5
where dewatering wells or wellpoints should be located.
Based on the pond deposition history, it is possible to pre-
dict with some accuracy how the pond will most effectively
be drained. A pilot test or a series of wells spaced around
the pond will help to confirm that prediction.
The best way to evaluate the drainability of a pond is
with real field data obtained from a pilot dewatering pro-
gram or pump test. This test usually consists of either a
pumped deep well or series of wellpoints and measurement
of the resulting drawdown in strategically located instru-
ments. Data obtainable from such a test could include
traditional aquifer parameters such as transmissivity and
storage coefficient, which are of minimal value in the midst
of highly variable conditions and when the objective is
pumping down a pond of trapped water. More importantly,
a pumping test provides an opportunity to measure the
yield of a representative dewatering device, or a device con-
structed and installed the way the wells or wellpoints will
be constructed and installed during production dewater-
ing. Early on in CCR pond closures, pumping tests would
be very elaborate in order to calculate aquifer parameters
at a representative or worst-case scenario location within
a pond. The authors’ thinking shifted very quickly to con-
ducting numerous yield tests at widely spaced locations
around a pond in order to evaluate (qualitatively) the vari-
ability of the pond.
Following a geotechnical investigation and possibly
pilot testing and test pits, one still has to consider that
the conditions on an ash pond are still highly changeable
because of the sensitivity of the material to water. The sin-
gle most significant factor with changing conditions is due
to variation in water level and saturation conditions and
the subsequent impacts on ash shear strength. The specific
yield of ash is relatively low, on the order of 8 to 12 percent
of total volume. This is advantageous in that the behavior
of the ash will change dramatically with a relatively small
amount of drainage (i.e. removal of the specific yield). A
small amount of recharge, however, can undo a lot of drain-
age effort which means a reversal of the shear strength gain
that occurred with drainage. So, the characteristics of the
ash can change dramatically with a relatively small change
in water content. Those changes within a pond can occur
with precipitation, how process water may be handled on a
pond, the influence of the pool, the addition of construc-
tion water, or infiltration into a pond from permeable natu-
ral ground in contact with the ash.
IMPROVING THE ASH
Dewatering, in the opinion of the authors, is the most read-
ily available, well proven, and cost-effective way to improve
ash conditions. Historically dewatering of ash ponds has
not been too successful. Prior to the CCR rule there were
many failed attempts, with only two ash ponds that were
actively and successfully dewatered. Conventional dewa-
tering methods improperly applied have been problematic
to the point where many people believe that pre- drain-
age techniques just don’t work in ash. Building effective
dewatering devices such as wellpoints or wells in ash is
complicated. The traditional well filter pack design used in
conventional construction dewatering doesn’t always apply
when the particles resemble ball bearings. Previous use of
fabric filters in lieu of conventional sand filters has proven
to restrict water flow and resulted in plugging of the wells
or wellpoints. In light of these past difficulties, when it
has been necessary to dewater fly ash, most operators have
chosen to dig a network of ditches leading to a sump or
sumps. Lowering the water with rim ditches and sumps
requires taking the excavation down in very thin lifts. This
technique is suited to long term pond maintenance activ-
ity or to closure projects with very long schedules where
the slow drainage of the pond is not critical. Placing land-
based equipment on a saturated ash surface in order to dig a
drainage ditch presents a significant safety hazard, certainly
with operators unfamiliar with ash.
Wellpoints (Figure 7) have been used extensively in fly
ash dewatering. The main advantage of a wellpoint system
is that it is a relatively economical system for finer ash where
many low yield devices on close centers are necessary. The
wellpoint system with closely spaced devices is also advan-
tageous where it is necessary to lower the phreatic surface as
closely as possible to the bottom of a pond with an underly-
ing impermeable layer (either clay or rock). The main dis-
advantage of wellpoints is that they work on vacuum and
are therefore limited in how high they can lift water. Typical
drawdowns that are observed in fly ash would be approxi-
mately 4.5 to 5.5 m (15 to 18 ft).
Wellpoints are ideal for dewatering shallow ponds and
for creating a dry crust that can support heavy equipment.
On deeper ponds, multiple stages of wellpoints can be uti-
lized, or deep wells can be installed that are not restricted
with depth. They lend themselves to deeper impoundments,
particularly where coarser, more transmissive ash may exist
at greater depths. A series of widely spaced wells in this con-
dition may be able to significantly lower pore water levels
with minimal equipment on the surface that may interfere
where dewatering wells or wellpoints should be located.
