Laboratory Tests - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 410

by horizontally arranged strain gauges. For a specimen with isotropic deformability a
possible arrangement of strain gauges is illustrated in Fig. 14.2 (left) while Fig. 14.2
(right) shows an adequate arrangement of strain gauges for a specimen with transverse-
ly isotropic deformability. In this case the tangential strains ε y should be recorded

perpendicular and parallel to the strike direction of the isotropic plane. The measured
strains in such a case strongly depend on the angle

ε x and

β between the end faces of the spec-

imen and the isotropic plane.

Figure 14.2 Examples for the arrangement of strain gauges

Alternatively, the specimen's strains may be determined by displacement measurements
using axial, circumferential and lateral displacement transducers or extensometers
(DGEG 1979a, ISRM 1979a, ISRM 1999a, DGGT 2004b). It is recommended to at-
tach the axial displacement transducers to the specimen 180° apart and contact the
specimen at approx. 25 and 75% of the height (ISRM 1999a). The axial strain

ε a is

calculated from the displacement Δ l of axial measuring points:

(14.12)

where l 0 is the initial spacing of the axial measuring points (Fig. 14.3, above). Owing to
friction the uniaxial stress state in the specimen is disturbed at the specimen's ends. It
is therefore essential that the axial measuring points are arranged at suffi cient distance
from the specimen's ends. It is inadmissible therefore to measure the axial strain over the
entire height H of the specimen.