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Unknown Foundations

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FHWA Unknown Foundations Summit

Experiences with Unknown Foundations Investigations at LADOTD
Steven Sibley, bridge maintenance engineer, LADOTD

Not a maintenance engineer, his boss is

LA's experience go back about 3 years. Problem has been known for while.

1990s started assessing bridges for scour susceptibility
Bridges without plans rated U (unknown)
Keeping an eye on the uncertain ones
2002—5,460 unknown foundations

1989 maintenance issued 2 directives—1) prompt notification of bridges in danger of collapse; 2) monitoring of bridges prone to drift and scour

Inspectors have intimate knowledge of bridges and how fair during storms. Inspectors were assigned to bridges for monitoring during flooding.

2003 approached by FDH-SE. Department had compared shorter structure on pile with idea that string could move. Bridge rated on that. Have pile-supported footings in the channels. Selected FDH for pilot project. Also looked at parallel seismic, cross bore-hole logging, sonic echo. Dispersive wave method and its lower cost was good.

FDH tested 3 bridges 2 concrete, 1 timber

Pilot Project 107 bridges to test, mostly interstate, US bridges, also some non-fed aid bridges for comparisons
11 control structures. Consultant didn't have access to files to find links
Test piles selection—didn't want to test every pile on every bridge. Wanted to statistically select piles on the bridge. Used factors such as drift build up

Issues Identified
Inconsistent results for steel piles. 1 bridge was good, others would have some piles good, others 12 –15 feet off. Hoped for error of +/- 10 percent, weren't meeting it.
Larger errors in timber piles
Concrete errors average 2.7 percent
Determined that bridges selected had dense sand layers, and bridge construction forces hit sand layers at blows per foot over 200. consistency of their results were higher. When retested came back with same result, gave good feeling about technology.

Timber results demo
Not concerned with 4 to 5 ft difference in piling. Some states been looking for nondestructive ways to get to 1 ft or 1-2 inches. We not concerned with that exactness for that data

Data useage
Bridge scour analysis to be conducted
Scour susceptibility rated based on predicted scour and tested pile length
Statistical approach to selecting future testing sites. Wanted to reduce number of bridges to test.

Current testing
LADOTD will have eliminated
FDH has promised to find problem with steel pile testing
LADOTD will assist them in verifying a testing procedure for steel piles.


Q: Dick Osman: Had problems with steel piles, hit rocks.
SS: Most of steel piles in southern part of state, no rock, normally pretty deep.

Q: Scott Sabol: Percentage of bridges got off of unknown status just thru record tracking?
SS: Better than half. Did records search, if found did analysis, if not then "unknown"

Q: xxx: How many people die in your states due to scour related bridge collapsed?
SS: No fatalities due to scour, but have lost bridges to scour. Lost a concrete bridge due to lots of drift in storm.
AK: None in Indiana.
JB: Don't know, need check. Lot of failures are in bents taken out, are washed around, 80% of failures due to that. Lots of debris related failures. Scour is gradually in coastal area and can be fixed in time. In Plains, xxx.
SS: Louisiana is flat, so scour prone bridges are xxx. Roads are typically in flat areas, so flooded, no one on bridge.

Q: Sayeed: relation between scour and phase 3.
SS: No phase 3 with bridges with unknown foundations.

Q: Current guidelines
A: Concerned with good results, need look at area to keep consistent with results.

Q: Changing recording after tests?
SS: Yes we are, will change to 6, but need to test for scour.

Q: Confidence in tests?
SS: Confident with 10%. Will use

Q: Factor to multiply results?
SS: Will ask to retest bridges. Only problem has been with steel. Steel bridges in pilot contract removed them with more concrete and timber bridges, not as confident with steel bridges to be tested.

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Updated: 03/09/2015
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