Civil Engineering Reference
In-Depth Information
13.2 Types of slope movement
In setting up a movement monitoring program
it is useful to have an understanding of the type
of movement that is occurring. This information
can be used to select appropriate instrumentation
for the site, and assist in interpretation of the
results. For example, if the slope were undergo-
ing a toppling failure, then crack width monitors
at the crest would provide direct measurement
of horizontal movement. In comparison, if an
inclinometer were to be installed, it may not be
certain that it extended to a depth below the zone
of movement, which would result in erroneous
readings. Furthermore, the type of movement is
related to the failure mechanism and this inform-
ation can be used to ensure that an appropriate
type of stability analysis is used. That is, out-
ward and downward movement at the crest and
bulging at the toe would indicate a plane or circu-
lar failure, whereas horizontal movement at the
crest only would be more indicative of a toppling
failure.
The following is a discussion on common types
of slope movement, and their implications for
slope stability.
showed that total displacement varied from
150 mm in a strong massive rock mass at Palabora
in South Africa to more than 500 mm in highly
fractured and altered rock at the Goldstrike Mine
in Nevada. The rates of movement during ini-
tial response periods decreased with time and
eventually showed no movement. Based on the
monitoring carried out at Palabora, the following
relationship has been established between the rate
of movement
V
(mm/day) and the time
t
(days):
A
e
−
bt
V
=
(13.1)
where
A
and
b
are constants that are a function
of the rock mass properties, the slope height and
angle, the mining rate, external influences and the
ultimate failure mechanism. The reported values
of
A
range from 0.113 to 2.449, while values for
b
range from 0.0004 to 0.00294.
The critical property of the relationship shown
in equation (13.1) is that the rate of movement
decreases with time, indicating that the slope is
not at risk from failure.
Another characteristic of initial response type
of movement is that it can occur within a large
volume of rock. For example, during the steep-
ening of the 150 m deep Berkeley Pit from a
slope angle of 45
◦
to an angle of 60
◦
, movement
measurements in two adits showed that rebound
occurred at a distance of up to 120 m behind
the face at the toe of the slope (Zavodni, 2000).
This rebound and relaxation mechanism has been
modeled using the FLAC and UDEC codes (Itasca
Group, MN) with the objective of predicting such
behavior on similar pits.
13.2.1 Initial response
When a slope is first excavated or exposed, there
is a period of initial response as a result of elastic
rebound, relaxation and/or dilation of the rock
mass due to changes in stress induced by the
excavation (Zavodni, 2000). This initial response
will occur most commonly in open pit mines
where the excavation rate is relatively rapid. In
comparison, the expose of slopes by the retreat
of glaciation and the gradual steepening of slopes
due to river erosion at the toe will occur over
time periods that may be orders of magnitude
longer. However, the cumulative strain of such
slopes can be considerable. Elastic rebound strain
takes place without the development of a definite
sliding surface, and is likely the result of dilation
and shear of existing discontinuities.
Martin (1993) reports on the initial response
measurements of three open pit mines, which
13.2.2 Regressive and progressive movement
Following a period of initial response and then
possible stability, slope “failure” would be indic-
ated by the presence of tension cracks at, or near
the crest of the slope. The development of such
cracks is evidence that the movement of the slope
has exceeded the elastic limit of the rock mass.
However, it is possible that mining can safely
continue under these conditions with the imple-
mentation of a monitoring system. Eventually, an
