Page 64 - Rock Mechanics For Underground Mining

3 Rock mass structure and

characterisation

3.1 Introduction

Rock differs from most other engineering materials in that it contains fractures of
one type or another which render its structure discontinuous. Thus a clear distinction
must be made between the rock element or rock material on the one hand and the
rock mass on the other. Rock material is the term used to describe the intact rock
between discontinuities; it might be represented by a hand specimen or piece of drill
core examined in the laboratory. The rock mass is the total in situ medium containing
bedding planes, faults, joints, folds and other structural features. Rock masses are
discontinuous and often have heterogeneous and anisotropic engineering properties.
The nature and distribution of structural features within the rock mass is known
as the rock structure. Obviously, rock structure can have a dominant effect on the
response of a rock mass to mining operations. It can influence the choice of a mining
method and the design of mining layouts because it can control stable excavation
spans, support requirements, subsidence, cavability and fragmentation characteris-
tics. At shallow depths and in de-stressed areas, structurally controlled failures may
be the prime concern in excavation design (Figure 3.1). At depth and in areas of

Figure 3.1 Sidewall failure in a
mine haulage aligned parallel to the
line of intersection of two major dis-
continuities (photograph by E. Hoek).

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