3
increases in saturation through capillary action will be more
vulnerable to instability. We see this consistently between
ponds occurring from 1–3 ft (0.3–0.9 m) above the water
table. Little is truly quantified about the strength behavior
of ash in the range of moisture contents between drained
and saturated, but it is well known that there is a “transition
zone” in ash above the water table.
The vibrations of construction equipment can also
rearrange the grains of fly ash into a slightly denser con-
figuration and a barely unsaturated ash can be rearranged
into a saturated ash and the water level in the material will
seem to rise due to the repetitive motion or vibration of the
equipment. This is what used to be referred to as “pump-
ing,” observed with repeated passes of construction equip-
ment over an access road. When pumping occurs in ash,
there is a corresponding drop in shear strength and there
have been instances of sudden engulfment of construction
equipment.
With continued construction activity, repetitive
motion and/or vibration of equipment, liquefaction
(dynamic liquefaction) can occur in what is initially nearly
saturated ash. The vibration will rearrange the grains, which
generates free water. If the water cannot dissipate quickly
enough, the pore pressure will rise. When the pore pres-
sure rises to the point where it is greater than the buoyant
weight of the overlying ash, the material will lose all shear
strength. The greatest potential for liquefaction to occur in
ash is when the ash is highly layered and stratified with thin
layers of finer, lower permeability or superfine ash. These
layers hinder the free water from flowing to the surface and
the pressure dissipating. Construction equipment can be
suddenly engulfed without any warning if the liquefaction
occurs just beneath the surface, as shown in Figure 4.
Figure 2. Saturated ash
Figure 3. Dewatered ash supporting equipment
Figure 4. Equipment engulfment as a result of vibration-induced liquefaction
increases in saturation through capillary action will be more
vulnerable to instability. We see this consistently between
ponds occurring from 1–3 ft (0.3–0.9 m) above the water
table. Little is truly quantified about the strength behavior
of ash in the range of moisture contents between drained
and saturated, but it is well known that there is a “transition
zone” in ash above the water table.
The vibrations of construction equipment can also
rearrange the grains of fly ash into a slightly denser con-
figuration and a barely unsaturated ash can be rearranged
into a saturated ash and the water level in the material will
seem to rise due to the repetitive motion or vibration of the
equipment. This is what used to be referred to as “pump-
ing,” observed with repeated passes of construction equip-
ment over an access road. When pumping occurs in ash,
there is a corresponding drop in shear strength and there
have been instances of sudden engulfment of construction
equipment.
With continued construction activity, repetitive
motion and/or vibration of equipment, liquefaction
(dynamic liquefaction) can occur in what is initially nearly
saturated ash. The vibration will rearrange the grains, which
generates free water. If the water cannot dissipate quickly
enough, the pore pressure will rise. When the pore pres-
sure rises to the point where it is greater than the buoyant
weight of the overlying ash, the material will lose all shear
strength. The greatest potential for liquefaction to occur in
ash is when the ash is highly layered and stratified with thin
layers of finer, lower permeability or superfine ash. These
layers hinder the free water from flowing to the surface and
the pressure dissipating. Construction equipment can be
suddenly engulfed without any warning if the liquefaction
occurs just beneath the surface, as shown in Figure 4.
Figure 2. Saturated ash
Figure 3. Dewatered ash supporting equipment
Figure 4. Equipment engulfment as a result of vibration-induced liquefaction