Sustainable Land Development Today - July/August 2008 - (Page 32) Subsidence potential can be defined as having two components: I Risk I Severity The risk of subsidence is determined by the likelihood of mine and overburden failure which would result in surface subsidence within expected time span of the development. The severity is assessed by the extent of the area prone to subsidence, as well as subsidence type, size, and magnitude which are a function of potential breadth and mode(s) of mine failure. The main factors which determine the subsidence type, size, and magnitude characteristics are: if the mine has collapsed and to what extent, the expected mine failure mechanism(s), and empirical subsidence correlations to the mining conditions. ROOF FAILURE ABOVE ROOM PILLAR CRUSHING PILLAR PUNCHING geology is too weak to support the mine over time. For large project sites the integrity of the mine structure can vary across the site. Project sites investigated where there was a significant variation in the mine stability, have involved the presence of greater coal support over a portion of the project, or the roof or floor geology had changed from weak to resistant. Therefore on some undermined sites it was found that it was only necessary to stabilize the portion of the mine workings of unacceptable risk instead of the common practice of grouting the entire mine workings under a proposed structure. Significant savings to the owner on stabilization costs is therefore realized. FIGURE 5 SKETCHES OF THE THREE PRINCIPAL MODES OF FAILURE OF MINE WORKINGS IN ILLINOIS WHICH CAN RESULT IN SURFACE SUBSIDENCE Mineable coal seams The evaluation of subsidence potential should also consider the possibility of future mining beneath the project site. If the mineral rights are not owned by the property owner and mineable seam(s) exists beneath the site, future subsidence risk may exist even if the project site may presently have no underground coal mining. From our experience, this can occasionally occur. For example a gas processing plant was placed over mineable coal reserves about 10 years ago. Today there are plans to mine these reserves with a high extraction technique which will cause immediate subsidence of the ground as the coal is mined. The importance of establishing accurate subsidence risk The most critical and driving factor for land development is the assessment of the risk of mine subsidence related damage above underground coal mines. However, where potential damage is of concern then establishing the risk of mine subsidence becomes a controlling factor. This does not mean that any undermined site will be a serious subsidence risk. Based on an in-house survey, only about 50 percent of the undermined sites investigated for mine subsidence were found to have significant risk. Half of the serious risk sites required only partial mine grouting to reduce them to low risk. The risk of land development above coal mining is established by: I The integrity of the mine structure against failure; I The nature and magnitude of the subsidence based on past events; I The tolerance of the proposed structure(s) to the estimated subsidence movements. The evaluation of the integrity of the mine structure must consider the various ways the underground old works can collapse resulting in surface subsidence. Although there may be other concerns, a mine stability investigation should consider that: I The rock roof above the mine can fail; I The coal pillars can crush over time; I The coal pillars can sink into the roof or floor over time. The greater the experience and knowledge of the investigative engineer the more accurate and certain he is of his assessment of the mine’s integrity. In other words, it is easy and safe for the investigative engineer who is less confident, and who has less knowledge, training and experience to conclude that significant risk exists and that the mine should be stabilized. From the numerous investigations and engineering analyses of old mine works performed, the risk of mine subsidence can range from very low to high. This is because the geologic and mining conditions vary from site to site. From a stability standpoint, very low to low risk project sites are determined to have adequate coal mine support. This determination takes into consideration the surrounding geologic conditions or when the mine is found to be already collapsed. Obviously higher risks are present where analyses indicate the rock Mitigation measures Where possible a cost effective option can be to design the proposed structure(s) for the potential subsidence movements. The assessment of less expensive surface mitigation compared to mine stabilization involves the consideration and integration of a number of areas of engineering including mining, geotechnical, structural, and architectural. This becomes evident when realizing the important factors that must be assessed: I Mine stability and the nature of existing mine conditions; I Ground stabilization alternatives; 32 July/August 2008 Sustainable Land Development Today
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