Civil Engineering Reference
In-Depth Information
Mining applications
Alan F. Stewart, P. Mark Hawley, Nick D. Rose
and Brent W. Gilmore
∗
15.1 Introduction
Rock slope engineering of open pit mines requires
careful application and adaptation of the full
range of tools that have been presented in earlier
chapters of this topic. Each ore body and host
rock mass is unique, and comprises distinct-
ive mineralogical assemblages and rock types.
In many instances, stratigraphy may be com-
plexly deformed by geologic forces. Geologic
and geomechanical characteristics, such as litho-
logy, mineralogy, alteration, rock strength,
in situ
stress, geologic structure and fabric, and ground
water conditions may vary widely between differ-
ent deposits, and even within a given deposit. The
challenge for the slope designer is first to deter-
mine which of these characteristics are important
in terms of stability. The next step is to plan
and execute focused investigations to obtain the
information required to define the key stability
parameters. Stability analyses are then conducted,
and results are used in conjunction with experi-
ence and judgment to develop slope design criteria
for use by mine planners and operators.
In open pit mining, the optimum slope design
is usually one that maximizes overall slope angles
and minimizes the amount of waste stripping.
At the same time, it must effectively manage the
risk of overall slope instability, and provide for
safe and efficient movement of personnel, equip-
ment and materials during mining operations.
The general methodology for designing open pit
mine slopes is described in this chapter by way of
four hypothetical examples. These examples rep-
resent a range of mine design and rock mechanics
issues in a variety of geologic environments.
Most open pit mines are developed using
benches that are designed to contain and control
rock falls and small failures. The geometry of the
pit and slopes is defined by the shape of the ore
body, the height and width of the benches, and the
locations of haul roads and stepouts; Figure 1.5
illustrates a typical pit slope geometry. As dis-
cussed in the following examples, inter-ramp
slopes are defined as slope sections comprised of
multiple benches between haul roads or stepouts.
Haul roads are necessary to provide access to the
ore and waste, and stepouts may be required for
reasons of stability or to accommodate the shape
of the ore body. Overall slopes incorporate inter-
ramp slopes as well as haul roads and stepouts,
and extend from the crest to the toe of the pit wall.
15.2 Example 1—porphyry deposits
This example describes a preliminary slope design
investigation conducted as part of a feasibility
study for a new porphyry copper deposit. Preli-
minary mine plans indicated a maximum open pit
depth of 250 m. No mining activity had occurred
in the deposit, and no previous design stud-
ies had been conducted, other than exploration
drilling, mapping and sampling related to ore
reserve definition.
A geotechnical investigation program was
conducted that incorporated site reconnais-
sance, structural mapping of available outcrops,
∗
Piteau Associates Engineering Ltd, North Vancouver, BC,
Canada.
