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Unknown Foundations

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FHWA Unknown Foundations Summit

State DOT Concerns and Strategies for Dealing with Unknown Bridge Foundations
Panel Discussion
Cynthia Nurmi, moderator

Panel members:
Jerry Beard
Mohammed Mulla
Athar Khan
Steven Sibley
Kerry Petrasic

[note: when audience members asked questions, they are identified by their State when possible]

Q: Why did your state choose to investigate unknown foundations?

Sibley: Had lots of unknown foundations, had to determine whether bridges were scour critical. Concerned about motorists safety and infrastructure. Funds come from replacement funds. Set up $3 million for bridge inspections, inspect large high-tressed bridges.

Q:Was it team effort and how get them to agree to spend money?

Sibley: Team effort. Raised concerns about UFs with boss. Consultant came and visited at same time.

Nurmi: What is Alabama doing?

Alabama: Not doing anything, have some unknown foundations but no current problems with those bridges. There are lots of county bridges, don't have jurisdiction, not much cooperation.

Nurmi: This is to understand why some states are doing things and why others are not.

Nurmi: [Asks another audience member]

Q: Talked with FHWA, go do assessments, look at 1990 to 2005, trying to reduce number of bridges on list. Will have to prioritize, re-channel some of our funds. Biggest focus will be to look at our records and clean up list.

Nurmi: Vermont only has 100 on list. That's a low number.

Vermont: started with 800 UFs, able to reduce through research. We haven't been involved with developing a UF program. Haven't tapped our fundings. The superstructures of our bridges are in bad shape, money is gone for UF quickly.

Sibley: The topic isn't on a lot of radar screens. Have to talk it up, make them believe there is a significant risk here.

Q: Most of this has been on scour issues, with a little bit on Karst issues. Also have other conditions, lateral loading conditions, barge impacts, seismic problems, debris. Want your thoughts on direction of UF, which way we need to go, where focus needs to be. Flow chart looked like ti was all scour-based. Thoughts on other modes of failure?

Athar Khan: In Indiana, geotech people didn't get involved until xxx. We tried to do a few projects. But, for UF, don't think should limit one way over another. In Indiana, have areas in a seismic zone. Committee had a corridor selection, going to focus on structures in that area. Has nothing to do with scour, based on condition of structure. Came up with 800 structures in area. Asked AK to do work using consultants. We own a CPT rig, have other projects pending, done 15. Will be spending a lot of money on other investigations. Trying to reduce cost, but cost will be $300,000 to $400,000 per year, for geophysical applications, bridge replacements, etc.

Petrasic: Not involved with PennDOT's UF program. Our problems with foundations come from 3 areas, all have some man-induced component: sink hole issues; mining; and xxx. For most part, we get into reactionary mode. Funds get tied up in solving problem not preventing.

Q: What is the process the States use to deal with rebarge?

Mohammed: Administrators listen to our concerns, they'll believe us, we're on the front line. Not all UF are bad. Need use stats, risk analysis, total cost failures. In NC, using 90s info. Need to ask questions.

Q: Spending money discovering UF, do you have a system in place not to create more UF? Or do we need something in place?

Sibley: La. Assumes that where piles are is where they are. Since '90 all bridges been designed for scour, [but just state bridges].

Mohammed: Variety of information available.

Q: Alabama is building bridges, but info not going into database. We're entering information in. If don't have it, going and getting records now. Need to have someone tracking that.

Q: Florida spent money on Electronic Data Management. Taken several years to clarify the issues involved. Half of our bridges are locally owned and maintained. Starting a tracking process is tough, we have to follow through to get info. Look for larger efforts for you to piggy back onto, to solve your problems with their help.

Nurmi: Good point.

Hillsboro County, Florida: Tried to establish system of not accepting any bridge. Bridges built became part of our inventory. Bridge has to be designed to follow specifications. Every month we report to FDOT what we are counting, and what's our goal.

Nurmi: Let's talk about accuracy. What is accuracy and competency when doing these measurements? What do you need to know?

