7
stacking of ash during a hybrid closure (consolidation of
ash on smaller footprint). The stability of the slope is driven
by the shear strength of the ash and will be based upon
water level assumptions as the excavation is brought down
(and ash is stacked behind). The CPTs will verify drained
strengths and saturated, undrained strengths.
CPTs will provide a detailed log of strength vs depth at
one point in time. However, instrumentation is necessary to
continuously monitor saturation conditions and sound an
alert if changes are problematic. Water level in the ash must
be considered constantly changing. Where wells and well-
points are utilized to provide stability for the deep slope cut
on the hybrid closure, the monitoring of the water levels is
critically important. Water level rises must be immediately
recognized. Vibrating wire piezometers are particularly use-
ful when installed in conjunction with an automated data
acquisition system connected to the internet. These systems
allow for monitoring of ash conditions in real time and are
easily configured to send alerts at predefined trigger levels.
In situations where the water level will be lowered signifi-
cantly, or where there is potential for perched water levels,
piezometers at several depths may be warranted.
Several sizable slope failures have occurred on ash
ponds concurrent with lowering of the pond open water
(Figure 10). Ponds are pumped down slowly so as to not
induce a “rapid drawdown failure.” The thinking is that
by pumping the pond down slowly, the water level out
underneath the beach or the adjacent ash stack drops at
the same rate as the open pool. That is not necessarily the
case. Typically, there is no instrumentation out in the ash
to confirm that the water level within the ash is at the same
elevation as the water level in the pool.
Figure 9. Floating equipment for well installation
Figure 10. Examples of rapid drawdown failure
stacking of ash during a hybrid closure (consolidation of
ash on smaller footprint). The stability of the slope is driven
by the shear strength of the ash and will be based upon
water level assumptions as the excavation is brought down
(and ash is stacked behind). The CPTs will verify drained
strengths and saturated, undrained strengths.
CPTs will provide a detailed log of strength vs depth at
one point in time. However, instrumentation is necessary to
continuously monitor saturation conditions and sound an
alert if changes are problematic. Water level in the ash must
be considered constantly changing. Where wells and well-
points are utilized to provide stability for the deep slope cut
on the hybrid closure, the monitoring of the water levels is
critically important. Water level rises must be immediately
recognized. Vibrating wire piezometers are particularly use-
ful when installed in conjunction with an automated data
acquisition system connected to the internet. These systems
allow for monitoring of ash conditions in real time and are
easily configured to send alerts at predefined trigger levels.
In situations where the water level will be lowered signifi-
cantly, or where there is potential for perched water levels,
piezometers at several depths may be warranted.
Several sizable slope failures have occurred on ash
ponds concurrent with lowering of the pond open water
(Figure 10). Ponds are pumped down slowly so as to not
induce a “rapid drawdown failure.” The thinking is that
by pumping the pond down slowly, the water level out
underneath the beach or the adjacent ash stack drops at
the same rate as the open pool. That is not necessarily the
case. Typically, there is no instrumentation out in the ash
to confirm that the water level within the ash is at the same
elevation as the water level in the pool.
Figure 9. Floating equipment for well installation
Figure 10. Examples of rapid drawdown failure