The current state of highway infrastructure across the country is cause for concern. Many highway-related structures, including culverts, are deemed to be in a state of disrepair or obsolescence. Culverts serve the dual purpose of conveying water under the highway while carrying traffic over streams and ditches. As such, they affect and influence the safety of the public traversing these highways while also ensuring proper drainage and hydraulic performance. (Photos 1 and 13)
Culverts must be properly inspected, assessed and evaluated for structural and functional performance on a regular basis as part of a logical highway maintenance and inspection program. It is important that all those who inspect highways are trained to understand the key factors related to culvert inspection and maintenance. Alternatively, the option of contracting with outside experts should be considered.
This article addresses practical issues associated with the inspection, assessment, evaluation and load rating of such culverts. The focus is on factors that influence the structural integrity of the culvert and items affecting efficient hydraulic performance. Methods of evaluation and discussion of potential solutions such as replacement, repair and maintenance also are presented.
Culverts typically are defined as conduits for conveying water through an embankment and serve as grade separations for the waterway and the surface traffic or facility above. The embankment may be used for a roadway, highway, street, railroad, access to an industrial facility or a private business, etc.
It is very common to find culverts under many state, county and municipal roadway systems. In most states, there are three to five times as many culverts in service as bridges. For states such as Ohio with more than 30,000 bridges, or Texas with more than 50,000 bridges, the number of culverts is overwhelming.
The Federal Highway Administration (FHWA) defines a culvert as a structure with a total span less than 20 feet. This can be a single span or several openings passing the same body of water, added together. (Photo 2) Conversely, the FHWA considers structures with a span equal to or greater than 20 feet to be a bridge. (References 5b) All bridges should be listed in the National Bridge Inventory (NBI), a database comprised of each state's department of transportation (DOT) bridge inventory. (References 5c)
Culverts come in many shapes, sizes and material types, and will vary in their functionality and design accordingly. Culverts are classified as rigid, semi-rigid or flexible based on their material type, how they carry load and to what degree they rely on the surrounding soil envelope for support.
While flexible culverts rely to a much greater extent on soil-structure interaction principles, the support of the underlying foundation and surrounding soil envelope is an important factor in the stability of any culvert and its traffic surface above. This article provides an overview of the practical design and performance differences to consider. However, detailed culvert design factors and procedures are readily available through the listed references. Those designing, inspecting, evaluating and specifying culverts must understand the functional differences between culvert types and the degree of dependence upon the in-place strength and physical characteristics of the underlying and surrounding soil.
Photo 1: Steel Structural Plate Culvert
Inspection and evaluation
Culverts present unique challenges when it comes to proper inspection. Unlike the well-established National Bridge Inventory (NBI) and the required inspection intervals for all U.S. bridges, culverts have had less regular attention paid to their in-place performance. In many cases, culvert inspection happens as a result of problems with the roadway above.
Very few states have a routine, proactive, comprehensive and measureable culvert inspection program. Although the FHWA suggests that states develop and implement such a program, a mandate has not been issued to state DOTs. As such, culvert inspection often falls to established bridge and highway inspection personnel, some of who may not have the extensive knowledge specifically needed to understand and appreciate the nuances and key, relevant factors associated with culvert inspection.
The proper evaluation of any given culvert is completely reliant on the data collected during a field reconnaissance. Collecting relevant data leads to a reliable evaluation, and ultimately a reliable load rating. A trained and competent culvert inspector will observe, and, when possible, measure and obtain all of the following key culvert data:
- The culvert's length, size, age, height of cover, wheel loading and material type
- Number of barrels (Photo 2)
- Type and condition of end treatment, including end sections, headwalls, rip-rap, nothing, etc.
- Waterway condition, such as inlet blockage/debris affecting hydraulic performance (Photo 3)
- Inlet and outlet scour condition, including erosion parallel to the culvert
- Shape geometry of flexible culverts, requiring detailed measurements (Photo 4)
- Culvert wall thickness and condition, such as cracking, extent of corrosion, spalling and exposed rebar (Photos 5, 6, 7 and 8)
- Infiltration and loss of structural backfill through joints, cracks or inlets
- Evidence of roadway settlement or repair, such as pavement distress, cracks or signs of recent repairs and repaving efforts, and guardrail sagging over the culvert
- Installation records to obtain backfill requirements or specifications
Depending on the length, size and number of barrels of the culvert to be inspected, it typically will take most of a day to collect this required data. Imagine a state DOT having 100,000 culverts and devoting the required man hours to accomplish this comprehensive approach on a routine basis. Consequently, a detailed approach to culvert inspection is often not taken or is outsourced to non-DOT consultants.
Some culverts, due to their size and/or limited accessibility, may be inspected by camera or video monitoring. Others may be inspected by deflection probes or other types of equipment that allow for basic evaluation and assessment where entry by inspectors isn't practical, possible or safe.
Photo 2: Multiple Barrel CMP
Photo 3: Inlet Blockage and Debris
Culvert inspection poses certain safety and risk issues. The inspection team needs to be properly trained in confined space entry. Tripping and slipping hazards exist. Conditions may be hazardous. Slope stability and negotiating embankments with potentially unstable footing and vegetation calls for proper safety measures. Working in teams or in tandem is recommended. This article will not attempt to list or describe all of the relevant issues and necessary safety precautions. It is recommended that inspection personnel follow OSHA and other safety guidelines for such work.
The safe live-load carrying capacity of a highway bridge or culvert is its "load rating". It is expressed as a rating factor (RF) or expressed in terms of tonnage for a particular vehicle. The RF is simply the structure's capacity at material yield (Capacity) less the dead load (DL) divided by the live load (LL) plus Impact as appliable. Restated, the structure must have enough strength remaining after the dead load is subtracted to carry the live load. Otherwise, this rating factor is less than one (1) and problematic, requiring a reduction in the applied load to keep the culvert in service. This process is called "posting," where a reduction in the allowable load is established and posted on the roadway to match the in-service capacity of the culvert. (Photo 9)
Determining the in-service capacity of a bridge or culvert is the cornerstone of an accurate load rating. The longer a culvert has been in service, the more likely the field characteristics show signs of deterioration that ultimately reduce the culvert's design capacity, resulting in a lower load rating than originally designed. The question becomes: Is there still enough remaining capacity for the culvert to safely carry the required in-service loads?
The American Association of State Highway and Transportation Officials (AASHTO, References 3a) provides three ways to calculate the rating factor:
- Allowable Stress Rating (ASR) — uses unfactored loads and the allowable working stress of the construction material
- Load Factor Rating (LFR) — uses factored live and dead loads and a slight resistance factor applied to the capacity based on the construction material
- Load and Resistance Factor Rating (LRFR) — uses factored live and dead loads, and condition, system and resistance factors based on the construction material's nominal capacity.
Most DOTs use AASHTO guidelines to determine which of these methods to use to calculate the rating factor. In general, new culvert and bridge designs completed since October 1, 2010, are rated using the LRFR method. The NBI ratings are based on the HL-93 culvert and bridge design loads, and should be reported as a rating factor (References 3b). For new bridges designed prior to October 1, 2010, where the design was based on Allowable Stress Design (ASD) or Load Factor Design (LFD), the NBI ratings can be based on either LFR or LRFR. The rating factors are based on AASHTO HS-truck loading and reported in tons when using the LFR method. Culverts that have been in the ground at least 30 years also use LFR or LRFR even though they were designed using allowable stress design.
Photo 4: Detailed Shape Measurement
Photo 5: Concrete Cracking, