2
the measured movement rates or visual observations, the
haul road could either only be used with spotters in place
or access closed altogether. A secondary, light vehicle access
was developed to ensure alternative, safe access in and out
of the open pit for personnel. A plan was developed to
establish a new primary access to the west of the existing
haul road and slope instability however, when slope move-
ment rates expectedly decreased during the Summer and
Fall of 2016 (i.e., drier weather conditions) mining the new
access was delayed.
Concurrent to the development of the west pit slope
failure that impacted the haul road, a local, previous failure
mass that developed within a fault zone exposed in the east
highwall also began displacing. This failure mass began fill-
ing the catch bench above the portal and eventually spilling
onto the pit floor. This led to portal extensions being con-
structed to minimize the exposure to employees.
During the winter of 2016–17, the region again expe-
rienced increased precipitation compared to the previous
several years. The resulting snow melt, rain, and run-off
infiltrated the failure inducing increased pore pressure.
Which in turn destabilized the area and caused the slope
movement rates to significantly increase. This increased
slope movement again resulted in intermittent closures of
the haul road following the SOP and TARP requirements.
A large increase during a rapid snow melt and rain event
in early February 2017 pre-emptively required the closure
and withdrawal of personnel from the underground opera-
tion due to the uncertainty of the stability of the access
road, and the potential for any slide run-out to impact the
portals.
During the middle of February, following the closer
of underground operation, mining activity associated with
establishing a new primary access has been accelerated by
reallocating surface mining equipment to the project. Also,
the underground mine engineers developed a plan for a new
portal location to be established to the south of the cur-
rent slope failures. With the shutdown of the underground
operation, there was urgency in evaluating all aspects of this
project in a timely manner, to identify and mitigate any
additional potential risks to begin executing on construc-
tion of the new portal. One of the obvious potential risks
being continued or new geotechnical slope instability in
other areas of the pit.
An internal geotechnical review of the highwall above
the proposed location(s) for the new portal was requested
and conducted in an accelerated timeframe in support capi-
tal funding request for completing the project. The limita-
tions of the assessment defined for the project team were
that there would be no new subsurface investigation (i.e.,
drilling) and there would be no external technical resources
(i.e., consultants) due to the required timeline. The evalua-
tion did include a review of the local geology, an assessment
of the current highwall condition and previously observed
performance during surface mining, and a rockfall analy-
sis to evaluate existing catchment above the new portal
location. The remainder of this paper briefly discusses the
aspects for the highwall review.
SITE GEOLOGY
Based on the current surface geology model and mapping
information, this area of highwall is primarily composed of:
Rodeo Creek Formation, Popovich Formation, and Roberts
Mountains Formation. The bedding and formation con-
tacts tend to dip toward the northeast around as observed
from exposed pit slopes. Since, the proposed portal location
would be located in an area of highwall that trends north-
west to southeast, the formation contacts are dipping back
into the highwall. Typically, this combination of pit slope
and structural intersections lead to a more stable configura-
tion (see Figures 2).
At the time of the assessment, there were no major fault
structures modeled in this proposed portal location high-
wall. Inherently, with the basin and range tectonic activity
Figure 1. Map of the open pit with the on-going slope
failures identified in red. The current portal locations are
indicated, as is the location of the proposed new portal.
the measured movement rates or visual observations, the
haul road could either only be used with spotters in place
or access closed altogether. A secondary, light vehicle access
was developed to ensure alternative, safe access in and out
of the open pit for personnel. A plan was developed to
establish a new primary access to the west of the existing
haul road and slope instability however, when slope move-
ment rates expectedly decreased during the Summer and
Fall of 2016 (i.e., drier weather conditions) mining the new
access was delayed.
Concurrent to the development of the west pit slope
failure that impacted the haul road, a local, previous failure
mass that developed within a fault zone exposed in the east
highwall also began displacing. This failure mass began fill-
ing the catch bench above the portal and eventually spilling
onto the pit floor. This led to portal extensions being con-
structed to minimize the exposure to employees.
During the winter of 2016–17, the region again expe-
rienced increased precipitation compared to the previous
several years. The resulting snow melt, rain, and run-off
infiltrated the failure inducing increased pore pressure.
Which in turn destabilized the area and caused the slope
movement rates to significantly increase. This increased
slope movement again resulted in intermittent closures of
the haul road following the SOP and TARP requirements.
A large increase during a rapid snow melt and rain event
in early February 2017 pre-emptively required the closure
and withdrawal of personnel from the underground opera-
tion due to the uncertainty of the stability of the access
road, and the potential for any slide run-out to impact the
portals.
During the middle of February, following the closer
of underground operation, mining activity associated with
establishing a new primary access has been accelerated by
reallocating surface mining equipment to the project. Also,
the underground mine engineers developed a plan for a new
portal location to be established to the south of the cur-
rent slope failures. With the shutdown of the underground
operation, there was urgency in evaluating all aspects of this
project in a timely manner, to identify and mitigate any
additional potential risks to begin executing on construc-
tion of the new portal. One of the obvious potential risks
being continued or new geotechnical slope instability in
other areas of the pit.
An internal geotechnical review of the highwall above
the proposed location(s) for the new portal was requested
and conducted in an accelerated timeframe in support capi-
tal funding request for completing the project. The limita-
tions of the assessment defined for the project team were
that there would be no new subsurface investigation (i.e.,
drilling) and there would be no external technical resources
(i.e., consultants) due to the required timeline. The evalua-
tion did include a review of the local geology, an assessment
of the current highwall condition and previously observed
performance during surface mining, and a rockfall analy-
sis to evaluate existing catchment above the new portal
location. The remainder of this paper briefly discusses the
aspects for the highwall review.
SITE GEOLOGY
Based on the current surface geology model and mapping
information, this area of highwall is primarily composed of:
Rodeo Creek Formation, Popovich Formation, and Roberts
Mountains Formation. The bedding and formation con-
tacts tend to dip toward the northeast around as observed
from exposed pit slopes. Since, the proposed portal location
would be located in an area of highwall that trends north-
west to southeast, the formation contacts are dipping back
into the highwall. Typically, this combination of pit slope
and structural intersections lead to a more stable configura-
tion (see Figures 2).
At the time of the assessment, there were no major fault
structures modeled in this proposed portal location high-
wall. Inherently, with the basin and range tectonic activity
Figure 1. Map of the open pit with the on-going slope
failures identified in red. The current portal locations are
indicated, as is the location of the proposed new portal.