Abstract：The Muzhailing tunnel，a key project of reconstruction of the national highway G212，is located in the northeastern Qinghai-Tibet plateau and traverses the Muzhai mountain. The very complex geological environments，such as high earth stress，deep-buried，rich groundwater，five faults zone with total width of 375 m (21.9% of the tunnel) and very weak and cracked rock masses influenced by groundwater and above-mentioned faults，make self-stability of the tunnel be unfavorable. During construction，intense deformation and serious failure happened. The monitoring data show that deformation was rapid with long duration. The maximum cumulative vertical and horizontal displacements reached to 1.712 m and 1.081 m，respectively in fault F2. Many serious failures happened due to large deformation，for example，roof subsidence，wall crushing，shotcrete flaking，S-shape twisted steel support，broken lining，floor upheaval，invert crazing and lots of cracks distributed everywhere. The surrounding rocks were reinforced and the tunnel was repaired for many times，but large deformation and bad damage did not stop；and previous intrinsic characteristics continued. By analyzing surrounding rocks，groundwater，earth stress as well as deformation characteristics，the reason and mechanism of large deformation of this tunnel were discussed. It is plastic flow and swell of surrounding rocks that are resulted in large deformation jointly. Based on correct understanding of dynamic evolution mechanism of the surrounding rocks，a new reinvention and reinforcement program was adopted from the viewpoint of controlling surrounding rocks. Those parts including large deformation and serious failure were repaired successfully. The construction of the tunnel through the rest of fault F2 and other faults benefited from the research results. The Muzhailing tunnel is achieved safely and successfully at last.