Q: Every new bridge built we are asking for records. We hold a meeting every 3 months with regard to scour, those issues coming up and we are trying to address them. With bridges already been built, trying to find pile records. Have used some of that information to make those bridges known. Trying to find average length of pile in ground. Want to put up bridges in low-risk category.

Nurmi: Accuracy?

Q: We are trying to find some information, going through era records, etc., to see what kind of length information. Don't have lot of confidence. Use other things, but good indication that Darrin Holt did some tests. Test results were better than several years ago. We can't place 100% confidence in that system yet.

Nurmi: Jerry Beard, how do you determine confidence levels?

Beard: Depends, varies. Find some after testing has been done. Rod drives are good for rock barrier. Double-testing [2 different testing methods]. More testing, the better you feel. Might ask consultant to redo it. If use all info available, over time gain confidence and get good results. Might be some variation in pile lengths. Critical point is most shallow pile in bin. Lot of variability built into piles.

Sibley: Don't have much to add. Haven't done lots of testing.

Nurmi: Pile length matter much?

Sibley: I think we do. If take 30 ft pile and calculations are off by 6 ft, when go compare to max predicted scour that 6 ft may play a lot. Have look at all of data.

Mohammed: There is, confidence level not as high as with the geotech engineers. Each method should its have own limits and be calibrated or a reduction factor should be used similar to factor of safety, lots of factors need to be addressed. 100% confidence is skeptical. Judgment is always all the way.

Nurmi: DOTs, is that what you're looking for.

Q: If calculate piles at 20 ft and scour at 25 ft, you should recheck those lengths. Sounded like accuracy in blind tests was highest in timber piles. I think that's bizarre, timber has highest variability. Seems like highest accuracy should be in steel piles, most uniform, then concrete, but would be cracked. Would somebody address that?

Athar: In Indiana, tested on H-piles. Don't know from what actual length they are. For steel piles, had UF and then testing so knew how close they came.

Sibley: The demo had close results, and other states had close results. I don't know why, structurally it's "slimy fish." But H-piles were either very close or way off.

Q: We are concentrating on length and condition of piles. We're missing forest for the trees. Each bridge has specific vibrations. If don't change can say it's stable. If parameters start changing then can say bridge might be more stable. Also, number of bridges recently built still have problems with UF, don't have all info. Need to keep record on vibrations. Quantify which bridges.

Sibley: What do you think cost comparison will be?

Q: NDT investigation should be a part of it. Cost will be less than what we are facing right now. Need to have information

Holt: Good question about timber results. On a wave propagation standpoint, timber is limber, allows motion. Lose more. Concrete is a volume issue, so much inertia that wave energy stays in concrete. Steel, is an H section, wave propagation in a plate. Lose lot of energy due to damping. When we were right we were right, when we were wrong we were very wrong. It's a wave propagation issue.

Q: Do I get better results for friction with steel?

Holt: Better results with skin friction.

Hawaii: Can't make one test fits all scenarios, need test as many as possible. The NDT tests aren't that high. Just do it right and get it done. That's problem with scour. Got to go in and get job done right and get experts. Money is no issues, do quality is key.

Florida: Talking about states vs. Feds, we're just doing experimentation. Waiting to see if we will get Fed funding. Other issue is we're looking for direction from Feds, could be guidelines from Feds or they'll accept what we come up with. We did some tests in '99, not too successful. One test on one method. They randomly failed. No thought of doing right test on right bridge. But goes back to who's going to pay.

Nurmi: To panel, how did you go about? Pick easy ones? Screening processes for selecting bridges?

Beard: Showed criteria for picking which bridges first. ADT, replacement project, construction date (not after 1990). It gave us 814 to look at. Started with critical major structures on major routes. Spread out, lots of travel involved. Working to come up with consensus. Determined which test for which bridge. We have a geologist on our committee. He told us about ground conditions. Varied their testing based on factors. Assigned consultants to single counties, reduced travel costs. We learned how to reduce costs by these methods.

