construction · infrastructure · family: it worked in the lab
bridges withno bottom
Tens of thousands of U.S. Bridges over water stand on foundations nobody can describe — so their scour vulnerability cannot be evaluated and owners fall back on plans of action
Problem statement
Scour — the erosion of streambed around piers and abutments during floods — is the leading cause of bridge failure in the United States, and evaluating a bridge's scour vulnerability requires knowing how deep its foundation goes. For a large population of older bridges the plans are lost, were never drawn, or omit the as-built pile lengths, so the foundation is "unknown": FHWA defines these as bridges "having insufficient details such that they cannot be evaluated for vulnerability to scour." As of December 2012 the National Bridge Inventory identified 36,076 bridges over waterways with unknown foundations — and the 2025 NBI release still carries 24,250 highway bridges coded "U" in Item 113 (verifier's count from FHWA's 2025 all-states delimited file, 2026-08-18) — and FHWA notes the count for bridges over land "is not known because this qualifier is not a reportable item." State and local bridge owners — the practitioners who must code these structures in NBI Item 113 — are told that non-destructive methods "can" determine the needed characteristics but that "currently available technology has limitations and the cost in some circumstances may prove prohibitive to many bridge owners."
Why this matters
Bridges that remain coded "U" cannot be moved into the regular scour program; since November 2010 each requires a Plan of Action — typically flood monitoring, inspection triggers or closure criteria — "until properly designed countermeasures are installed to protect the bridge foundations or until the bridge is replaced." That converts a knowledge gap into an open-ended operational burden for bridge owners, and it leaves genuinely vulnerable structures indistinguishable from safe ones. FHWA's stated goal is explicitly "to reduce, not eliminate the inventory of bridges coded U," conceding that "there will be an inventory of bridges that remain coded U because sufficient information could not be obtained." The same characterization problem blocks a second professional need: reusing existing foundations when a bridge is replaced, where "the main issues … are their condition assessment, their load-carrying capacity, their remaining service life."
What’s been tried and why it hasn’t worked
FHWA's risk-based procedure (2009) sorts unknown-foundation bridges into risk categories and allows three routes: positive discovery (field determination of embedment depth), inference (assuming foundation type and depth from era, construction practice, neighboring bridges and pile-driving records), or an accepted-risk Plan of Action. Positive discovery can use test pits or borings, or geophysical NDT methods — parallel seismic (which needs a borehole drilled alongside the foundation), sonic/ultrasonic echo, induction-field, ground-penetrating radar, resistivity — catalogued since Geotechnical Guideline No. 16 (1998) and rehearsed again at FHWA's 2013–2014 foundation-characterization workshops. They work in favorable conditions but are limited by access (piers in water, pile caps under fill), by ambiguous signals in stiff soils or with battered or spliced piles, and by cost per bridge that is hard to justify for a low-traffic rural structure — hence FHWA's own caveat about prohibitive cost. Inference is cheap but carries exactly the uncertainty the scour evaluation was meant to remove, and owners differ on how much inference to accept. On the related question of reusing foundations, FHWA's 2013 workshop found that "States have their own individual procedures; there is no ready means of assessing the present practice." The research agenda the 2013 workshop recommended — better field-evaluation guidelines, load testing of existing foundations, and instrumentation of new foundations ("smart piles") or existing ones — was still a list of research needs in 2014, when the report on that year's workshop said FHWA was "just beginning a research program that focuses on foundation characterization."
What would unlock progress
Progress needs a low-cost, access-tolerant way to estimate embedment depth from the accessible parts of a pier — for example, dispersive-wave or vibration-based methods interpreted with modern signal processing, or fusion of several cheap partial measurements with a Bayesian prior built from era-specific construction practice — packaged so that a county engineer can apply it without a specialist crew. Adjacent fields with transferable methods: pile-integrity testing in new construction, borehole-free seismic characterization in geotechnics, and utility-locating practice that combines weak signals from multiple sensors.
Entry points for student teams
A civil/geotechnical team could build a scaled test bed (driven model piles of known lengths in a soil box or field plot) and quantify how accurately a surface-only method — impact-echo or flexural-wave dispersion with modern spectral analysis — recovers embedment depth, then test on an accessible instrumented pier. A data team could build the inference side: a model that predicts pile type and depth from bridge age, owner, region, and structure type using state records where plans do exist, and validate it against known bridges to give owners a defensible prior. A systems team could analyze one state's inventory of U-coded bridges to show where positive discovery is worth its cost. Skills: structural/geotechnical engineering, signal processing, statistics, GIS.
Genes
Genes marked “+” were added in a later review.
Structural cousins: stuck the same way, in other fields
Sources
"Foundation Characterization Program (FCP): TechBrief #1 — Workshop Report on the Reuse of Bridge Foundations," James G. Collin & Frank Jalinoos, FHWA-HRT-14-072, Federal Highway Administration Turner-Fairbank Highway Research Center, accessed 2026-08-17; "Additional Guidance for Assessment of Bridges Over Waterways with Unknown Foundations," M. Myint Lwin, FHWA Office of Bridge Technology memorandum HIBT-20, 2009-10-29, accessed 2026-08-17; "Frequently Asked Questions — Bridges Over Waterways with Unknown Foundations," FHWA, accessed 2026-08-17 go to source 1 ↗ go to source 2 ↗ go to source 3 ↗
Working notes
Notes made while writing and checking this brief, shown as written. They use working shorthand.
