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FHWA Unknown Foundations Summit

NDT Evaluation of Unknown Bridge Foundations for NCDOT
Ned Billington, Schnabel Engineering

Contract Summary
Teamed up with Olson engineering for bridge scour testing
Tested 100 bridges

Types of foundations tested
Steel concrete & timber posts on concrete sills/footings
Mass concrete abutments and piers with shallow footings

Testing Methods
Joint PS/conePT

Evaluation criteria
Is pile tip at 15 ft below thalweg?
Is footing on scour-resistant material—sit on rock or weather rock

SE/IR testing of timber piles
Impulse response at 15 ft

Ultraseismic and sonic echo used hand in hand
US—can take data and do sonic echo analysis and come up with supporting analysis.

PS evaluation of concrete abutment footings with buried piles.
Drill and install PVC casing, look for energy travel down abutment, gives pile tip depth.

Did 2 parallel assessment depths

PS/CPT evaluation of piles
Drive down cone at 2 ft intervals, then strike. Look for break and slope and change in energy. Less costly, just drive the cone and collect the data.

PS/CPT evaluation
What's great is get subsurface soil information, can figure out what kind of soil are in.

Using Geoprobe for PS testing Less costly than drill rig drill a bore
Energy comes off of pile, pick up energy below the pile
Not always able to push down deep enough to get cone below pile depth, but can compare to thalweg.

SASW and SE tests on concrete sills
Drop below concrete sill, shows less strength, might not be sitting on rock, might be mat to be scoured out

US and SE tests on concrete abutments

Driving steel rods count blows/foot
Wanted to see if tip length corresponded to xx layer
Used for abutments and sills near rock to see if on rock.

Typical sequence of work
Research existing info
Return to bridges as needed and collect more info
Prepare draft for NCDOT

Example report
Shows kinds of tests, schematics of elements, results, summary data, tables

Summary—Results
177 analyses on 66 bridges, including 55 rod drives
8% had no confidence
Low confidence had some accuracy
Most had high confidence SE/IR

Summary
PS & CPT/PST very effective, 81 % high confidence
PS in cased hole has highest cost/test of available NDT methods

Summary of SE/IR methods
Very effective on exposed and simple pilings
73% high confidence
Sometimes SE provided approximate depth
Low cost, quick tests

Summary US
Useful for exposed and simple foundations elements
Results mixed 25% high and 12% low confidence

SASW
Effective for exposed concrete footing/sills
Use in conjunction with SEIR
92% high confidence
Can provide info on stiffness of soil/rock below footing

Conclude
Methods effective and useful
Research existing info is important
Driving rods,
Field observation
Common sense

Questions

Q: Ian Friedland: how define confidence bands?
NB: More subjective, not quantitative

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