Tunneling induced geohazards in mylonitic rock faults with rich groundwater: A case study in Guangzhou

This paper presents a case history of geological hazards and treatment measures during the shield tunneling of Metro Line No. 7, in water-rich mylonitic faults in Guangzhou, China. The site selection of Metro Line No. 7 is introduced based on the land use and the geology in Guangzhou. Metro Line No. 7 passes through a number of dead faults, which are unavoidable due to the presence of urban residences and geological formations. The mylonitic rock faults feature rock fractures, high rock permeability, complex formation structure, susceptibility to collapse, and rich groundwater. Geohazards during tunneling in groundwater-rich mylonitic faults include ground collapse, groundwater in-flows, land subsidence, and long-term instability of tunnel. These types of hazards would delay the construction of tunnels and potentially cause significant damage to the tunnels. Preventions and treatments for these hazards aforementioned are analyzed based on the tail grouting material optimization, advanced ground monitor, grouting treatment and shield machine modification, with monitoring the feedback data. The mechanism of ground collapse is discussed according to the cavity expansion principle.