The article describes the hydrogeological problems related to water influx to the mine and water sulphate content association with underground oil shale mining in an environmentally sensitive area of Estonia. The water balance method was used to calculate the ratio of mine water outflow/inflow. Water balance calculations are needed for estimating the groundwater transversal flow into closed mines. Before oil shale mining groundwater geochemical processes are characterised by low sulphate content. Dewatering of oil shale mines lowered groundwater level in the Keila-Kukruse aquifer, and caused an about 50-fold increase in sulphate by intensive oxidation of pyrite of natural origin. The mobile sulphate ion may serve as an indicator of changes occurring in the sulphate content of groundwater stored in the Keila-Kukruse and Lasnama¨e-Kunda aquifers of the Ordovician system in the area of closed and working mines. Using the method of inverse mixing, sulphate ion distribution is a possibility to study the movement of groundwater in lateral and transversal directions.
This paper analysis the stability o f the mining blocks in Estonian oil shale mines, where the room-and-pillar mining system is used. The pillars are arranged in a singular grid. The processes in overburden rocks and pillars have caused the subsidence of the ground surface. Statistical analysis o f the pillars cross-sectional area evaluated the calculations. Normal distribution control allows determing the stability of a mining block. By normal distribution of the pillars cross-section area a potential collapse of a mining block can be expected. Theoretical and in situ investigations in Estonian oil shale mines showed that their results are close to the modeling ones. The surface subsidence parameters will be determined by conventional calculation schemes. Presented method suits well for mining block stability analysis and spontaneous failure prognosis.
The processes of immediate roof exfoliation and pillars collapse accompanies by significant subsidence of the ground surface. Ground surface subsidence causes soil erosion and flooding, swamp formation, agricultural damage, deforestation, changes in landscape, ground water level decreasing and the formation unstable cavities. During experimental measurement of immediate roof absolute deformation on "Estonia" mine three earthquakes were registered. The main reason of investigation has served jumping characteristic of absolute deformation near a pillar after earthquake. Method of final elements for analysis of deformation modelling is used. Seismic risk assessment for underground constructions stability is presented in this study.
The paper deals with the improved mining design method for Estonian oil shale mines, where the room-and-pillar mining system is used. Design of mining block parameters is based on the instruction used in Estonian oil shale mines. The factor of safety is very large. Consequently, the design method does not take into consideration all the influence factors. It is determined the supplementary influence factors and given the mathematical formulas. In this case the factor of safety is reduced up to 1.2. The improved mining design method is of particular interest for practical purposes.
The processes of immediate roof exfoliation and pillars collapse is accompanied by significant subsidence of the ground surface. 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 analysis 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.
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