Page 564 - Rock Mechanics For Underground Mining

MONITORING ROCK MASS PERFORMANCE

surfacesubsidenceassociatedwithundergroundextraction.Satellite-basedGlobalPo-
sitioning Systems (GPS) may also be used for the latter purpose (e.g. Collier, 1993,
Windsor, 1993). In the past, considerable use was made of photoelastic plugs and discs
to monitor stress changes in the rock surrounding excavations and in support elements.
Details of several of these photoelastic instruments are given by Roberts (1977).
Hydraulic and pneumatic diaphragm transducers are used for measuring water
pressures, support loads, cable anchor loads, normal components of stress and set-
tlements. In all cases the method of operation is the same. The quantity measured
is a fluid pressure which acts on one side of a flexible diaphragm made of a metal,
rubber or plastic. Twin tubes connect the read-out instrument to the other side of the
diaphragm. To take readings, air, nitrogen or hydraulic oil pressure is supplied from
the read-out unit through one of the tubes to the diaphragm. When the supply pressure
is sufficient to balance the pressure to be measured, the diaphragm acts as a valve and
allows flow along the return line to a detector in the read-out unit. The balance pressure
is recorded, usually on a standard Bourdon pressure gauge or a digital display.
Electrical devices probably provide the most common basis of the instruments
presently used to monitor the performance of rock masses surrounding mining struc-
tures, although mechanical systems still find widespread use in displacement moni-
toring. Electrical systems generally operate on one of three basic principles.
ElectricresistancestraingaugesoperateonaprinciplediscoveredbyLordKelvin,
namelythattheresistanceofawirechangesproportionatelywithstrain.Instraingauge
systems, strain gauges made from thin wire or foil, are bonded to a rock, concrete or
steel surface. Changes in the strains in the host material are accompanied by changes
in the strains, and resistances, of the strain gauges which are read using a Wheatstone
bridge circuit.
The use of electrical resistance strain gauges in the measurement of in situ stresses
was discussed in section 5.3. Most load cells, many pressure transducers and some
types of inclinometer also use electrical resistance strain gauges. The major disadvan-
tages associated with the use of electrical resistance strain gauges for rock mechanics
applications are
(a) it is difficult to obtain and maintain a good bond between the strain gauge and
the rock,
(b) the strains are measured over relatively short gauge lengths, and
(c) temperature effects cannot always be eliminated.
Vibrating wire sensors are based on the fact that the natural frequency of vibration,
f , of a tensioned wire of length l w and density is related to the tensile stress in the
wire, w , by the equation
1/2

1 w
f =
2l w
If this frequency is measured by electromagnetic plucking, the tension w can be
determined. This value may be used to determine the pressure acting on a diaphragm
to which one end of the wire is attached, or the axial load on a load cell in which
the vibrating wire is mounted. Thus vibrating wire systems are used in piezometers,
soil pressure cells, stressmeters and load cells. A vibrating wire instrument used for
monitoring stress changes in rock is described in section 18.2.6.
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