Analysis on influence of existing shallow tunnel due to asynchronous excavation of bilateral closely attached deep foundation pits

Abstract

The urban underground development typically involves the excavation of foundation pits adjacent to subway lines. Conventionally, the excavation on both side of existing tunnel generally adopts synchronous symmetrical excavation, however it's difficult to execute synchronous symmetrical excavation in whole process due to the influence of external environment. Asynchronous excavation is detrimental to tunnel deformation control, in severe cases, it will affect operational safety. Furthermore, in order to get more space, the foundation pit is getting closer and closer to the existing tunnel and the depth of the pit exceeds the depth of existing tunnel is more and more, the shallow tunnel is formed suspended between the foundation pits on both sides, in this case asynchronous excavation is more detrimental to tunnel deformation. Therefore it is necessary to study the tunnel deformation rule to provide a basis for controlling tunnel deformation. Taking Guangzhou Subway Line 12 project Chigang station bilateral deep foundation pits which close contact with both side of operating subway tunnel of Line 8 as a study case, the deformation monitoring date was analyzed combined with different working conditions, excavation speed, geology etc. The result shows that: The tunnel began to shift to the foundation pit of excavation in advance, the maximum translation is 3.14 mm, which occurs when the excavation in advance reaches the bottom of the exist tunnel; With the excavation on another side of the tunnel, the translation speed slows down and the tunnel translation shift the moving direction to another side which foundation pit was subsequently excavated when excavation below the existing tunnel, the maximum translation is 6.12 mm, which occurs before the foundation pits is bottom closed. Asynchronous excavation can restrain tunnel settlement to some extent, but it can lead to transverse differential settlement, the transverse differential settlement is 3.38 mm before bottom closed, but after bottom closed and stabilized is 2.51 mm. The above implementation fully demonstrates that: in the case of asymmetrical excavation condition, the horizontal displacement on both sides of the tunnel is unequal and the displacement direction will be reversed during the excavation process, it is highly related to the deformation of foundation pit; Asynchronous excavation cause the tunnel to tilt towards the foundation pit of excavation in advance; The maximum deformation occurs before the foundation pit is closed, casting bottom slab of the foundation pit can effectively stabilize the deformation of the tunnel, deformation tends to be stable after the foundation pits on both side of the tunnel is bottom closed.

Keywords

deep foundation pit / asynchronous excavation / deformation analysis / shallow tunnel / closely attached