Field Tests - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 477

larger displacements to initiate failure of the specimen. Therefore rectangular fl at jacks
(500 mm 1000 mm) are proposed to be

used for this test arrangement which allow slot widening of several cm (DGEG 1984a).
These must be inserted and cemented into slots formed by overlapping boreholes. Fig-
ure 15.15 shows the drilling of such a slot using a drilling rig.

⋅ 1000 mm) or square fl at jacks (1000 mm ⋅

Figure 15.15 Drilled slot of overlapping boreholes (Interfels GmbH)

In both tests, the strains of the specimen in the direction of loading applied by
the fl at jacks (x direction and z direction in Fig. 15.14) need to be measured.
This can be accomplished by slope measuring devices installed in four small-di-
ameter boreholes located near the slots. Defl ectometers are recommended for
this purpose, which measure the relative displacements

w in the x di-
rection and the z direction due to fl at jack pressures of p x and p z , respectively.
From

Δ u and Δ

w the mean strains and of the specimen can be calculat-
ed (Fig. 15.14, center). In test BFJ1 the mean strain of the specimen in the
y direction, due to p x and p z , respectively, must also be measured. For this purpose,
an extensometer is proposed to be installed in a small-diameter borehole in the mid-
dle of the specimen, which measures the relative displacement

Δ u and Δ

v (Fig. 15.14, lower).
Extensometers and defl ectometers will be described in Sections 17.4.1 and 17.4.2,
respectively.

Δ

Parametric studies by means of three-dimensional fi nite element analyses were carried out
in which test BFJ1 was simulated and the elastic constants were varied (Kiehl 1985). As a