Monitorindg of micro-crack formation around galleries in salt rock

In the Asse salt mine in Northern Germany, large-scale experiments are being carried out in order to test methods for the safe final disposal of radio-active waste. Two of such long-term experiments were monitored using up to 29 Acoustic Emission (AE) sensors installed in boreholes. The AE transducers were sensitive in the high-frequency region of 5 to 100 kHz, thus enabling the detection and accurate location of comparatively small cracks in the centimeter range. Nearly all AE events were found to originate from zones close to walls of galleries or, in case of a heater experiment, near boreholes. The extent of such micro-cracking zones was studied during a thermomechanical experiment on a series of five heater boreholes and during the drifting of two galleries. We found that changes in the stress distribution will enhance the AE activity considerably.