Civil Engineering Reference
In-Depth Information
is stick-slip behavior, which is related to the
difference between the static and dynamic coef-
ficients of friction on rock surfaces (Jaeger and
Cook, 1976).
Operations can be continued below slopes
experiencing regressive movement, but it is neces-
sary that the mining be conducted for short
periods with frequent pullbacks, with care being
taken to identify the transition to a progressive
failure (Zavodni, 2000). As shown in Figure 13.2,
geological conditions that may be associated with
progressive failure are discontinuities that dip out
of the face at a steeper angle than the friction angle
(Type II). Also, a slide surface on which the shear
strength gradually diminishes with displacement
may experience progressive failure. The duration
of the progressive stage of a failure has varied
from 4 days to 45 days, with no obvious cor-
relation between the time and the site conditions
(Zavodni and Broadbent, 1980). However, more
rapid failure would be expected where there is a
well-defined slide surface.
As shown by curve C in Figure 13.2, a regress-
ive failure may transition into a progressive fail-
ure and rapidly lead to collapse. Causes of this
change in behavior can include where mining day-
lights a sliding surface, break up of the rock at the
toe of the slope, an increase in water pressure,
or continued mining causing the slope to acceler-
ate beyond recovery. It is obviously important to
recognize the onset of progressive failure, which
will require a diligent monitoring program and
careful analysis of the results.
earthquakes that each cause displacement, and
climatic changes that result in periods of high
precipitation and increased water pressures in
the slope. The Downie and Dutchman's Ridge
Slides in British Columbia, which experienced
tens of meters of ancient, downslope creep prior
to reservoir filling at the base, are both examples
of long-term creep (Moore and Imrie,
1993;
Moore
et al
., 1997).
The authors have examined several dozen land-
slides in western North America where a series
of tension cracks at the crest indicate that tens
of meters of movement has occurred. In most of
these cases, there is no evidence of recent move-
ment because the rock surfaces are weathered and
there is undisturbed soil and vegetation filling
the cracks. It is possible that very slow creep is
occurring, but no long-term monitoring program
was available to determine if this was occurring.
In one case in Alaska, comparison of historic
photographs in the local museum showed no sub-
stantive change in the appearance of the slope
over a period of 120 years. From these observa-
tions, it has been concluded that the presence of
tension cracks does not necessarily indicate that
there is risk of imminent collapse. However, the
hazard may be significant if there is evidence of
recent movement such as disturbance to the soil
and movement of blocks of rock, or there is a
proposed change to the forces acting on the slope,
because of excavation at the toe, for example.
13.3 Surface monitoring methods
This section describes common procedures for
making surface measurements of slope move-
ment. In general, monitoring of the surface of
a slide is likely to be less costly to set up and
maintain than sub-surface measurements that
will require drilling holes to install the instru-
ments. However, surface measurements can only
be used where the surface movement accurately
represents the overall movement of the slope.
For example, it would not be appropriate to
make surface measurements where loose blocks
of rock on the surface were toppling and rotating
independently of the main slide movement. Other
13.2.3 Long-term creep
In contrast to the rapid excavation, and the con-
sequent large scale, relatively fast movements
that take place in open pit mines, mountain
slopes may creep over periods of hundreds of
years. Long-term creep may occur where there
is no defined failure surface, such as a top-
pling failure (Type III, Figure 13.2), or where
the change in slope geometry is very slow, for
example, due to stress relief following glacial
retreat or erosion at the toe by a river. Other
causes of such long-term movement are historical
