Page 65 - Rock Mechanics For Underground Mining

MAJOR TYPES OF STRUCTURAL FEATURES

high stress concentration, the influence of structure may be less marked, and lim-
iting the induced boundary stresses or energy release rates may be more important
considerations (Chapters 7 and 10).
This chapter describes the types and important properties of structural features
found in rock masses, methods of collecting, processing and presenting data on rock
structure, and the incorporation of such data into rock mass classification schemes.
The uses of these data and rock mass classifications in selecting mining methods and
designing excavations will be described in subsequent chapters.

3.2 Major types of structural features

Structural features and their origins are well described in several textbooks on general,
structural and engineering geology. From an engineer’s point of view, the accounts
given by Hills (1972), Hobbs et al. (1976), Blyth and de Freitas (1984), Price and
Cosgrove (1990) and Goodman (1993) are particularly helpful. The reader who is not
familiar with the elements of structural geology should study one of these texts. All
that will be given here is a catalogue of the major types of structural feature and brief
descriptions of their key engineering properties.
Bedding planes divide sedimentary rocks into beds or strata. They represent inter-
ruptions in the course of deposition of the rock mass. Bedding planes are generally
highly persistent features, although sediments laid down rapidly from heavily laden
wind or water currents may contain cross or discordant bedding. Bedding planes may
contain parting material of different grain size from the sediments forming the rock
mass, or may have been partially healed by low-order metamorphism. In either of
these two cases, there would be some ‘cohesion’ between the beds; otherwise, shear
resistance on bedding planes would be purely frictional. Arising from the depositional
process, there may be a preferred orientation of particles in the rock, giving rise to
planes of weakness parallel to the bedding.
Folds are structures in which the attitudes of the beds are changed by flexure
resulting from the application of post-depositional tectonic forces. They may be major
structures on the scale of a mine or mining district or they may be on a smaller local
scale. Folds are classified according to their geometry and method of formation (Hills,
1972, for example).
The major effects of folds are that they alter the orientations of beds locally, and
that certain other structural features are associated with them. In particular, well-
defined sets of joints may be formed in the crest or trough and in the limbs of a fold.
Figure 3.2 shows the typical development of jointing in one stratum in an anticline.
During the folding of sedimentary rocks, shear stresses are set up between the beds
where slip may occur. Consequently, the bedding plane shear strength may approach,
or be reduced to, the residual (section 4.7.2). Axial-plane or fracture cleavage may
also develop as a series of closely spaced parallel fractures resulting from the shear
stresses associated with folding.
Faults are fractures on which identifiable shear displacement has taken place. They
may be recognised by the relative displacement of the rock on opposite sides of the
fault plane. The sense of this displacement is often used to classify faults (Hills, 1972,
for example). Faults may be pervasive features which traverse a mining area or they
may be of relatively limited local extent on the scale of metres; they often occur in
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