5
Two model scenarios were evaluated. The first evalu-
ated the condition of the open pit slope in its current
condition at the time of the assessment these results are
depicted in Figure 9. The second analysis was conducted
with a 3-ft barrier (berm or fence) along the crest of the
catch bench immediately above the proposed portal. This
allowed a comparison of the estimated rockfall events that
could be retained above the portal and surrounding area,
which is shown in Figure 10, with the results of the current
condition analysis.
The results of the analyses indicated that:
• A total of 14 rockfall events could reach the floor/toe
of the open pit slope (14/500*100% =2.8%) for the
existing conditions.
– Most of these events occurred at the southern
extend of the proposed portal area, where back
break during active surface mining reduced athe
catch bench width and allowable containment.
• A small barrier (i.e., berm) constructed along the
bench above the portal mitigated all modeled rockfall
events from reaching the floor/toe of the highwall.
It is important to note that while the Trajec3D soft-
ware, at the time of this study, indicated a percentage of
rockfall events that would or would not be contained prior
to reaching the toe of a highwall, variations in the size and
number of rocks, and variations in field conditions beyond
the assumptions incorporated into the model result in only
an approximation of expected outcomes. Nevertheless, the
analysis provided a guide to understand the potential risk
to personnel working in the area around a portal. In any
scenario, vigilance by geotechnical staff to regularly inspect
and evaluate work areas is always required to identify
changing conditions.
The example scenario below briefly explains the prob-
ability of a rockfall passing all the catch benches and caus-
ing a harmful event:
As there will be human exposure and that the software
cannot predict all possible field conditions or scenarios,
even for models that predict all rockfall events would be
contained should not assume a 0.00% probability.
DISCUSSION OF FINDINGS
This operational geotechnical review did not identify
critical geotechnical issues of concern with the proposed
portal location. The geologic contacts are at a more favor-
able orientation with the geometry of the open pit slope.
Furthermore, there are no known major structures visually
observed in the open pit slope nor within the geology model.
The open pit slope, in this area anyway, had performed
very well for the past several years. This is an example when
collecting regular photographs, during visual inspections,
of normal conditions are beneficial. Geotechnical staff of
course collect photographs of geohazards, slope instabilities,
and slope failures as they develop. But the normal condition
photographs can be used internally to verify no significant
Figure 9. Three dimensional rockfall analysis conducted on
the existing highwall geometry
Figure 10. Three dimensional rockfall analysis conducted on
the current highwall geometry with a 3-ft barrier along the
crest
Two model scenarios were evaluated. The first evalu-
ated the condition of the open pit slope in its current
condition at the time of the assessment these results are
depicted in Figure 9. The second analysis was conducted
with a 3-ft barrier (berm or fence) along the crest of the
catch bench immediately above the proposed portal. This
allowed a comparison of the estimated rockfall events that
could be retained above the portal and surrounding area,
which is shown in Figure 10, with the results of the current
condition analysis.
The results of the analyses indicated that:
• A total of 14 rockfall events could reach the floor/toe
of the open pit slope (14/500*100% =2.8%) for the
existing conditions.
– Most of these events occurred at the southern
extend of the proposed portal area, where back
break during active surface mining reduced athe
catch bench width and allowable containment.
• A small barrier (i.e., berm) constructed along the
bench above the portal mitigated all modeled rockfall
events from reaching the floor/toe of the highwall.
It is important to note that while the Trajec3D soft-
ware, at the time of this study, indicated a percentage of
rockfall events that would or would not be contained prior
to reaching the toe of a highwall, variations in the size and
number of rocks, and variations in field conditions beyond
the assumptions incorporated into the model result in only
an approximation of expected outcomes. Nevertheless, the
analysis provided a guide to understand the potential risk
to personnel working in the area around a portal. In any
scenario, vigilance by geotechnical staff to regularly inspect
and evaluate work areas is always required to identify
changing conditions.
The example scenario below briefly explains the prob-
ability of a rockfall passing all the catch benches and caus-
ing a harmful event:
As there will be human exposure and that the software
cannot predict all possible field conditions or scenarios,
even for models that predict all rockfall events would be
contained should not assume a 0.00% probability.
DISCUSSION OF FINDINGS
This operational geotechnical review did not identify
critical geotechnical issues of concern with the proposed
portal location. The geologic contacts are at a more favor-
able orientation with the geometry of the open pit slope.
Furthermore, there are no known major structures visually
observed in the open pit slope nor within the geology model.
The open pit slope, in this area anyway, had performed
very well for the past several years. This is an example when
collecting regular photographs, during visual inspections,
of normal conditions are beneficial. Geotechnical staff of
course collect photographs of geohazards, slope instabilities,
and slope failures as they develop. But the normal condition
photographs can be used internally to verify no significant
Figure 9. Three dimensional rockfall analysis conducted on
the existing highwall geometry
Figure 10. Three dimensional rockfall analysis conducted on
the current highwall geometry with a 3-ft barrier along the
crest