Tests on intact rock 177Figure 11.4 Automated discontinuity location measuring equipment (after
Nordqvist, 1984).
11.3 Tests on intact rock
Tests on intact rock are associated with describing the character of the rock
material, measuring the mechanical properties, and measuring other
properties such as permeability. As rock mechanics has developed,
following geological guidelines and soil mechanics experience, the
description of rock has tended to be of a 'field-book entry' type of approach
during borehole logging. One wishes to know the lithology, colour, etc, of
the rock, but in classifymg for rock engineering the mechanical properties
are also required.
In Fig. 11.5, there are photographs of the two most common field tests
conducted in association with exposure examination and borehole
logging in the field: these are the Schmidt rebound hammer and the point
load test. Both of these are index tests in that they provide a measure of the
'quality' of the rock; the fundamental properties are estimated from tables
using the measured index values.
The Schmidt rebound hammer is a portable device, by which a spring-
driven cylindrical hammer rebounds off the rock surface; the rebound
distance is considered to be a measure of the rock quality. The hammer can
be used directly on a rock surface, or on a rock core: in the latter case, a
special support cradle is required. When the in situ block size is large, the
Schmidt hammer does measure the properties of the intact rock; when the
rock is fragmented, the use of the Schmidt hammer on the exposed rock
surface will be a measure of the rock mass quality rather than the intact
rock per se. In addition, by the very nature of the test, the condition of the
tested rock surface will have a significant effect on the result, because of
geometrical irregularities or because the surface itself has deteriorated and
is not representative of the fresh, intact rock. For this reason, it is
recommended that the Schmidt hammer is used repeatedly within the
immediate vicinity of a measurement location. If it is suspected that
variation in the results is occurring because of geometrical irregularity, the
lower readings should be discarded. If the surface is weathered, then all
the values will be significant. When measurements are made on