Embankment Load Testson a Coal Ash Basin

Embankment Load Testson a Coal Ash Basin

Thomas G. Pace, PEAlan F. Rauch, PE, PhD

Geohazards in Transportation 2009

Outline

Site/Facility Overview

Project Background

Field Test Introduction

Instrumentation Program– Inclinometers– Observation System– Surveying– Settlement Arrays– Piezometers

Findings

E.W. Brown Generating Station Ash Treatment Basin

2000

1990

1980

1970

1960- Basin created

- Basin raised

- Basin raised

- Basin full

Cross-Section through Planned Embankment

Future Gypsum Embankment

Original Dam

1989 Overlay

Existing Ash Deposits

Future Ash Deposits

72 ft

1973 Overlay

Future Flexible Membrane Liner

2005 Field Test

Characterize ash as foundation material

Unscheduled interruption of operations

Dewatered ash basin

1 month to prepare, 1 month to conduct test

Aggressive plan to obtain maximum data

Instruments abandoned in place

Cold, wind, dust

In-situ Testing• Vane shear• Cone penetrometer• T-bar penetrometer• Downhole seismic• Nuclear density probe• Neutron probe

Instrumentation Program

Installed Instruments

Inclinometers

-0.1 0.0 0.1 0.2 0.3Lateral Displacement (ft)

East Fill

-0.1 0.0 0.1

840

850

860

870

880

890

Elevation (ft)

West Fill

Observation System

Purpose– Record of construction progress– Capture events engineers miss

Construction– Color security camera– Time-Lapse VCR

Lessons Learned– Automatic digital stills better– Vantage point difficult– No scale reference

Surveying

Differential GPS

Uses– Construction Layout– Locating Instruments– Monitoring Fill Height

20 30 40 50 60 70 80 90 100 110 120Horizontal Offset from Baseline (ft)

880

890

900

910

Elev

atio

n (ft

)

20-Nov 2:00 pm20-Nov 4:30 pm21-Nov 2:30 pm21-Nov 5:15 pm23-Nov 8:18 am29-Nov 2:05 pm

Surface Profile - East Test Fill

Slope = 1.4:1repose = 35.8o

100 90 80 70 60 50 40 30 20 10 0Horizontal Offset from Baseline (ft)

880

890

900

910

Elev

atio

n (ft

)

23-Nov 10:00 am23-Nov 4:15 pm28-Nov 9:50 am28-Nov 12:22 pm29-Nov 1:30 pm

Surface Profile - West Test Fill

Settlement Arrays

Installation: Magnetic Extensometers

Challenges– Keep Strings Untangled, Organized– String Strength– Backfill Bridging

0 1 2 3 4Vertical Compression (%)

East Fill

2 1 0

840

850

860

870

880

890

Elevation (ft)

West Fill

Vibrating Wire Piezometers

Purpose: pore pressure

Installation– Direct Burial– Unvented Transducers

Lessons Learned– Measure Elevation– Backfill, Joint Seals– Influence of Weather

Piezometer Readings

0 1 2 3 4 5 6 7 8 9Elapsed Time (days)

0

0.1

0.2

0.3

Exc

ess

Por

e W

ater

Pre

ssur

e / I

nitia

l Ver

tical

Effe

ctiv

e S

tress

West Fill0 1 2 3 4 5 6 7 8 9

0

0.1

0.2

0.3

0.4 East Fill= Periods of Placing Fill

Removing FillFill Height = 23 feet

Fill Height = 9 ftFill Height = 20 feet

Findings

Vibrocompaction reduced settlements ~50%

Excess pore pressures dissipate within a few days

Settlements take place over same time

Staged construction rates

Vibrocompaction not needed

Construction Cost Savings– Less conservative– Better construction bids– ~ 50 times field test cost