6
highwall, that was observed to contain more silt and sand
material (compared to recent pits) and a greater volume of
groundwater flow from the slope face.
Although, there had previously been several geology
borings near the current mining area, they were focused on
understanding the depth to coal and coal quality sampling.
The documentation consisted of driller logs, where the
driller and/or helper had made notes of soils encountered
based on experience in the region. The nearest, detailed
geotechnical study had been completed three years prior
at a location 1,000 feet east of the observed bench failures.
Following the dragline bench failure, an attempt was
made to back-analyze the conditions to understand the
conditions leading to instability. The limitation in the
interpretation is with the general, basic soil descriptions
provided on the driller logs. As a result, the interpretation
and analysis simply classified the dragline bench geology as
either silt and sand dominated and a second case of being
clay dominated. The actual conditions when examined for
detailed inter-bedding intervals, while core logging, would
have characterized a mix of granular and fine-grained soils.
The methodology applied examined the bookend soil
conditions expected to be excavated and exposed in the
dragline bench. This was necessary to try and correlate
material types to geotechnical properties determined in
previous studies. Under the observed conditions, with the
available data for interpretation, a model could be devel-
oped to depict a failure however, it required a nontypical
approach of assigning the drained (long-term) strength to
the clay dominate layers, instead of the undrained strength
properties.
Following this instability, a limited geotechnical inves-
tigation was conducted for the purpose of obtaining more
detailed subsurface information near the instabilities. This
would provide a more detailed, accurate geology compared
to the general stratigraphy in the desktop study. The geol-
ogy, with the observed increased groundwater elevation,
proved key to provide the information necessary to more
accurately model the dragline bench stability.
The slope stability model was updated accordingly.
When the elevated, local groundwater elevation was
included, the model suggested a marginal stability condition
(i.e., FOS ~1.0) with extents that were like site observations
(see Figure 4). Subsequent iterations were completed where
the intermediate and lower groundwater elevations were
incorporated into the models. The mine operation had pre-
viously not observed overall bench instability at these lower
elevations. These model comparisons demonstrated the sig-
nificant impact that the highest observed groundwater had
on slope stability. Whereas the higher groundwater resulted
in unstable or marginally stable slopes for various slope
Figure 4. Cross-sections depicting marginal stability when incorporating the actual site
conditions of the failures
highwall, that was observed to contain more silt and sand
material (compared to recent pits) and a greater volume of
groundwater flow from the slope face.
Although, there had previously been several geology
borings near the current mining area, they were focused on
understanding the depth to coal and coal quality sampling.
The documentation consisted of driller logs, where the
driller and/or helper had made notes of soils encountered
based on experience in the region. The nearest, detailed
geotechnical study had been completed three years prior
at a location 1,000 feet east of the observed bench failures.
Following the dragline bench failure, an attempt was
made to back-analyze the conditions to understand the
conditions leading to instability. The limitation in the
interpretation is with the general, basic soil descriptions
provided on the driller logs. As a result, the interpretation
and analysis simply classified the dragline bench geology as
either silt and sand dominated and a second case of being
clay dominated. The actual conditions when examined for
detailed inter-bedding intervals, while core logging, would
have characterized a mix of granular and fine-grained soils.
The methodology applied examined the bookend soil
conditions expected to be excavated and exposed in the
dragline bench. This was necessary to try and correlate
material types to geotechnical properties determined in
previous studies. Under the observed conditions, with the
available data for interpretation, a model could be devel-
oped to depict a failure however, it required a nontypical
approach of assigning the drained (long-term) strength to
the clay dominate layers, instead of the undrained strength
properties.
Following this instability, a limited geotechnical inves-
tigation was conducted for the purpose of obtaining more
detailed subsurface information near the instabilities. This
would provide a more detailed, accurate geology compared
to the general stratigraphy in the desktop study. The geol-
ogy, with the observed increased groundwater elevation,
proved key to provide the information necessary to more
accurately model the dragline bench stability.
The slope stability model was updated accordingly.
When the elevated, local groundwater elevation was
included, the model suggested a marginal stability condition
(i.e., FOS ~1.0) with extents that were like site observations
(see Figure 4). Subsequent iterations were completed where
the intermediate and lower groundwater elevations were
incorporated into the models. The mine operation had pre-
viously not observed overall bench instability at these lower
elevations. These model comparisons demonstrated the sig-
nificant impact that the highest observed groundwater had
on slope stability. Whereas the higher groundwater resulted
in unstable or marginally stable slopes for various slope
Figure 4. Cross-sections depicting marginal stability when incorporating the actual site
conditions of the failures