A simple design chart for ditch design based upon

This preview shows page 123 - 126 out of 316 pages.

A simple design chart for ditch design, based upon work by Ritchie (1963), is reproduced in Figure 9.7. Hanging nets or chains for blocks tumbling from above Supports stayed by rock anchors or deadmen Loose blocks to be scaled from any face without nets Bench as rockfall collector Free hanging mesh suspended from above Warning signs Fence or wall Gravel bed Rock trap
Rockfall Hazard Rating System 123 Free fall Bounce Roll Motion of a falling rock Slope gradient 0:1 0.25:1 0.3:1 0.5:1 0.75:1 1:1 120 100 80 60 40 20 0 W=25’ W = 20’ 90 80 70 60 50 40 Overall slope angle - degrees Slope height - feet W = 15’ D = 3’ W = 10’ D = 4’ Ditch Design Chart Slope height Roll 30 deg - slope angle Roll 45 deg Bounce 60 deg Depth (D) Fall Width (W) Rock falls on slopes Figures taken from FHWA Manual ’Rock Slopes’ November 1991. USDOT Chapter 12 Page 19. Figure 9.7: Rockfall ditch design chart based upon work by Ritchie (1963). 9.4 Rockfall Hazard Rating System Highway and railway construction in mountainous regions presents a special challenge to geologists and geotechnical engineers. This is because the extended length of these projects makes it difficult to obtain sufficient information to permit stability assessments to be carried out for each of the slopes along the route. This means that, except for sections which are identified as particularly critical, most highway slopes tend to be designed on the basis of rather rudimentary geotechnical analyses. Those analyses which are carried out are almost always concerned with the overall stability of the slopes against major sliding or toppling failures which could jeopardise the operation of the highway or railway. It is very rare to find an analysis of rockfall hazards except in heavily populated regions in highly developed countries such as Switzerland. In recognition of the seriousness of this problem and of the difficulty of carrying out detailed investigations and analyses on the hundreds of kilometres of mountain highway in the western United States and Canada, highway and railway departments have worked on classification schemes which can be carried out by visual inspection and simple calculations. The purpose of these classifications is to identify slopes which are particularly hazardous and which require urgent remedial work or further detailed study. In terms of rockfall hazard assessment, one of the most widely accepted 5 is the Rockfall Hazard Rating System (RHRS) developed by the Oregon State Highway Division (Pierson et al 1990). Table 9.1 gives a summary of the scores for different 5 This system has been adopted by the States of Oregon, Washington, New Mexico and Idaho and, in slightly modified form, by California, Colorado and British Columbia.
124 Chapter 9: Analysis of rockfall hazards categories included in the classification while Figure 9.8 shows a graph which can be used for more refined estimates of category scores. The curve shown in Figure 9.8 is calculated from the equation y x = 3 where, in this case, x = (Slope height- feet)/25.

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture