Pol. J. Environ. Stud. Vol. 25, No. 2 (2016), 777-785 DOI: 10.15244/pjoes/61117 Original Research Mine Subsidence as a Post-Mining Effect in the Upper Silesia Coal Basin Grzegorz Strozik1, Rafał Jendruś1,2*, Anna Manowska1, Marcin Popczyk1 1Silesian University of Technology, Faculty of Mining and Geology, Akademicka 2A, 44-100 Gliwice, Poland 2GeoRock Geotechnical Services Company, Armii Krajowej 14/I/10, 41-943 Piekary Śląskie, Poland, Received: 6 November 2015 Accepted: 23 December 2015 Abstract On the basis of three selected examples, some problems attributed to the presence of voids resulted from mining operations in already commissioned underground coal mines are discussed. Continuous and discontinuous deformations of the ground surface (sinkholes and subsidence troughs) of anthropogenic origination implicate hazards for the ground surface, even many decades after completion of mining work. Nowadays, when decommissioning and fl ooding of numerous mines happens, ground movements as well as seismic events related to fl ooding of abandoned underground mines can be observed. The fi rst case study draws attention to the problems of effective voids localisation methods as well as the reliability of grout injection operations for road construction purposes. The second case emphasizes a problem occurring often in areas with long and complex mining histories, meaning appropriate determination of origin, process and protective measures regarding occurred ground deformations, or damage to buildings. The third case describes specifi c effects on the surface (shocks and ground surface elevation) generated by the fl ooding of decommissioned mines. Keywords: sinkholes, subsidence, abandoned underground mines, post-mining areas 200·106 Mg in 1980 to nearly 80·106 Mg in 2012), and the total surface of active mining areas has been reduced by Introduction half. But the environmental impacts from the already com- pleted mining works still generate damage. Hundreds of years of underground mining operations The USCB in its Polish part has an area of 5,400 km2. in the Upper Silesia Coal Basin (USCB) has resulted in The whole area includes coal beds with total capacity of heavy degradation of the surface of the ground, which is round 70 billion tons, from which 10-15% have already frequently densely urbanized. Since the 1990s, though, been extracted [1]. Currently active mine areas have a to- mining activities have been strongly curtailed (in the case tal surface of 1,100 km2 (19%). Decommissioned mines of hard coal mining annual production decreased from and exhausted coal reserves represent a total surface of 635 km2 (11%). The next 27% of the USCB area contains coal beds at a depth below 1,000 m, which may be consid- *e-mail: [email protected] ered for future mining operations. Depth of the coal beds 778 Strozik G., et al. is changing from 0 (outcrops) down to maximally 4,600 infl uence urbanised areas that expect to be free from m below sea level. The USCB was rich also in other min- further negative impacts of decommissioned mines. eral beds, from which the most important were silver-zinc- lead ore beds located in the northern part of the region, which were the subject of the oldest mine operations in Subsidence as a Late Effect this area in the Middle Ages, due to their shallow depth of Terminated Mines (down to 100 m below sea level maximally). Mining of silver was completed in the beginning of the 19th century In the USCB, ca. 98% of coal output is produced and zinc-lead beds were depleted in the second half of the using the longwall system with caving. Because most of 20th century. In some areas, shallow residues from older the shallow deposits have already been extracted, most metal mines have been later undermined by deeper coal of the subsidence resulting from the current mining op- extraction and resulted in dramatic development of mine erations occur in the form of subsidence troughs (continu- subsidence problems in densely populated and urbanised ous subsidence). Due to high rock mass pressure at large areas. The population of the USCB area is approaching depths, collapse of the roof strata takes place immediately 3.5 million people, with average density of 645 per km2. after coal extraction happens, then the ground surface be- The most important and most frequent problems of ar- comes stable in a relatively short time after completion eas of completed mining operations include mine subsid- of the mining works (most of the ground movement ence and risks to buildings, and the resulting infrastructure occurs between one and three years after completion min- damage. ing) [3]. Results of current, mostly deep coal bed extractions At shallow depths, however, where the pressure of frequently induce or intensify geomechanical impacts the rock mass is signifi cantly lower, voids (abandoned from previous shallow coal mining, and historical residues workings and cavings) can be present in quasi-stable from metal ore mining or quarrying. conditions even many decades after mining is completed. In the case of