A case study on severe damage at a tunnel in serpentinite rock mass

Face squeezing, floor heave, and buckling of invert were found after a 10-day holiday in 2014 around the serpentinite face of a tunnel in Hokkaido, Japan. The damage continued for over 2 months, extending 400 m toward the entrance causing a massive roof fall. The tunnel was excavated again with a circular section and extra-thick shotcrete, and the face had crossed the damaged part 6 years after the damage occurred. Uniaxial and triaxial compression tests were carried out to obtain the mechanical properties of the serpentinite to clarify the severe damage mechanism at the tunnel. The main experimental findings are as follows. The uniaxial compressive strength of the serpentinite samples was very low, and the ratio of the strength to the estimated overburden pressure was extremely low. The parameter n indicated that the time-dependent deformation of the serpentinite was not large but the same as ordinary rocks. All specimens showed strain-hardening in the triaxial compression test, and the friction angle was very low by the brucite content. Only primary creep was observed in the multistage triaxial creep test. The pressure on the shotcrete from rock mass for the damaged tunnel was enough to cause creep deformation and failure of shotcrete. From the above findings, designing the concrete lining that can support the earth and water pressure is recommended for tunnel excavation in such a weak serpentinite rock mass, particularly with a very low friction angle by brucite. Highlights Face squeezing, floor heave, and buckling of invert were found after a 10-day holiday around the serpentinite face in Hokkaido, Japan. The time-dependent deformation of the serpentinite was not large but the same as ordinary rocks. All specimens showed strain-hardening in the triaxial compression test, and the friction angle was very low by the brucite content. Only primary creep was observed in the multistage triaxial creep test. The severe damage to the tunnel was not a brittle creep failure of the serpentinite rock mass itself but the shotcrete lining.