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FHWA Unknown Foundations Summit

Unknown Subsurface Bridge Foundation Testing for Depth Determination Larry Olson, President, Olson Engineering

Unknown US Bridge Foundations
Everything varies in our bridges, from foundation to superstructure

Unknown Foundation Surface NDE Methods
Sonic echo/impulse response—first thing people have tried, can be used for integrity and length. Testing timber piles, hit the top and watch the wave travel down.

Get the depth of it, not going to see surface of piles. Periodic echoes visible in data. Impulse response also resonated because there are multiple modes of resonation.

Bending Waves method
Just hit horizontally, hit the side and track down and back. Found if piles were too long and soil too stiff and deep not much echo was recorded
Can look at travel times and reflections in data. It's very access-friendly, almost always see the sides of piles. That was the basic idea, to make sure that when hit it wave went down and back to bottom.

As hit it vertically and watch go down and back, hoping to see reflections here. Concluded that 23 ft was a material change.

Ultraseismic testing in NC.
Important to get wave separation or it's harder.

Horizontal profiling
Spectral analysis of surface waves, only use to get depth of piers when not that deep.
Concrete foundation in Conn. If have wall shape surface waves can be useful if not too deep.

Borehole NDE methods
Parallel seismic
Induction field

Parallel Seismic method
Determination of foundation depths, typically with superstructure on top of foundation. Want drill about 15 ft deeper than the anticipated depth of the foundation tip.
Have usually used hydrophones in xx casings. When goes beyond tip it's slower, weaker signal.
Can plot arrival time vs. depth. When went into limerock signal was slower and weaker.

Parallel Seismic
PC based signal analyzer
xxx
xxx

NCHRP 21-5

Concrete pile pier, old Bastrop bridge
This example of partially saturated site
Radial receiver of tri-axial receiver. Hydrophone data showed fast arrival time. When partially saturated can see longer and earlier arrival times.

Coors Bridge, highway 58, made interesting case study for research. Dominated by 2 waves. Will see lot of energy go from footing into the ground. Caused tube wave go up/down, indicates tip.

Length Determination of Timber Piles with Parallel Seismic xxx [railroad bridge in California]
Sonic echo tests—large timber piles.
Drilled borings after echo tests didn't work, could hit each one individually, six were exposed.

[Data results]

Induction method for steel piles.

No strong magnetic field when go beyond tip.

Borehole radar method, won't see much of reflection for radar, sometimes.

Induction field, 1 site worked, 1 didn't

Combine parallel seismic When push cone in can learn something about soil. Getting cone data, strength of soil, etc.

Data examples of energy and cone measurement. Weaker and slower

NC

Can give info when want to

Faster, more economical testing than with borings fro PS test.

Conclusions
Parallel seismic (PS) advanced for all foundations, Ultraseismic good for first element depth or piers with compressional

Questions

Q: PS is above 90 percent accuracy, 95 in real world. Borehole method is higher. When soils stiffer may fall off or not see bottom echo. [statement, no response]

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