building safety, the problem not only In USCB mining-geological conditions, the minimal results from the residual effects of mining activities (in a depth required to generate a fully developed subsidence form of subsidence or sinkholes), but also from the prop- trough after mining extraction is 200 m; however, the most erties of the grounds, which create a direct foundation for frequently observed shallow voids occur at depths of up to buildings. Due to the intensive mining and overall indus- 80 m. The last observation may result from the fact that trial activities at the USCB area, a large amount of terrain heavy surface structures could reach such a depth. In the was subject to mine waste disposal. In contrast to the mod- case of the loss of stability of such a void and its sudden ern approach to the waste disposal procedures, in histori- collapse, a sinkhole subsidence (discontinuous subsidence) cal times any industrial waste was disposed of freely in the appears on the surface, which can cause severe damage to most convenient locations, mainly very close to the facto- existing structures and create public safety issues. ries and mines, and located far from settlements at these The most important factors that can infl uence the gen- times, but now representing vivid areas of modern cities. eration of sinkholes are [4]: As a result of these implications as well as territorial – The thin layers of loose rock mass close to the surface expansion of the cities, many contemporary buildings are of the ground. built on areas that were not intended to be urbanised and – Mining in close proximity to geological faults. allowed for uncontrolled storage of waste. This waste – – Mining in a close proximity to bed outcroppings or from exploration workings, mining operations, metallur- other geological strata. gical processes, and other activities – were formerly dis- – The high rate of advance of the mining front. posed of without consideration of the future. – Overlaying mining area borders. Nowadays, due to the territorial development of ur- A sinkhole may occur as a result of an underground banised areas, the construction of roads, and overall mod- void collapsing. However, it can happen suddenly even ernisation processes, many new investments in the UCSB after many years in quasi stable conditions. Sinkholes can area must cope with problems related not only to current be generated by [5]: mining issues, but also to historic and frequently poorly – Changes of structure in the rocks surrounding a void, documented industrial operations (much archival informa- i.e., infl uence of water, fracturing, weathering, suffu- tion was destroyed during World War II) [2]. sion, and clogging processes, etc. On the basis of the three selected case studies, the prob- – Movement of rocks resulting from deeper, current lems of the late subsidence effects and their infl uence on mining operations. the safety and functionality of building structures will be – Changes in hydrogeological conditions. presented. Due to the complexity of problems addressed – Static or dynamic load on the ground surface. in this paper, the intent of the authors is to show how sur- – Seismic activity (also related to mining operations). prisingly and unexpectedly these phenomena can arise and Each collapse of voids does not necessarily result in a infl uence local life in any aspect, including environmental sinkhole. A discontinuation of fi rm layers of the rock mass protection, business, public safety, living standards, etc. must occur fi rst, followed by a transfer of loose rock mate- Also, decommissioning mines and the side-effects rial toward the voids. Typical for the processes of sinkhole of terminating underground activities may unfavourably generation is that there are no warning symptoms before Mine Subsidence as a Post-Mining... 779 their occurrence – the creation of sinkholes is sudden and fast, and their dimensions are large in relation to typical buildings. Sinkholes affect surface structure heavily, but in small areal range. Another subsidence-related issue results from the fl ooding of decommissioned mines, which is a relatively new phenomena being observed in the USCB area. His- torically, closure of mines was unique due to the rich coal deposits and adequately long lifetimes of the mines. Even today the fl ooding of abandoned mines does not happen often because there is a strong need to protect operation- al mines against fl ooding. The main protective measure against the increasing water hazard in active mines is drainage of already closed mines and keeping its voids and workings empty to avoid building a water level near an active mine. Fig. 1. Sinkhole subsidence in an area of shallow mining works Case Study I: Grouting of the Shallow Voids at the JK mine in Jaworzno (photo: R. Jendruś). in the Course of a Motorway in Jaworzno Like railways and roadways, linear structures are es- voids and zones of lowered density, the presence of which pecially vulnerable to ground deformations resulting from creates a hazard for the motorway.