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• 2.3 SAIC Data SAIC stereographic projection data was replotted prior to the field work. These plots were reviewed in the field in the course of carrying out the scanline surveys described below, and were found to be representative of conditions in situ. 2.4 scanline Survey Significant lengths of the main cliff face of the Twentymile sandstone outcrop present an apparently massive, unjointed appearance. However, recent failures of the face in response to undermining by Panel No. 6 confirm that significant jointing is present, resulting in the formation of a variety of block sizes, some of which are very large. Closer inspection reveals that jointing is generally present in the Twentymile sandstone outcrop, although weathering or surface patina tend to obscure these features in many areas. • 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 in the cliff faces, to ensure that the boulder catalog was representative and, in particular, that the potential for block sizes significantly larger than indicated by the catalog was minimal. Conventional scanline surveys involve logging all of the discontinuities which intersect a selected line between known points. This line is typically provided by a tape measure or string run between the end points. In the present case this approach was precluded by the irregular nature of the topography at the base of the cliff, and the relatively wide joint spacing which dictated the use of long scanlines to provide representative data. In view of these limitations, the approach adopted was simply to walk sections of the cliff and assign each joint encountered to one of the sets identified on the SAIC stereoplot (Figure 2-1). 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