Civil Engineering Reference
In-Depth Information
Figure 15.9
Horizontal double plate-loading test in an exploration adit, Landrücken tunnel in Germany
(GIF 2004)
Before starting the test a pre-load is applied to the rock mass. As in the case of borehole
expansion tests a loading program including several loading and unloading cycles with
increasing peak values is recommended (DGEG 1985a). The test results are plotted in
the form of stress-displacement curves (Fig. 15.3).
If the underground test location is excavated according to the drill and blast method,
only test layouts that allow displacement measurements at some distance from the loos-
ened zone near the rock surface (disturbed rock zone in Fig. 15.8) can be recommended,
since these enable deformations of the undisturbed rock mass to be measured. Young's
modulus is then calculated assuming circular loading of a homogeneous, isotropic, elas-
tic half-space:
(15.13)
where
Δδ
2
are the displacements at two extensometer measuring points z
1
and
z
2
in a central instrumentation hole due to the stress increment
Δδ
1
and
p. Also, z
1
and z
2
must
be located outside the disturbed rock zone (Fig. 15.8). If uniform loading is assumed,
the function K takes on the following form (ISRM 1979c):
Δ
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