2015-02-26_REPORT - C1982056 (3)

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Table 2: Surface Descriptions for Tangential Coefficient Selection CRSP3 Description Equivalent Site Condition Smooth hard surfaces such as pavement or smooth bedrock Sandstone cliff face; surfaces road pavement Most bedrock surfaces Sandstone cliff face; road and talus without pavement vegetation colluvium over Most talus slopes with Talus covered upper some low vegetation foreslope; thin colluvium Soft soil slopes over marine shales on upper foreslope. Vegetated talus slopes Thin colluvium overlying and soil slopes with marine shales on upper scarce vegetation foreslope; colluvium covered lower foreslope Brush covered soil Colluvium covered lower slope foreslope. The foreslopes have minimal surface roughness compared to the dimensions of the blocks analyzed. Under these circumstances the irregularity of the block becomes a controlling factor, and Pfeiffer and Bowen suggest the effective roughness is then equivalent to 25% to 50% of the block diameter. Table 3: Surface Descriptions for Restitution Coefficient Selection CRSP3 Description Equivalent Site Condition Smooth hard surfaces and paving Most bedrock and Sandstone cliff face; boulder fields road pavement Talus and firm soil Talus covered upper slopes foreslope; thin colluvium over marine shales on upper foreslope. Soft soil slopes Colluvium covered lower foreslope. A large number of runs were carried out for each slope using worst case, average, and best case slope parameters, and effective roughness values in the range 25 % to 50 %n of block diameter. These runs demonstrated sensitivity to effective roughness but relative insensitivity to slope parameters in the ranges investigated. Plots of predicted stopping distances against observed stopping distances for various block sizes indicated that an effective roughness of about 33% of diameter was appropriate for the design block size. Effective roughness appears to increase slightly as block size decreases, presumably in response to the increasing influence of slope roughness and vegetation at these smaller block sizes. However, a comprehensive investigation of this relationship was not undertaken. 3. HAZARD ASSESSMENT A number of slope morphological zones were established from the morphological mapping. Each zone represents an area of reasonably consistent slope morphology, providing a convenient basis for subdividing the slope for analysis. CRSP models were developed based on selected sections in each zone, and run using average slope parameters, the design block size described above, and an effective roughness of 33 %. From the CRSP output, cumulative probability plots of stopping distances were produced and used to prepare the hazard contours shown on Figure 1. These represent the lines beyond which 50 %n, 5 %© and 0% of fallen blocks would be expected to travel based on the conservative assumptions outlined above. The 0% contour thus represents a conserv- ative limit of the hazard zone, and delineates areas in which rockfall hazard mitigation measures needed to be applied. 4. ROCKFALL HAZARD MITIGATION Several alternative rockfall mitigation measures were considered including; construction of catch fences; temporary or permanent re- routing of sections of the road; excavation of rock catch areas and/or construction of soil berms; and combinations of the above options. The use of catch fences was ruled out at an early stage due to the predicted boulder size. The final configuration included components of all the other options. CRSP runs were performed for each of the sections located within the study area and for each of the applicable alternative catch arealberm configura- tions. These runs formed the basis for determining the size and lateral extent of mitigation measures and areas where the road would need to be relocated to accommodate those measures. The area of mitigation is shown on Figure 1.