Page 265 - Introduction to Mineral Exploration

248 M.K.G. WHATELEY & B. SCOTT

Drilling rig using a suitable epoxy resin. Overcoring with a
core barrel of larger size causes stress relief in
the vicinity of the strain cell which induces
changes on the strains registered in the gauges
(Brady & Brown 1985). These measurements
are used to help in mine design.
Additional tests are carried out during min-
ing using various types of strain gauges. Some
Borehole mines, e.g. Kidd Creek, have their own rock
mechanics, backfill, and mine research depart-
ments whose rock mechanics instruments are
regarded as an indispensable tool for detecting
and predicting ground instability (Thiann 1983,
Potential Hannington & Barrie 1999). Hoek and Brown
mineable (1980) and Hoek and Bray (1977) give excellent
deposit examples of the use of rock mechanics in both
underground and surface mining respectively.

10.5.2 Hydrogeology
End of
borehole During an exploration program it is important
to note where the water table is encountered in
a borehole because if one were to mine this
deposit later, then it is essential to know where
the water is, how much water there is, and the
water pressure throughout the proposed mine.
Useful tests that can be performed on boreholes
Strain cell
include measuring the draw down of the water
level by pumping, and upon ceasing pumping
measuring the rate of recovery. Packer tests
in boreholes are used to obtain a quantitative
estimate of the contribution a particular bed
or joint may make to the water inflow at a site
(Price 1985). A good description of the basic
fieldwork necessary to understand the hydro-
geology of an area is given by Brassington
(1988). Two properties of the rocks that can be
measured are the porosity and the permeabil-
Overcoring ity. Porosity describes how much water a rock
can hold in the voids between grains and in
joints, etc. Permeability is a measure of how
easily that water can flow through and out of
Strain cell
the rock. The flow of water is described empir-
ically by Darcy’s law, and the relative ease of
the flow is called the hydraulic conductivity.
Darcy’s law is a simple formula:

v = Ki
FIG. 10.29 A diagrammatic representation of the where v = the specific discharge or the velo-
principle of overcoring.
city of laminar flow of water through a porous
medium (the critical Reynold’s number at