Geology Reference
In-Depth Information
These examples illustrate our uncertainty and the dif
culties in judging the degree of hazard by
examination alone. It is highly likely that even after ground investigation, our ability to judge the
severity of the situation is often rather poor. The conclusion must be that consequence should be
the priority when assessing the risk of slope failure. If there is a major risk to life, then works
should be done. This is the underlying philosophy behind the Landslide Preventive Works (LPM)
strategy in Hong Kong where the catalogue of tens of thousands of slopes, prepared in the 70s and
80s, has been compiled and ordered in terms of perceived risk (a function of height, angle
and proximity to vulnerable facilities). Each slope is being checked and upgraded in order. Most
of these are dealt with using essentially prescriptive engineering works, including soil nails and
inclined drains installed to a pattern.
If there is clear danger from the hazard, then it should be dealt with. In the Korean case discussed
above, despite the apparently slow retreat of the rock exposure above the petroglyphs, visitors to the site
should be protected against the evident rockfall hazards.
Quantitative risk assessment of rockfall to roads
At the site shown in
Figure B6-4.3,
while it might be intuitively obvious that there is some risk to life from
rockfall along the road and some history of such rockfalls, the cost of preventive works may be very
expensive. One way to deal with this quandary is to try to quantify the risk and compare this to the cost of
reducing the risk.
To do this requires the following data to be measured or estimated:
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Frequency and size of rockfall incidents (per day).
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Number of vehicles per day, average length and velocity.
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Vulnerability of persons in vehicles to rockfall (depends on size of falls).
