Field Tests - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 469

(15.16)

where R is the radius, Δδ C is the displacement at the center and is the mean dis-

placement at the boundary of the plate.
To indicate anisotropic behavior of the rock mass, the load can be applied to the wall of
an adit in different directions (Oberti et al. 1979). However, so far, no method has been
developed to determine the elastic constants of an isotropic rock mass from the results
of plate loading tests. For transversely isotropic rock, the determination of E 1 , E 2 and
G 2 in principle may be possible, combining the results of plate loading tests carried out
with different orientations with respect to the isotropic plane, similarly to the method
described for dilatometer tests in the previous section.

15.4 Flat Jack Tests

15.4.1 Isotropic Rock Mass
In a large fl at jack (LFJ) test a slot is sawn into a rock wall and subsequently widened
by a uniform pressure applied by one or more hydraulic jacks that are inserted into the
slot. From the measured widening, Young's modulus can be calculated under idealiz-
ing assumptions (Fig. 15.10). A detailed description of this testing method is given in
DGEG (1984a) and ISRM (1986).

Figure 15.10 shows the preparation and arrangement of an LFJ test that was originally
developed by LNEC (Rocha & Da Silva 1970). The wall of an exploration adit or a test
chamber usually serves as a test location. The cross-section must be large enough to
allow the mounting of drilling and cutting equipment.
The test preparation begins with the drilling of a ca. 1.5 m deep pilot hole with a diam-
eter of 168 mm (Fig. 15.10, upper). The core of this allows access to information on the
rock condition at the test location. Then a 7 mm wide slot is sawn normal to the rock
wall using a diamond disk saw (Fig. 15.10, center, and Fig. 15.11).
In this way, one or more slots are cut in line. In each slot a ca. 6 mm thick LFJ of a
suitable form with dimensions ranging from 1.25 to 1.5 m in length and from 1 to 1.2 m
in width is inserted in such a way that the disturbed rock zone behind the rock's surface
of at least 0.25 m remains unloaded (Fig. 15.10, lower left). The pilot hole must be fi lled
with mortar or half cylinders of hard wood before testing (ISRM 1986).
The slot widening p is measured at four different points

of each LFJ by means of deformeters consisting of displacement transducers or steel
springs equipped with strain gauges that are installed into the LFJ (Fig. 15.10, lower).
The measuring range for

Δ s due to a pressure increment Δ

Δ s is less than 10 mm (DGEG 1984a, ISRM 1986).