7
height and angle geometries, the intermediate and lower
groundwater cases were well above the typical FOS of 1.2.
Example #5. Spoil and highwall variability
Geologic variability occurs on all sites. Characterization
of the variability through investigation efforts and obser-
vation are critical to document conditions and adjust the
mine dragline operations to plan for and maintain stable
ground control conditions. Two areas where site variability
has been observed to impact ground control stability are 1)
basal instability or block failure of spoil overlying over-con-
solidated clay stone and 2) geologic structural features on
the highwall side which can create instable blocks and are
discussed in previous examples. The focus of this example is
on block failure of the soil side which can pose risks to the
dragline when it is operating spoil-side.
In surface coal operations, layers adjacent to the coal
seam are often over consolidated or stiff claystone materials.
When operations advance, the spoil material is often placed
over the top of these over consolidated claystone layers
which serve as basal layers to the spoil. If these basal layers
are conducive to softening, spoil side ground control can
be controlled by fully softened or residual shear strengths.
Understanding this possible mechanism and characteriza-
tion of these layers is important when assessing ground
control and the impacts of variability across an advancing
mine operation.
When over-consolidated clay materials are unloaded,
as occurs during removal of the overburden, a decrease in
load allows for swelling and fissuring of the over consoli-
dated claystone which initiates a process that can lead to
a fully-softened state of the clay (see Figure 5). This soft-
ening reduces the shear resistance of the clay. If clay par-
ticles within the material are orientated such that they are
aligned, a residual strength condition can occur which con-
sists of a lower shear resistance than the fully-softened con-
dition. Mobilization of either the fully-softened or residual
strength (both drained strengths) of over-consolidated clay
material can control slope stability (Mesri and Shahien,
2003 Stark and Eid, 1994).
These strengths can be significantly less than intact
strengths. Many factors can impact the reduction of
shear strength associated with fully-softened and residual
strengths. Factors can include to varying degrees: unloading
of the material because of the mining process, exposure to
wetting and drying periods (weathering) which can acceler-
ate the softening process, variation of plasticity within the
clay unit, and time that the pit is open. Figure 5 presents
a depiction of the clay particles and how realignment leads
to a lower strength which can lead to a spoil side block fail-
ure. The comparison of shear strength decrease is plotted
in Figure 6.
In one observed failure, small spoil toe failures had
occurred in previous pits near similar stations. On the
third pit, a major spoil side block failure occurred which
nearly extended across the pit width and halted operations.
Stability back-analyses utilizing limit equilibrium method-
ology, indicated a block failure had occurred through the
basal layer which consisted of over consolidated claystone.
Utilizing a residual strength for the basal layer under
the spoil, and conducting a back-analysis, resulted in a
model FOS of approximately 1.0. The stability model
Figure 5. Depiction of progression of strength decrease for over consolidated claystone which exhibits fully softened and
residual potential
height and angle geometries, the intermediate and lower
groundwater cases were well above the typical FOS of 1.2.
Example #5. Spoil and highwall variability
Geologic variability occurs on all sites. Characterization
of the variability through investigation efforts and obser-
vation are critical to document conditions and adjust the
mine dragline operations to plan for and maintain stable
ground control conditions. Two areas where site variability
has been observed to impact ground control stability are 1)
basal instability or block failure of spoil overlying over-con-
solidated clay stone and 2) geologic structural features on
the highwall side which can create instable blocks and are
discussed in previous examples. The focus of this example is
on block failure of the soil side which can pose risks to the
dragline when it is operating spoil-side.
In surface coal operations, layers adjacent to the coal
seam are often over consolidated or stiff claystone materials.
When operations advance, the spoil material is often placed
over the top of these over consolidated claystone layers
which serve as basal layers to the spoil. If these basal layers
are conducive to softening, spoil side ground control can
be controlled by fully softened or residual shear strengths.
Understanding this possible mechanism and characteriza-
tion of these layers is important when assessing ground
control and the impacts of variability across an advancing
mine operation.
When over-consolidated clay materials are unloaded,
as occurs during removal of the overburden, a decrease in
load allows for swelling and fissuring of the over consoli-
dated claystone which initiates a process that can lead to
a fully-softened state of the clay (see Figure 5). This soft-
ening reduces the shear resistance of the clay. If clay par-
ticles within the material are orientated such that they are
aligned, a residual strength condition can occur which con-
sists of a lower shear resistance than the fully-softened con-
dition. Mobilization of either the fully-softened or residual
strength (both drained strengths) of over-consolidated clay
material can control slope stability (Mesri and Shahien,
2003 Stark and Eid, 1994).
These strengths can be significantly less than intact
strengths. Many factors can impact the reduction of
shear strength associated with fully-softened and residual
strengths. Factors can include to varying degrees: unloading
of the material because of the mining process, exposure to
wetting and drying periods (weathering) which can acceler-
ate the softening process, variation of plasticity within the
clay unit, and time that the pit is open. Figure 5 presents
a depiction of the clay particles and how realignment leads
to a lower strength which can lead to a spoil side block fail-
ure. The comparison of shear strength decrease is plotted
in Figure 6.
In one observed failure, small spoil toe failures had
occurred in previous pits near similar stations. On the
third pit, a major spoil side block failure occurred which
nearly extended across the pit width and halted operations.
Stability back-analyses utilizing limit equilibrium method-
ology, indicated a block failure had occurred through the
basal layer which consisted of over consolidated claystone.
Utilizing a residual strength for the basal layer under
the spoil, and conducting a back-analysis, resulted in a
model FOS of approximately 1.0. The stability model
Figure 5. Depiction of progression of strength decrease for over consolidated claystone which exhibits fully softened and
residual potential