95 abandoned old roadways up to 50 years old and 300m deep were observed at Kushiro Coal Mine. Large plastic deformations as roof deflection and/or floor heave closed most roadways although a few roadways were remain open. Such in-situ measurements as Rayleigh wave velocity, permeability etc. were carried out. Rayleigh wave velocity in the closed roadway was approx. half the virgin rock mass. Permeability of closed roadway was 40 times greater than that for virgin rock mass. This suggests that 1/40 sealability can be expected even a rock cavern in clastic rocks completely collapsed. This would be much better than rock cavern in crystalline rocks in which much less sealability is expected when completely collapsed.

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