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Gd Baker <br />April 8, 2002 <br />Page 9 <br />still possible as close as 3H to 6H, or one-fourth to one-half the distance to the Mahogany Zone, <br />with a corresponding increase iu the severity and frequency of ground control problems. <br />Solution mining plans for Panel 2 are not likely to preclude successful mining of the <br />Mahogany Lone. The severity and areal extent of future ground control problems at the mining <br />horizon, if any, will be controlled largely by the ultimate size of the solution cavities. Wider <br />cavities will more closely approximate the behavior of longwall panels. However, it is expected <br />that practical constraints will prevent cavities from reaching the size of longwall panels. Cavity <br />widths, even for very high extraction solution cavities, aze expected to be small by comparison <br />and overburden disturbance less than exemplified by longwall mining experience. <br />Ground control problems during future oil shale mining could include pillar weakness, <br />rib sloughing, roof falls, floor heave, offsets in the mining horizon, and generally less predictable <br />conditions. Such conditions may or may not necessitate additional ground support, modified <br />mining practices, or mine layout modifications, similar to the conventional approach taken when <br />faults or other naturally occurring disturbance zones are encountered. However, problem areas <br />are more likely to be infrequent, rather than ubiquitous over a cavity, corresponding to localized <br />subsidence features. Generally, the risk to future mining is considered low. <br />Sincerely, <br />~~xsCX/~_-~ <br />Leo Gilbride <br />Associate <br />LG/pg <br />Cc: M. Hardy (AAI) <br />C <br />Agapito Associates, Inc. <br />