In 1991, there were approximately 580,000 highway bridges in the National Bridge Inventory. For a large number of older non-federal-aid bridges, and to a lesser extent federal-aid bridges, there was no design or as-built bridge plans, and there was little or no information available to document the type, depth, geometry, or materials incorporated into the foundations. The Federal Highway Administration had announced it would begin to require state transportation departments to perform screenings and evaluations of all bridges over rivers or streams to determine their susceptibility to scour; however, information on bridge foundations was necessary to perform such evaluations. Similar information on foundations was also needed for seismic evaluation of bridges.

Research was needed to develop practical methods and functional equipment to investigate subsurface bridge foundations when information was unavailable. There was a need to determine the practicality, feasibility, and initial and operational costs for concepts, methods, and equipment that would provide a means to determine subsurface foundation characteristics where this information was not available, was unknown, or was uncertain.

The final report for this project has been approved but not published because a continuation project (21-05(2), Unknown Subsurface Bridge Foundation Testing) was awarded to Olson Engineering, Inc. to carry out further research and field test the most promising methods for determining unknown foundations. The findings of this project have, however, been published as "NCHRP Research Results Digest Number 213, Nondestructive Testing of Unknown Subsurface Bridge Foundations -- Results of NCHRP Project 21-5."