1
24-005
A Surface Geotechnical In-Pit Underground Portal Relocation:
An Operations-Based Case Study
Russell J. Sheets
Barr Engineering Co., Bismarck, ND, USA
ABSTRACT
Reactivation of highwall slope failures during adverse
weather had intermittently impacted haul road access into
a previously mined open pit. These impacts ranged from
limited access to full closures however, the greater impact
was to an underground mine that was developed within
the open pit. Following several significant precipitation
events, a rapid snow melts greatly accelerated movements
finally closing the haul road and destabilizing other past
failures. In addition to re-designing the overall pit access,
the decision was made to relocate the primary portal to
a more stable highwall. An accelerated surface geotechni-
cal review was required to advance the portal relocation
however, only existing information from past investiga-
tion, existing geology model, visual inspections, and slope
monitoring data were available for the assessment -no new
investigations or data collection could be completed within
the allowable timeframe. The evaluation included a review
of the geology, an assessment of the current highwall condi-
tion and previously observed performance, and a rockfall
analysis to evaluate existing catchment above the proposed
new portal location.
BACKGROUND
The mine operation discussed in this operational case study
is in northern Nevada, USA. Although gold mineralization
associated with this deposit was identified in the 1960s,
surface mining did not begin until 2004. The open pit was
mined in 5 separate, adjacent phases from 2004 until the
completion of the final layback in late 2010. Immediately
following completion of surface mining, development
for an underground portal operation began in December
2010 in the northern end of the pit floor.
The most significant open pit slope instability occurred
during the second layback, or phase, which encompassed
the southernmost portion open pit slope. Active mining of
this phase occurred between 2005 and 2007. The southern
highwall was established within a structural zone bound
by two defined, typical basin and range faults. In 2006,
the region experienced above average spring precipitation
which adversely affected highwall stability. The natural
topography and run-off essentially drained toward the open
pit slope. This highwall proved to be sensitive to rainfall,
snow melt, and run-off in the years following mining activ-
ity. The south pit slope exhibited continued slope failure
after underground operation commenced, during wetter
than normal Spring months. These internal failures would
impact a laydown yard that had been developed below the
pit slope. Remediation activities generally required reloca-
tion of materials and equipment, cleaning back of failure
material, and building larger containment berms.
Throughout mining of subsequent phases, the pit slope
generally performed very well no significant slope failures
that impacted mining activity were observed. There were
some local bench-scale failures in the upper most benches
which were established in weathered Vinnini Fm. or Rodeo
Creek Fm. that were managed with clean-up or contained
on existing benches.
However, in December 2015, slope instability began to
develop in the west highwall of the final open pit phase (see
Figure 1). A slope failure of the lower extent of the highwall
impacted the use of the batch plant. Crest displacement
included the pit side outer portion of the haul road/access
road into the pit.
The slope movement impacted the available width
of the haul road. Access was narrowed to signal lane by
berming the outer edge. Furthermore, the observational
geotechnical risk mitigation approach outlined in a site
specific, standard operating procedure (SOP) and trigger
action response plan (TARP) specified that depending on
24-005
A Surface Geotechnical In-Pit Underground Portal Relocation:
An Operations-Based Case Study
Russell J. Sheets
Barr Engineering Co., Bismarck, ND, USA
ABSTRACT
Reactivation of highwall slope failures during adverse
weather had intermittently impacted haul road access into
a previously mined open pit. These impacts ranged from
limited access to full closures however, the greater impact
was to an underground mine that was developed within
the open pit. Following several significant precipitation
events, a rapid snow melts greatly accelerated movements
finally closing the haul road and destabilizing other past
failures. In addition to re-designing the overall pit access,
the decision was made to relocate the primary portal to
a more stable highwall. An accelerated surface geotechni-
cal review was required to advance the portal relocation
however, only existing information from past investiga-
tion, existing geology model, visual inspections, and slope
monitoring data were available for the assessment -no new
investigations or data collection could be completed within
the allowable timeframe. The evaluation included a review
of the geology, an assessment of the current highwall condi-
tion and previously observed performance, and a rockfall
analysis to evaluate existing catchment above the proposed
new portal location.
BACKGROUND
The mine operation discussed in this operational case study
is in northern Nevada, USA. Although gold mineralization
associated with this deposit was identified in the 1960s,
surface mining did not begin until 2004. The open pit was
mined in 5 separate, adjacent phases from 2004 until the
completion of the final layback in late 2010. Immediately
following completion of surface mining, development
for an underground portal operation began in December
2010 in the northern end of the pit floor.
The most significant open pit slope instability occurred
during the second layback, or phase, which encompassed
the southernmost portion open pit slope. Active mining of
this phase occurred between 2005 and 2007. The southern
highwall was established within a structural zone bound
by two defined, typical basin and range faults. In 2006,
the region experienced above average spring precipitation
which adversely affected highwall stability. The natural
topography and run-off essentially drained toward the open
pit slope. This highwall proved to be sensitive to rainfall,
snow melt, and run-off in the years following mining activ-
ity. The south pit slope exhibited continued slope failure
after underground operation commenced, during wetter
than normal Spring months. These internal failures would
impact a laydown yard that had been developed below the
pit slope. Remediation activities generally required reloca-
tion of materials and equipment, cleaning back of failure
material, and building larger containment berms.
Throughout mining of subsequent phases, the pit slope
generally performed very well no significant slope failures
that impacted mining activity were observed. There were
some local bench-scale failures in the upper most benches
which were established in weathered Vinnini Fm. or Rodeo
Creek Fm. that were managed with clean-up or contained
on existing benches.
However, in December 2015, slope instability began to
develop in the west highwall of the final open pit phase (see
Figure 1). A slope failure of the lower extent of the highwall
impacted the use of the batch plant. Crest displacement
included the pit side outer portion of the haul road/access
road into the pit.
The slope movement impacted the available width
of the haul road. Access was narrowed to signal lane by
berming the outer edge. Furthermore, the observational
geotechnical risk mitigation approach outlined in a site
specific, standard operating procedure (SOP) and trigger
action response plan (TARP) specified that depending on