2
crack is believed to be a discontinuity dipping nearly ver-
tical for which only the surface expression was observed.
This discontinuity is believed to have formed the back-
plane of the failure, thereby creating an unpinned block.
Observations along the pit highwall showed a few other
traces of discontinuities, but these were limited in number
and not consistent in spacing.
A thin rider seam was located at the bottom of a mud-
stone layer. It was about 1 to 1.5 inch thick and likely
would not have been identified from e-logs performed in
exploration borings.
Immediately above the thin rider was a high-plasticity
clay about 3 feet in thickness. The contact between the clay
and coal was glossy with minor striations. The clay layer
was at the approximate depth where the bottom failure
plane was estimated to be located by observation.
Tests showed that this clay layer was of highplasticity
such that shear strength degradation could occur to a fully
softened or residual strength. While not excessive, there was
water present in the highwall and may have contributed to
strength reduction in the clay material forming the base
plane.
To avoid future problems, the recommendations
included inspections of the highwalls and mapping any
structural geology discontinuities observed to assist in
projecting where such structural features might occur
in subsequent pits. Another adjustment would be to use
side-cutting to lower the dragline bench to an elevation
below the plastic clay layer. Finally, the mine could con-
sider working the dragline from a leveled spoil-side bench
instead of the highwall.
Example #2. Glaciation induced structures
In this example a dragline partially slid into the pit while
operating from the highwall. The machine had created a
front cut and was removing overburden from above a coal
seam at the time of the slide. There had been no evidence of
instability while uncovering coal in prior pits.
To characterize this failure and make recommendations
for the future we first needed to determine the cause of
the failure. The determination of causes was made based
on field observation, investigative drilling, installation of
vibrating wire piezometers, laboratory testing and geotech-
nical modeling. It was determined that the failure occurred
due to the presence of geologic structure (jointing/faulting)
and a corresponding reduction in material strength. The
reduction in material strength was induced by the proxim-
ity to a plastic zone along the fault and a potential reduc-
tion of strength in the clay formation based on observation
of slickensides in core samples from this unit.
The observed geologic structure in the end wall of the
pit and measured alignment indicates that the fault struc-
ture passed through the vicinity of the back portion of the
dragline tub and dipped toward the open front cut. The
Figure 1. Schematic diagram of structural-controlled dragline bench failure
crack is believed to be a discontinuity dipping nearly ver-
tical for which only the surface expression was observed.
This discontinuity is believed to have formed the back-
plane of the failure, thereby creating an unpinned block.
Observations along the pit highwall showed a few other
traces of discontinuities, but these were limited in number
and not consistent in spacing.
A thin rider seam was located at the bottom of a mud-
stone layer. It was about 1 to 1.5 inch thick and likely
would not have been identified from e-logs performed in
exploration borings.
Immediately above the thin rider was a high-plasticity
clay about 3 feet in thickness. The contact between the clay
and coal was glossy with minor striations. The clay layer
was at the approximate depth where the bottom failure
plane was estimated to be located by observation.
Tests showed that this clay layer was of highplasticity
such that shear strength degradation could occur to a fully
softened or residual strength. While not excessive, there was
water present in the highwall and may have contributed to
strength reduction in the clay material forming the base
plane.
To avoid future problems, the recommendations
included inspections of the highwalls and mapping any
structural geology discontinuities observed to assist in
projecting where such structural features might occur
in subsequent pits. Another adjustment would be to use
side-cutting to lower the dragline bench to an elevation
below the plastic clay layer. Finally, the mine could con-
sider working the dragline from a leveled spoil-side bench
instead of the highwall.
Example #2. Glaciation induced structures
In this example a dragline partially slid into the pit while
operating from the highwall. The machine had created a
front cut and was removing overburden from above a coal
seam at the time of the slide. There had been no evidence of
instability while uncovering coal in prior pits.
To characterize this failure and make recommendations
for the future we first needed to determine the cause of
the failure. The determination of causes was made based
on field observation, investigative drilling, installation of
vibrating wire piezometers, laboratory testing and geotech-
nical modeling. It was determined that the failure occurred
due to the presence of geologic structure (jointing/faulting)
and a corresponding reduction in material strength. The
reduction in material strength was induced by the proxim-
ity to a plastic zone along the fault and a potential reduc-
tion of strength in the clay formation based on observation
of slickensides in core samples from this unit.
The observed geologic structure in the end wall of the
pit and measured alignment indicates that the fault struc-
ture passed through the vicinity of the back portion of the
dragline tub and dipped toward the open front cut. The
Figure 1. Schematic diagram of structural-controlled dragline bench failure