Geology Reference
In-Depth Information
before the slope
finally collapsed in a disastrous manner. There were
fundamental misconceptions about the geological conditions by all of
the investigators. The potential for self-delusion that such methods of
analysis truly represent actual stability conditions is expressed a little
cynically in the song
in
Box 6-6.
Lerouiel &
Tavernas (1981) used various classic examples of slope failures and
their analysis to demonstrate how different assumptions can lead to
different results and explanations.
'
Slopey, Slopey, Slopey
'
A decision needs to be made on whether the risk from slope
failure is acceptable or not and whether the cost of engineering
works can be justi
ed. A modern approach to assessing the need
for preventive measures is to use quanti
ed risk assessment, as
described by Pine & Roberds (2005). The project described
involved remediation and stabilisation of several sections of high
cut and natural slopes dominated by potential sheeting joint fail-
ures and by the potential for failure of rock blocks and boulders
bouncing down exposed sheeting joints to impact the road below.
Design of slope cut-backs and stabilisation measures was based on
a combination of reliability criteria and conventional FoS design
targets aimed at achieving an ALARP (as low as reasonably prac-
ticable) risk target, which, in actuarial terms, translated to less
than 0.01 fatalities per year per 500m section of the slopes under
remediation. Further examples of quantitative risk calculation are
given by Fell
et al
. (2005).
Remediation of stability hazards on slopes is often not trivial,
especially where the works are to be conducted close to existing infra-
structure and implementation of the works can itself increase the risk
levels, albeit temporarily. Factors that will in
uence the decision on
which measures to implement include the speci
c nature of the
hazards, topographic and access constraints, locations of the facilities
at risk, cost and timing. The risks associated with carrying out works
next to active roads, both to road users and to construction workers
themselves, need to be addressed (GEO, 2000a). Pre-contract stabili-
sation works might be needed to allow site access and preparation.
Preventive measures such as rock bolting may be carried out at an early
stage to assist in the safe working of the site and designed to form
part of the permanent works. Options for the use of temporary
protective barriers and catch nets to minimise disruption to traf
c
during the works also need to be addressed, as do contractual controls
and alternatives for supervision of the works. Traf
c controls may be
