Geology Reference
In-Depth Information
will stand safely provided that the ground model is correct
probably in
part because of inherent conservatism in most assessments of mass
strength parameters (see discussion on disturbance in
Chapter 4).
More
sophisticated analyses can be carried out using generalised representa-
tions of soils and their properties in both two and three-dimensions
(e.g. FLAC SLOPE and FLAC3D
-
Itasca), and these software packages
allow the engineer to see how the failure develops in a time-stepping
manner, which is very helpful. The best use of stability analyses is to test
the signi
-
cance of the various assumptions to the outcome. Lumb (1976)
addressed some of the problems of the Factor of Safety approach and
advocated that engineers think instead in terms of probability:
forcing
the designer to consider the reliability of all his data and to face up to the
consequences of his being wrong
'
. If, for example, water level is shown as
critical to stability, then that should lead to a careful assessment of the
need to prevent in
'
ltration and to install drainage systems.
In the partial factor approach of Eurocode 7, each part of the
analysis
-
-
are factored in a prescrip-
tive manner. Commonly used software packages can cope with this.
This approach might be regarded as rather limiting and perhaps giving
an incorrect impression that everything is understood and that
all factors are always the same. For example, the Eurocode partial
factor for cohesion is the same as for friction (1.25), whereas it is
common experience that friction can generally be measured or esti-
mated with far more con
forces and strength parameters
dence than cohesion and changing
assumptions on cohesion can have a disproportionate in
uence on
calculated FoS.
All analyses are of course only as valid as the input parameters and
especially the geological and hydrogeological models; if the model is
wrong, so will be the analysis. In a study of the failures of several
engineer-designed slopes, Hencher (1983e) concluded:
'
Six of the eight cut slopes that failed had been investigated by drilling
in recent years. In
five of these cases, important aspects that controlled
the failure were missed. In only one case were the true geological
conditions recognised, but even then the groundwater levels were
underestimated considerably. In all cases, where piezometric data
were available and the groundwater level was known by other means,
albeit approximately (e.g. observed seepage), the piezometric data did
not re
ect peak water pressure at the failure surface. This was princi-
pally due to failure to observe rapid transient rises and falls in water
levels. A further problem was that many of the piezometers were
installed at levels where they could not detect the critical perched
water tables which developed.
'
More recently, Lee & Hencher (2009) document a case study where
a slope was subject to numerous ground investigations and analy ses
(often in response to some relatively minor failure) over many years,
