Mechanical properties of abandoned and closed roadways in the Kushiro Coal Mine, Japan

The objective of this research is to clarify the mechanical properties and self-healing ability of the Excavation Damaged Zone (EDZ) around rock caverns in clastic-rock. Observations of nearly one hundred closed roadways up to fifty years old, which can be regarded as very severe EDZs with no initial sealability and are up to 300 m deep in clastic-rock, were made at the Kushiro Coal Mine, Japan, to accomplish the objective. Most old roadways were closed, though a few remain open. Closure of old roadways was mainly due to roof deflection and/or floor heave. Large plastic deformations dominated; however, severe fractures were seldom observed in closed old roadways. Rayleigh wave velocity and hydraulic conductivity in the closed old roadways were in the range of 0.3–1.2 km/s and 5×10−7–1×10−7 m/s, respectively, and those in EDZ and EdZ (Excavation disturbed Zone) around recently excavated roadways were 1.1–1.8 km/s and 1×10−8–5×10−8 m/s, respectively. The extent of EDZ around the present tailgate was in the range of 1–5 m. Mechanical excavation and prevention from water are suggested as the key points for long-term maintenance of rock repositories. Pressurization from inside the cavern to decrease the permeability of EDZ is proposed for maintenance of rock repositories in medium-hard clastic-rock masses at similar depths for long periods.

► Most abandoned roadways in a coal mine up to 50 years old and 300 m deep were closed.
► Large plastic deformations as roof deflection and/or floor heave closed the roadways.
► Rayleigh wave velocity in the closed roadway was approximately half the virgin rock mass.
► Permeability in the closed roadway was approximately 40 times the virgin rock mass.