Non-Destructive Testing of Unknown Bridge Foundation Lengths Using Parallel Seismic Testing-Paper

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The paper evaluates Parallel Seismic Testing (PST) as a non-destructive method for estimating the lengths of undocumented bridge foundations—information needed for scour assessment, service-life evaluation, and decisions about reuse or replacement.<br /><br />PST involves installing a water-filled, cased borehole close to the foundation and recording seismic-wave arrival times at regular depths after impacts are applied to the structure. On a time-versus-depth plot, a change in slope indicates the approximate foundation tip. Results depend on wave velocities, soil conditions, foundation inclination, and test geometry; nearby components in complex foundation systems can produce misleading signals.<br /><br />Two Florida bridge investigations demonstrate the method’s range. At the Green Bridge, PST was used on conventional pile bents and waterline footings. Results were generally high quality, with estimated pile-length accuracy of about 1–2 feet at the site. Low Strain Integrity Testing (LSIT), calibrated using PST results, provided useful supplementary estimates, though its accuracy varied.<br /><br />At the SR 80 Southern Boulevard Bridge, PST estimated East Bascule Pier piles at approximately 27 feet, consistent with soil conditions and plan quantities. West Pier results were inconclusive because retained cofferdam sheet piles may have generated the observed signal, illustrating the need to account for complex foundation components and select borehole locations carefully. During demolition six years later, extracted East Pier piles were estimated at 28–30 feet, corroborating the earlier PST estimate—a rare field confirmation of a non-destructive prediction.<br /><br />The authors conclude that PST is a robust and versatile method for estimating unknown foundation lengths across different bridge configurations. Its effectiveness is greatest when testing is carefully planned, results are interpreted with knowledge of the structure and ground conditions, and complementary methods are used where appropriate.

Keywords

Parallel Seismic Testing

PST

bridge foundations

non-destructive testing

scour assessment

foundation length estimation

seismic-wave arrival times

Low Strain Integrity Testing

Florida bridge investigations

foundation tip detection