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may be precursors to major rock collapses. Methods of mitigating
rockfalls and other potential landslides are discussed below.
Box 6-4 Judging the severity of rockfall hazards and the associated risks
'
People
-
even experts
-
rarely assess their uncertainty to be as large as it usually turns out
to be.
'
Baecher & Christian (2003).
The assessment of hazard of rock slope failure is always rather subjective, as illustrated by a visit to the
petroglyphs at Anhwa-ri, Goryeong, Korea, in February 2008. The rock exposure shown in Figure
B6-4.1, above the rock carvings, appears to be on the brink of failure and one would be tempted to
Figure B6-4.1 Petroglyphs at Anhwa-ri, Goryeong, Korea. Rock slope above petroglyphs (with small
protective fence) shows signs of vegetation wedging, with loose blocks resting against trees.
fence off the area, immediately followed by removal of any blocks that cannot be stabilised by
dowelling and dentition works. However, the fact that the precarious open-jointed rock is directly
above the ancient rock carvings, is evidence that this rock face has not retreated very far over a
period of more than 2,000 years. The process of deterioration and collapse is actually quite slow and
judgment of the risk as immediate and obvious, requiring urgent action, would therefore err on the
conservative side.
Conversely, the slope shown in Figure B6-4.2 is in the Cow and Calf Quarry at Ilkley, Yorkshire, in the
UK, and was used to teach MSc engineering geology students to map rock discontinuities for several
years. The collapse to the left of the photograph occurred unexpectedly between mapping exercises,
despite its repeated examination and systematic logging on scan lines, without the failure mechanism
having been identi
ed.
 
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