Several decades of faulty exploitation of salt through solution mining at Ocnele Mari (Romania) led to the creation of world’s largest artificial underground cavern containing several millions of cubic meters of brine. The presence of this amount of brine above a densely inhabited area created a huge hazard for personal property and human life.

Following two accidental roof collapses occurred in September 2001 and July 2004 a technical solution was implemented to address this instability concern through the controlled collapse of the entire roof while pumping the brine out and filling the cavern with sterile. The geotechnical design required that microseismic monitoring be employed continuously. An area of over 1 km2 was monitored with a staggered array of 36- uniaxial 15 Hz geophones installed in 12 boreholes about 160 to 360 m deep. A total of 2392 seismic events with Mw between -2.8 and 0.2 occurred from July 2005 to March 2006, located with average location accuracy (in a vectorial sense) of 18 m. The cumulative frequency-magnitude distribution exhibited a b-value of 1.5 with a time variation from 0.5 to 1 and from there to 1.5 over the course of this monitoring, suggesting that the collapse initiated as a linear fracture pattern, followed by shear planar fragmentations and finally a 3D failure process. The brunching ratio of seismicity is indicative of a super-critical process, except for a period in mid-February when temporary stability existed shortly. Event relocation through the use of a collapsing technique outlines that major clusters of seismicity were associated with the main cavern collapse, whereas smaller clusters with the fracturing of smaller size nearby caverns, as a result of stress re-distribution. Focal mechanism solutions indicate that normal and reverse shear were statistically the most encountered failures.

C-I. Trifu and V. Shumila
ESG, Kingston, Ontario, Canada