All three sources are FHWA agency documents (tier 1). The 36,076 figure is as of December 2012 (FHWA-HRT-14-072); the current count was computed by the verifier from FHWA's 2025 highway-bridge delimited NBI file (2025HwyBridgesDelimitedAllStates.txt, SCOUR_CRITICAL_113 field): 24,250 of 624,193 records coded 'U' (a further 108,103 coded 'N', not over waterway) — a one-third decline since 2012 but still a large residual population, so the framing stands. The specific NDT limitations (access, ambiguous signals in stiff soils, battered/spliced piles) are the brief author's summary of standard practice knowledge rather than quotations from these documents; the FHWA FAQ supplies only the general statement that "currently available technology has limitations" and cost "may prove prohibitive." NCHRP Web-Only Document 107 (risk-based management guidelines) is referenced by FHWA but was not read. `failure:lab-to-field-gap` reflects that geophysical methods demonstrated at test sites do not transfer economically to the field inventory; `failure:not-attempted` was ruled out (a decade-long federal initiative exists). Related collection briefs: `construction-bridge-visual-inspection-subjectivity` (superstructure condition rating), `construction-post-tensioned-tendon-corrosion-detection` and `construction-rebar-cover-depth-field-verification` (other NDT gaps in existing structures); none addresses foundation depth. Verified at intake 2026-08-17: gate (net) + adversarial source check + contested-tag second coding. Verifier note: 36,076 (Dec 2012), 'not a reportable item', 'reduce, not eliminate', 'remain coded U', the November 2010 POA trigger, 'insufficient details', 'currently available technology has limitations … cost … prohibitive', 'States have their own individual procedures; there is no ready means of assessing the present practice', 'main issues for foundation reuse', load-testing and 'smart piles' recommendations all confirmed in the three FHWA documents.
Reconciliation 2026-09-29: The whole write-up was read against its sources. All three cited FHWA documents were fetched and read in full on 2026-09-29 from the addresses on the Source line: the TechBrief FHWA-HRT-14-072 (24 pages), the memorandum of October 29, 2009, and the FAQ memorandum of June 3, 2009. Two more FHWA documents, both linked from FHWA's unknown foundations page (https://www.fhwa.dot.gov/unknownfoundations/), were fetched the same day and read in part: Geotechnical Guideline No. 16, "Determination of Unknown Subsurface Bridge Foundations," issued August 27, 1998 (https://www.fhwa.dot.gov/engineering/geotech/policymemo/gt-16.pdf), which this write-up names, and Hydraulic Engineering Circular No. 18, "Evaluating Scour at Bridges," Fifth Edition, April 2012, FHWA-HIF-12-003 (https://www.fhwa.dot.gov/engineering/hydraulics/pubs/hif12003.pdf). (1) The text said the Plan of Action requirement is "a permanent operational burden for thousands of small owners." No document gives a number of owners, and the memorandum says the plan lasts until countermeasures are installed or the bridge is replaced. The sentence now says "an open-ended operational burden for bridge owners." (2) The text said positive discovery "relies on geophysical NDT methods." The 2009 memorandum describes positive discovery "using NDT, borings or test pits." The sentence now includes test pits and borings. (3) The text used the quotation "States have their own individual procedures; there is no ready means of assessing the present practice" as a finding about how much inference owners accept. In the TechBrief that sentence is about the reuse of foundations and comes from the summary of the 2013 workshop. The text now says so. (4) The text said the research agenda "addresses future bridges more than the legacy inventory." The TechBrief's recommendations include load testing of existing foundations, field evaluation guidelines for unknown and known foundations, and instrumenting new foundations ("smart piles") or existing ones, so the comparison does not hold. The sentence now lists the recommendations of the 2013 workshop as the TechBrief gives them and quotes its statement that FHWA was "just beginning a research program that focuses on foundation characterization." Confirmed and left as written: the Source line for all three documents; 36,076 bridges as of December 2012; the "not a reportable item" statement; the definition beginning "having insufficient details"; the "limitations" and "prohibitive" quotation; the November 2010 Plan of Action trigger and the "until properly designed countermeasures" quotation; "to reduce, not eliminate"; "remain coded U"; the three risk categories and the routes of positive discovery, inference and Plan of Action; the "main issues" quotation on foundation reuse; the 2013 and 2014 workshops. The 2025 count was recomputed from FHWA's file 2025HwyBridgesDelimitedAllStates.txt, downloaded on 2026-09-29 from https://www.fhwa.dot.gov/bridge/nbi/2025hwybronefiledel.zip and the counts match the text. There are 24,250 records coded U and 108,103 coded N out of 624,193. Three statements are not in the three cited documents but are supported by the two other FHWA documents and are left as written. Hydraulic Engineering Circular No. 18 says "The most common cause of bridge failures is from floods scouring bed material from around bridge foundations" and lists a bridge closure plan among the parts of a Plan of Action. Geotechnical Guideline No. 16 says that for many older bridges "there are no design or as-built bridge plans available," describes sonic echo, parallel seismic, ground penetrating radar and induction field methods, says parallel seismic needs a drilled and cased borehole, and lists buried pile caps, stiff soils and access among the limits of the methods. NOT checked: "spliced" piles as a source of ambiguous signals, which was not found in any document fetched, and the statement that the cost is hard to justify for a low-traffic rural bridge, which is the brief author's reading. NCHRP Web-Only Document 107 was not opened. These are left as written. Genes untouched.