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Empirical Approach: <br />While not strictly applicable to the problem under investigation in this note, some related <br />empirical guidelines are available for mining underneath water bodies. As documented in the <br />`Guidance for Evaluating the Potential for Breakthroughs from Impoundments into Mine <br />Workings and Breakthrough Prevention measures ( Revised May 2003)', the following empirical <br />criteria have been suggested to avoid the development of sink holes and pillar failures when <br />mining under water bodies: <br />1. For `first mining only' with respect to sinkholes, the amount of `solid strata' should be <br />equal to at least 5 times the entry width, or 10 times the mining height, whichever is <br />greater, <br />2. The affected zone of the mine should have at least a 100 ft of cover below the water body <br />to avoid any chance of sinkhole development, <br />3. The pillar safety factor (using ARMPS) should be at least 2.0 for the long term support, <br />4. The immediate floor below the pillar should be competent enough to avoid any pillar <br />punching into the floor and causing the roof to sag. <br />When the above criteria are applied to the Wolf Creek seam development workings, the <br />geological and mining conditions satisfy them all. The proposed gate entry width is 20 ft and the <br />maximum mining height would be around 12 ft, so the minimum `solid strata' cover needed is <br />120ft which is less than the interburden thickness of 135ft. Hence, conditions 1 and 2 are <br />satisfied and thus show there will not be any chance of sinkhole formation. <br />The ARMPS safety factor for the proposed gateroad configuration (figure 8) for development <br />under default parameters comes to 4.46 which far exceedsthe minimum required safety factor of <br />2.0. <br />The immediate floor is quite competent and has an average uni-axial compressive strength of <br />8000 psi. Therefore the imemdiate floor will not lead to any pillar punching or excess roof <br />sagging. <br />In summary, even based on the above empirical guidelines, the gateroad development in the <br />Wolf Creek seam will not have any safety issues from the presence of the water pool located in <br />the Wadge seam workings above. <br />Conclusions: <br />Based both on the numerical modeling work described above and some empirical guidelines, it <br />can be concluded that the Wolf Creek seam gateroad development in Panel 1 below the water <br />pool in the Wadge seam workings will not cause interburden stability issues and thus can be <br />carried out safely. <br />Page #10 <br />