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Paper: Mapping potential areas of ground subsidence in Estonian underground oil shale mining district

txt: Mapping potential areas of ground subsidence in Estonian underground oil shale mining district Ingo Valgma www.minest.ee/mining Northeast part of Estonia has been subjected to oil shale mining since 1916. Oil shale as main source for power industry in Estonia is mined in amount of 12 million tonnes per year. The underground production rate is about 6 million tonnes of the mineral annually. Currently three open casts and six underground mines are operating. In past 6 underground oil shale mines have been closed. Totally 979 million tonnes of rock, including oil shale has been mined underground. Today, about 305 km2 area has 512 million m3 abandoned mine workings in the depth of 10 to 70 m below the surface. The problem is influence of underground mining and mine workings on ground subsidence and ground- and surface water regime. Map of the Baltic oil shale area Baltic oil shale area covers about fifty thousand square kilometers, its oil shale, from Middle Ordovician age is named kukersite oil shale. The Baltic area includes the Estonia and Leningrad deposits and Tapa occurrences, of which the first two are commercially exploited. Estonia deposit is one of the largest commercially exploited oil shale deposits in the world with its total resources exceeding five billion tons of oil shale. Oil shale mining in Estonia In Estonia, oil shale was broken in surface mines close to the outcrop until 1922. Open cast mining technology was used from the beginning. Underground mining started already in the depth of ten meters because of the stable roof. Drill and blast methods were applied both for drifting and stoping. First stripping shovels and locomotives appeared in thirties. At the same time electric drilling began. Transition to the mechanized mining began in fifties. Until this, all mines used strait works technology, which meant handwork. After that, longwall mining, which was widely used by Russian coal mining, was applied. For oil shale mining, double unit face method was used. Longwall mining with caving in Estonian oil shale underground mines, causing ground subsidence Drilling in underground oil shale mine, room and pillar mining method Distribution of mined oil shale quantity in 1997 by technologies in Baltic area The prognoses show inescapable decrease in the capacities. In this connection, collecting old mines data has special value today. Oil shale production in Estonian and Russian deposits Estonian Map Centre has supported the study with making available to use base map of Estonia for reference. The study is part of the ESF Grant G3403 Posttechnological Processes in mined out areas.