8
findings were supported through field observations of slick-
ensides, laboratory testing to determine Atterberg Limits,
ring shear testing to obtain strength values, and inclinom-
eter monitoring to determine the depth of the failure plane.
Post failure activities required ground control modeling to
develop a mine sequence acknowledging a residual strength
was present. Forward looking identification of this layer
was also undertaken by reviewing e-log data from explora-
tion borings and looking for high gamma zones beneath
the coal seams.
DISCUSSION OF PAST INCIDENTS
A common thread throughout the examples provided
above is that each case included components of the fail-
ure mechanism that had not been previously encountered
and would have been unlikely to have been discovered in a
more global slope stability study covering a large area. Each
case described included elements in the failure mechanism
that were either local in nature or relatively random. In at
least two of the cases the failure was driven by how the
dig sequence interacted with the lithology and structural
geology.
In several cases the failure mechanism included a com-
bination of structural features and soil characteristics. This
typically involved joints or faults in the wall and clays that
can become fully softened. Since the intersection of these
features can be uncertain it suggests that diligence on an
ongoing basis will be important for recognizing potential
issues and maintaining a safe condition for operation of the
dragline.
For most of these cases, examination of extensive
lengths of highwall and spoil benches on the same pit did
not show evidence of failure. This provides observational
support for failures involving the dragline often being the
result of localized and situational circumstances.
RECOMMENDATIONS
This section provides a variety of practical tips related to
ground control for dragline operations. This is based on
past experiences from a variety of sites the authors have
been involved with, drawing on nearly 100 years of com-
bined professional expertise.
• Awareness and understanding of the geotechnical
investigation and analysis completed to support the
dragline bench design.
• What geologic conditions were identified and
included?
• What conditions were observed, but limited and
potentially not considered in the assessment?
• Why were conditions included or not included?
• Appropriate investigation and assessment of soil and
rock present at the mine for material characteriza-
tion, representative strength properties, groundwa-
ter conditions and changes in different units, and
defined structural geology (eLogs).
• Review of e-log data from exploration to observe:
• Granular versus cohesive overburden and interbur-
den material.
• High gamma reading zones adjacent to coal seams.
• Incorporation of fully softened and/or residual
strength for ground control slope stability analyses.
• Typically, mines consider open pits to be temporary
exposures particularly when backfilled with spoil,
such that the initial, intact strength condition will
apply.
• Certain geologic conditions can exist where cohesive
soils actually exhibit reduced strength “in situ”.
• Where information on larger scale geological condi-
tions is identified, has the dig sequence plan consid-
ered adverse bedding or structural orientations when
daylighted?
• Mine engineering staff needs to be sufficiently con-
versant with slope failure types and causes and local
conditions to recognize possible problems when
these are observed.
• Is there regular discussion and collaboration between
mine engineers and mine operations?
• Help all understand the plan objectives and identify
areas that may be a challenge to execute based on
previous mining of similar geology.
Figure 6. Stress-strain (shear) behavior of various clay
materials (drained)
findings were supported through field observations of slick-
ensides, laboratory testing to determine Atterberg Limits,
ring shear testing to obtain strength values, and inclinom-
eter monitoring to determine the depth of the failure plane.
Post failure activities required ground control modeling to
develop a mine sequence acknowledging a residual strength
was present. Forward looking identification of this layer
was also undertaken by reviewing e-log data from explora-
tion borings and looking for high gamma zones beneath
the coal seams.
DISCUSSION OF PAST INCIDENTS
A common thread throughout the examples provided
above is that each case included components of the fail-
ure mechanism that had not been previously encountered
and would have been unlikely to have been discovered in a
more global slope stability study covering a large area. Each
case described included elements in the failure mechanism
that were either local in nature or relatively random. In at
least two of the cases the failure was driven by how the
dig sequence interacted with the lithology and structural
geology.
In several cases the failure mechanism included a com-
bination of structural features and soil characteristics. This
typically involved joints or faults in the wall and clays that
can become fully softened. Since the intersection of these
features can be uncertain it suggests that diligence on an
ongoing basis will be important for recognizing potential
issues and maintaining a safe condition for operation of the
dragline.
For most of these cases, examination of extensive
lengths of highwall and spoil benches on the same pit did
not show evidence of failure. This provides observational
support for failures involving the dragline often being the
result of localized and situational circumstances.
RECOMMENDATIONS
This section provides a variety of practical tips related to
ground control for dragline operations. This is based on
past experiences from a variety of sites the authors have
been involved with, drawing on nearly 100 years of com-
bined professional expertise.
• Awareness and understanding of the geotechnical
investigation and analysis completed to support the
dragline bench design.
• What geologic conditions were identified and
included?
• What conditions were observed, but limited and
potentially not considered in the assessment?
• Why were conditions included or not included?
• Appropriate investigation and assessment of soil and
rock present at the mine for material characteriza-
tion, representative strength properties, groundwa-
ter conditions and changes in different units, and
defined structural geology (eLogs).
• Review of e-log data from exploration to observe:
• Granular versus cohesive overburden and interbur-
den material.
• High gamma reading zones adjacent to coal seams.
• Incorporation of fully softened and/or residual
strength for ground control slope stability analyses.
• Typically, mines consider open pits to be temporary
exposures particularly when backfilled with spoil,
such that the initial, intact strength condition will
apply.
• Certain geologic conditions can exist where cohesive
soils actually exhibit reduced strength “in situ”.
• Where information on larger scale geological condi-
tions is identified, has the dig sequence plan consid-
ered adverse bedding or structural orientations when
daylighted?
• Mine engineering staff needs to be sufficiently con-
versant with slope failure types and causes and local
conditions to recognize possible problems when
these are observed.
• Is there regular discussion and collaboration between
mine engineers and mine operations?
• Help all understand the plan objectives and identify
areas that may be a challenge to execute based on
previous mining of similar geology.
Figure 6. Stress-strain (shear) behavior of various clay
materials (drained)