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<br />-~~~:-~. ~=~~ - SubTerra, Inc. <br />r <br />Mr. M. Middleton <br />Cyprus-Amax <br />Empire Corporation <br />P.O Box 68 <br />Craig, CO 81626 <br />Civil and Mining Rock Mechanics <br />Mine Subsidence <br />Solid Waste Disposal <br />Rapid Excavation/Support Systems <br />Concrete/Shotcrete Technology <br />Construction Bid Documents <br />Sunday, April 02, 1995 <br />Re: Potential for Subsidence over the 5-Entry Mains at the Empire Mine, Craig, Colorado. <br />Dear Mr. Middleton: <br />This letter addresses the potential for subsidence over Cyprus Empire's 5-entry mains currently <br />being driven in the E-seam. The 5-entry system consists of 20 ft wide entries driven on 85 tf <br />centers. Pillars are 65 ft wide, 105 ft long, and nine to ten feet high. The extraction ratio for each <br />5-entry system is about 36 percent.. Overburden ranges from~about 10.00 to 1400 feet in the E- <br />seam entry extension. <br />Stability of this system is governed by the strength of the pillars, mine floor, and roof. The system <br />is considered to be stable, and therefore not likely to lead to surface subsidence, as: <br />• • Pillar sizes have been selected so that they will be stable under the redistributed loads. In <br />addition, experience gained during development for E-seam longwall panels has shown that :. <br />smaller pillars are stable under similar cover (e.g., even though the redistributed stresses are <br />higher). <br />• The potential for pillar punching into the floor is considered to be small based on an evaluation <br />of the bearing capacity of the mudstone floor. <br />• Roof support will be installed to ensure that the openings are stable for the life of the mine. <br />Furthermore, if the 5-entry system is modeled as a 385 ft wide panel, the maximum surface <br />subsidence would be 10% of the maximum room convergence. Therefore, It is also unlikely that <br />there will be any measurable subsidence at the surface even if convergence is experienced in the <br />main entries (1 foot of convergence would be required to permit detection of 0.1 ft of subsidence <br />directly over the 5-entry mains centerline). <br /> <br />Please let me know if you require further clarification of the above. <br />Yours Sincere) <br />~.C- <br />Chris D. Breeds <br />President <br />Enc.: Calculation Sheets <br />5106 272nd Avenue N. E., Redmond, WA 98053 (206) 868-8327 fAX (206) 868-75?4 <br />