2015-02-26_REPORT - C1982056 (3)

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Task 4: Engineering and Design: The scope of work for Task 4 included an engineering interpreta- tion of the hazard map, boulder catalog and CRSP output and development and modeling of alternative mitigation techniques. 2. SITE INVESTIGATION AND DESIGN PARAMETER SELECTION The objectives of this work were to provide data on slope morphology, surface characteristics and block - size frequency- distribution. Specific tasks, which were carved out concurrently, included; mapping of slope morphology and surface characteristics, including slope breaks, soil and vegetation type, and man -made features. compilation of a boulder catalog containing details of blocks from earlier falls including location, approximate dimensions, source area (if determinable), and failure mode (as determinable). scanline mapping to assess the spacing of the various discontinuity sets present. 2.1 Slope Morphology During the site investigation, morphological features were recorded by SubTerra personnel to supplement Twentymile Coal Company's ] -inch to 200 -feet topographical maps and to aid in developing CRSP3 model parameters. Features of interest included upper and lower slope breaks, cliff base and crest lines, and man -made features such as leveled areas, drainage ditches, and CAT roads. The general distribution of the surficial materials appeared to be related to the slope breaks, however, they were mapped only in sufficient detail to enable appropriate rebound parameters to be assigned in the CRSP3 models. These surficial materials consisted predominantly of talus and colluvium. Talus appeared to be restricted to the upper part of the slope, and included some buried or partially buried material. The colluvium is considered to be a widespread deposit consisting mainly of fine sand or silty fine sand with a variable amount of sandstone pebbles, cobbles, and boulders, In areas where the marine shale horizon is close to the surface, the colluvial matrix contained a higher proportion of silt and clay sized material. 2.2 Structural Reghims And Boulder Characteristics The Twentymile sandstone outcrop had a massive, unjointed appearance for significant lengths. However, the earlier cliff failures, in response to undermining by Panel 6, confirmed that significant jointing was, in fact, present. This jointing resulted in the formation of a variety of block sizes, some of which were very large. Closer inspection revealed that jointing was present in the outcrop, although weathering or surface patina tended to obscure these features in many areas. 2.2.1 Scanline Survey Although the intent was to base the design block size primarily on the boulder catalog, it was deemed necessary to make an estimate of the block size distribution based on jointing in the cliff face to ensure that the boulder catalog was representative of the size of boulders likely to be encountered. Conventional scanline surveys were precluded by the irregular nature of the topography at the base of the cliff and the relatively wide joint spacing. In view of these limitations, the approach adopted was simply to walk sections of the cliff and assign each joint encountered to one of four previously mapped joint sets. In the case of S2 (bedding plane) joints, absolute spacings were estimated in a number of cliff sections and the results combined to obtain spacing frequency distribution directly. For all other joint sets, mean apparent spacings were estimated to provide an indirect estimate of frequency distribu- tion. Four scanlines, with a total length of 2,000 feet were recorded in Zones 2, 3/4, 617, and 8/9. 2.2.2 Boulder Surrey Boulder dimensions were measured in the direction of the slope, normal to the slope and across the slope, respectively. The upper part of the cliff face had numerous rounded exfoliated blocks, many of which were detached or partially detached from the substrate. These exfoliated blocks were probably the source of many of the smaller blocks located on the slopes below. Source area elevations were estimated from the topographical maps. 2.2.3 Block Size The general approach to design block -size selection was to review the data on historical rockfall events summarized in the boulder catalog, and to compare this to the block sizes estimated from the scanline data. The scanline data included estimated absolute spacing for bedding plane discontinuity (S2) and apparent mean spacing for S ] , S3 and S4. Minor