2
followed by uplift, oxidation, erosion, and mineral enrich-
ment (Yong, et al 2022).
Faulting is significant at the mine and generally delin-
eates the geological and geomechanical domains of the pit.
The fault zones are characterized by extremely fractured
and/or altered rock that is often clay-rich with RQD val-
ues near zero. Three main joint sets can be identified from
investigations and generally include a sub-horizontal set
and two dipping (25-degree and 55-degree) towards the
east (Yong, et al, 2022).
The rock mass domains of the pit are comprised of six
main lithologies and two alteration grades. The six units
include porphyry (intrusive), limestone, sandstone, shale,
rhyolite (volcanic), and fault zones. Significant degrada-
tion of the rock mass quality and strength are attributed
to phyllic, argillic, potassic, propylitic, and gossan altera-
tion or Grade (i), with examples shown in Figure 1. The
second alteration Grade (ii) has little adverse impact on the
rock mass quality or strength and represents unaltered rock.
Figure 2 illustrates the alteration grades and known faults
in the 2016 open pit (Yong, et al 2022).
Numerous slope instabilities at various locations in
the open pits have occurred because the pit walls were
comprised of highly altered and fragmented weak rock
mass of the Monzonite Porphyry, Rib Hill Sandstone, and
Chainman Shale units.
The pit has high pore water pressures located in the
northern, eastern, and southern high walls, although the
highest pore water pressures are seen in the lower walls of
the pit.
The northern wall is comprised of low permeability
material, including faults, shales, and bedding planes. Seeps
have been observed but were generally located on the lower
benches of the pit. It was observed that pore water pressures
lagged significantly behind the pit dewatering activities.
The southern and eastern walls were also comprised
of low permeability materials, although to a lesser degree
than the northern wall. This is likely due to less faulting,
and fewer adverse bedding planes located in the high-wall.
Additionally, a historic canyon existed behind the southern
and eastern wall but had been backfilled with dump mate-
rial. However, this geological feature is likely still funneling
meteoric and groundwater towards the pit.
Aggressive pumping of the standing water in the pit,
horizontal drains, and diverting surface water run-off were
utilized to manage and reduce the impact of water in the pit.
MINING SEQUENCE AND SLOPE
PERFORMANCE
The mining company started developing the pit in early
2012. The northern wall was originally developed with an
inter-ramp angle of approximately 37 degrees. The Factor
of Safety (FOS) for the high wall was considered marginally
stable at about 1.1–1.2. This was expected to drop if the wall
could not be successfully depressurized from groundwater.
By late 2012, tension cracks were observed in a haul
road located directly above the north wall. A dewatering
well also had to be abandoned in the area due to the ongo-
ing slope deformation. Slope monitoring radar registered
that the northern wall had an average velocity of about
2 inches per day.
Mining continued in this pit throughout 2013 and
2014. The northern wall continued to degrade, with the
Figure 1. Visual comparison of the different alteration grades
on core samples (Yong, et al, 2022).
Figure 2. Map of the entire pit indicating the two different
alteration grades (Yong, et al, 2022).
followed by uplift, oxidation, erosion, and mineral enrich-
ment (Yong, et al 2022).
Faulting is significant at the mine and generally delin-
eates the geological and geomechanical domains of the pit.
The fault zones are characterized by extremely fractured
and/or altered rock that is often clay-rich with RQD val-
ues near zero. Three main joint sets can be identified from
investigations and generally include a sub-horizontal set
and two dipping (25-degree and 55-degree) towards the
east (Yong, et al, 2022).
The rock mass domains of the pit are comprised of six
main lithologies and two alteration grades. The six units
include porphyry (intrusive), limestone, sandstone, shale,
rhyolite (volcanic), and fault zones. Significant degrada-
tion of the rock mass quality and strength are attributed
to phyllic, argillic, potassic, propylitic, and gossan altera-
tion or Grade (i), with examples shown in Figure 1. The
second alteration Grade (ii) has little adverse impact on the
rock mass quality or strength and represents unaltered rock.
Figure 2 illustrates the alteration grades and known faults
in the 2016 open pit (Yong, et al 2022).
Numerous slope instabilities at various locations in
the open pits have occurred because the pit walls were
comprised of highly altered and fragmented weak rock
mass of the Monzonite Porphyry, Rib Hill Sandstone, and
Chainman Shale units.
The pit has high pore water pressures located in the
northern, eastern, and southern high walls, although the
highest pore water pressures are seen in the lower walls of
the pit.
The northern wall is comprised of low permeability
material, including faults, shales, and bedding planes. Seeps
have been observed but were generally located on the lower
benches of the pit. It was observed that pore water pressures
lagged significantly behind the pit dewatering activities.
The southern and eastern walls were also comprised
of low permeability materials, although to a lesser degree
than the northern wall. This is likely due to less faulting,
and fewer adverse bedding planes located in the high-wall.
Additionally, a historic canyon existed behind the southern
and eastern wall but had been backfilled with dump mate-
rial. However, this geological feature is likely still funneling
meteoric and groundwater towards the pit.
Aggressive pumping of the standing water in the pit,
horizontal drains, and diverting surface water run-off were
utilized to manage and reduce the impact of water in the pit.
MINING SEQUENCE AND SLOPE
PERFORMANCE
The mining company started developing the pit in early
2012. The northern wall was originally developed with an
inter-ramp angle of approximately 37 degrees. The Factor
of Safety (FOS) for the high wall was considered marginally
stable at about 1.1–1.2. This was expected to drop if the wall
could not be successfully depressurized from groundwater.
By late 2012, tension cracks were observed in a haul
road located directly above the north wall. A dewatering
well also had to be abandoned in the area due to the ongo-
ing slope deformation. Slope monitoring radar registered
that the northern wall had an average velocity of about
2 inches per day.
Mining continued in this pit throughout 2013 and
2014. The northern wall continued to degrade, with the
Figure 1. Visual comparison of the different alteration grades
on core samples (Yong, et al, 2022).
Figure 2. Map of the entire pit indicating the two different
alteration grades (Yong, et al, 2022).