1. The document investigates the engineering properties of slurried fly ash through laboratory tests that aim to replicate field conditions. 2. Sedimentation tests did not achieve proper density for further testing, but consolidation tests using a batch consolidometer were successful in replicating field densities of slurried ash. 3. Triaxial compression tests on the consolidated samples found that slurried ash is very soft and weak, with a stiffness of 6,000 psi and a strength of 80 psi at a confining stress of 40 psi, displaying dilative behavior under shear.

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Background Sedimentation & Consolidation Tests Sampling CU Triaxial Test Results
Engineering Behavior of Slurried Ash
Jalila Elfejji1
, Dr. Y. Park2, and Dr. M. Pando3,
1SPIDUR REU at UNC Charlotte, Department of Civil Engineering, University of Wisconsin-Madison
2 co-Mentor, EPIC, UNC Charlotte
3Mentor, Department of Civil & Environmental Engineering, UNC Charlotte
Fly ash is a residual product of burning coal and other fossil
fuels. There are different methods to help mitigate the
potentially harmful effects of fly ash including reusing the ash
for construction materials or using wet storage of this material
in “ash ponds.” In recent years there has been major
concerns related to the stability and possible failure of these
ponds. The chemicals contained in the fly ash have the
potential to seep into the soil and reach the groundwater
which can cause major health and environmental risks. From
a stability standpoint, the properties of the wet fly ash are of
concern.
On December 22 2008, the TVA Kingston Plant in Harriman,
TN failed and released more than 5.4 million yd3 of ash
spreading across 400 acres. The spill ravaged 12 homes,
caused a train accident, and contaminated the Emory River.
Later investigations revealed that the underlying layers of the
ash slurry were weak, and hadn’t been noticed in previous
TVA inspections. The ash underwent a significant amount of
static liquefaction and creep.
Left: Kingston TVA plant before the spill. Right: Kingston TVA plant post spill.
Sedimentation Test: Wet disposal of fly ash involves pumping
wet ash into a pond. The deposition in the field is a slow
sedimentation process. Lab tests needed to replicate slurried
fly ash field conditions (unit weight and moisture content).
Method A: Sedimentation tests (see below).
Consolidation Test: Method A did not achieve field unit
weight. The second approach used a batch consolidometer
with positive pressure and bottom drainage (Method B).
Engineering Behavior of Slurry Consolidated Fly Ash Samples
0
0.2
0.4
0.6
0.8
1
1.2
0 2000 4000 6000 8000 10000 12000 14000 16000 18000
H(t)/H0(in)
Time (s)
Normalized Height vs Time of Duke Belews Creek Fly Ash
Fluid Unit Weight= 132.5 pcf
Fluid Unit Weight=97.4 pcf
Fluid Unit Weight= 80 pcf
Fluid Unit Weight= 114.9 pcf
35
45
55
65
75
85
95
105
115
125
0 2000 4000 6000 8000 10000 12000 14000 16000 18000
SaturatedUnitWeight(pcf)
Time (s)
Saturated Unit Weight vs Time of Duke Belews Creek Fly Ash
Fluid Unit Weight= 132.5 pcf
Fluid Unit Weight= 97.4 pcf
Fluid Unit Weight= 80 pcf
Fluid Unit Weight= 114.9 pcf
Field Saturated
Field Saturated
Unit Weight
1. Saturation 2. Consolidation 3.Shearing
Purpose:
Ensures all voids
are filled with
water
Purpose:
Brings sample to
effective stress
required for
shearing
Purpose:
Find out what
stress causes
failure
Method:
Increase pore &
cell pressure
until sample is
fully saturated
Method:
Increase cell
pressure and
maintain constant
back pressure
Method:
Apply deviator
stress until failure
Deviator
Stress (q)
Confining
Stress (σc)
Pore
Pressure
Effective Stress (σ’) Pore Pressure (u)
Left: Ash sample after consolidation test. Middle: Sample taken from
shelby tube. Right: Sample trimmer.
Pressure Valve Top Rod
Top Porous Stone
Bottom Porous Stone
Drainage Tube
Left: Dry Belews Creek fly ash. Right: Fly Ash Slurry.
Engineering Behavior of Ponded Ash
(CU triaxial with cell pressure = 40 psi)
0
10
20
30
40
50
60
70
80
90
0 20 40 60 80 100 120 140 160
q(psi)
p' (psi)
Ф= 35º
0
10
20
30
40
50
60
70
80
90
0 5 10 15 20 25
q(psi)
Axial Strain (%)
-25
-20
-15
-10
-5
0
5
10
15
0 5 10 15 20 25
ExcessPorePressure
(psi)
Axial Strain (%)
I would like to thank Dr. Pando and Dr. Park for their
mentorship and laboratory assistance as well as the SPIDUR
Program at the University of North Carolina Charlotte.
Broader Impact
The ultimate goal of this research is to prepare in the lab
slurried ash samples that represent realistic field conditions.
These samples can be used to measure engineering
properties of the slurried ash to help assess the stability of
typical ash pond facilities. Future research on fly ash
samples could not only prevent future spills, but also provide
economical solutions to help improve ash pond stability while
maintaining a safe and clean environment.
Es=6,000 psi
Conclusions
The sedimentation tests (Method A) did not produce
samples dense enough for Triaxial testing, but it did give
us a good representation of how the slurry will settle
overtime in an ash pond. Method B, involving a batch
consolidometer, was successful in replicating field
densities. A CU Triaxial test on a slurried ash confirmed
wet pond ash is very soft and weak (as per stiffness and
strength obtained). Additionally the fly ash specimen
exhibited a dilative behavior under undrained shear.
Acknowledgments
Presented at the 2015 Charlotte Summer Research Symposium
Left: Ash pond. Right: Sedimentation test (Method A) to
prepare slurried ash.
Schematic of batch consolidometer (Method B).
Engineering behavior of slurried ash was investigated by
means of consolidated undrained (CU) Triaxial compression
tests.
CU Triaxial Compression Testing of slurried ash.
Engineering behavior: Stiffness (Es): 6,000 psi which is quite
soft (similar to rubber). Strength: qult = 80 psi (for 40 psi
confining stress) which is very weak; f’ = 35o.