GEOTECHNICAL PROCESSES IN CLOSED OIL SHALE MINES

Abstract

During the years 1998–2001 geotechnical processes in closed underground oil shale mines and opencasts were investigated. In addition to kukersite oil shale deposit, the closed Sillamae uranium (Dictyonema shale) mine was prospected. The main tools and methods included mine plans, aerial photographs and Geographical Information System data. 290 km 2 of underground and 130 km 2 of strip-mined areas were studied. The mining maps of Estonian underground and surface mines were created. The stability of underground mined area, where room-and-pillar method was used, was the main objective of the study. It was studied with the help of aerial photographs, mine drawings, maps of quaternary sediments and mathematical modeling of rock failure. The main results are: 20 % of subsidences remain undiscovered and 42 % of subsidence occurrences have no remarkable influence to the land cover; the probability of subsidence remains and may increase in the case of mine drowning. As several mines will be closed during the next few years, the problems of drowned waste (which were not subjects of this study) are going to be more actual than before: increasing underground water level, pollution of underground water, formation of technogenic water sources, overflooding of reclaimed areas, etc. There are two kinds of oil shale in Estonia: kukersite as the principal one and Dictyonema argillite (black oil shale, alum shale) as uranium shale. Both oil shales are deposited in low depth and their fields are large. Kukersite oil shale deposits cover more than 5 % of Estonian mainland. There are fourteen closed and/or abandoned underground mines (including two Sillamae uranium mines) and nine partly reclaimed kukersite open casts today. The total area of mined-out lands exceeds 400 km 2 that is less then 1 % of the Estonian area. In Estonia since the year 1916 almost 1 billion tonnes kukersite oil shale has been extracted, approximately 2 billion m 3 overburden has been excavated and about 200 million m 2 underground workings have been formed.