| The deformation caused by the subway tunnel passing through the bridge foundation will be transmitted to the superstructure of the bridge to form additional stress.When the superstructure is a sensitive irregular-plate structure,there are potential hazards such as cracking at the bottom of the bridge plate that affect the safe operation of the bridge.This kind of engineering problem has been encountered many times in Beijing subway construction.In this paper,based on a large number of engineering problems of cracking at the bridge plates in the construction of subway tunnel crossing the irregular-plate bridges,a wide range of domestic and foreign literature has been investigated.Based on the case study of Beijing metro line 7 and line 10 tunnel passing through the sensitive irregular-plate bridges,the control standard of pile foundation settlement,identification and evaluation of construction risk,key technology of construction control and analysis of cracks after construction were carried out by the combined method of theoretical analysis,field test,simulation calculation.Through the above researches,the following innovative results were obtained.(1)The results of simulation calculation and field monitoring show that the deformation of pile foundation should be controlled within 3mm when the subway tunnel passes through the sensitive irregular-plate bridges,which can effectively inhibit the emergence and development of cracks at the bottom of the bridge plates,and ensure the normal use of the superstructure.(2)The displacement and stress of steel underpinning beam are the control index of controllable active pile underpinning,which can control the deformation of pile foundation within 3mm during underpinning.However,there are many working procedures,so it is necessary to strictly control the engineering risks in three main working procedures: underpinning pile construction,underpinning jacking and tunnel excavation.(3)Compensation jacking technology makes full use of the deformation ability of the irregular plate upward.The superstructure of the irregular-plate bridge is jacked in advance before the subway tunnel construction,which compensates the settlement of the irregular plate and reduces the difficulty of controlling the ground settlement during the subway underpass construction,so as to solve the difficulties of shield tunnel successfully crossing the sensitive irregular plate bridge in the sandy pebble stratum.(4)The location of the cracks in the irregular plate bridge appears at midspan baseplate,which is obviously different from the deformation and failure characteristics of the continuous beam.The new cracks are mainly transverse and oblique cracks,which are closely related to the differential settlement of pile foundation.However,there is a difference between the distribution of additional stress and cracks after construction.The reduced calculated settlement value is used as the index of reverse jacking to compensate the settlement deformation of irregular plate caused by tunnel excavation.(5)It is difficult to compensate the settle deformation in time by controllable active pile underpinning.Compensation jacking technology can only be used in shield tunnel construction,and the risk is great.In view of the shortcomings of two kinds of risk control technology,a new technology of subway tunnel crossing sensitive irregular-plate bridge is proposed.Before construction,the irregular plate is jacked in advance to reduce the midspan stress of the affected part,and the deformation space for tunnel underpass construction is reserved.At the same time,when the deformation of irregular plate reaches 70% of the control value in the construction process,the compensation jacking is started to reduce the additional stress of irregular plate in time,so that the major risk of crossing the sensitive irregular-plate bridge is more controllable. |
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