Abandoned roadways aged up to 50 years observed in Kushiro Coal Mine, Japan

摘要

Excavation Damaged Zones (EDZs) appear around deep rock repositories for geological disposal. Permeability in EDZs would be much higher than in intact rock masses, and the sealability of a rock cavern is supposed to be significantly compromised by high permeability. Mechanical properties of medium-hard clastic rocks in EDZs are unknown. To clarify the sealability of abandoned and closed rock caverns in clastic rocks, old roadways were observed at 95 sites during the mining of longwall panels in Kushiro Coal Mine. Rayleigh wave velocities and permeabilities in abandoned and closed roadways as old as 50 years, which can be regarded as very severe EDZs with zero initial sealability, and those in EDZs and EdZs (Excavation disturbed Zones) around recently excavated roadways in clastic rocks as deep as 300 m were measured and compared. Most roadways were completely closed by deflection of roof (Fig. 1) and/or floor heave due to plastic deformation of clay and siliceous rock. There observed no significant fractures in the closed parts. On the other hand, a few roadways were left open by the wooden support. No back filling was reached to the open roadways. Rayleigh wave velocity in the closed roadway EDZ and EDZ around the present roadway are in the range of 0.3-1.2 km/s and 1.1-1.8 km/s, respectively. Permeability in the closed roadway EdZ, closed roadway EDZ, EdZ and EDZ around the present roadway were 5.1 to 6.0 x 10~14, 2 to 10 10~13, 1.4 10~15 and 5.3 x 10~15 m2, respectively. It is estimated that the rock mass within 4 m from the sidewall was an EDZ from permeability distribution along a borehole in roof sandstone. The EDZ boundary was estimated to be between 1 m and 1.5 m from the deformability distribution along a borehole in the sidewall. In the triaxial compression test on roof sandstone from the site, permeability increased with specimen failure. It gradually decreased with time in the residual state and reached almost the same level as that for the intact rock specimen. Permeability decreased with the increase in confining pressure and then increased with its decrease. Fracture closures and poorly connected tortuous fractures were seen in the micrograph of the thin section. The sealabil-ity which is recovered to approx. 1/40 the virgin rock mass would be much better than rock cavern in crystalline rocks in which much less recovery in sealability is expected when collapsed.