3
presence of the front cut, and the orientation and dip of
the geologic structure, along with possible strength reduc-
tion in the clay formation, is believed to have triggered
the failure. This type of failure is not typically expected at
this location given past operational experience and typical
expected strengths of highwall materials. It is hypothesized
that the failure would not have happened if the digging face
was not created in this specific location, but this is only rel-
evant to this specific instance when the joint alignment and
dip created a block from which the dragline was operating.
The geologic structure (jointing and faulting) observed in
the end wall after the slide had minimal offset but enough
to create a feature through the highwall materials and the
presence of slickensides in the core samples indicate the
possibility of strength reduction. The presence of slicken-
sides may be associated with deformation developed within
the clay at the time of the fault formation.
Based on the geologic age of the clay deposition, and
the location, typical tectonic activity was not the cause of
the fault. The proximate location of previous glacial ice
flows induced a local stress differential that “failed” the sub-
surface resulting in a local fault structure. An advancing
flow terminated within the present mine operation which
would have induced significant loading on the clay along
with subsequent release of load and rebound to impart the
observed structural deformation (see Figure 2).
To avoid future problems several recommendations
were made. These included regular inspection of the exposed
highwalls to look for any fault or joint structures that exist.
Doing so on progressive pits may allow the projection of
possible structures to future pits and pit progression can be
planned accordingly. In critical areas, as identified through
field observations noted above, a close inspection of the
highwall immediately adjacent to where the dragline will
operate should be made to look for any indication of faults
or joints.
Intersection of faults and joints with the highwall
slope can create kinematically unstable blocks. Once geo-
logic structures are mapped, the potential for blocks can be
reviewed. Kinematic instability would occur when a par-
ticular fault has a strike and dip which intersects the pit
highwall such that sliding toward the open pit or the exca-
vated front cut is kinematically feasible. Steeply dipping
beds in the highwall stratigraphy may also increase the risk
of block-type failures associated with faults. Typically, the
geologic bedding is relatively horizontal however, if steeply
dipping beds are encountered and are accompanied by sur-
rounding faults, a review of the area is recommended.
Figure 2. Diagram of glacial conditions near the dragline bench failure (red star)
presence of the front cut, and the orientation and dip of
the geologic structure, along with possible strength reduc-
tion in the clay formation, is believed to have triggered
the failure. This type of failure is not typically expected at
this location given past operational experience and typical
expected strengths of highwall materials. It is hypothesized
that the failure would not have happened if the digging face
was not created in this specific location, but this is only rel-
evant to this specific instance when the joint alignment and
dip created a block from which the dragline was operating.
The geologic structure (jointing and faulting) observed in
the end wall after the slide had minimal offset but enough
to create a feature through the highwall materials and the
presence of slickensides in the core samples indicate the
possibility of strength reduction. The presence of slicken-
sides may be associated with deformation developed within
the clay at the time of the fault formation.
Based on the geologic age of the clay deposition, and
the location, typical tectonic activity was not the cause of
the fault. The proximate location of previous glacial ice
flows induced a local stress differential that “failed” the sub-
surface resulting in a local fault structure. An advancing
flow terminated within the present mine operation which
would have induced significant loading on the clay along
with subsequent release of load and rebound to impart the
observed structural deformation (see Figure 2).
To avoid future problems several recommendations
were made. These included regular inspection of the exposed
highwalls to look for any fault or joint structures that exist.
Doing so on progressive pits may allow the projection of
possible structures to future pits and pit progression can be
planned accordingly. In critical areas, as identified through
field observations noted above, a close inspection of the
highwall immediately adjacent to where the dragline will
operate should be made to look for any indication of faults
or joints.
Intersection of faults and joints with the highwall
slope can create kinematically unstable blocks. Once geo-
logic structures are mapped, the potential for blocks can be
reviewed. Kinematic instability would occur when a par-
ticular fault has a strike and dip which intersects the pit
highwall such that sliding toward the open pit or the exca-
vated front cut is kinematically feasible. Steeply dipping
beds in the highwall stratigraphy may also increase the risk
of block-type failures associated with faults. Typically, the
geologic bedding is relatively horizontal however, if steeply
dipping beds are encountered and are accompanied by sur-
rounding faults, a review of the area is recommended.
Figure 2. Diagram of glacial conditions near the dragline bench failure (red star)