Model Test Study On Urban Shallow-buried Underground Tunnel Crossing Existing Bridge Piles

With the development of China's economic construction and urbanization process,the buildings in urban centers are becoming more and more intensive,especially as some areas of the second-tier cities have almost exhausted the ground space.A large number of underground passages and metro tunnels are constrained by the existing buildings around them.As a result,many newly planned underground passages and metro tunnels can only be built under the existing road lines,interlaced between the pile groups and underground pipelines of bridges,or even at the bottom of the pile ends of bridges.If a tunnel is excavated around or below a dense building,it will inevitably affect the surrounding soil and buildings.At the same time,excavation of tunnels results in the release of stratum stress,breaking the original balance,and the redistribution of soil stress will lead to uneven settlement of soil,resulting in different degrees of damage to bridge piles,pipelines and superstructure in soil layer.Some damages can be repaired,but more damages are irreversible.Therefore,how to rationally plan the relative position of the tunnel with existing bridge piles,underground pipelines and other existing structures and reduce the surrounding environment to a minimum is a problem that could not be ignored.Based on the project of Shenzhen under-crossing the ramp of Longgang Avenue,this paper studies the influence of tunnel excavation on pier,pile,surrounding soil and surface through model test.The main work and achievements are as follows:(1)By simulating tunnel excavation,the influence of different excavation procedures and the distance between bridge pile and tunnel excavation boundary on the change of pier top inclination angle is studied;the influence of different excavation procedures and different excavation methods on the soil deformation on both sides of excavation boundary is studied,and it is pointed out that the soil deformation on both sides mainly concentrates on the right and left sides of excavation section.(2)The influence of three factors,namely,different excavation procedures,different excavation methods and the distance between the point on the pile and the excavation boundary,on the deformation of bridge piles is studied.It is pointed out that the influence of excavation method is more obvious than that of the point on the pile and the distance between the excavation boundary in this test condition.At the same time,it is suggested that attention should be paid to the construction procedure and the excavation footage to protect the pile foundation when the tunnel face approaches the pile in practical engineering.(3)The influence of different excavation procedures,different burial depth and different excavation methods on the settlement of soil layer above the excavation boundary is studied.It is found that the negative settlement around the ground settlement trough occurs due to the skidding effect of rock beams.After the initial support is removed,the surface settlement decreases in the initial direction of excavation.At the same time,the surface settlement is larger than that of the soil below the surface.It is suggested that the actual construction should be carried out.When Cheng demolishes the initial support,attention should be paid to the observation of surface subsidence.