27 th Oil Shale Symposium Colorado School of Mines 15-17 October, 2007 1 Risk Assessment of Pillars Stability for Experimental Mining Blocks in Estonian Oil-Shale Mines Sergei Sabanov Tallinn University of Technology, Department of Mining, Ehitajate tee 5, VII-202, Tallinn, 19086, Estonia, e-mail: sergei.sabanov@ttu.ee ; sergei.sabanov@mail.ru Abstract This study addresses risk associated with pillars dimension using new room-and-pillar min- ing technology with modern machines at Estonian oil shale mines. Processes in overburden rocks and pillars have caused unfavourable environmental side effects accompanied by sig- nificant subsidence of the ground surface. The aim of this work was to determine the dam- age of new technology on pillars dimension and to define the coefficient of blasting opera- tion influence (q). During the last three years, oil shale mining at an experimental mining block introduced a new blasting technology with great entry advance rates (EAR). With such improved technol- ogy the EAR reached 4 m, two times greater than conventional technology can guarantee. However, explosive volume increased up to two times and explosions occur during 4.5 sec- onds (~ 15 times longer than old technology). In places with complicated mine-geological conditions in the mining blocks, deviation of pillars dimension from project value was up to 16%, because pillar parts broke during the three to four week period after the blasting op- erations. Observation of two experimental mining blocks showed that pillars dimension was reduced an average of 7%. Experimental data for different mine-geological conditions allow consideration of the influ- ence of blasting operation on pillars dimension. A coefficient to improve accuracy parame- ters for new technology was added to the formula for calculating pillars. Design of pillars parameters for old technology is based on the instruction used in Estonian oil shale mines, where the coefficient of blasting operation influence is q = 0.6. Using the improved formula, it is possible to reduce the disturbance of new mining technology on pillars dimension. The correct choice avoids collapse in a mining block, and guarantees stable parameters and minimal losses of the oil-shale reserves. Introduction For 90 years, oil shale has been a mineral resource in the economy of Estonia. This study addresses risk associated with pillars dimension using new room-and-pillar min- ing technology with modern machines at Estonian oil shale mines. The processes in overburden rocks and pillars have caused unfavorable environmental side effects ac- companied by significant subsidence of the ground surface. The processes cause a large number of technical, economical, ecological and juridical problems. The first spontaneous collapse of pillars and surface subsidence in an Estonian oil shale mine took place in 1964. Up to present, 73 col- lapses on the area of 100 km 2 (about 400 mining blocks) have been recorded. Col- lapse in a mining block also stops the mining works (Pastarus et al., 2005, Pas- tarus et al., 2006.) Underground oil shale extraction by the room-and-pillars method with blasting is rather inexpensive, highly effective and easily mechanized. It gives an extraction factor of 70–80%. The field of an oil shale mine is divided into panels, which are sub- divided into mining blocks, each approxi- mately 300-350m in width and 600-800m in length. A mining block usually consists of two semi-blocks. The height of the room is 2.8m or 3.8m. The room is very stable when it is 6-10m wide. However, bolting must still support the immediate roof. The pillars in a mining block are arranged in a