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24-013
Can Fly Ash Pond Closure Expertise be Applied to Mine Tailings?
Paul C. Schmall
Keller North America, Rockaway, NJ
ABSTRACT
In recent years, several geotechnical methods have been
successfully applied to ash pond closure. These include
modified dewatering techniques with wellpoints and wells
to physically drain the material, soil mixing to create a
soil-concrete material for improved accessways as well as
improved ground structures within ponds to act as barriers
or gravity dams, and geotechnical instrumentation to serve
as an early warning of the potential for liquefaction. Given
the parallels between fly ash and sluiced in place mine tail-
ings, the application of fly ash remediation method to mine
tailings shows significant potential, as illustrated in this
paper.
SHARED ORIGINS
In 2008, a fly ash containment stack containing millions
of cubic yards of coal ash from a Kingston, Tennessee coal-
fired plant failed, resulting in the release of over five million
cubic yards of ash into an adjacent river, disruption to elec-
trical power, natural gas, and water lines, and covering local
roadways and a rail track. Community and environmental
damages were widespread. This was the biggest environ-
mental disaster in the history of the United States.
In 2014, a drainage pipe burst at a coal ash contain-
ment pond, part of a closed coal-fired plant in Eden, North
Carolina. Thirty-nine thousand tons of coal ash and 27
million gallons of wastewater was released into the Dan
River before the event ended. Ash containing metals was
deposited up to 70 miles (110 km) from the spill site.
The sheer magnitude and far-reaching consequences of
these two events precipitated the 2015 enactment of “The
Disposal of Coal Combustible Residuals (CCRs) from
Electric Utilities Rule,” the purpose of which was to provide
a comprehensive set of requirements for the safe disposal of
CCRs from coal-fired power plants. The ruling addressed
the risk from unregulated coal ash disposal, such as the
leaking of contaminants into the groundwater and the
potential for catastrophic failure of surface impoundments.
CCRs consist primarily of fly ash but also include
bottom ash and what we will refer to as “superfine” ash.
A typical pond is probably 80% fly ash (Figure 1a), with
approximately 80% silt-sized particles, and is absolutely
non-cohesive. Although the particles are predominantly
silt-sized, the material tends to behave more like a fine
uniform sand than a silt. Ash does not consolidate like a
natural silt or a clay. Bottom ash (probably 10% of a typi-
cal pond) is a coarse sand, free draining, with more angular
particles (Figure 1b). From an earthwork perspective, it’s an
excellent, problem free material to work with and within.
Superfine ash (Figure 1c) is the finest ash that finds its way
to the far end of the pond. It is so fine that it will hold onto
water and will not respond to drainage methods (such as
dewatering). Superfine ash constitutes probably 10% of the
typical pond.
Depending upon the condition of the impound-
ment or if ash is stored in a sensitive area, pond closure
may require the complete excavation and removal of the
CCRs, partial excavation and consolidation of ash over a
smaller footprint, or just regrading and capping. The spe-
cific requirements for closure are in part dictated by the
CCR Rule and in part decided by each state.
The experience gained in CCRs since the ruling was
first enacted is directly applicable to tailings storage facili-
ties of similar characteristics. The similarity starts with their
origins, considering only the mine tailings that, like CCRs,
are sluiced in place. They have similar resulting traits. If
the sluice pipe never moves over the life of a pond, coarser
particles would, predictably, settle first and the finer par-
ticles would travel further in the pond. Many ponds are
intended to be filled evenly, so the pipe is moved around
the periphery of the pond to achieve this. This creates a
24-013
Can Fly Ash Pond Closure Expertise be Applied to Mine Tailings?
Paul C. Schmall
Keller North America, Rockaway, NJ
ABSTRACT
In recent years, several geotechnical methods have been
successfully applied to ash pond closure. These include
modified dewatering techniques with wellpoints and wells
to physically drain the material, soil mixing to create a
soil-concrete material for improved accessways as well as
improved ground structures within ponds to act as barriers
or gravity dams, and geotechnical instrumentation to serve
as an early warning of the potential for liquefaction. Given
the parallels between fly ash and sluiced in place mine tail-
ings, the application of fly ash remediation method to mine
tailings shows significant potential, as illustrated in this
paper.
SHARED ORIGINS
In 2008, a fly ash containment stack containing millions
of cubic yards of coal ash from a Kingston, Tennessee coal-
fired plant failed, resulting in the release of over five million
cubic yards of ash into an adjacent river, disruption to elec-
trical power, natural gas, and water lines, and covering local
roadways and a rail track. Community and environmental
damages were widespread. This was the biggest environ-
mental disaster in the history of the United States.
In 2014, a drainage pipe burst at a coal ash contain-
ment pond, part of a closed coal-fired plant in Eden, North
Carolina. Thirty-nine thousand tons of coal ash and 27
million gallons of wastewater was released into the Dan
River before the event ended. Ash containing metals was
deposited up to 70 miles (110 km) from the spill site.
The sheer magnitude and far-reaching consequences of
these two events precipitated the 2015 enactment of “The
Disposal of Coal Combustible Residuals (CCRs) from
Electric Utilities Rule,” the purpose of which was to provide
a comprehensive set of requirements for the safe disposal of
CCRs from coal-fired power plants. The ruling addressed
the risk from unregulated coal ash disposal, such as the
leaking of contaminants into the groundwater and the
potential for catastrophic failure of surface impoundments.
CCRs consist primarily of fly ash but also include
bottom ash and what we will refer to as “superfine” ash.
A typical pond is probably 80% fly ash (Figure 1a), with
approximately 80% silt-sized particles, and is absolutely
non-cohesive. Although the particles are predominantly
silt-sized, the material tends to behave more like a fine
uniform sand than a silt. Ash does not consolidate like a
natural silt or a clay. Bottom ash (probably 10% of a typi-
cal pond) is a coarse sand, free draining, with more angular
particles (Figure 1b). From an earthwork perspective, it’s an
excellent, problem free material to work with and within.
Superfine ash (Figure 1c) is the finest ash that finds its way
to the far end of the pond. It is so fine that it will hold onto
water and will not respond to drainage methods (such as
dewatering). Superfine ash constitutes probably 10% of the
typical pond.
Depending upon the condition of the impound-
ment or if ash is stored in a sensitive area, pond closure
may require the complete excavation and removal of the
CCRs, partial excavation and consolidation of ash over a
smaller footprint, or just regrading and capping. The spe-
cific requirements for closure are in part dictated by the
CCR Rule and in part decided by each state.
The experience gained in CCRs since the ruling was
first enacted is directly applicable to tailings storage facili-
ties of similar characteristics. The similarity starts with their
origins, considering only the mine tailings that, like CCRs,
are sluiced in place. They have similar resulting traits. If
the sluice pipe never moves over the life of a pond, coarser
particles would, predictably, settle first and the finer par-
ticles would travel further in the pond. Many ponds are
intended to be filled evenly, so the pipe is moved around
the periphery of the pond to achieve this. This creates a