Geology Reference
In-Depth Information
Figure B6-4.3
Road cut through limestone with very little engineering support or protective measures,
Tailuko Gorge, Taiwan.
The annual probability of risk of death can then be calculated and compared to published guide-
lines on acceptable risk (e.g. Fell
et al
., 2005). Different sections of road will be shown to have
different risk levels, which will allow decisions to be made on where to carry out mitigation works.
Quite often such a calculation will show that risks are acceptable even if, judgmentally, the hazard
is still intolerable (the situation looks very worrying). It may well be found that relatively simple
measures, such as scaling off the most obvious loose rock and providing netting or cheap barriers
such as gabions locally, will reduce risk considerably whilst also making the situation feel safer.
Further guidance on judging rockfall hazards and the use of rockfall rating systems is given by
Bunce
et al
. (1997) and Li
et al
. (2009).
6.6.1.2 Structural
The distinction of failure mechanisms into planar, wedge and top-
pling, and the discontinuity geometries and conditions responsible
for each style of failure, are set out clearly by Hoek & Bray (1974),
and this has been updated by Wyllie & Mah (2004). The most
common type of failure is sliding on a single discontinuity, and this
is simple to analyse. The main dif
culties are in assessing shear
strength of the rock discontinuities, as set out in
Chapter 5,
and
how to deal with groundwater pressures. Generally, a simple analysis
is done in which it is supposed that water pressure at the slope face is
zero, increasing back within the slope, to some height below ground
surface at the rear of the slope
(Figure 6.23)
. This is often a conser-
vative assumption, in that water pressure will be localised, not acting
throughout the whole slope at the same time. Richards & Cowland
(1986) discuss a well-investigated site where it would have been
unrealistic to design the slope to withstand the maximum water
