Page 562 - Rock Mechanics For Underground Mining

MONITORING ROCK MASS PERFORMANCE

(a) fracture or slip of the rock on the excavation boundary (observed visually);
(b) movement along or across a single joint or fracture (either monitored by a simple
mechanical ‘tell-tale’ or measured more accurately);
(c) the relative displacement or convergence of two points on the boundary of an
excavation;
(d) displacements occurring within the rock mass away from the excavation pe-
riphery;
(e) surface displacements or subsidence;
(f ) changes in the inclination of a borehole along its length;
(g) groundwater levels, pressures and flows;
(h) changes in the normal stress at a point in the rock mass;
(i) changes in loads in support elements such as steel sets, props, rock bolts, cables
and concrete;
( j) normal stresses and water pressures generated in fill;
(k) settlements in fill;
(l) seismic and microseismic emissions; and
(m) wave propagation velocities.
Although it may appear from this list that a wide range of variables may be moni-
tored, only two basic physical responses, displacement and pressure, can be measured
relatively directly using current technology. Measurements can be made of the abso-
lute displacements of a series of points on the boundaries of an excavation or, with
more difficulty, within the rock mass. The relative displacement, or convergence,of
two points on the boundary of an excavation is easier to measure than absolute dis-
placement. Because the relative displacement of two points can usually be measured,
a measurement of normal strain can be obtained by assuming that the strain is uni-
form over the base length of the measurement. Pressures in groundwater and normal
stresses at rock-support contact or in fill can be measured by the pressures induced in
fluid-filled pressure cells, often using a null method. An average pressure normal to
the surface of the pressure cell sensor is obtained. Time is always recorded as a fun-
damental variable. Temperature may be an important variable in some applications
and for some measurement methods.
It is important to recognise that the ‘measurement’ of most other variables of inter-
est, notably forces and stresses, requires the use of a mathematical model and material
properties (e.g. elastic constants) to calculate the required values from measured dis-
placements, strains or pressures. As a general rule, it is preferable to use directly mea-
surable parameters for purposes of comparison and decision making rather than pa-
rameters calculated from a mathematical model using measured parameters as input.

18.2 Monitoring systems

18.2.1 General features of monitoring systems
The instrumentation system used to monitor a given variable will generally have three
different components. A sensor or detector responds to changes in the variable being
monitored. A transmitting system which may use rods, electrical cables, hydraulic
lines or radiotelemetry devices, transmits the sensor output to the read-out location. A
read-out and/or recording unit such as a dial gauge, pressure gauge, digital display,
magnetic tape recorder or computer, converts the data into a usable form and presents
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