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FHWA Unknown Foundations Summit

Detection Of Unknown Bridge Foundations For Bridge Scour Analysis Using Electrical Surface Geophysical Methods
Wade Kress, hydrologist, US Geological Survey

Purpose
Use commercially available software and equipment to solve unknown bridge foundations
Done this over last 7 years
Helped identify contaminants

Equipment & Software
AGI, Iris, Zonge, Abem, others
RES2DINV

Concept of electrical resistivity data

Resistivity and induced Polarization
Induces polarity
Turn it on, then off time to measure the decay of voltage over time
Different properties of earth hold charges for different amounts of time

Wenner / Schlumbarger Arrays
Tell computer which

Data Collection
Deploy cables and electrodes
Connect resistivity unit and batteries
Data processed in field on site

Site map
North Dakota
Had 4 lines on 1 side of bridge pier, then additional 4 lines
8 2D lines
Collect data on different lines

Line Location Map
Shows bridge pier, only 2 lines on top of footing
Looked at several different things. Inject current into ground
Looked at feasible/economic method to image bridge footer—wanted to know how close
Do you have to be to it.
Processed as quasi-3D set

Resistivity
Plotted in depth and feet, in ohm-meters. Red is more resistant, dark is less resistant
Used 2.5 meter electrode spacing
Have electrical signature

Induced polarization data
[strong contrast of colors]
Structure holds charge longer than surrounding geology
Depending on need of project, could get a high-resolution image
Could collect this data and process in couple of hours

3D data processing

Conclusions of pilot study
Couple of days in field to collect data, couple of days in office to process
Thought it was promising, but more work needs to be done

Where do we go from here? Collected data from 2 bridges
Report coming next month
Want to model and simulate different geological environments
Have several resistivity tools, try different ones, evaluate different geologies
Evaluate at different bridge structures
Define best approaches—help DOTs use in their states, use as estimation tool
Define optimal environmental approaches

Questions

Q: Had 30 ft depth. Did you know where water table was and what are WT effects?
Kress: Moisture does affect results. Water in river, so ground water elevation was probably couple feet below land surface. Able to image conductor inside of another conductor. Footer less resistive than geology was. Depends on construction of structure and environment. Modeling can help. Plug in values, give good estimation of end product.

Q: How do this in flowing water?
Kress: We can float electrodes behind a boat. These electrodes are waterproof; but in flowing water can be different. Successful in the past for geological mapping. Tools available for water-borne methods.

Q: Try use xxx?
Kress: Used simplest approach. Next, try different array types. Depth of investigation is limited to spacing of electrodes. Have to work within confines of collection area.

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