A comparison study of face stability between the entering and exiting a shallow-buried tunnel with a front slope

摘要

Worldwide, it is a great challenge to tunnel in portal section, where excessive deformation, cracking, or even collapse often occurs during the construction process. This paper presents a comparison study of face stability between entering and exiting a shallow-buried tunnel with a front slope. Firstly, the theoretical solutions of support pressure sigma ( T ) and safety coefficient FS of the excavation face considering surface slope are derived by upper-bound limit analysis method. Secondly, for different slope angles, buried depth and surrounding rocks of the exit and entrance sections, the sigma ( T ) and FS are obtained. The results show that when the burial depth of the tunnel crown h remain constant, the sigma ( T ) increased first and then decreased while the FS increased gradually, and both become steady when the slope angle alpha arrive at a certain value. In addition, the thicker the h is, the smaller the certain value alpha is. When the h and alpha remain constant, the loose area in front of the excavate face will decreased apparently with the increase of the internal friction angle phi, thus the sigma ( T ) will decrease and FS will increase. Moreover, the sigma ( T ) decreased linearly with the increase of cohesion c and unit weigh gamma of surrounding rock, while the FS is the opposite. Compared with the entrance section, the construction risk at the exit section is greater.

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