1997-07-28_GENERAL DOCUMENTS - M1973021 (9)

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' III. AREAS MINED IN 1996 During1996, active mining occurred along the east side, and at the south end and southwest g g corner of the Central Quarry, as well as in the northern and eastern portions of the North ' Quarry. 3.1 East Highwall of Central Quarry ' By the end of the 1995 mining year, the upper approximately 160 ft(vertical) of the East Highwall of the Central Quarry was mostly reconfigured into four 40 ft ' highwalls and 50 to 60 ft wide benches. This slope configuration was consistent with the proposed slope remediation for reestablishing stability of the East Highwall. This slope configuration was defined and shown in Mine Plan Amendments Nos. 3 and 4, and represented the first phase of stabilizing the slope after which slope conditions would be reevaluated and, if desirable, reconfigured. ' All blasting of the permanent highwalls in this area employed precision(perimeter control)blasting methods. The upper two highwalls were drilled and blasted vertically. In response to localized block and wedge instabilities on these highwalls, ' the third and fourth highwalls were drilled and blasted at an angle of approximately 20 degrees from vertical (70 degree dip), resulting in significantly fewer localized instabilities and acceptable conditions. ' Early in 1996, while removing blasted rock to complete the southern portion of the fourth bench and highwall, a slope failure occurred through the second and third ' ' benches in the vicinity of an earlier failure on a previously developed East Highwall slope in a zone of faulted, relatively poor quality rock. Excavation of the fourth bench exposed a shallow dipping, preexisting shear surface that mobilized almost ' immediately upon removing material that had buttressed the mass above the shear surface. This occurrence resulted in a special geotechnical failure investigation and remedial repair both of which were completed during the 1996 mining year. ' A Technical Revision was also submitted and approved, on 28 May 1996 and 24 June 1996, respectively, to accommodate the repair. The Technical Revision modified the Mining Limits, but not the Limits of Disturbance along the East Highwall, so that excavation to unload the slide driving mass could be performed at the top of the failure area. More detailed descriptions of the special geotechnical investigation and slide repair are provided in subsequent sections of this narrative. ' 3.2 South End of Central Quarry ' The south end of the Central Quarry was mined by extending this end of the quarry to the south and southwest. This was accomplished via new mining in the southwest corner and by reconfiguring interim benches in the central part of the south end. The Central Quarry floor remained at approximate elevation 6,490 ft, and was not extended farther to the south during 1996. In general, 40 ft highwalls were present ' 3 G:\PROJECTS\20200\002\96ANNRPT.WPD