1992-02-10_GENERAL DOCUMENTS - C1981017 (2)

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Memo to Candace Thompson Re: Coal Basin Mines - Sugsidence page 2 structures or renewable resource lands to possible material damage." (Excerpts from Memo from im Pendleton to Candace Thompson, dated 14 January 1986)] In approving MCR's revision request to delete subsidence monitoring above the Coal Basin Mines the Division, at my request, imposed four stipulated conditions. (1 ) If any unanticipated surface disturbance (cracking, cratering or landsliding) m nifests itself within the area potentially affected by subsidence (angle of draw = 20.5 degrees ), the operator will be required to i ediately reinstitute the subsidence monitoring program. (2 ) All existing m�numents must be left in place and maintained. (3) All existing m numents must be resurveyed at the conclusion of the current five y ar permit period, to determine whether additional subsidence manifests itself. (4) A subsidence monitoring program will be required above any double-pass to gwall operations, in order to determine the ultimate subsidence fac;or above the relatively untested extraction technique. Compliance with Stipulation 0 ) In a letter report dated October 25, 1988, Dr. Bruce Collins of MCR reported that two ground cracks were discovered on Huntsman Ridge while performing the subsidence resurvey required by revision stipulation (3). The larger of these two cracks "extends almos without interruption for approximately 3,500 feet in length, along or closely on either side of the ridge line to the saddle between the south fork of Coal Creek and South Twin Creek". Dr. Collins presents the opinion that these cracks are the result of reactivation of an ancient slip-plane relate) block landslide movement. He believes that this mass movement is the result of natural processes, "primarily the greater-than-normal preci itation experienced in the period from 1981 through 1986", and that it is "re ated to mining only indirectly if at all ". (Excerpts from Dr. Collins October 25, 1988 letter. ) European and North American observations have documented that it is not uncommon for ground subsidence to contribute to the reactivation of landslide movement, which was one f the the reasons the Division imposed Stipulation (1 ) upon approval of the 1986 revision. The purpose of subsidence monitoring programs is to verify su sidence projections included in the permit application, in order to perfect projections so that material damage can be avoided. In the event that MCR proposed an expansion of the permit area the ability to accurately pr .ect areas of potential subsidence occurrence could be important, if structu es or renewable resources were to exist within the potentially affected are