Laboratory Tests - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 435

of the shear box and, if required, encapsulated therein using materials such as synthetic
resin, mortar or gypsum. The specimen must be positioned in the shear box so that the
applied normal force acts normal to the shear plane passing through its center of area
in order to avoid applying a moment to the discontinuity during shearing. After hard-
ening of the encapsulating material the specimen is mounted into the upper half of the
shear box. A zone of at least 5 mm at either side of the discontinuity should be left free
from encapsulating material (Fig. 14.18).
Figure 14.17 Sampling of specimens for
direct shear tests on discontinuities to be
carried out in the laboratory (DGEG 1988)
The shear force can be applied by means of either a hydraulic system or a mechanical
gear drive. The equipment should be designed for a shear displacement greater than
10% of the specimen's length. Between the lower half of the shear box and the base,
a low-friction system should be arranged in order to reduce resistance of the equip-
ment to shear displacement (Fig. 14.18). Alternatively, the low-friction system can
be placed between the normal load system and the upper half of the shear box. The
normal force can be applied by a hydraulic, pneumatic or mechanical system (ISRM
1975a, DGGT 1988).