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FHWA Unknown Foundations Summit

Investigating Unknown Foundations in North Carolina
Jerry Beard, Hydraulics unit, NCDOT

IN 1989 FHWA requested each state establish scour program
NC set up committee to code bridges
committee included disciplines from hydraulics, soil & foundation

NC currently has 13,751 bridges over water
Most bridges in NC are owned by state, not counties
5641 have unknown foundations
UF may have piles with unknown length
May have footings with bottom footing elevation not known

Picture analysis

Photo of pile bent

Why is foundation depth important
NC has 3 different regions—mountains, plains, coastal
This creates different kinds of soil foundation, mixes of sand, dirt, rocky, depending on which area it is in.

One key point is material under foundations, whether scourable or unscourable. Most Important, depends on whether your bridges will withstand a flood. Material and geology is very important to foundation of bridges

[picture] bridge closed
Debris is big issue, timber takes out bridges

Prioritize unknown foundations to test
Eliminate bridges scheduled for replacement
Eliminate bridges with ADT < 1500 average daily traffic count
Eliminate bridges built after 1990, built for scour
This left 814 bridges to test first

Consultants providing services to NCDOT

Methods used by NCDOT consultants
Sonic echo, ultra seismic, bending wave, parallel seismic, induction field, ½" rod

Sonic Echo testing
Hit pile with hammer, measure waves. Want to make it a known foundation

SE testing of timber piles. [Photo]

Ultraseismic testing

Bending wave
Uses flexible wave, (sonic echo uses compression waves)

Drilling for parallel seismic test Expensive, several thousands of dollars to get drill out into river
Equipment dropped down into hole

Angle drilling for parallel seismic
Had to put in at angle get close to footing

Cone rig with Parallel seismic
Went in behind the cone
Good for sandy materials,
Can test on shoulder, good for access
Good for testing under bridges if have access and can get down bank

16 lb. Hammer & ½" rod
If have piles and think tipped on rock, put rod right beside it.

Testing challenges
High ground lines, footings under ground then piles on top
Parallel seismic only thing that works, expensive
Need to devise testing methods without drilling bore hole

Coding process

  1. Assessment—search all records, field test bridge; scour committee to code bridge, members of 5 disciplines
  2. Evaluation—field survey, HEC-ras, calculate scour, obtain borings, analyze structure, code bridge. All this is expensive, try to avoid if can.
  3. Countermeasures—provide drawings. Even if bridge is unknown doesn't mean is not scour critical.

Forms for bridge scour assessment

  1. Bridge scour report—any info to give to maintenance folks, hydraulic info, stream channel, wide or narrow
  2. Thalweg—lowest point of stream. If stream migrates, and foundations are below thalweg then foundations are safe from scour risk.

Assessment criteria
Lot of judgment involved in coding bridges. Lot of reading between lines.
Who's coded bridges? [about 1/3 to ½]
it's good for doing research

Assessment criteria
If answer questions yes, then bridges are considered low risk.

Final Comments
Comments, then assessment code.


Q: Dan Billington: Number of bridges?
JB: Number changes every month. Some are replaced and taken out of UF, other reasons for fluctuation.

Q: Scott Sabol: How much of as builts could we find and percentage?
JB: Info hard to find, searches got more diligent. Maintenance have found this info over the years. These are microfilm plans. 25-30 percent are finding records on.

Q: Cynthia Nurmi: Cost and number of bridges?
JB: Started with low number of bridges. As progressed with consultants, cost dropped as work progressed. Started with $1 million to do 400 bridges. Cost has continued to drop; as knowledge has increased it has gotten better.

Q: John Clark, FL DOT: Different techniques with different bridges used? Any reporting on this?
JB: Sonic echo/impulse response used on timber piles, concrete piles, steel piles. Ultraseismic used in combination with sonic echo: these would alternate higher confidence levels. Don't know which was better. Bending wave good for timber piles and concrete. Induction good for steel.

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Updated: 06/27/2017
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