Civil Engineering Reference
In-Depth Information
The main limitations of crack width monitoring
are that the upslope pin or reference point must
be on stable ground, and that it is necessary
that people access the crest of the slide to make
the measurements. This work could be hazard-
ous where the slope is moving rapidly. These
limitations can be overcome to some degree by
automating the system using vibrating wire strain
gauges and data loggers to automatically read the
distance and record the measurements.
(a)
400
400
300
200
N
100
Pit
Bottom
50
0
Limit of
movement
13.3.2 Surveying
Reference station
Instrument stations
Haul road
On large slides where access to the slope is haz-
ardous and/or there is a need to make frequent
and precise measurements and rapidly analyze
the results, surveying using EDM (electronic dis-
tance measurement) equipment is the most suit-
able monitoring method (Vamosi and Berube,
1987; ACG, 1998). There are usually three com-
ponents of a survey system (Figure 13.4). First,
one or several reference points are required on
stable ground, but that can be viewed from the
instrument stations closer to the slide. Second,
a number of instrument stations are set up on
reasonably stable ground at locations from which
the slide is visible. If the co-ordinate positions
of the movement stations are to be measured, then
the instrument stations should be arranged such
that they form an approximately equilateral tri-
angle. Third, a series of stations are set up on,
and possibly just outside the slide area, which are
then located relative to the instrument stations.
It is preferable that the measurement direction
be in the likely direction of movement so that
the distance readings approximate the actual slide
movement. For example, in Figure 13.4(a), it is
preferable to measure stations on the north por-
tion of the slide from instrument station 1, and
those on the south side from instrument station 2.
The stations on the slide can be reflectors used on
heavy equipment, or survey prisms, depending on
the sight distance and the accuracy required.
The survey arrangement shown in Figure 13.4
can be used to measure at any desired frequency
or level of accuracy. For example, for slow
moving slides the readings may be made every
Movement stations
on slide
(b)
Tension crack
Reflector prism
Vertical
angle
EDM
Slope
failure
Figure 13.4
Survey system to remotely measure slope
movement: (a) typical arrangement of reference,
instrument and monitoring stations; (b) measurement
of vertical angle and distance to determine vertical
displacement (Wyllie and Munn, 1979).
few weeks or months, while for a rapidly moving
slide above an active mining operation, an auto-
mated system can be set up that takes a series
of readings at pre-set intervals, and records and
plots the results. Also, quick checks of stabil-
ity can be made by making distance measure-
ments only, and these can be followed up with
triangulation measurements to determine the co-
ordinates of each station at less frequent intervals.
Figure 13.4(b) shows that measurement of the
vertical angle and the slope distance allows the
vertical displacement to be measured, which is of
value in determining the mechanism of failure (see
Section 13.5).
