3
that had occurred, there was potential for minor to mod-
erate faults to be present with the formations. In the sev-
eral years since surface mining was completed, there had
not been deformation in this area of the highwall which
would have exhibited deterioration along such secondary
structures.
Fortunately, these conditions are different from the
geologic conditions that were observed in the north end
of the open pit and associated with bounding the on-
going slope failures. The west wall failure had been devel-
oping along fault and formation contacts. The contacts
between the Rodeo Creek, Popovich, Roberts Mountains
Formations are in proximity, with an overall pitward dip-
ping orientation with respect to the highwall. Infiltration
from precipitation and surface run-off over time, acceler-
ated deterioration along structures and contacts. The weak-
ening resulted in the instability that began to develop in
December 2015. This area is shown in Figure 3.
Similarly, the local failure above the existing por-
tals was occurring along a modeled fault structure that is
also the contact between the Rodeo Creek and Popovich
Formations. The projection of the fault/contact in the high-
wall was near vertical above the northern portal. Failure
along this structure did begin during surface mining activ-
ity of the final phase. Over the past several years, the existing
failure and weak material along the contact has continued
to deteriorate particularly when there have been winters or
springs with above average precipitation. The area is indi-
cated in Figure 3.
HIGHWALL CONDITION
A visual inspection of the highwall above the proposed por-
tal location was conducted to identify if any major local
instabilities had or were developing. Pictures are included as
Figures 4, 5, and 6. The upper two benches do exhibit more
substantial deterioration and weathering (Figure 4). This
was observed in the Vinnini and Rodeo Creek Formations
as surface mining was developing each phase of the open
pit. There were some localized failures that were the result
of weak material not able to remain stable when excavated
at 65-degree bench face angles however, these had been
contained on the existing catch benches.
The remainder of the of the open pit slopes had per-
formed very well, considering the local geological and
geotechnical conditions exposed in the instabilities. There
is, of course, minor ravel and sloughed material that had
accumulated at the toe of each bench face however, when
examining photographs of the same area taken in July
2010 (see Figure 7) and January 2011(see Figure 8) it does
appear that most of the material accumulated either during
Figure 2. Geologic cross-section through the proposed
portal location. There were no major faults according to
the geology model geologic formation contacts are dipping
away from the pit toward the northeast.
Figure 3. Geologic map of the northern end of the open pit.
The observed slope failures were controlled by both geologic
formation contacts and known faults.
Figure 4. View of the highwall above the proposed portal
location. The catch benches contain material along each
bench toe however, there are no signs of slope movement.
that had occurred, there was potential for minor to mod-
erate faults to be present with the formations. In the sev-
eral years since surface mining was completed, there had
not been deformation in this area of the highwall which
would have exhibited deterioration along such secondary
structures.
Fortunately, these conditions are different from the
geologic conditions that were observed in the north end
of the open pit and associated with bounding the on-
going slope failures. The west wall failure had been devel-
oping along fault and formation contacts. The contacts
between the Rodeo Creek, Popovich, Roberts Mountains
Formations are in proximity, with an overall pitward dip-
ping orientation with respect to the highwall. Infiltration
from precipitation and surface run-off over time, acceler-
ated deterioration along structures and contacts. The weak-
ening resulted in the instability that began to develop in
December 2015. This area is shown in Figure 3.
Similarly, the local failure above the existing por-
tals was occurring along a modeled fault structure that is
also the contact between the Rodeo Creek and Popovich
Formations. The projection of the fault/contact in the high-
wall was near vertical above the northern portal. Failure
along this structure did begin during surface mining activ-
ity of the final phase. Over the past several years, the existing
failure and weak material along the contact has continued
to deteriorate particularly when there have been winters or
springs with above average precipitation. The area is indi-
cated in Figure 3.
HIGHWALL CONDITION
A visual inspection of the highwall above the proposed por-
tal location was conducted to identify if any major local
instabilities had or were developing. Pictures are included as
Figures 4, 5, and 6. The upper two benches do exhibit more
substantial deterioration and weathering (Figure 4). This
was observed in the Vinnini and Rodeo Creek Formations
as surface mining was developing each phase of the open
pit. There were some localized failures that were the result
of weak material not able to remain stable when excavated
at 65-degree bench face angles however, these had been
contained on the existing catch benches.
The remainder of the of the open pit slopes had per-
formed very well, considering the local geological and
geotechnical conditions exposed in the instabilities. There
is, of course, minor ravel and sloughed material that had
accumulated at the toe of each bench face however, when
examining photographs of the same area taken in July
2010 (see Figure 7) and January 2011(see Figure 8) it does
appear that most of the material accumulated either during
Figure 2. Geologic cross-section through the proposed
portal location. There were no major faults according to
the geology model geologic formation contacts are dipping
away from the pit toward the northeast.
Figure 3. Geologic map of the northern end of the open pit.
The observed slope failures were controlled by both geologic
formation contacts and known faults.
Figure 4. View of the highwall above the proposed portal
location. The catch benches contain material along each
bench toe however, there are no signs of slope movement.