Laboratory Tests - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 434

Hoop test
The hoop test, fi rst described by XU et al. (1988), is intended to determine the uniaxial
tensile strength of intact rock using a hoop-shaped specimen formed by coaxial coring.
A pair of half-cylindrical-shaped platens are inserted into the hole in the hoop and
pressed diametrically against the hoop until the specimen fails in tension.
The results of analytical and numerical investigations carried out by John et al. (1993),
and tests conducted by Butenuth et al. (1993), confi rmed that the uniaxial tensile
strength of isotropic intact rocks can be simply calculated from the applied force at
failure Ff f and the failure surface A f obtained in hoop tests as

(14.47)

Hoop tests were also conducted on anisotropic intact rocks (gneisses and amphibolites)
taken from cores by the deep drilling program in Germany (KTB). It was observed that
the specimens often broke into two or more pieces, and failure occurred consecutively
in different directions (Butenuth et al. 1994). Thus, anisotropic rocks are not suitable to
be tested by means of hoop tests.

14.5 Shear Strength of Discontinuities

The shear plane investigated by direct shear tests on discontinuities carried out
in the laboratory should preferably be square with a minimum area of 2500 mm2
(ISRM 1975a).
Careful sampling is required to obtain specimens containing an undisturbed disconti-
nuity. Figure 14.17 shows the sampling method proposed in DGEG (1988) in which a
drill core is extracted with a discontinuity running through the core's axis. After extrac-
tion, the mechanical integrity of the core during transportation should be maintained
by binding with steel wire or tape.
Larger specimens can be obtained using the sampling method proposed by Londe
(1973a), in which block-shaped specimens containing a horizontal discontinuity in the
middle are worked out by overlapping boreholes.

The specimen should be immediately encapsulated using cement, plaster, resin or other
encapsulating materials. Specimens that are not encapsulated should be given a water-
proof coating to prevent drying out during transportation to the laboratory and storage
(ISRM 1975a).
The test apparatus consists of a shear box and systems to apply the shear force F S and
the normal force F N . The placing of the specimen into the shear box with a minimum
of disturbance is of particular importance. After removal of the protective packaging,
with the exception of the steel wire or tape, the specimen is placed into the lower half