Acoustic emission and rock deformation measurements as tools for evaluating pillar stability in the Asse salt mine

AE activity in a rock salt pillar has been monitored since 1980 simultaneously with pillar deformation. The pillar is loaded by the pressure of the overburden and the adjacent strata, and by stress redistribution from the surrounding mined rooms. As a result of the loading, the pillar exhibits creep deformation, especially compression in the direction of maximum principal stress and expansion in the direction of minimum principal stress. This deformation is accompanied by emission of high-frequency acoustic energy. Event rates, as well as amplitude frequency distribution of the signals, were determined using 2-channel AE equipment. Approximately 1.5 years after the mining activities were completed about 250 m below the pillar at the end of 1988, the horizontal compressive deformation rate in the pillar decreased and reached a constant value in 1994, whereas AE rates are still increasing. AE event rate and deformation correlate well over the long term. The increase in measured values from a low to a high level is explained by redistribution of stresses in the rock surrounding the salt structure due to mining activity. At the present time, no evidence for a sudden loss of stability of the rock salt pillar is indicated.