Based on the pond deposition history, it is possible to pre-
dict with some accuracy how the pond will most effectively
be drained. A pilot test or a series of wells spaced around
the pond will help to confirm that prediction.
The best way to evaluate the drainability of a pond is
with real field data obtained from a pilot dewatering pro-
gram or pump test. This test usually consists of either a
pumped deep well or series of wellpoints and measurement
of the resulting drawdown in strategically located instru-
ments. Data obtainable from such a test could include
traditional aquifer parameters such as transmissivity and
storage coefficient, which are of minimal value in the midst
of highly variable conditions and when the objective is
pumping down a pond of trapped water. More importantly,
a pumping test provides an opportunity to measure the
yield of a representative dewatering device, or a device con-
structed and installed the way the wells or wellpoints will
be constructed and installed during production dewater-
ing. Early on in CCR pond closures, pumping tests would
be very elaborate in order to calculate aquifer parameters
at a representative or worst-case scenario location within
a pond. The authors’ thinking shifted very quickly to con-
ducting numerous yield tests at widely spaced locations
around a pond in order to evaluate (qualitatively) the vari-
ability of the pond.
Following a geotechnical investigation and possibly
pilot testing and test pits, one still has to consider that
the conditions on an ash pond are still highly changeable
because of the sensitivity of the material to water. The sin-
gle most significant factor with changing conditions is due
to variation in water level and saturation conditions and
the subsequent impacts on ash shear strength. The specific
yield of ash is relatively low, on the order of 8 to 12 percent
of total volume. This is advantageous in that the behavior
of the ash will change dramatically with a relatively small
amount of drainage (i.e. removal of the specific yield). A
small amount of recharge, however, can undo a lot of drain-
age effort which means a reversal of the shear strength gain
that occurred with drainage. So, the characteristics of the
ash can change dramatically with a relatively small change
in water content. Those changes within a pond can occur
with precipitation, how process water may be handled on a
pond, the influence of the pool, the addition of construc-
tion water, or infiltration into a pond from permeable natu-
ral ground in contact with the ash.
IMPROVING THE ASH
Dewatering, in the opinion of the authors, is the most read-
ily available, well proven, and cost-effective way to improve
ash conditions. Historically dewatering of ash ponds has
not been too successful. Prior to the CCR rule there were
many failed attempts, with only two ash ponds that were
actively and successfully dewatered. Conventional dewa-
tering methods improperly applied have been problematic
to the point where many people believe that pre- drain-
age techniques just don’t work in ash. Building effective
dewatering devices such as wellpoints or wells in ash is
complicated. The traditional well filter pack design used in
conventional construction dewatering doesn’t always apply
when the particles resemble ball bearings. Previous use of
fabric filters in lieu of conventional sand filters has proven
to restrict water flow and resulted in plugging of the wells
or wellpoints. In light of these past difficulties, when it
has been necessary to dewater fly ash, most operators have
chosen to dig a network of ditches leading to a sump or
sumps. Lowering the water with rim ditches and sumps
requires taking the excavation down in very thin lifts. This
technique is suited to long term pond maintenance activ-
ity or to closure projects with very long schedules where
the slow drainage of the pond is not critical. Placing land-
based equipment on a saturated ash surface in order to dig a
drainage ditch presents a significant safety hazard, certainly
with operators unfamiliar with ash.
Wellpoints (Figure 7) have been used extensively in fly
ash dewatering. The main advantage of a wellpoint system
is that it is a relatively economical system for finer ash where
many low yield devices on close centers are necessary. The
wellpoint system with closely spaced devices is also advan-
tageous where it is necessary to lower the phreatic surface as
closely as possible to the bottom of a pond with an underly-
ing impermeable layer (either clay or rock). The main dis-
advantage of wellpoints is that they work on vacuum and
are therefore limited in how high they can lift water. Typical
drawdowns that are observed in fly ash would be approxi-
mately 4.5 to 5.5 m (15 to 18 ft).
Wellpoints are ideal for dewatering shallow ponds and
for creating a dry crust that can support heavy equipment.
On deeper ponds, multiple stages of wellpoints can be uti-
lized, or deep wells can be installed that are not restricted
with depth. They lend themselves to deeper impoundments,
particularly where coarser, more transmissive ash may exist
at greater depths. A series of widely spaced wells in this con-
dition may be able to significantly lower pore water levels
with minimal equipment on the surface that may interfere