Page 565 - Rock Mechanics For Underground Mining

MONITORING SYSTEMS

The outputs of vibrating wire sensors are frequencies rather than voltages or cur-
rents. This is an advantage in that frequency is easier to transmit over long distances
without distortion than are analogue signals. Frequency counts can usually be trans-
mitted and detected reliably even in the presence of heavy background noise. Vibrating
wire sensors also have excellent long-term stability.
Self-inductance instruments are based on the mutual inductance of a pair of coaxial
solenoids forming a circuit of resonant frequency
(LC) −1/2
f =
2
where L is the self-inductance and C the capacitance of the solenoids. A relative
displacement, d, of the inner and outer solenoids produces a change in the measured
resonant frequency, f , such that
f 0
2
1/2
d = K 1 −
f 2
where f 0 is the calibrated frequency for the null position and K is a constant. Londe
(1982) describes a series of borehole extensometers and inclinometers that are based
on this principle. These instruments may be read remotely using radio-telemetry. The
self-inductance multiple-point extensometer is described in section 18.2.4.
Linear variable differential transformers (LVDTs) are sometimes used as sen-
sors in displacement monitoring instruments. The LVDT also operates on a mutual
inductance principle. However, it produces as the output signal not a frequency, but
a voltage which is directly proportional to the linear displacement of an iron core
along the axis of the instrument. LVDTs operate using an alternating current. Direct
current differential transformers (DCDTs) operating using direct current have some
advantages for geotechnical applications. Linear potentiometers which operate on
the electrical resistance principle provide an alternative to LVDTs and DCDTs for the
remote measurement of linear displacement (Dunnicliff, 1988).
In the following sections, some examples are given of instruments used to monitor
displacements of various kinds, pressure, stress change and seismic emissions. The
selection of instruments described is far from exhaustive, but examples of the major
instrument types used in practice are included. Dunnicliff (1998) provides a fuller
account of the wide range of instruments used for geotechnical monitoring. Table
18.1 provides a listing and broad assessment of the methods used to measure the most
commonly monitored variable, displacement.

18.2.3 Convergence measurement
Convergence, or the relative displacement of two points on the boundary of an excava-
tion, is probably the most frequently made underground measurement. The measure-
ment is made variously with a telescopic rod, invar bar or tape under constant tension,
placed between two measuring points firmly fixed to the rock surface (Figure 18.1).
A dial gauge, micrometer, or an electrical device such as an LVDT, potentiometer or
vibrating wire gauge is used to obtain the measurement of relative displacement.
Figure 18.2 shows a high precision convergence measuring system developed by
Kovari et al. (1974) and used by Rachmad and Widijanto (2002) in convergence
measurements in Freeport Indonesia’s Deep Ore Zone block caving mine, Indonesia
(see section 18.3.2 below). The instrument is used to provide a constant tension of
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