FHWA Unknown Foundations Summit
NDT of Unknown Foundations Bridge in Indiana
Athar Khan, Chief geotechnical engineer, Indiana DOT
No precise methods for measuring. Magic, voodoo, wheel of fortune.
Can use geophysics for developing unknown foundations.
Case Histories, 3 in Indiana
Case No. 1, S.R. 933/US 31 in St. Joseph County, Indiana, city of south bend.
two routes go together in city
needed way to deal with this
- 3-span, earth-filled concrete arch bridge
- Built in 1914
- Various repairs to super structure
- Scour protection added in 1988
- Existing pier no. 3 in poor condition w/severe scaling
- 1-inch wide vertical crack extending thru pierstem and footing to channel bottom
Purpose of Investigation
- Assess structure conditions
- Determine type of foundation
- Determine depth and size of foundation
Proposed Exploratory program and method of analysis October 2002
- 2 SPT BORING AT PIER NO. 3
- Parallel seismic survey to evaluate the possible presence of piles, type and length.
- Ground penetrating radar
[photo] crack in pier no. 3 going all way down and up
[photo] pier no. 3
Pier no. 3 Plan view
Boring Log B-1
Parallel seismic sketches from consultant
GPR survey, north side of pier and south side of pier, 5 ft and 10 ft from pier
10 ft away, don't see much scour there.
Underneath pier can see scoured area
Results and/or findings
- Parallel seismic survey revealed that the structure is supported by timber piles based on following seismic surveys
- GPR survey: north side 7 ft; south side 8 ft
Conclusions & recommendations
- Given subsurface conditions and assuming pile diameter 12 inches, working pile capacity for shaft length of 15 ft = 20 tons
- Design info estimated pier no. is anticipated at 5,000 tons
- Historical evidence suggests that scour depth has not been sufficient to significant xxx
- Extend coffer dam to depth of 5 feet below existing foundation grade.
- Modify existing river channel to avoid potential risk of scour beneath coffer dam
Bridge is going out for letting.
Case history #2 Michigan Rd bridge, Indianapolis: this was surprise, were doing regular survey for widening of bridge
Bedding plane voids observed during bridge improvement investigation
Voids were karstic in nature
Problem was, now purpose of investigation changes. This was big b ridge, lots of traffic.
Methods used during investigation
If on 1 side of pier multi layer of void, but 1 layer of void if on other side of pier.
Single void one west side pier no 3,4,5
Use drilled shaft foundation in area of pier 3,4,5
Drilled shaft would extend at least 6 ft. below voids
Based on cost analysis and risk factor, decided to replace entire bridge, replaced 2004
Case history #3
Indianapolis, Shadeland Avenue
Purpose and Scope
Bent supported on xx
Q: Mohammed Mulla: how much investigations cost?
A: $10,000 for first; $25,000 for second.
AK: City did not want to take risk. Estimated scour got from investigation