Risk assessment of seismic impact on the roof and pillars stability in Estonian underground

The processes of immediate roof exfoliation and pillars collapse is accompanied by significant subsidence of the ground surf ace. Ground surface subsidence causes soil erosion and flooding, swamp formation, agricultural damage, deforestation, changes in landscape, ground water level decreasing and the formation of unstable cavities. For the last four years a new blasting technology with great entry advance rates (EAR) has been introduced in an experimental mining block. By improved blasting technique the EAR reached 4 m; it is twice greater in comparison with usual technology, but emulsion explosive volume is twice higher and explosion occurs for 4.5 seconds (about 15 times longer than with the old technology). As a result of such greater advance rates, unsupported room lengths up to 5.5 m with decreasing stability of the immediate roof (IR) can be expected. In this paper the an alysis of the IR stability using the deformation criteria for a new room-and-pillar mining technology with modern machinery at "Estonia" mine is presented. The analysis of the IR stability is based on the on-site underground testing by using benchmark stations and convergence measurements. The target of this study is to determine the impact of the vibration on the roof and pillars stability using the risk assessment method. Risk analysis on the basis of available earthquake data is also carried out.