Abstract: Large deformation was observed in excavation of Baogaishan Tunnel through the granite and tuff contact zone. Although the excavation supporting structures were in good conditions, excessive deformation at crown of the tunnel was accumulated to 1.2 meters in a short period of time. Through a thorough geological reconnaissance and analyses of petrographic and minerals of rock mass near the contact zone, results showed that large deformations occurred in a location between granite and tuff contact zone. Due to the special magmatic evolution and diagenesis process near the contact zone, soft rock mass that consisting of kaolinite, montmorillonite and other hydrophilic soft rock minerals were well developed in this area. The presence of a large number of hydrophilic soft rock minerals not only provides material conditions for the development of large deformations, but also results in the significant deterioration of the physical and hydrological characteristics of the surrounding rock mass. With the disturbance by the tunnel excavation, the stress redistribution and groundwater seepage, these factors result in the reservation of the surrounding rock mass deformation. The rock mass therefore experiences of cyclic flooding-water loss flooding process. This cycle of groundwater saturation not only causes the expansion of hydrophilic soft rock minerals, but also contributes to the deformation of the larger extent surrounding rock mass and the excessive earth pressure acting on the support structures beyond their design capacity, which in turn result in the large deformation of the surrounding rock mass. A series of mitigation measures such as drainage path, reinforcement of surrounding rock mass and early support were implemented, and the deformation of surrounding rock was successfully controlled to ensure the safety of the project.

Key words: tunnel excavation, granite contact zone, mechanism of large deformation, mitigation measures