Abstract: Tunnel collapse hazards were frequently in the Luojiazhai highway tunnel that is shallow buried in the completely or strongly weathered granite rock mass. Factors that might induce these collapses were evaluated with the combination field investigation, laboratory testing and theoretical approach. The results indicate that, the characteristics of the completely and strongly weathered granite are strength softening, slaking and poor stability when water is encountered. The main factors that caused the collapses are: geotechnical subsurface conditions: characteristics of the completely and strongly weathered granite, heavy precipitation, rich groundwater supply, abundant groundwater conduits and depressed groundwater collection shape; tunnel excavation under inappropriate advanced supporting measures caused by the insufficient geotechnical investigation data and inaccurate geological condition predictions during the excavation. Under the combination of the construction disturbance and the infiltration of the groundwater, the tunnel excavation area had the slaking failure and then rapidly developed into the global stability failure of the entire excavated area. The tunnel is then collapsed, and the ground sinkhole expression was also presented. With the understanding of above potential factors, the tunnel excavation sequence and advanced supporting method were adjusted, and the collapse hazards were able to be eliminated during the tunnel excavation.

Key words: tunnel engineering, completely and strongly weathered granite, tunnel collapse, risk factors, hazard factors