176 Testing techniquesperformed quickly, but may not determine an intrinsic property-the
point load test and Schmidt hammer rebound tests are examples. A funda-
mental test, on the other hand, measures a conventionally accepted
intrinsic property, such as the compressive strength. One can estimate
the compressive strength via the point load test in the field, or one
can conduct direct compressive strength tests in the laboratory. The latter
are more expensive and time consuming but do measure the property
directly. Alternatively, one could conduct many tests using the point
load apparatus, with fewer direct tests in the laboratory, or rely mainly on
the point load test calibrated occasionally against direct tests in the
laboratory.
The example in the previous paragraph is illustrative of all rock mechan-
ics testing. Consider the measurement of the in situ rock deformation
modulus. Should one use a dilatometer in a borehole, or a tunnel jacking
test, or estimate the modulus using an analytical model and laboratory-
determined values for the component parameters? In Fig. 11.3, we illustrate
a 6 MN tunnel boring machine tunnel jacking test in chalk. The modulus
determined by this means was between 7 and 10% of the laboratory,
determined value, and of the same order as that determined from a finite
element analysis back-calculation of ground settlement, back-analysis being
yet another method available to the engineer.
In the majority of cases, the rock mechanics information is obtained from
tests on borehole core, so it is essential that the drilling report and
borehole core logs are correctly completed and available. In this book we
will not deal with borehole core logging: instead we refer the reader to
the Geological Society of London recommended procedures, the
relevant British Standard, and, of course, all other relevant National
Standards. With the advent of sigruficantly increased microcomputing
power, there is a move to provide more automation and immediate data
acquisition and presentation in the field. In Fig. 11.4, one idea for auto-
mated discontinuity location recording is presented; this was developed
by Nordqvist (1984).
Up to 8 MN applied /
through 8 jacks Reaction ring secti,Figure 11.3 A 6 MN tunnel loading test for estimating in situ rock modulus, from
Hudson et al. (1977).