2
complex pattern of deposition with a high degree of inter-
mixing of finer and coarser horizontal layering.
Because the materials are sluiced in place, they are very
loose. Loose corresponds to contractive and when a highly
contractive material is disturbed (sheared) in an undrained
(or undrainable) condition, the grains will rearrange them-
selves into a denser configuration, leaving abundant free
water which will allow the material to flow. Class F fly ash
(which is what most of these ponds consist of) is slightly
pozzolanic so the ash, once sluiced in place, will undergo
very light cementation, which allows the ash to remain in
a very loose condition, even after the pond height increases
significantly. This metastable condition is rather fragile
and leaves the ash susceptible to collapse. In an ash stack,
(like Kingston), the tipping point can be reached when the
stresses in a stack are greater than the strength of the mate-
rial, and what has been called “static liquefaction” occurs.
This failure can be initiated with very slight ground move-
ments, or disturbances, which is why these materials are
sometimes referred to as “brittle.” The potential for static
liquefaction is probably the greatest common concern
between CCRs and mine tailings.
Mine tailings are more variable than ash, and generally
exhibit better behavior and more favorable characteristics.
Tailings will vary by the type of mining, the ore itself, and
the particle size that results from the crushing and grind-
ing process. Mine tailings typically tend to be more angular
than fly ash particles. Despite this, they are still suscepti-
ble to uncontrollable flow events (i.e., static liquefaction)
where once the material is mobilized and movement begins,
it continues as long as unrestrained tailings have a place
to go. The rearrangement of the particles upon movement
generates enough free water to keep the material in motion.
The particle make-up and hydraulic deposition is in essence
what makes these two materials from very different sources
behave in similar fashion geotechnically. We can, therefore,
consider ash as a worstcase condition and apply the lessons
learned to mine tailings.
FLY ASH CHARACTERISTICS AND BEHAVIOR
Complex horizontal layering of bottom, fly, and super-
fine ash is possible. The presence of bottom ash will facili-
tate drainage of ash and the presence of superfine ash will
inhibit vertical percolation of water (which is necessary for
drainage). Add other materials to a pond (such as gypsum
which is also a byproduct of power generation) and the
resulting drainage conditions can be very complex. An ash
pond is hydrogeologically significantly more complex than
any natural soil formation.
When saturated (Figure 2), the ash is a very dangerous
material to work on with conventional construction equip-
ment. Saturated fly ash exhibits very low undrained shear
strength which is why it cannot support equipment or even
foot traffic. Ash that is sluiced in place but drained will
still have low shear strength, but this will be significantly
higher than that of saturated ash. The presence or absence
of water makes all the difference in the behavior of ash. The
light cementation of the ash contributes (to some unknown
extent) to its drained strength. The strength of the ash, and
the subsequent ability to support construction equipment
or be stable in a slope cut, hinges on the presence or absence
of water (Figure 3).
Fly ash is silt-sized material that is highly susceptible
to capillary action (or wicking of moisture up into the ash
above the phreatic surface, or “water table”). It has a rela-
tively high permeability for silt sized particles. Coal ash that
Figure 1. Types of ash
complex pattern of deposition with a high degree of inter-
mixing of finer and coarser horizontal layering.
Because the materials are sluiced in place, they are very
loose. Loose corresponds to contractive and when a highly
contractive material is disturbed (sheared) in an undrained
(or undrainable) condition, the grains will rearrange them-
selves into a denser configuration, leaving abundant free
water which will allow the material to flow. Class F fly ash
(which is what most of these ponds consist of) is slightly
pozzolanic so the ash, once sluiced in place, will undergo
very light cementation, which allows the ash to remain in
a very loose condition, even after the pond height increases
significantly. This metastable condition is rather fragile
and leaves the ash susceptible to collapse. In an ash stack,
(like Kingston), the tipping point can be reached when the
stresses in a stack are greater than the strength of the mate-
rial, and what has been called “static liquefaction” occurs.
This failure can be initiated with very slight ground move-
ments, or disturbances, which is why these materials are
sometimes referred to as “brittle.” The potential for static
liquefaction is probably the greatest common concern
between CCRs and mine tailings.
Mine tailings are more variable than ash, and generally
exhibit better behavior and more favorable characteristics.
Tailings will vary by the type of mining, the ore itself, and
the particle size that results from the crushing and grind-
ing process. Mine tailings typically tend to be more angular
than fly ash particles. Despite this, they are still suscepti-
ble to uncontrollable flow events (i.e., static liquefaction)
where once the material is mobilized and movement begins,
it continues as long as unrestrained tailings have a place
to go. The rearrangement of the particles upon movement
generates enough free water to keep the material in motion.
The particle make-up and hydraulic deposition is in essence
what makes these two materials from very different sources
behave in similar fashion geotechnically. We can, therefore,
consider ash as a worstcase condition and apply the lessons
learned to mine tailings.
FLY ASH CHARACTERISTICS AND BEHAVIOR
Complex horizontal layering of bottom, fly, and super-
fine ash is possible. The presence of bottom ash will facili-
tate drainage of ash and the presence of superfine ash will
inhibit vertical percolation of water (which is necessary for
drainage). Add other materials to a pond (such as gypsum
which is also a byproduct of power generation) and the
resulting drainage conditions can be very complex. An ash
pond is hydrogeologically significantly more complex than
any natural soil formation.
When saturated (Figure 2), the ash is a very dangerous
material to work on with conventional construction equip-
ment. Saturated fly ash exhibits very low undrained shear
strength which is why it cannot support equipment or even
foot traffic. Ash that is sluiced in place but drained will
still have low shear strength, but this will be significantly
higher than that of saturated ash. The presence or absence
of water makes all the difference in the behavior of ash. The
light cementation of the ash contributes (to some unknown
extent) to its drained strength. The strength of the ash, and
the subsequent ability to support construction equipment
or be stable in a slope cut, hinges on the presence or absence
of water (Figure 3).
Fly ash is silt-sized material that is highly susceptible
to capillary action (or wicking of moisture up into the ash
above the phreatic surface, or “water table”). It has a rela-
tively high permeability for silt sized particles. Coal ash that
Figure 1. Types of ash