Abstract:

Muzhailing tunnel on LanzhouChongqing railway is a 19 kmlong twintube tunnel. Due to the high ground stress and the soft rock, large deformation occurs during the construction of the tunnel, which has great influence on the safety and successful construction of the tunnel. By drilling largediameter boreholes in advance to release the stress, the stress on the support structure is reduced, the deformation is brought under control and the support system becomes safe. In the paper, the construction and the effect of the largediameter boreholes are analyzed. Conclusions drawn are as follows: The largediameter boreholes drilled in advance can restrict the development of the deformation，but has some negative influence on the tunnel construction; The stress releasing effect and the construction organization of the largediameter boreholes should be further studied and improved.

Key words: Muzhailing tunnel, largediameter boreholes drilled in advance, high ground stress, soft rock, stress releasing