Sibley: Used ADT, pile type, bridge length, location of piles. Didn't just want to test only piles in stream or not in stream. Varied the selection. Older bridges had shorter piles.

Athar Khan: Not lot of scour issues. Xxx.

Nurmi: I'm going to ask FHWA division people. Your concerns? What do you want, want to push, not push?

Colorado FHWA: I haven't been pushing lots of issues. Be nice to get summary of best practices from UF and with NDE testing.

New Mexico FHWA: haven't pushed hard to address UF. Have 76 scour bridges, focusing there first. For what we're looking for, the state of the practice has improved. Encouraging that there are technologies out there. Don't think we'll spend lot of money evaluating bridges. Not go spend $2,000 to $5,000 to evaluate a bridge, we don't have a lot of traffic. I could see us doing high priority bridges, but not lower ones. The high profiles could be done with records searches, eliminate half, the rest be slow process.

Nurmi: Strategy?

NM FHWA: NM not high tech for scour critical bridges. Might expand it. Do investigate after extreme events. Might do unknowns, but regular basis.

Colo FHWA: have 38 UF in states systems. Do treat UF as scour critical bridges. Supposed to close bridge if high water event.

Jorge Pagan: I have some strategies. Glad to hear this discussion. FWHA has been asking for actions for scour critical bridges and UF. Took action after a report. Accuracy, technology. We developed applications that put together different types of NDE regarding cost and accuracy. After memo issued, realized we weren't ready to force DOTs to develop plan. That's reason FHWA granted exception. Also directed DOTs to take exception to interstate bridges. In mid-90s, granted request for interstate bridges. Allowed dots to gather numbers of bridges for scour. Had to come up with strategy to evaluate interstate that serve national defense yet to be evaluated. Strategies should address interstate bridges. Use tools to develop priority list, materials, collectors. Then determine number to apply NDEs. Can't apply NDEs to all of them. Message is to prioritize interstate bridges then use risk-based assessment to determine different priorities.

Nurmi: Panelists give advice?

Mohammed: I think we are going in the right direction. Need risk analysis and confidence factor in the NDT methods. Continue to utilize and encourage developing different methods. Our program could be better in NC. Communication, info sharing would be good.

Khan: Always need improvement. In Indiana, have created argument for item. We can use item for investigation. Don't think should limit to scour. If want to utilize whole foundation, can do so. Pile length doesn't mean much to me. We are designing xxx. We create one program, side manager, keeping track of it. We get the info before we drive the pile. Afterward, they send that info on pile to us, add it to our database. There are always drawbacks, but try these methods. FHWA should create certification program so that know info is more reliable, consistent nationwide. Certification for private industry. Training for DOTs, better use info from private sector. Experts supposed to provide better recommendation.

Sibley: I look at NDE methods as a tool. Each specific for specific job. Will use each depending on what outcome you're looking for. To States that are considering NDE tools, don't take anything for granted. Use blind tests, mix them up to challenge consultant. Look at your results, use your own judgments on how to apply these things. Try to collect raw data from field. Analysis of data might improve.

Beard: Is material under foundation scourable? In NC, in piedmont have rock at certain depths, in mountains closer to surface. The key is material. Know your geology, your material, where your foundations are to that material. May have footings on half material that's scourable. Have lots of footings with problems in mountains. Need methods to analyze to see if scourable. That's key to making bridges safe. The problem is when pile is embedded or encased in concrete, less than 5 ft embedded. Need to look for these things. 14 bridges have been coated scour critical. We developing countermeasures to solve that problem. To FHWA, it all involves money. If states have separate money for UF, scour analysis, might be willing to do it. Also need money for research. DOTs do lot of own research, academia do research. Lot of smart DOT engineers here, maybe you could solve problems if have money. In-house DOT research money.

Petrasic: Going to be resource limits. Best thing is a good plan. Fit plans to your DOT. Make sure plan fits into total system. Couple with good risk assessment to efficiently target resources.

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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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