4
LESSONS LEARNED FROM CCR
Every pond exhibits somewhat different behaviours and
“has a unique personality.” The personality of the pond
comes from:
• the source of the coal,
• the characteristics of the burner,
• the manner in which the ash was transported to the
pond,
• the proportions and frequency of mixing bottom and
fly ash,
• the shape of the pond,
• the location(s) of the sluice pipe(s),
• the thickness of the ash, and
• the chemistry of the ash and the water.
The variables are seemingly endless. What ultimately
matters is how the ash behaves on an exposed slope cut or
under the tracks of an excavator. To some extent, a geotech-
nical investigation can characterize a pond, but ultimately
it is going to reveal itself during construction.
Prior to the CCR rule, there were hundreds of ash
ponds with absolutely no geotechnical characterization.
Access was not just difficult but also considered dangerous.
There was never any need to understand the composition of
the material if there was never any intention to do anything
with it. Once there was a CCR rule and a need to under-
stand the composition of the material, the cone penetrom-
eter quickly became the tool of choice.
The CPT provides much finer resolution of the mate-
rial than standard SPT borings. CPTs can provide a picture
of the variability of the ash with depth and also variabil-
ity of the ash across the pond and, with the near continu-
ous logging, indicate where there are thin zones of coarser,
more drainable ash. These are the places where the perme-
ability of the ash is locally higher and are opportune places
to provide drainage of the overlying materials with dewater-
ing devices.
These more transmissive zones are generally only a small
portion of the overall ash thickness but represent most of
the ash’s ability to transmit or release water. In general, high
transmissivity is identified on a CPT log by comparing the
measured pore pressure response (u) to the hydrostatic pres-
sure. As the CPT probe is advanced, it creates a pressure
wave in front of it. In low permeability zones, the measured
pressure will be higher than hydrostatic pressure because
the pressure induced with the driving of the probe dissi-
pates slowly. In more permeable zones, no pressure builds
up with the driving of the cone and the measured pressure
will tend to fall right on the hydrostatic line. These pore
pressure response zones also tend to correlate to zones of
relatively high tip resistance (qt) which would be expected
from coarser, more granular material. This is to be expected
since most saturated ash columns will be very soft, while
thin layers of coarser material will tend to be more pro-
nounced to the cone. Figure 5 is a CPT log showing zones
of high permeability. Figure 6 shows an amphibious CPT
rig.
Prior to draining an ash pond, the pond filling history
and CPTs should be reviewed to better understand what
parts of the pond are likely to be coarser and what parts
are likely to be finer. Historical photos are necessary for
revealing the deposition history of a pond and identifying
potential problem areas, which are usually where finer ash
is likely to be deposited or materials are mixed. Historic
photos can also reveal areas of coarser ash which are areas
Figure 5. CPT log
Figure 6. Amphibious CPT rig (courtesy R. B. Jergens)
LESSONS LEARNED FROM CCR
Every pond exhibits somewhat different behaviours and
“has a unique personality.” The personality of the pond
comes from:
• the source of the coal,
• the characteristics of the burner,
• the manner in which the ash was transported to the
pond,
• the proportions and frequency of mixing bottom and
fly ash,
• the shape of the pond,
• the location(s) of the sluice pipe(s),
• the thickness of the ash, and
• the chemistry of the ash and the water.
The variables are seemingly endless. What ultimately
matters is how the ash behaves on an exposed slope cut or
under the tracks of an excavator. To some extent, a geotech-
nical investigation can characterize a pond, but ultimately
it is going to reveal itself during construction.
Prior to the CCR rule, there were hundreds of ash
ponds with absolutely no geotechnical characterization.
Access was not just difficult but also considered dangerous.
There was never any need to understand the composition of
the material if there was never any intention to do anything
with it. Once there was a CCR rule and a need to under-
stand the composition of the material, the cone penetrom-
eter quickly became the tool of choice.
The CPT provides much finer resolution of the mate-
rial than standard SPT borings. CPTs can provide a picture
of the variability of the ash with depth and also variabil-
ity of the ash across the pond and, with the near continu-
ous logging, indicate where there are thin zones of coarser,
more drainable ash. These are the places where the perme-
ability of the ash is locally higher and are opportune places
to provide drainage of the overlying materials with dewater-
ing devices.
These more transmissive zones are generally only a small
portion of the overall ash thickness but represent most of
the ash’s ability to transmit or release water. In general, high
transmissivity is identified on a CPT log by comparing the
measured pore pressure response (u) to the hydrostatic pres-
sure. As the CPT probe is advanced, it creates a pressure
wave in front of it. In low permeability zones, the measured
pressure will be higher than hydrostatic pressure because
the pressure induced with the driving of the probe dissi-
pates slowly. In more permeable zones, no pressure builds
up with the driving of the cone and the measured pressure
will tend to fall right on the hydrostatic line. These pore
pressure response zones also tend to correlate to zones of
relatively high tip resistance (qt) which would be expected
from coarser, more granular material. This is to be expected
since most saturated ash columns will be very soft, while
thin layers of coarser material will tend to be more pro-
nounced to the cone. Figure 5 is a CPT log showing zones
of high permeability. Figure 6 shows an amphibious CPT
rig.
Prior to draining an ash pond, the pond filling history
and CPTs should be reviewed to better understand what
parts of the pond are likely to be coarser and what parts
are likely to be finer. Historical photos are necessary for
revealing the deposition history of a pond and identifying
potential problem areas, which are usually where finer ash
is likely to be deposited or materials are mixed. Historic
photos can also reveal areas of coarser ash which are areas
Figure 5. CPT log
Figure 6. Amphibious CPT rig (courtesy R. B. Jergens)