Testing tech n iques
77
In this chapter, the first two sections concern the practical aspects of obtaining
access to the rock mass for testing, and the general philosophy of the testing
requirements-bearing in mind the engineering objective. In the three sub-
sequent sections, there are overviews of tests on intact rock, discontinuities
and rock masses. We conclude with a discussion on standardized tests.1 1.1 Access to the rock
In Fig. 11.1, the main ways in which there is physical access to a rock mass
are indicated. These are a rock exposure (whether at the surface or under-
ground), lengths of borehole core, and the borehole wall itself after the hole
has been drilled. Because the amount of exposed rock is limited and possibly
locally altered, testing has tended to be concentrated on the cylindrical
lengths of borehole core that are obtained by drilling during a site investiga-
tion and by measurements made within the borehole. When studying the
illustrations in Fig. 11.1, recall our discussions about intact rock, discon-
tinuities, rock masses, stresses, and the inhomogeneity and anisotropy factors
in the previous chapters. What, exactly, is it that we wish to know about the
rock mass in order to design and construct a rock engineered structure?
From a rock exposure, it is relatively easy to measure any property of the
intact rock. Similarly, the rock mass structure is evident and a good estimate
of most discontinuity properties can be obtained. Also, flat jack stress
determination tests and larger-scale modulus and permeability tests can be
conducted.
Considering borehole core, it is again evident that any mechanical
property of the intact rock can be measured. The measurements of dis-
continuity properties are, however, immediately severely constrained. An
excellent estimate of the discontinuity frequency in the direction of the
borehole can be obtained, but virtually no information on the persistence
of the discontinuities is revealed. One cannot measure the in situ stress from
the borehole core (techniques such as anelastic strain recovery, differential
strain analysis and the Kaiser effect are still questionable). Also, there is no
possibility of measuring rock muss properties from the rock core.