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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-Madison2 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 2000 4000 6000 8000 10000 12000 14000 160000

0.2

0.4

0.6

0.8

1

1.2

Normalized Height vs Time of Duke Belews Creek Fly Ash

Fluid Unit Weight= 132.5 pcfFluid Unit Weight=97.4 pcfFluid Unit Weight= 80 pcfFluid Unit Weight= 114.9 pcf

Time (s)

H(t)

/H0

(in)

0 2000 4000 6000 8000 10000 12000 14000 1600035

45

55

65

75

85

95

105

115

125

Saturated Unit Weight vs Time of Duke Belews Creek Fly Ash

Fluid Unit Weight= 132.5 pcfFluid Unit Weight= 97.4 pcfFluid Unit Weight= 80 pcfFluid Unit Weight= 114.9 pcf

Time (s)

Satu

rate

d Un

it W

eigh

t (pc

f) Field Saturated Unit Weight Upper Bound

Field Saturated Unit Weight Lower Bound

1. Saturation 2. Consolidation 3.ShearingPurpose:

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)

PorePressure

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 StoneDrainage Tube

Left: Dry Belews Creek fly ash. Right: Fly Ash Slurry.

Engineering Behavior of Ponded Ash(CU triaxial with cell pressure = 40 psi)

0 20 40 60 80 100 120 140 1600

10

20

30

40

50

60

70

80

90

p' (psi)

q (p

si)

Ф= 35º

0 5 10 15 20 250

10

20

30

40

50

60

70

80

90

Axial Strain (%)

q (p

si)

0 5 10 15 20 25

-25

-20

-15

-10

-5

0

5

10

15

Axial Strain (%)

Exce

ss P

ore

Pres

sure

(p

si)

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 ImpactThe 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

ConclusionsThe 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.