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10: EVALUATION TECHNIQUES 245

or in situ. Subjective data helps with the 10.5.1 Quantitative assessment of rock
decision making fairly early in a project, but
these data will always need quantification. It There are numerous good textbooks which
is therefore necessary to study and record the cover the properties of rocks from an engineer-
direction and properties of joints, cleavage, ing point of view (Krynine & Judd 1957,
cleats, bedding fissures, and faults, and to study Goodman 1976, 1989, Farmer 1983, Brady &
and record the mechanical properties (mechan- Brown 1985), the latter reference being particu-
ics of deformation and fracture under load), larly relevant to underground mining. The
petrology, and fabric of the rock between the main properties which are used in the engineer-
discontinuities. ing classification of rocks for their quantitative
It is clear from the above that structural assessment are porosity, permeability, specific
information is extremely important in mine gravity (bulk density), durability and slakabil-
planning. In the initial phases of a program ity, sonic velocity, and rock strength.
most of the structural data come from core. Porosity and permeability are important in
The geologist can log fracture spacing, attitude, the assessment of subsurface water. Specific
and fracture infill at the rig site. A subjective gravity is required to determine the mass of
description of the quality of the core can be the rock. Durability and slakability tests reveal
given using Rock Quality Designation (RQD) what affect alternate wetting and drying will
(Table 10.8). Pieces of core greater than 10 cm have on surface or near-surface exposures.
are measured and their length summed. This is Under conditions of seasonal humidity some
divided by the total length of the core run and rocks have been known to disaggregate com-
expressed as a percentage. A low percentage pletely (Obert & Duval 1967). Sonic velocity
means a poor rock while a high percentage tests give an indication of the fracture intensity
means a good quality rock. of the rock. Probably the most important prop-
Bieniawski (1976) developed a geomechan- erty of rock is its strength.
ical classification scheme using five criteria:
strength of the intact rock, RQD, joint spacing, Rock strength
conditions of the joints, and ground water con-
ditions. He assigned a rating value to each of The fundamental parameters that define rock
these variables, and by summing the values strength are stress and strain, and the rela-
of the ratings determined for the individual tionship between stress and strain. Stress (σ)
properties he obtained an overall rock mass is a force acting on a unit area. It may be hydro-
rating (RMR). Determination of the RMR of an static when the force is equal in all direc-
unsupported excavation, for example, can be tions, compressional when the force is directed
used to determine the stand up time of that towards a plane, tensional when directed away
excavation. from a plane, torsional (twisting), or shear
In order to provide information that will stress (τ) when the forces are directed towards
assist the geotechnical and mining engineers it each other but not necessarily in line. Strain
is necessary to understand the stress–strain (ε) is the response of a material to stress by pro-
characteristics of the rocks. This is especially ducing a deformation, i.e. a change in shape,
important in relation to the mining method length, or volume. The linear relationship
that is proposed for the rocks, the way in which between stress and strain is known as Hooke’s
the rocks will respond in the long term to sup- law and the constant (E) connecting them is
port (in underground mining), or in deep open Young’s modulus. It is also called the modulus
pits how deformation characteristics affect the of elasticity, and gives a measure of stiffness.
stability of a slope. As these rock properties The strength of a rock sample can be tested
become better understood, it is possible for the (usually in the laboratory) in several ways.
geotechnical and mining engineers to improve Unconfined (uniaxial) compressive strength
the design of underground mines, the way in is measured when a cylinder (usually core) of
which excavations are supported, the effici- rock is loaded to a point of failure. The load im-
ency of mining machinery, and the safety of the mediately prior to failure indicates the rock’s
mine environment. strength. Tensile strength is measured by