Kaiser effect performance in anthracite as an indicator of the rock resistance to cryo-thermal treatment

The article presents the experimental studies into the stress memory in acoustic emission (Kaiser effect) in anthracite samples subjected to different number of freeze-thaw cycles. All in all, 6 groups of relatively uniform samples were tested; one group (0) was a reference set unexposed to cryo-thermal effects; the number of a group conformed with the number of treatment cycles. After the cryo-thermal action, all samples were subjected by two cycles of mechanical loading up to maximum stresses of 10 and 14 MPa, respectively. During a loading cycle, acoustic emission activity of a sample was measured, and the Kaiser effect in the second cycle of loading was assessed by two characteristics. These characteristics were the factor of retention FR of the stress memory, or the ratio of the stress of emission initiation in the second cycle to the maximal stress of the first cycle, and the index Δ, or the ratio of the averaged acoustic emission activities before and after the moment of the Kaiser effect. The values of FR and Δ are obtained and analyzed as functions of the number of anthracite freeze-thaw cycles. The value of FR drops but Δ grows with increasing cryogenic disintegration of coal. The maximal change in FR and Δ, and, accordingly, the highest dynamics of damage in anthracite takes place in the first cycles of cryo-thermal treatment.