Civil Engineering Reference
In-Depth Information
geomechanical logging of drill cores, and a test-
ing program involving point load index testing of
core, and direct shear testing of selected discon-
tinuities. In addition, six geotechnical coreholes
were drilled to obtain oriented core. Piezometers
were targeted for various holes throughout the
property to monitor ground water levels and
obtain an indication of potential pit dewatering
requirements.
NW trending fault
Approximate
outline of
intrusive
complex
II
I
III
IV
VI
Structural
domain
boundary
V
15.2.1 Design issues
The proposed pit would have a modest overall
depth of 250 m, and would be excavated in a com-
petent rock mass with a consistent, pervasive set
of joints and faults related to the genesis of the
deposit. Open pit slope design was expected to be
controlled by the stability of individual benches,
and the need to optimize bench geometry to min-
imize waste stripping. Due to the combination of
moderate overall slope height and a competent
rock mass, inter-ramp and overall slope stability
were not significant concerns.
Figure 15.1
Distribution of structural domains.
system that deposited the ore. The strike of the
tangential set (Set 2) was approximately normal
to Set 1 and dipped at 45-60
◦
towards the cen-
ter. Set 2 was probably formed during collapse
of the hydrothermal system. Peak orientations of
these two principal sets varied depending on their
position in relation to the intrusive center.
Based on the distribution of discontinuity ori-
entations, the deposit was divided into six struc-
tural domains distributed radially around the
deposit, as illustrated in Figure 15.1. Within
each structural domain the geologic structural
fabric was expected to be reasonably consistent.
Figure 15.2 is a stereonet that shows the distribu-
tion of discontinuities in Structural Domain I.
Regionally, northwest trending sub-vertical
faults were present throughout the area. In partic-
ular, a large fault zone with a width of about 10 m
was interpreted to intersect the northeast corner
of the proposed pit.
15.2.2 Engineering geology
The porphyritic intrusion was dacitic in compo-
sition, hosted by tertiary andesites and andesite
breccias, and was hydrothermally altered with
a distinctive alteration zonation ranging from
potassic to phyllic to propylitic. In terms of
rock mass competency, the potassic altera-
tion increased the overall competency of the
rock, whereas the phyllic alteration significantly
weakened the rock and reduced discontinuity
shear strength.
Propylitic alteration appeared
to
have
had
little
influence
on
overall
rock
15.2.3 Rock strength and competency
competency.
Results of the structural mapping and core
orientation indicated a pattern of radial and
tangential jointing and faulting that appeared
to be centered around the intrusive core. The
radial joint set (Set 1) dipped sub-vertically and
with a strike approximately radial to the cen-
ter of the intrusive complex. These structures
were probably related to the original intrusion
and facilitated development of the hydrothermal
Field estimates of hardness (ISRM, 1981b)
obtained during geomechanical logging of the
drill core were correlated with point load index
results. Both of these measures of rock strength
indicated a moderately hard rock mass, with
unconfined compressive strengths (UCS) ranging
from about 40 to 100 MPa. Local zones of
phyllic alteration had an average UCS as low as
about 5 MPa.
