8
Concurrent with construction activities, the practice of
real time pore pressure monitoring for working platform
safety is increasing. Pore pressure monitoring can be per-
formed as an early indicator of liquefaction. A combination
of highly layered ash, low shear strength material at shal-
low depth and a construction vibration induced pore pres-
sure rise is the likely setting for a liquefaction (dynamic)
failure. In theory, if the pore pressure rises to the effective
stress at transducer depth all shear strength is lost. Real time
pore pressure monitoring and communication of the data
has been practiced successfully to watch for that sudden,
construction vibration-induced rise in pressure in pore
pressure (3).
Where field activities are dependent upon the shear
strength of the ash, hand-held vane shear tests (VST) can
be performed in the upper 10 ft (3 m) of ash, to evalu-
ate conditions for the use of heavy equipment. This can
provide the best indication of dry crust thickness and
strength. Low shear strength measurements or elevated
and/or perched water levels can suggest increased potential
for instability. Water levels should be monitored at regular
frequency in areas of activity and VST should be repeated
when it is suspected that conditions have been altered,
either by construction activity, precipitation/infiltration/
wetting, or other occurrence which may result in a change
in water levels.
Geostructural Improvements in Ash
Although drainage (dewatering) is the most readily achiev-
able improvement that can be made in ash, sometimes it
is inappropriate, or a greater geo-structural improvement
is needed. In those instances, soil mixing has been utilized
with great success.
Soil mixing (Figure 11) involves the mechanical blend-
ing or mixing of a binding agent, typically Portland cement,
with the in-situ soils (or ash) to create a soil-concrete prod-
uct similar to weak concrete. There are a number of dif-
ferent basic techniques that can be implemented with wet
soil mixing, where the binder is introduced as a fluid grout.
Those techniques are single axis soil mixing, multi-axis,
Cutter Soil Mixing (CSM) and shallow mass mixing. The
same techniques are also used for ISS (in-situ stabilization),
a process that is increasingly considered for stabilizing ash
in place in a pond where it will remain in contact with
groundwater (4).
Ash is actually an ideal material in which to perform
soil mixing. The difficulty is using the massive equipment
that is typically needed. To date, within the confines of ash
ponds, soil mixing has been used to construct a deep grav-
ity dividing wall bifurcating an active pond, a large and
very high retaining wall, shear panels for seismic retrofit-
ting of an ash impoundment built by upstream construc-
tion methods, and construction of a soil mixed shallow
working platform.
CONCLUSIONS
When sluiced in place, mine tailings can be similar to CCR.
Removal of the pore water from both CCR and mine tail-
ings is the single most significant measure one can take to
mitigate the potential for instability and liquefaction (1).
Much has transpired in the CCR world in the last seven
years with the pond closures that have occurred. In CCRs,
we have refined ground improvement techniques such as
dewatering and soil mixing to improve the characteristics of
the ash. We also have engineered controls to verify condi-
tions and facilitate safe work conditions.
Figure 11. Soil mixing in ash
Concurrent with construction activities, the practice of
real time pore pressure monitoring for working platform
safety is increasing. Pore pressure monitoring can be per-
formed as an early indicator of liquefaction. A combination
of highly layered ash, low shear strength material at shal-
low depth and a construction vibration induced pore pres-
sure rise is the likely setting for a liquefaction (dynamic)
failure. In theory, if the pore pressure rises to the effective
stress at transducer depth all shear strength is lost. Real time
pore pressure monitoring and communication of the data
has been practiced successfully to watch for that sudden,
construction vibration-induced rise in pressure in pore
pressure (3).
Where field activities are dependent upon the shear
strength of the ash, hand-held vane shear tests (VST) can
be performed in the upper 10 ft (3 m) of ash, to evalu-
ate conditions for the use of heavy equipment. This can
provide the best indication of dry crust thickness and
strength. Low shear strength measurements or elevated
and/or perched water levels can suggest increased potential
for instability. Water levels should be monitored at regular
frequency in areas of activity and VST should be repeated
when it is suspected that conditions have been altered,
either by construction activity, precipitation/infiltration/
wetting, or other occurrence which may result in a change
in water levels.
Geostructural Improvements in Ash
Although drainage (dewatering) is the most readily achiev-
able improvement that can be made in ash, sometimes it
is inappropriate, or a greater geo-structural improvement
is needed. In those instances, soil mixing has been utilized
with great success.
Soil mixing (Figure 11) involves the mechanical blend-
ing or mixing of a binding agent, typically Portland cement,
with the in-situ soils (or ash) to create a soil-concrete prod-
uct similar to weak concrete. There are a number of dif-
ferent basic techniques that can be implemented with wet
soil mixing, where the binder is introduced as a fluid grout.
Those techniques are single axis soil mixing, multi-axis,
Cutter Soil Mixing (CSM) and shallow mass mixing. The
same techniques are also used for ISS (in-situ stabilization),
a process that is increasingly considered for stabilizing ash
in place in a pond where it will remain in contact with
groundwater (4).
Ash is actually an ideal material in which to perform
soil mixing. The difficulty is using the massive equipment
that is typically needed. To date, within the confines of ash
ponds, soil mixing has been used to construct a deep grav-
ity dividing wall bifurcating an active pond, a large and
very high retaining wall, shear panels for seismic retrofit-
ting of an ash impoundment built by upstream construc-
tion methods, and construction of a soil mixed shallow
working platform.
CONCLUSIONS
When sluiced in place, mine tailings can be similar to CCR.
Removal of the pore water from both CCR and mine tail-
ings is the single most significant measure one can take to
mitigate the potential for instability and liquefaction (1).
Much has transpired in the CCR world in the last seven
years with the pond closures that have occurred. In CCRs,
we have refined ground improvement techniques such as
dewatering and soil mixing to improve the characteristics of
the ash. We also have engineered controls to verify condi-
tions and facilitate safe work conditions.
Figure 11. Soil mixing in ash