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FHWA Unknown Foundations Summit

Dealing with Unknown Foundations: North Carolina Strategy
Mohammed Mulla, Assistant State Geotechnical Engineer

Bridge failure (See picture of scour failure of Schoharie Bridge NY 1987). NCDOT history progress with UF since 1988.

Took us almost a year to establish a committee (July 25, 1989), The committee contacted the FHWA to clarify some of the issues with the Technical Advisory requirements and time.

Interdisciplinary committee was established to include the following units:

Bridge Maintenance Unit

Structure Design Unit

Geotechnical Engineering Unit

Hydraulics Unit


  1. Program includes the following steps Initial Screening
  2. Prioritization for Structure Scour Evaluation
  3. Assessment
  4. Detailed Study
  5. Plan of Action

The Soils and Foundation Design Section and the Geotechnical Unit combined together on 2003 to create a new Unit called Geotechnical Engineering Unit.

Screened structures
UF—technical advisory committee didn't say anything about unknown foundations

Scour Evaluation Process (Flow Chart)
Bridge inspection program

  • Low Risk
  • Scour Susceptible
  • Unknown Foundation
  • Scour Critical

The screening of all 11800 bridges to be completed by March 31, 1991 (Initial screening only)

Classify bridges as low risk or not

If not then prepare plan of action

Data on existing Structures: April 1990

Open/Posted Bridges (see chart)
Most bridges were built after World War II, built in 1950s

Scour Evaluation Process
The interdisciplinary committee selected 10 projects, as a pilot project to estimate the cost and time to complete the study for scour. The preliminary cost was $6,000 to $8000 per bridge
Problem was not easy. We had 11800 bridges over water. The cost was high. We have a short budget (no money was allocated for the scour study from the FHWA).

NCDOT Plan received national recognition
FHWA gave NCDOT an award by publishing their bridge screening program on HEC 18 1991.
Technical Advisory T5140.20 had no discussion about the UF. Beginning of UF discussion and plan of action came from our plan in NC, (see HEC 18- 1991) then FHWA made improvements as we go.

Whole idea was to minimize the damage to the nation's bridges

In early 1990s
Developed NDT test for timber piles, most bridges in NC used timber piles
Technology developed by NCSU to test timber piles from side.
Did about 40 or 70 bridges in mid-90s
Some improvement needed for program. 60% accuracy. The program depends on who collects the data and who runs the data, must be highly experienced data collector.

Non Destructive Testing by Dispersive Wave Propagation [photo]

In my opinion the dept. should always be able to run the NDT tests, and to evaluate the results to build an expertise within the dept. to be able to check the private firms and to use the technology at any time it is needed.

Screening Process Revealed
7,800 bridges with UF
All through 90s we hired consultants to rate bridges for scour. We did some UF but all bridges were studied for scour

Unknown Foundation Bridges—Plan of Action
Directed to do so by FHWA, memorandum issued to encourage States to develop plan of action

North Caroline Bridge Inventory
5,636 bridges with UF—The reduction in the number of UF is due to bridge replacement program and NDT for the last 2 years.
UF, unknown piles lengths, footings, embedded pile footings, concrete, steel, timber, masonry

UF bridges Plan of Action
What we did to reduce number—eliminated bridges schedule for replacement, (TIP)

ADT <1500;

Built after 1990; (Diesel hammer used to drive the piles)

Found records;

Test bridges at risk; priority for bridges with ADT >1500

Sonic echo/impulse response
Bending wave
Ultra seismic
Parallel seismic
Have home made device

NCDOT program
Phase I = field testing

Sometimes we pay more money for terrain, bridges not easy to get to

The number of unknown bridges in our database did not match the NBI number?

I don't know how often the national bridge inventory list is updated, every April and November.
[photo] Need to ask the right question of consultants. They will do what you ask them to.
If you ask for pile length they will give a pile length. You need to ask for the integrity of the pile in addition to embedment to be able to evaluate the stability of the pile.

Phase II

Questions to ask

After getting the pile embedment how do we analyze it?
Are we using new AASHTO loads? No criteria whatsoever. FHWA should give us this criteria and help us.
Using historical info of scour before & after flood?
Is scour permanent?


Going at right speed with NDT technologies? The same progress since 1990

Need to get out of the box.

NCDOT are in progress to develop a fuzzy logic and neural networks (FLNN) model to use in many different areas to keep the experience in place. Want to replace the hummer approval process by using FLNN model. Have a huge amount of data we could learn from.

Why do we call UF? Why not call dangerous bridges? You'll get the money faster.

Questions for FWHA and consultants
Ways for states to ensure they are getting a competent vendor for NDT?
What are foundation types

Big picture is good. Companies do good work, States get good results. Just need to refine technologies.

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Updated: 06/27/2017
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