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FHWA Unknown Foundations Summit

Dispersive Wave Methods for Unknown Foundations
Darrin Holt, President, FDH Engineering Inc.

Dispersive wave methods = bending wave

Just another tool to go into arsenal of methods. Sometimes it works, sometimes other things need be used.

Spread footings and piles.

Some NDT methods
Pulse echo
Borehole
Dispersive wave methods

Dispersive wave testing for pile length
Strike pile with hammer, create wave, that's what tracking.
Strike pile on end have dispersive wave.
Tracking flexural wave motion

Mount transducers
Mounted six sensors onto pile, then strike with hammer.
Collect stress wave data
Analyze data

Octagonal piles are fine. Take variety of hammers for field testing. Since 90 every pile has a fingerprint, each pile has own signature.

Timber piles
[same]

Instrumental piling. Equipment set up

Pile supported footings
PS my methodology of choice. Did dispersive wave test on piles.

Dispersive wave propagation is not bending waves exclusively, just how frequencies travel within certain elements. Waves change shape when travel for a while.

Underwater testing
Less than 10 percent error, steel piles and concrete piles. Scuba diver cleans pile, mounts sensor equipment.

Case studies, very important
Need do blind tests, only way to determine accuracy.
In Fla., district 6. tested some pilings in Miami. Only ran for 3-5 ft. foundations went below that. Got to work with digital signal processing feed.

Over the past year…
Tested 117 bridges in NC, approx 75% exposed, 25% pile supported pilings.
La. 100 bridges, all exposed piles.

Dispersive wave won't help with spliced pile. Systematic approaches to problem come in handy.

Systematic approach
Bridge ID
Lit review
Site visit
Determine testing procedures
Field work/reporting/discussion

Open lines of communications extremely important.

Have pulled piles out of ground for testing. Agency said half of links are correct, xxx. broken piles, deteriorated piles. Internal deterioration of one of piles. Pile condition could be problem below grade.

Research into Timber pile condition.
Do lots of this research. Bending waves dependent upon material.
Structural speed of waves is dispersive

Other stuff, PS testing
Knowledge of the subsurface materials is helpful for most NDE evaluations

Rod and hammer information.
Drive rods into ground to figure out if calculated

Radar methods, GPR when soil is perfect

NDT is not solution to every problem. Have to solve problems old-fashioned way.

Use experienced people. Do this since '89. info is highly subjective, be sure to train people properly.

Questions

Q: DH: foundation depth is top of pile to pile tip.

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More Information

Contacts

Silas Nichols
Office of Bridges and Structures
202-366-1554
E-mail Silas

Joe Krolak
Office of Bridges and Structures
202-366-4611
E-mail Joe

 
 
Updated: 04/07/2011
 

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United States Department of Transportation - Federal Highway Administration