Geology Reference
In-Depth Information
for example, where building a road or railway, then the size of the design
event may be predicted by historical studies or from
first principles.
Hungr et al. (2005a) present a useful review of landslide characteristics
that might be considered for design. In assessing existing facilities and
structures, sometimes these can be protected by barriers and other
engineering devices, but occasionally the risks are so high and cost of
mitigation too expensive, so that relocation is the only real solution.
Open hillside landslides and rockfalls have a much more limited
distance of travel. For most landslides in Hong Kong, debris travel
distance is less than 100m, so the area of concern is quite obvious, both
in terms of source of landslides and structures at risk. It does not make
them less dangerous, just that the nature of the hazard and focus for
analysis is clear. Hazard assessment can follow standard methods of
investigation, analysis and design, as outlined in
Chapter 6.
Risk
review can be used to justify cost of mitigation works or taking no
action
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despite there being a clear and obvious danger as, for example,
along many roads through mountainous areas. The practical applica-
tion of risk concepts are reviewed by many authors in Hungr et al.
(2005b). Fell et al., (2005) and Wong (2005) are particularly useful.
The concept of ripening of slopes prior to failure has been a useful idea
for many years, but recently evidence for progressive deterioration of
slopes prior to detachment has become better documented. This
applies to both natural slopes and cut slopes (Malone, 1998;
Hencher, 2000; Parry et al., 2000). Factors involved in slope deteriora-
tion are illustrated schematically in
Figure 3.58,
and some of the
factors triggering natural terrain failures are illustrated in
Figure 3.59.
The gradual deterioration can be represented by a curve in which the
Factor of Safety reduces over a period of time, which may be hundreds
of years (
Figure 3.60).
The vertical lines represent temporary reduc-
tions in Factor of Safety caused by relatively short-term, transient
events (days). In the course of time, the slope will deteriorate to the
point where it is vulnerable to a transient event
causing a reduction in
the Factor of Safety below 1.0. Whether that event results in cata-
strophic failure, or only minor movement and internal deformation,
depends on many factors, including the severity of the triggering event
and how long it lasts. The concept of ripening and progressive failure
is discussed in more detail in Hencher (2006). Similar concepts are
discussed for claystone slopes by Picarelli & Di Maio (2010).
Signs of gradual deterioration or, more likely, the cumulative effect
of intermittent triggering events, can be seen in many exposures and
these can be used during ground investigation to help judge whether a
failure is imminent, although this may still not be straightforward, as
discussed in
Chapter 6,
Box 6-4.
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