Mechanisms of large deformation in soft rock tunnels: a case study of Huangjiazhai Tunnel

Typical large-deformation phenomena of soft rock occurred frequently during the tunneling process in Huangjiazhai Tunnel, Hubei Province, China, including extrusion at the tunnel wall and severe damage of the primary support and secondary lining. To reveal the mechanisms of these anomalies, comprehensive investigations have been undertaken combining engineering, laboratory, and microscopic analyses. Since the monitoring results show that there might be a close relationship between the large deformation phenomena and water infiltration into the tunnel, the water–rock interaction is the research focus in the investigations. The experimental results reveal that the deforming resistance and strength of shales exposed at the excavation face weaken quickly in the first 20 days after the shales have contact with water. The results obtained by mineral composition detection and microstructure observation illustrate the microscopic reasons for the decreasing strength and deforming resistance of the tunnel surrounding rock after water infiltration. The results from in situ geostress tests indicate that as a result of high tectonic stress and low rock strength, the field of Huangjiazhai Tunnel is an extremely high geostress area. By combining analyses from the mechanical and geological perspectives, there are two main reasons for the large deformations in the Huangjiazhai Tunnel: the plastic flow caused by tunnel excavation under high geostress and low rock strength and a hydrated-mechanical coupling process between the shales and water.