Analysis Of Deformation Effect And Characteristics Of Bridge Pile Foundation Caused By Deep Foundation Pit Excavation

With the booming of infrastructure construction in China,cities are often limited by terrain and site conditions,and the comprehensive problems of underpass or adjacent viaducts of foundation pits are becoming increasingly prominent,The protection standards for similar problems are often stricter.This paper relies on Xiamen Second West Passage(Haicang Subsea Tunnel)underpass Hualong Road Elevated Section Project.During the excavation of 12 m deep foundation pit,it is necessary to expose one of the bridge foundations of Hualong Road viaduct,including the caps and pile foundations.The minimum distance between the piers outside the foundation pit and the retaining structure of the foundation pit is about 1.5 m.The diameter and length of bridge pile foundation(bored cast-in-place pile)are 1.8m and 27 m respectively.Therefore,in the process of deep foundation pit construction,it is particularly important to ensure the safety of bridge structure.On the basis of summarizing the existing research and according to the actual engineering characteristics,this paper systematically studies the influence of foundation pit excavation on the pile foundation of viaduct bridge by means of field measurement and numerical simulation.The main research contents and conclusions are as follows:1)Before excavation of foundation pit,through theoretical analysis,the bridge piles in the pit and the upper piers are assumed to be hinged at one end and slender equilateral rods with fixed length of 30 m at the other end.The maximum horizontal displacement of the exposed Bridge Piles in the pit is calculated to be 5.47 mm.According to the existing codes and regulations,the safety control indexes of the bridge are determined.2)Make a detailed and reasonable measurement plan to obtain the monitoring data of foundation pit and bridge.The maximum horizontal displacement of retaining structure of foundation pit is about 0.11%~0.21%H;Foundation pit excavation will lead to vertical uplift and horizontal displacement of pier.The exposed pier in the pitis a fixed support.Its deformation inclines towards the pit,and the position of the cap is the largest.The pile foundation of the bridge under the cap is "fish belly";The pier outside the pit is a movable support,and the excavation of the foundation pit results in its translation towards the pit.The maximum deformation of the pier outside the pit is 11.9 mm during the excavation of the pit,and the maximum deformation of the bridge pile inside the pit is 3.8 mm from the beginning of monitoring to the end of excavation.3)Flac3D finite difference software is used to carry out numerical simulation research.The influence of deep foundation pit excavation on supporting structure and bridge pile foundation under actual geological and practical conditions is analyzed.The rationality of numerical simulation parameters and reliability of measured data are verified by comparing and analyzing measured data and numerical calculation results.On this basis,the maximum deformation of bridge pile in pit is about 10.61 mm during excavation.The deformation model predicted by numerical calculation is consistent with that predicted by measured results.4)The influence of bridge pile location,support stiffness,boulder and excavation depth of soil behind bridge pile in pit on the deformation of bridge pile in soil-rock composite stratum is studied.The results show that when the unloading depth is constant,the deformation of the bridge pile outside the pit decreases gradually with the distance from the foundation pit increasing,and decreases gradually with the increase of the stiffness of the retaining structure;The deformation of bridge piles in pits decreases with the increase of unloading depth of soil behind piles.Under the same volume of solitary stones,the height of solitary stones can restrain the deformation of bridge piles more effectively than the depth of solitary stones.On the basis of this study,some suggestions on deformation control of bridge piles during construction are put forward.