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<br />I~ <br />U <br />• <br />3.0 Rockfall Hazard Assessment <br />3.1 Design Block Size Selection <br />The general approach to design block-size selection was to review the data on historical <br />rockfall events summarized in the boulder catalog, and to compare this to the block sizes <br />estimated from the scanline data. <br />The scanline data included estimated absolute spacings for bedding plane discontinuities <br />(S2), and apparent mean spacings for S1, S3 and S4. Minor sets (S5, S6 and S7) were <br />not included in the analysis, because their low frequency indicated that they would not <br />significantly influence block size. A Terzaghi Correction (Terzaghi, 1965) was applied to <br />the discontinuity spacing field data to convert the apparent spacing to true spacings. The <br />apparent and corrected spacings for each discontinuity set are summarised in Table 3-1. <br />Table 3-1: Summary of scanline Survey Data <br />scanline <br />(length, ft) Joint <br />Set Number of <br />joints Apparent <br />Spacing <br />(ft) Alpha <br />(degrees) Terzaghi <br />Corrected <br />Spacing <br />(ft) <br />7(8D0) S7 57 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(440) S1 14 31.4 75 8.1 <br /> S3 59 7.5 20 7.0 <br /> S4 52 8.5 15 8.2 <br />4(420) S1 18 23.3 75 6.0 <br /> S3 49 8.6 20 8.1 <br /> S4 39 10.8 15 10.4 <br />14 <br />