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<br />1• <br />1 <br />1 <br /> <br /> <br />1 <br /> <br />N <br /> <br />1 <br /> <br /> <br />1 <br /> <br />1 <br />'• <br />1 <br />SubTerra, Inc. Twentymile Coal Company 5/23/99 <br />Northern Mining District Rockfall Hazards Evaluation <br />cliff face to ensure that the boulder catalog was representative of the size of boulders likely to <br />be encountered. <br />Conventional scanline surveys were precluded by the irregular nature of the topography at the <br />base of the cliff and the relatively wide joint spacing. In view of these limitations, the approach <br />adopted was simply to walk sections of the cliff and assign each joint encountered to one of four <br />previously mapped joint sets. In the case of S2 (bedding plane) joints, absolute spacings were <br />estimated in a number of cliff sections and the results combined to obtain spacing frequency <br />distribution directly. For all other joint sets, mean apparent spacings were estimated to provide <br />an indirect estimate of frequency distribution. Four scanlines, with a total length of 2,000 feet <br />were recorded. <br />2.2.2 Boulder Survev <br />During Phase 1, boulder dimensions were measured in the direction of the slope, normal to the <br />slope and across the slope, respectively. The upper part of the cliff face had numerous rounded <br />exfoliated blocks, many of which were detached or partially detached from the substrate. These <br />exfoliated blocks were probably the source of many of the smaller blocks located on the slopes <br />below. The source area elevations were estimated from the topographical maps. <br />2.3 Block Size <br />The general approach to design block-size selection was to review the data on historical rockfall <br />events summarized in the boulder catalog, and to compare this to the block sizes estimated <br />from the scanline data. <br />The scanline data included estimated absolute spacing for bedding plane discontinuity (S2) and <br />apparent mean spacing for S1, S3 and S4. Minor sets (S5, S6 and S7) were not included in the <br />analysis, because their low frequency indicated that they would not significantly influence block <br />size. A Terzaghi Correction (Terzaghi, 1965) was applied to the discontinuity spacing field data <br />to convert the apparent spacing to true spacing. The apparent and corrected spacing for each <br />discontinuity set are summarized in Table 2.1. <br />Table 2.1: Summary of scanline Survey Data <br />scanline <br />(length in ft) Joint <br />Set Number of <br />Joints Apparent <br />Spacing (ft) Alpha <br />(degrees) Terzaghi <br />Corrected <br />Spacing (ft) <br />1 (600) S1 51 11.8 36 9.5 <br /> S3 28 21.4 76 5.2 <br />2 (560) S1 19 29.5 77 6.6 <br /> S3 54 10.4 20 9.7 <br /> S4 42 13.3 15 12.9 <br />3 PN: 9915 <br />