Civil Engineering Reference
In-Depth Information
Site investigation and geological
data collection
3.1 Planning an investigation program
The design of rock cuts is often an iterative
process that proceeds from initial reconnaissance,
through preliminary and final design followed by
construction. This process involves progressively
collecting more detailed design data, specific to
the site conditions and needs of the project. Typ-
ically, the three stages of a complete investigation
are as follows:
and existing cuts to study structural geo-
logy, test pits to obtain information on
overburden thickness and properties, and
diamond drilling to investigate rock condi-
tions at depth. Components of the drilling
could include core orientation to obtain struc-
tural geology information, and installation of
piezometers to measure ground water levels,
and possibly measure permeability. Rock
strength testing could comprise laboratory
testing of drill core to determine the friction
angle of discontinuities, uniaxial compres-
sion strength tests and slake durability tests.
Figure 3.1 shows a diamond drilling program
in progress where an inclined hole and triple
tube core barrel are being used to investigate
targeted geological structure and collect high
quality core in closely fractured rock. Some
of the current (2003) requirements for drilling
and sampling programs may include collec-
tion of all circulation water (“zero discharge
drilling”), and complete restoration of the drill
site and access roads.
•
Reconnaissance
: examination of published
geological maps and reports, study of air pho-
tographs, gathering of local experience, field
visits to examine, if possible, the performance
of existing slopes in similar geological condi-
tions, and geophysics studies if outcrops are
limited.
•
Route selection/preliminary pit slope design
:
if the project involves the evaluation of alter-
native routes, limited investigations could be
carried out of each route comprising out-
crop mapping, geophysics to find overburden
thickness and index tests of rock properties.
For an open pit mine, there will usually be
considerable geological information on the
property generated during the exploration
program. This will often include mapping,
geophysics and drilling from which geotech-
nical data can be obtained. It is beneficial to
the design of the pit slopes if geotechnical data
can be collected as part of the exploration
program.
Because of the wide variety of both site conditions
and slope designs, it is not considered appro-
priate to draw up any rules on the types and
quantity of investigation programs. That is, every
investigation is unique. The only general rule that
applies to investigation for rock slope design is
that information is required on geology, rock
strength and ground water. These three sets of
design parameters are discussed in the following
sections.
•
Detailed investigations
: final design would
usually require detailed mapping of outcrops
