Page 568 - Rock Mechanics For Underground Mining

MONITORING ROCK MASS PERFORMANCE

Figure 18.2 The Distometer in relatively large volumes of rock can be recorded. MPBX measurements are partic-
ISETH, a high-precision mechanical ularly useful in recording bed sag and separation in bedded strata above coal mining
convergence measuring system. roadways (e.g. Hebblewhite and Lu, 2004). These data are generally more useful than
the results of convergence measurements which only give relative surface displace-
ments and may be influenced by surface conditions. Sometimes convergence measure-
ments are made between the heads of MPBX installations as illustrated in Figure 18.3.
Suggested methods for monitoring rock movements using borehole extensometers
are given by the International Society for Rock Mechanics Commission on Stan-
dardization of Laboratory and Field Tests (1978b). Instruments using both electrical
and mechanical sensors are described, and guidelines for determining the precisions,
sensitivities and measuring ranges required for a number of applications are given. It
is suggested that for large underground excavations, the minimum measuring range
should be 50 mm (300 mm with reset), the precision should be in the range 0.25–
2.5 mm, and the instrument sensitivity should be typically 0.25–1.00 mm.
MPBX installations may use a variety of techniques and materials. The extensome-
ter may use stainless steel wires or rods made of aluminium, carbon fibre compos-
ites, fibreglass, invar, mild steel or stainless steel. The rods may be anchored with

Figure 18.3 Multiple-point bore-
hole extensometer (MPBX) installa-
tions with convergence measurement
between MPBX heads.

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