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

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-4- scale and nature of the slump itself, the sediment load of the south fork of Coal Creek might be temporarily increased . No other significant effects can be projected . It should be noted that the memorandum from Mr . Jim Pendleton to Ms . Candace Thompson dated January 14, 1986, and upon which the Technical Revision approved February 3, 1986, was based, contains a misinterpretation of data presented by MCR to establish an angle of draw of 180 for the Coal Basin Mines area. Mr . Pendleton interpreted the report written by the writer while acting as consultant to MCR as demonstrating " . . .that observed angles of draw have varied from 13 degrees to 21 degrees . . . " . In fact, the report established, given the data available at its writing, that 210 was the min mum adjusted angle between a mined-out area and a monitoring point that had not moved, given the monitoring point density. Thus the actual maximum angle of draw was less than 2 °, and based on the calculations made at the time was estimated at 180; this was intended as an approximate maximum, and not an average . MCR makes this point only for clari6ication and future reference, and does not dispute the 20 . 5 reference angle now included in the permit . In summary, local surface cracking in the general area of mining operations wa detected during the subsidence survey conducted as require; during the late summer of 1988 . All observed features we a on surface under which no coal has been mined, both wit in and outside of the maximum projected angle of draw of 20 . °. The major feature parallels topography rather th n mined-out area boundaries, is apparently related t an existing ancient slump block, and has its major develo ment at its greatest distance from mined-out area, dyin out at both ends as mined-out areas are approached . Bas d on the available data, the features are thus assumed to a natural ridge-line slumpage cracks, with movement trigge ed by natural processes, including excessive precipitation and possibly reactivation of old slippage plans related to the existing slump block . Worst- case effects of continued movement would be limited to resumption of slumping and possible increase of upper Coal Creek silt load, depending on scale and nature of slumpage . In reference to Stipulation No . 1 of its approved Technical Revision, MCR proposes to monitor the area by visual observation during subsequent summer field seasons; full- scale resumption of the subsidence monitoring program would be meaningless as th re are no subsidence monitoring points or control points in the area of concern. A sketch map showing the area discussed, the approximate location and extent of the two cracks discussed, subsidence monitoring and control points and subjacent mine workings is attached . Please note that weather conditions prohibited accurate mapping of these features . Specifically, the