Civil Engineering Reference
In-Depth Information
Quantitative description of
discontinuities in rock masses
II.1 Introduction
This appendix provides details of the parameters
used in geological mapping and diamond drilling
for the quantitative description of rock masses.
The information provided is based entirely on
the procedures drawn up by the International
Society of Rock Mechanics (ISRM, 1981a), and
which are discussed in more detail in Sections 4.2
and 4.3 of this topic. The objectives of using
the ISRM procedures for geological mapping and
drill core logging are as follows. First, these pro-
cedures are quantitative so that each parameter is
measured and the results can be used either dir-
ectly, or interpreted, in design. Second, the use of
standardized procedures allows different person-
nel to work to the same standards, and to produce
comparable information.
The following is a description of the parameters
that describe the rock mass, together with tables
listing values used to quantify these parameters.
Also provided are mapping forms that can be used
to record both geological mapping and oriented
core logging. Further information on geological
characterization and methods of data collection
are discussed in Chapter 4.
dimensions and shape of the blocks of rock, and
ground water conditions. Each of the parameters
is discussed in this appendix.
II.2.1 Rock material description
A Rock type
The value of including the rock type in describing
a rock mass is that this defines the process by
which the rock was formed. For example, sedi-
mentary rocks such as sandstone usually contain
well-ordered sets of discontinuities because they
are laid down in layers, and are medium to low
strength because they have usually only been sub-
ject to moderate heating and compression. Also,
the rock type gives an indication of the prop-
erties of the rock mass from general experience
of their engineering performance. For example,
granite tends to be strong and massive and
resistant to weathering in comparison to shale
which is often weak and fissile, and can weather
rapidly when exposed to wetting and drying
cycles.
Table II.1 shows a procedure for defining
the rock type. This procedure involves identi-
fying three primary characteristics of rock as
follows:
II.2 Rock mass characterization
parameters
Figure II.1 illustrates the parameters that char-
acterize the rock mass, and Figure II.2 shows
how they can be divided into six classes related
to the rock material and its strength, the dis-
continuity characteristics, infilling properties, the
1
Color, as well as whether light or dark
minerals predominate;
2
Texture or fabric ranging from crystalline,
granular or glassy; and
3
Grain size that can range from clay particles
to gravel (Table II.2).
