Study On The Construction Technology Of Shallow Buried Small-pitch Highway Tunnel Crossing Bridge In Urban

The design of highway tunnels in the city is severely restricted by terrain.In order to maximize the use of space,the structure of the tunnel is often selected to be a small clear distance tunnel.When this type of tunnel is excavated in weak surrounding rock,the deformation of the surrounding rock easily exceeds the allowable value,resulting in safety accidents.In order to ensure the safety of the tunnel with small clearance in the weak surrounding rock during the excavation of the bridge underneath,it is of practical significance to study the influence of different excavation methods on the surrounding rock and bridge pile.In this paper,based on the elastoplastic finite difference theory combined with actual engineering,a three-dimensional solid model was established using FLAC3 D software.According to the actual engineering situation,the deformation law of the weak surrounding rock shallow buried small distance highway tunnel under the existing bridge was simulated and analyzed.Mainly conducted a systematic study on the excavation method of the small-pitch tunnel,the excavation sequence of the predecessor tunnel,and the surface settlement law and bridge displacement law caused by different excavation footage and excavation steps,and the following results were obtained.(1)This paper summarizes the research status of the impact of underpass bridge tunnel construction on stratum deformation and adjacent existing bridge pile foundation at home and abroad,and summarizes the application cases of underpass bridge tunnel construction,and then discusses the stratum deformation and pile foundation deformation The basic principle.(2)Analyze the settlement law of surrounding rock and surface during the excavation of shallow-buried small-pitch highway tunnel.The excavation of the small-pitch tunnel with the expansion of the surface settlement trough,the surface settlement trough is located above the centerline position of the left-line tunnel from excavation to penetration.With the continuous excavation of the right-line tunnel,the surrounding rock surface settlement trough gradually moved towards the right-line tunnel.The maximum settlement channel is located above the centerline of the two tunnels and the maximum settlement channel conforms to the Peck curve law,which has obvious superposition effect.The size of the settlement at the measuring point is related to the excavation progress of the shallow buried small spacing tunnel.The monitoring results show that the maximum settlement value of the ground surface is 17.3mm which is less than the 30 mm control requirement stipulated in the specification.(3)Based on the geotechnical mechanics and finite difference theory,the numerical simulation method was used to study the deformation and settlement law of small clearance tunnel excavation under weak surrounding rock conditions,and the tunnel enclosure under different excavation methods of shallow buried small spacing tunnels was obtained.Rock deformation and the changing trend of the upper bridge.Combining with the actual situation of Zengjiayan Tunnel,from the perspective of safety and economy,it is recommended to use CD method for excavation.Simultaneously,through simulation calculation,under the premise of CD method excavation,the influence of small clear tunnel excavation footage,excavation staggering step and pre-cavity tunnel excavation sequence on the surrounding rock and the upper bridge was obtained.It is concluded that the double-holes have large staggering steps,and the safety of tunnel excavation can be effectively ensured when the unfavorable hole is excavated first.(4)The deformation law of the pile foundation caused by the shallow-buried small-pitch tunnel passing through the existing bridge structure is analyzed.The maximum horizontal displacement of the pile during excavation using the upper and lower steps is 3.85 mm,which exceeds the control value by 3mm.The maximum horizontal displacement of the pile when excavated by the CD method is 1.55 mm,and the maximum horizontal displacement of the pile during the excavation of the double-wall guide hole method is 1.4mm,which are all within the control value,so the CD method or the double-sided wall should be used Excavation with the guide hole method to ensure the safety of excavation.Compared with the numerical simulation values,the monitoring results of the surrounding rock settlement convergence,ground settlement and bridge displacement during the actual excavation process have the same change law,which shows the scientificity and reliability of the research results in this paper.