Laboratory Tests - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 428

True triaxial tests are mainly carried out to investigate the infl uence of the interme-
diate principal normal stress σ 2 on the shear strength of intact rock and to formulate

corresponding failure criteria that fi t the test results. In practice, generally, failure
criteria are used that are independent of the intermediate principal normal stress such
as the Mohr-Coulomb or the Hoek-Brown criteria. In addition, as stated in Section
3.2.2, failure criteria that depend on the intermediate principal normal stress, at least
at low to moderate σ 2 levels which are relevant for rock engineering structures, do

not always give a better agreement with test data. Thus, true triaxial tests so far are
conducted mainly for research purposes.

Point load test
The point load test is intended as an index test to estimate unconfi ned compressive
strength of intact rock. The test can be performed with portable equipment or using
a laboratory testing device. Thus, the test can be conducted in both the fi eld and the
laboratory (DGEG 1982, ISRM 1985, DGGT 2010).
The test apparatus consists of a force applying and measuring device, a pair of spheri-
cally truncated, conical platens and a system for measuring the spacing between the two
platen contact points. The conical steel platens with a spherical tip of radius 5 mm must
have an opening angle of 60° (Fig. 14.13).
The specimens may be drill cores, cut blocks or irregular shaped rock pieces. The re-
quirements with respect to size and shape of the specimens are specifi ed in Fig. 14.14,
following the recommendations given in DGEG (1982), ISRM (1985) and DGGT
(2010).
Brittle isotropic rocks are better suited for point load tests than others. Tests on weath-
ered or soft rocks or on rocks with anisotropic strength cannot be recommended
(Wittke 1990).
The specimen is inserted into the test apparatus and the spacing s between the two
platen contact points is measured. Then, the force F is applied to the specimen. F is
gradually increased until failure takes place and the failure load Ff f is recorded. The test
can be recognized as valid if the fracture surface passes through both loading points.
Otherwise the test should be rejected as invalid (ISRM 1985, DGGT 2010).
The point load strength or point load strength index I S (in MPa) was originally defi ned
as follows (Broch & Franklin 1972):

(14.40)

where F f and s need to be specifi ed in kN and mm, respectively. The s 2 can be regard-
ed as an idealized fracture plane with quadratic cross-section. In ISRM (1985) and
DGGT (2010) the defi nition of I S was modifi ed. In practice, however, the different
defi nitions of I S do not differ signifi cantly so that the original defi nition (14.40) can
still be used.