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A DEPARTMENT OF NATURAL RESOURCES David H. Getches, Executive Director MINED LAND RECLAMATION DIVISION DAVID C. SHELTON, Director Richard D. Lamm Governor MEMORANDUM 26 F,t?bruary 1986 T0: Jeff Deckler / 1 FROM: Jim Pendleton 1-"-`~~'1~ RE: Eastside Mine mi- nnual Subsidence Monitoring Report (C-84-063) Pursuant to your request, I have reviewed the December, 1985 semi-annual subsidence monitoring report submitted by Abraxas, Inc. for the Eastside Mine. I forward the following comment for your consideration. I agree with the operator's statement that the limited mining activity to date makes it unlikely that any monument other than No. 1 would be experiencing subsidence. Monument No. 1 is exhibiting no significant movement. However, the U. S. Department of Commerce specifies that surveys performed for the purpose of subsidence monitoring should conform with the standards of accuracy for Second Order, Class II surveys. These standards specify: Horizontal control accuracy to 1 part in 20,000 and Vertical control of 1.3 millimeters multiplied by the square root of K, Where K is the distance in kilometers between surveyed points. I recommend that you inform Steve Self that future subsidence surveys will be expected to conform with this required survey accuracy standard. Without this accuracy it is difficult to determine whether monuments are stable and whether subsidence is actually occurring. You should also inform Mr. Self that the operator is only required to monitor those monuments which may actually be affected prior to the next monitoring survey. The operator may determine this by projecting a conservative angle of draw to the surface and surveying only those monuments which fall within the area of potential subsidence. cc: Mike Long 423 Centennial Building, 1313 Sherman Street Denver, Colorado 80203 Tel. (303) 866-3567