184 Testing techniquesFrom the record, and the overview statistics, one can determine the
discontinuity frequency (and hence mean spacing), the discontinuity
spacing histogram and the RQD.
This style of log sheet is designed for direct input to a computer and
hence to be able to take advantage of modern developments in databasing
and information technology. The log sheet is also designed to assist a
structural geologist to interpret the information, because we advocate the
use of a 'geological approach' whereby intelligence can be incorporated
into the sampling process, resulting in a staged approach and hence
optimal use of resources. Databasing the information contained on such log
sheets is still in its infancy, as is the automated recording of values
illustrated in Fig. 11.4. However, both these subjects will develop in the
future.
Once the information is contained within a database, a multitude of
analyses can be conducted in various interrogative modes. For example,
from the log sheets compiled from granite core, one can assess the
occurrence of, say,
(slickensided discontinuities) n (~30" dip) n (green coating)
on the assumption that thrust faulting may be present. This is just one
example of how the speed associated with interrogation of a computer
database can assist-by showing the relevant relations contained within it,
as determined by the engineer and geologist. Moreover, algorithms can be
developed to produce the semi-variograms and associated criteria for
partitioning the rock mass into different structural regions.
We explained earlier that it is generally necessary to have 50 or so
discontinuities for a reasonable estimate of discontinuity frequency, and of
the order of 200 to provide a reasonably coherent histogram. If the
information is contained within a database, algorithms can be used to assess
directly the significance of the variety of discontinuity statistics that may
be output. These ideas also apply to measurements on exposures, where
other parameters such as trace length can be measured.
We are not discussing in any further detail the use of borehole television
cameras, as illustrated in the lower photograph of Fig. 11.1, except to say
that we anticipate greater use of these as the technology of the video images
improves.1 1.4.2 Mechanical attributes
There is a variety of testing procedures for the mechanical attributes of
discontinuities, ranging from the tilt test, through the field shear box and
standard triaxial procedures to sophisticated tests on servo-controlled
equipment.
The tilt test, illustrated in Fig. 11.8, is used to estimate the angle of friction
between the discontinuity surfaces, or indeed any two rock surfaces. The
test can be either carried out simply by hand, or with increased sophistica-
tion using an elevating cradle. The test is simple, with the angle of
inclination when slip occurs directly indicating q), but the process by which