United States Department of Transportation
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Foundation Reuse : Length, Condition, and Capacity of Existing Driven Piles
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2018-05-01
[PDF-8.15 MB]
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NTL Classification:NTL-HIGHWAY/ROAD TRANSPORTATION-Bridges and Structures;NTL-HIGHWAY/ROAD TRANSPORTATION-HIGHWAY/ROAD TRANSPORTATION;NTL-HIGHWAY/ROAD TRANSPORTATION-Materials;
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Abstract:Reusing existing bridge foundations is an appealing design alternative for many bridge projects, but methods for investigating and analyzing existing foundations vary widely. Researchers from the University of Missouri performed research to evaluate various methods for predicting the installed length, assessing the condition, and predicting the load capacity of existing foundations. The methods were evaluated via application to two MoDOT bridges, both built in the 1960s and replaced in 2017. One bridge was founded on driven closed-end steel pipe piles backfilled with concrete (CIP piles), and the other was founded on octagonal precast concrete piles. The foundations were not actual candidates for reuse, but were selected for study because of the age and type of piles. To perform the evaluations, researchers gathered and made predictions from historical records, performed various geophysical test methods to predict pile length and potentially identify deterioration, performed static load testing, performed high-strain dynamic analyses of pile restrikes, and exhumed the piles to assess true pile length and condition. The research found that historical pile driving records could be used to predict pile length within 3 percent of the exhumed length whereas values from as-built plans were as much as 30 percent less than the exhumed length. Results from parallel seismic testing produced estimates of pile length that were within 8 percent of the exhumed length. Parallel seismic results were more accurate when the sensor was located within 5 ft. of the pile. MoDOT’s seismic cone penetration test (SCPT) rig was effective for parallel seismic tests. Sonic Echo / Impulse Response test methods performed after bridge demolition were effective in predicting pile lengths for the CIP piles, but application to the precast piles underpredicted lengths by as much as 20 percent, most likely because of the taper in the precast piles. SE/IR tests performed prior to bridge demolition were mostly inconclusive, mostly because of complex vibrations between the pile and connected superstructure. Observations of exhumed piles for both CIP and precast piles revealed no significant deterioration. CIP piles had visible surface corrosion, but caliper measurements of wall thickness at sections cut through the most corroded portions were not significantly different from measurements through portions without visible corrosion. Estimates of axial load capacity of the existing foundations varied widely. Values listed on historical documents were the least, and quite conservative compared to values from static load testing. The CIP test pile was loaded to four times the historical design capacity, and the precast test pile was loaded to seven times the historical design capacity. Maximum load test values can be considered lower bound values since the tests terminated upon failure of the capping beam for the existing bridge, which was used as the reaction. Dynamic analysis of restrike data yielded estimates of axial capacity similar to the maximum loads applied during static load testing. The dynamic values are also lower bound values because the restrike hammer had insufficient energy to transfer significant load to the pile tips.
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