Since the dynamic load will decay during the propagation of soil,when the vibration propagates to the far tunnel side,the peak value has decayed to 3%～8%of the input acceleration,and the dynamic response of the far tunnel side is already very weak.In this paper,the response of the foundation support structure under dynamic load is analyzed for a specific engineering problem in a campus in Shenzhen,and the proposed composite artificial boundary method has some universality in the time domain analysis of fluctuation problems.The adopted method and the obtained conclusions are of theoretical value for relevant engineering applications.