Field Tests - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 488

In view of the high costs of in-situ shear tests they are often carried out as multiple
failure state tests as described in Section 14.4.2 (Fig. 14.10). Otherwise at least fi ve indi-
vidual tests would have to be conducted on the same test horizon, with each specimen
tested at different but constant normal stress (ISRM 1975a).
In each loading stage or individual test the normal force is increased up to a specifi ed
value while the vertical displacements of the specimen are recorded. During this oper-
ation, excess pore pressures may appear in discontinuities fi lled with cohesive soil. As a
consequence, time-dependent deformations due to consolidation of the fi lling material
may arise. Consolidation can be regarded as completed when the rates of the measured
vertical displacement fall below 0.005 mm/min (ISRM 1975a). Consolidation is essen-
tial because the frictional resistance of the shear plane in an unconsolidated stage will
be considerably lower than after consolidation.

Subsequently, shearing with a constant rate of shear displacement is initiated and con-
tinued until failure or immediately before failure, if a multiple failure state test is carried
out. Shear displacement rates of 0.1 - 0.5 mm/min are recommended (ISRM 1975a). In
tests on discontinuities fi lled with cohesive soil the shear rates must be reduced in order
to maintain drained test conditions (ISRM 1975a).
In a multiple failure state test the normal force is increased and the described procedure
is repeated for the next higher loading stage. In the last stage the discontinuity is sheared
off until residual strength is reached. Finally, the specimen is unloaded.
The maximum shear displacement is limited to 10 - 20% of the specimen's length. A
shear displacement of at least 70 mm should be possible, to be able to determine the
residual strength (ISRM 1975a).
After the test, the shear plane and the specimen should be described and photographed.
Samples of rock and fi llings should be taken for laboratory testing (ISRM 1975a).
The normal stress σ n and shear stress τ res acting on the discontinuity are calculated from

F N , F T and α as follows:

(15.38)

(15.39)

Owing to the decrease of the shear plane A during shearing, the normal stress increases
with increasing shear displacement. Thus, the applied normal force needs to be reduced
during shearing to keep the normal stress approximately constant (ISRM 1975a).
As an example, Figs. 15.21 and 15.22 show the shear stress versus shear displacement
plot and the shear stress versus normal stress plot, respectively, obtained in a multiple
failure state test carried out on a clay-fi lled, schistosity-parallel discontinuity of a clay
slate. This test was carried out with three loading stages (stages 1, 2 and 3) followed
by an unloading (stage 4). The fi lling of the discontinuity was mylonitized, indicating