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.
1. INTRODUCTION
The salt deposit of Ocnele Mari is located in the sub- Carpathian hills of southern Romania. About 8 km long, it runs from east to west, with a width of 3.5 km and a thickness of up to 400 m, dipping about 20° to the north (Figure 1). Salt has been exploited through dissolution in four fields which entered in production sequentially, beginning with 1954. Operations in Field II were initiated in 1969 and led to the extraction of 13.5 million tones of salt until a major collapse occurred in March 1991. This event underscored the risks associated with the presence of large dissolution chambers close to a densely populated area, prompting the decision to shutdown the field [1].
Fig. 1. Cross-section of the Ocnele Mari salt formation (spatial scales in meters). (available in full paper)
Cavernometry measurements carried out in 1993 by SOCON outlined the presence of a massive cavern formed by the complete dissolution of the inter-chamber pillars of six of the fifteen wells, containing 5.5 million m3 of compressed brine. Another major collapse took place towards the northern edge of the cavern in September 2001, when a part of the roof gave in, forming a quasi-circular crater of almost 200 m in diameter and leading to the spill of 1.7 million m3 of brine.
