6
with closure operations. To date, the authors have installed
wells on ash ponds at up to 45 m (150 ft) depths.
Deep wells (Figure 8) are typically the least expensive
dewatering solution where deep coarser layers of bottom
ash exist. They also present fewer piping obstructions to
the earthwork contractor than wellpoints. The main dis-
advantage of deep wells is that they are generally not eco-
nomical to install on close centers because the unit cost for
each well, equipped with a submersible pump, is relatively
high. Deep wells require safe access to the pond surface for
large drilling equipment. This type of equipment requires
a stable platform. Typically, this platform is created with
wellpoints as described above or by pushing material onto
the pond to create a floating road.
Horizontally directionally drilled (HDD) drainage
pipes can be installed from the periphery of a pond but have
had very limited success. On paper, this technique would
have the advantage of a long drain line, submerged in the
ash, with continuous contact with saturated ash. The reality
is that horizontal devices have been only partially effective
because of borehole smear effects and plugging of filter fab-
rics. HDD wells are very inefficient in pulling water from
the formation without a properly installed well filter pack.
With the distinct horizontal layering in sluiced-in materi-
als, a vertically installed device that can be installed with
drilling methods that minimize borehole smear effects, is
better oriented to tap all of the horizontal permeable layers.
ACCESS SAFETY CONSIDERATIONS
Safely accessing a sluiced-in pond is typically the first chal-
lenge with any pond closure. The irony is that dewatering
a dry crust on the pond to gain equipment access often
requires equipment access to install the dewatering system.
The author has had good success laying down a small access
lane using geogrid and plywood suitable for foot traffic
only. From this geogrid and plywood access lane, shallow
wellpoints can be installed by hand to create a dry crust for
access for equipment.
For deeper ponds where it is necessary to use a drill
rig to install dewatering wells, it will be necessary to build
floating access roads with geogrid or to dewater a dry crust
with wellpoints. In the case where the pond is flooded,
it is possible to put the entire installation operation on a
barge or flexi-floats to install either wellpoints or wells (see
Figure 9). This method eliminates the safety concern of
working directly on top of the ash.
Amphibious equipment can be utilized when there
is not a significantly thick dry crust to work from. The
amphibious equipment will not sink in the ash however,
amphibious equipment is not intended to be free-floating.
Amphibious excavators can tip when there is little traction.
ENGINEERED CONTROLS FOR WORKING IN
CHANGEABLE CONDITIONS
Because water level and saturation conditions in an ash
pond are highly changeable due to many climatological and
stormwater management factors, and the subsequent effects
on ash shear strength are very pronounced, engineered con-
trols should be implemented to evaluate conditions at criti-
cal times. Those engineered controls are the use of CPTs,
instrumentation, and field vane shear strength tests (2).
CPTs can be performed at various milestones during
pond closure. A good example is the intermittent per-
formance of CPTs during the excavation of a slope and
Figure 7. Wellpoints Figure 8. Deep wells
with closure operations. To date, the authors have installed
wells on ash ponds at up to 45 m (150 ft) depths.
Deep wells (Figure 8) are typically the least expensive
dewatering solution where deep coarser layers of bottom
ash exist. They also present fewer piping obstructions to
the earthwork contractor than wellpoints. The main dis-
advantage of deep wells is that they are generally not eco-
nomical to install on close centers because the unit cost for
each well, equipped with a submersible pump, is relatively
high. Deep wells require safe access to the pond surface for
large drilling equipment. This type of equipment requires
a stable platform. Typically, this platform is created with
wellpoints as described above or by pushing material onto
the pond to create a floating road.
Horizontally directionally drilled (HDD) drainage
pipes can be installed from the periphery of a pond but have
had very limited success. On paper, this technique would
have the advantage of a long drain line, submerged in the
ash, with continuous contact with saturated ash. The reality
is that horizontal devices have been only partially effective
because of borehole smear effects and plugging of filter fab-
rics. HDD wells are very inefficient in pulling water from
the formation without a properly installed well filter pack.
With the distinct horizontal layering in sluiced-in materi-
als, a vertically installed device that can be installed with
drilling methods that minimize borehole smear effects, is
better oriented to tap all of the horizontal permeable layers.
ACCESS SAFETY CONSIDERATIONS
Safely accessing a sluiced-in pond is typically the first chal-
lenge with any pond closure. The irony is that dewatering
a dry crust on the pond to gain equipment access often
requires equipment access to install the dewatering system.
The author has had good success laying down a small access
lane using geogrid and plywood suitable for foot traffic
only. From this geogrid and plywood access lane, shallow
wellpoints can be installed by hand to create a dry crust for
access for equipment.
For deeper ponds where it is necessary to use a drill
rig to install dewatering wells, it will be necessary to build
floating access roads with geogrid or to dewater a dry crust
with wellpoints. In the case where the pond is flooded,
it is possible to put the entire installation operation on a
barge or flexi-floats to install either wellpoints or wells (see
Figure 9). This method eliminates the safety concern of
working directly on top of the ash.
Amphibious equipment can be utilized when there
is not a significantly thick dry crust to work from. The
amphibious equipment will not sink in the ash however,
amphibious equipment is not intended to be free-floating.
Amphibious excavators can tip when there is little traction.
ENGINEERED CONTROLS FOR WORKING IN
CHANGEABLE CONDITIONS
Because water level and saturation conditions in an ash
pond are highly changeable due to many climatological and
stormwater management factors, and the subsequent effects
on ash shear strength are very pronounced, engineered con-
trols should be implemented to evaluate conditions at criti-
cal times. Those engineered controls are the use of CPTs,
instrumentation, and field vane shear strength tests (2).
CPTs can be performed at various milestones during
pond closure. A good example is the intermittent per-
formance of CPTs during the excavation of a slope and
Figure 7. Wellpoints Figure 8. Deep wells