The catchphrase “out of sight, out of mind” too often can be used to describe the culverts located under our nation's roadways. Once installed, many lie forgotten and inconspicuous, conveying discharges from one ditch or waterway to the next. However, any component neglected long enough will eventually make its presence known — many times in the form of a catastrophic failure and frequently resulting in large expenditures of emergency funds.
To gain a better understanding of the economic impacts of these failures, the Transportation Research Board (TRB) of the National Academies arranged for a study of culvert failures in North America. The resulting report, “The Economic Costs of Culvert Failure,” was written in November 2003 by Joseph Perrin Jr. and Chintan S. Jhaveri of the University of Utah. The American Concrete Pipe Association provided funding for the report.
By surveying U.S. and Canadian agencies, researchers were able to investigate design and selection practices. They also analyzed documented failures to quantify costs, identify the need for documentation, and determine if risk of failure influences culvert selection.
Reviewing existing literature
To determine existing standards of practice for pipe design life, Perrin and Jhaveri reviewed literature related to culvert design and selection. Referencing a U.S. Army Corps of Engineers report from March 1998, the researchers summarized their findings as follows:
Service life: For major projects, designers should use a minimum service life of 100 years when considering life-cycle design.
Concrete: Most studies estimated service life at 70 to 100 years. Of nine state highway departments, three listed 100 years, five said 70 to 100 years, and one said 50 years.
Corrugated steel: Usually fails due to corrosion of the invert or pipe exterior. Properly applied coatings can extend life to at least 50 years for most environments.
Aluminum: Usually affected more by soil-side corrosion than by corrosion of the invert. Long-term performance is difficult to predict because of the material's relatively short history of use, but designer shouldn't expect more than 50 years.
Plastic: Many materials fall under this general category, each of which may have unique applications where use is suitable or unsuitable. Performance history is limited. Designers shouldn't service life of greater than 50 years.
The American Association of State Highway and Transportation Official's (AASHTO) 1991 Model Drainage Manual also was referenced and recommendations for design and selection presented:
Material selection shall include consideration of service life that includes abrasion and corrosion.
Culverts shall be located and designed to present a minimum hazard to traffic and people.
The detail of documentation for each culvert site shall be commensurate with the risk and importance of the structure. Design data and calculations shall be assembled in an orderly fashion and retained for future reference.
Culverts shall be regularly inspected and maintained.
Material selection shall consider replacement cost and construction difficulty as well as traffic delay.
Selection shall not be made using first cost as the only criteria.
Select an alternative that best integrates engineering, economic, and political considerations.
The chosen culvert shall meet the selected structural and hydraulic criteria and shall be based on:
- construction and maintenance costs
- risk of failure or property damage
- traffic safety
- environmental or aesthetic considerations
- political or nuisance considerations
- land use requirements.
Because existing literature provides for a wide range of estimates for design life and method of culvert selection, the authors stress a need for documenting performance and failures on a national level as a way to better predict service life and costs.
Developing a cost analysis
Although initial installation is usually considered in a cost analysis, only a few respondents applied some type of life-cycle cost analysis (LCCA) when designing and selecting culverts. Neglecting to include future replacement costs won't reflect actual cost. The authors proposed an equation that accounts for replacement and user delay costs as well as installation.
Cost of culvert installation over 100 years
|Assumed life of culvert||25 years||50 years||100 years|
|Number of replacements in 100 years|
Perrin and Jhaveri sent out surveys to 57 agencies in the U.S. and Canada. Following are their observations based on the 25 agencies that responded:
Only four perform a least-cost analysis for pipe material selection.
Different agencies assumed different life-cycles for each material. Assumed life of reinforced concrete and non-reinforced concrete varied from 50 to more than 100 years, assumed life of corrugated metal varied from 35 to 50 years, assumed life of high-density polyethylene (HDPE) varied from 30 to 100 years, and assumed life of polyvinyl chloride (PVC) was 50 years. None of the respondents used vitrified clay pipe so no data were available.
Five agencies provided cost data. Results varied from agency to agency and by pipe material and size. Some agencies also mentioned that pipe cost was negligible compared to earthwork, traffic control, and installation/labor costs.
Only two agencies considered the risk of failure while performing a cost analysis as recommended by the AASHTO drainage manual.
Only five agencies considered emergency replacement costs. Seven agencies considered user costs that could be incurred in the event of a failure. This is confusing, as only four states responded that they perform a LCCA.
Fifteen agencies document failures within their jurisdiction. However, the level of documentation detail greatly varied. Most written documentation was unavailable.
Some agencies didn't allow the use of a certain pipe type above a particular annual average daily traffic (AADT) level. 42% of the agencies stated that AADT was a consideration in pipe material selection, often only allowing rigid pipe on high-AADT roads. This identifies that risk and traffic delay are indirectly considered in pipe selection, often without conducting an LCCA.
Failure: when, where, why
Agencies also were asked to submit case studies of failures. For the seven presented in the report, Perrin and Jhaveri found a wide range of traffic conditions, replacement costs, and user impacts and costs.
All failures took place in pipes that had met or exceeded expected design life.
Four failures occurred along interstates; three occurred along two state routes and a Canadian highway. AADT on these routes ranged from 5,000 to 300,000 vehicles with heavy commercial traffic between 3% and 30%.
All but one failure required detours that extended travel times 20 minutes to four hours.
Total replacement costs, including user costs, ranged from $265,000 to more than $8 million.
Based on responses, Perrin and Jhaveri report that “some of the failures had met the agencies' expected life for that pipe type but no planned replacement was scheduled. This raised the concern that there is a lack of inspection and/or tracking of useful life expectancy.”
Easy way to prevent failure
Perrin and Jhaveri conclude that the “reality is that pipes aren't being replaced as they approach their expected design life. Only one agency inspected all culverts within its jurisdiction every two years and there are no reported failures from that agency.”
Based on case studies and survey results, the authors offered the following observation. “By quantifying the additional costs of emergency replacement it's clear that an inspection/maintenance program provides an attractive cost benefit.”
In addition to setting up a regular inspection and maintenance programs, the authors stressed the importance of adding replacement costs and user costs to the LCCA to obtain a more accurate cost of culvert placement.
The authors suggest “a need to establish a standard procedure of documenting culvert failures and expand the existing limited data into a nationwide database.” They call for AASHTO or TRB to take the lead in developing such a report.
— This article was developed and portions of it excerpted from “The Economic Costs of Culvert Failures,” by Joseph Perrin Jr. and Chintan S. Jhaveri. The complete report is at the TRB site at www.trb.org or the ACPA site at www. concrete-pipe, org.