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SubTerra, Inc. Twentymile Coal Company 5/23/99 <br />Northern Mining District Rockfall Hazards Evaluation <br />' In the Phase 1 study, the foreslopes had minimal surface roughness compared to the <br />• dimensions of the blocks analyzed. Under these circumstances the irregularity of the block <br />' becomes a controlling factor, and Pfeiffer and Bowen suggest the effective roughness is then <br />equivalent to 25% to 50% of the block diameter. <br />A large number of runs were carried out for each slope using worst case, average, and best <br />case slope parameters, and effective roughness values in the range 25% to 50% of block <br />diameter. These runs demonstrated sensitivity to effective roughness but relative insensitivity to <br />slope parameters in the ranges investigated. Plots of predicated stopping distances against <br />' observed stopping distances for various block sizes indicated that an effective roughness of <br />about 33% of diameter was appropriate for the design block size. Effective roughness appears <br />to increase slightly as block size decreases, presumably in response to the increasing influence <br />' of slope roughness and vegetation at these smaller block sizes. However, a comprehensive <br />investigation of this relationship was not undertaken. <br />' 2.5 Damage to the Cliff in the Phase 1 Study Area <br />Cliff failure occurred several months after undermining where the cliff had been completely <br />' undermined. Plate 1 shows sections of the failed cliff and Plate 2 shows some of the blocks that <br />traveled down slope from each failure area. <br />Approximately twenty boulders were retained by the remedial trench (Plate 2). Retained <br />boulder size ranged from 10 to 900 tons with an average block weight of 115 tons (1,500 cubic <br />feet). The average boulder weight was 50 tons (600 cubic feet) when the largest boulder is <br />removed from consideration. <br />M <br />3. PARAMETER SELECTION FOR THE NORTHERN MINING DISTRICT <br />' The parameter selection process for work in the Northern Mining District was similar to Phase I. <br />An additional site visit was made to characterize the source area, evaluate slope characteristics <br />' and define the design block/boulder size. Three representative sections (see Drawing 1) were <br />identified and one critical Section, Section C was set up as a CRSP3 data file. <br />Lessons learned in the first phase of this project were incorporated into the second phase where <br />possible. For example, the results of the Phase I rockfall hazard assessment revealed that <br />there is a good correlation between the boulder catalogue and the frequency distribution of <br />block sizes determined by the scanline survey. Therefore, an extensive boulder catalog was not <br />' required in the Northern Mining District study area. <br />SubTerra personnel carried out supplemental fieldwork on March 16, 1999. The site, shown in <br />Plate 3, consists of outcropping ledges of Twentymile sandstone that dip gently along and into a <br />' slope above County Road 33. A comparison of Plates 1 and 3 clearly illustrates the significant <br />differences in topographic relief between the Phase 1 study area and this site. Principal <br />differences include: the height of the potential boulder release or source area; and conditions on <br />' the immediate foreslope below the source area. <br />The presence of a large quantity of blocks on the foreslope immediately below the cliff led to a <br />' revision of the Phase 1 surface roughness factor. In Phase 1, this factor was set at 33% of the <br />design block diameter for relatively smooth slopes. For the Northwest Mining District, surface <br />. roughness was estimated to be 50% of the design block diameter but was varied between 33% <br />and 50% for sensitivity analysis purposes. <br />' 6 PN: 9915 <br />