Influence Of Large Area Excavation Around Bridge Piers On Working Performance Of Existing Railway Bridges

With the development of urban rail transit,excavation under the bridge is often involved.The soil unloading caused by excavation around the pier will lead to changes in the displacement and stress of the soil around the pier,thereby affecting the bearing capacity and stability of the bridge.However,the current research on the influence of soil excavation around the bridge pier on the working behavior of railway bridges is not sufficient.Therefore,relying on the treatment of black and smelly water in the lower section of the Yuxi River and the sponge project,which are underneath the Kunyu Railway and the Kunyu River Railway,this paper studies the following contents by using the main research methods of on-site investigation,theoretical analysis,on-site detection,on-site experiment,and numerical calculation:1.The displacement variation of soil around a single pier affected by excavation under pile-soil interaction is analyzed by the analytical method.It is found that the deformation of soil is the largest when the distance from the pier is zero meters,and the maximum vertical displacement is 2.31 mm.The extreme value decreases with the increase of distance,and the deformation is zero above 6 meters.The results of the analytical method and numerical calculation are compared and analyzed.It is found that the mirror method has errors in the calculating settlement.The error comes from conservative simplification in the calculation process.2.MIDAS GTS is used to simulate the excavation under the bridge,and the interaction between the excavation under the bridge and the existing railway bridge is reflected through the construction process module.The effects of excavation on the exposed piers and abutments,different widening lengths,different excavation depths,different river positions,and excavation methods are studied.It is found that some piers and abutments originally buried in the soil will not produce additional displacement after excavation,and the displacement comes from the effect of unloading.When the widening width is 10 m – 20 m,the displacement change is the most obvious.Maintain stability when the widening value is 80 meters;with the deepening of the excavation depth,the vertical displacement of the bridge increases.When the excavation depth is less than 3 meters,the extreme value of the vertical displacement of the bridge changes into linear growth and decreases when the excavation depth is greater than 3 meters.The stress changes linearly with the increase of excavation depth.3.The displacement and stress changes of the bridge under train dynamic load before and after excavation are studied.It is found that the maximum vertical displacement curve of the bridge is similar during the train dynamic load.The horizontal displacement along the bridge and the horizontal displacement across the bridge increased significantly.The horizontal displacement along the bridge direction before the construction of the third span is 0.0017 mm,and 0.0051 mm after the construction.The maximum transverse horizontal displacement of the bridge is 0.025 mm before construction and 0.038 mm after construction.The maximum bridge stress is 0.027 MPa,which meets the requirements of the specification.Through the study,it is found that the influence of excavation on piles under train dynamic load is far less than that on the superstructure of the bridge.4.The changes of displacement and stress of bridge and pile before and after excavation under seismic dynamic load are studied.It is found that the relative vertical displacement of the bridge and the relative horizontal displacement along the bridge direction generated by the bridge decrease to a certain extent.The vertical displacement of the first span before excavation is 3.44 mm,and the vertical displacement after excavation is 2.63 mm.The vertical displacement of the sixth span is 4.05 mm before excavation and 2.71 mm after excavation.And from the degree of change,the change of relative horizontal displacement is more obvious,the extreme value decreases from90.72 mm before excavation to 79.06 mm after excavation.But the change rate of vertical relative displacement is greater.The relative horizontal displacement of the pile along the bridge is very small,and the relative horizontal displacement of the transverse bridge is relatively large.In addition,from the stress comparative analysis,it is found that the stress change of the pile is more obvious than that of the upper structure of the bridge.The maximum horizontal stress under seismic dynamic load before construction is 2.296 MPa,and the maximum horizontal stress under seismic dynamic load after construction is 3.416 MPa.