POWER December 2014 - 50

Coal Combustion Residuals
Construction Considerations Are
Key in Closure Planning for Coal
Ash Ponds
When faced with a variety of options for coal ash pond closure, use a holistic
approach rather than choosing what seems to be the quickest or cheapest
on paper.
Mark Johnson and Kent Nilsson
C
losure and remediation solutions for
coal ash ponds, or impoundments,
vary greatly. Assuming the U.S. Environmental
Protection Agency (EPA) promulgates
the expected proposed Subtitle D
regulations, technical options are plentiful,
and the vast majority will be viable for
most sites. Among the choices are capping,
dewatering and/or stabilizing, consolidating
into a new landfill, disposing off site,
converting to wetlands, or any combination
of these options.
There is no one-size-fits-all option, so it's
important to familiarize yourself with the advantages
and disadvantages of each. This is
especially important if the planning design
and construction activities occur at an open
and active facility. A successful closure approach
accounts for the size of the pond,
dewatering requirements and methods, final
site use, integration of any remediation efforts,
constructability, site layout, and longterm
costs.
Pond Size Matters
To better accommodate materials handling,
staging, and water management, divide
larger ponds into working subsections. A
large pond that is closed in place can be incrementally
covered so that the closed areas
shed precipitation as storm water rather than
as contact water that must be collected and
treated. Because storm water discharge is
normally handled through gravity flow, this
can substantially reduce water management
costs over a lengthy construction period.
Consolidation of pond solids into a reduced
footprint reduces cost. Because the
surface area is smaller, the cost for constructing
the permanent final cover will be
reduced. Similarly, the cost for long-term
maintenance of the cover system proportionally
drops (Figure 1).
Dewatering Requirements and
Methods
Water removal from an ash pond is critical
1. In-place closure. This schematic cross-section shows the three stages of in-place consolidation
and closure for ash pond solids. Courtesy: TRC Companies Inc.
Standing water
Ash
Current conditions
Perimeter ditch
for dewatering
Dewatering
Berm fill used
for cover
Ash
In-place closure/consolidation
50
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To wastewater
treatment
Wellpoints
Ash
(to be removed)
Affected soil to
be removed
Vegetative layer
Geomembrane
Perimeter
Berm
Depositional delta
at discharge location
Perimeter
Berm
to constructability, project economics, and
long-term performance. Without proper dewatering
of saturated ash materials before removal,
excavations risk caving and cut slope
collapse, creating safety hazards and potentially
costly repairs.
During closure, dewatering should be conducted
to limit disturbance and re-suspension
of a pond's solids, especially when construction
activities take place in the immediate
vicinity of the discharge structure. In some
cases this may require the use of a temporary
settlement structure prior to discharge.
Dewatering is also important for controlling
closure costs. The cost per ton of solids
for hauling ash by truck for off-site disposal
is substantially higher if the ash moisture
content is high, which usually translates
into higher disposal costs, ranging from
hundreds of thousands to millions of dollars,
depending on the size of the project.
Moisture content of the ash solids will likewise
have a financial impact on hauling to
an on-site disposal cell.
Because dewatering factors into the
success of a pond closure, planners need
to allot sufficient time for solids to drain.
Sumps, well points, and dewatering ditches
have a limited radius of influence, so as ash
is drained and stabilized, dewatering must
progressively advance. Drainage properties
of the ash may dictate that dewatering be
initiated weeks, if not months, in advance
of inaugural activities. Seasonal surface
drying and precipitation also affect dewatering
(Figure 2).
Final Pond Site Use
The end use of the pond site factors into
the choice of closure options. If the overall
property will be retained for future power
generation or similar use, options for site
use include:
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POWER December 2014

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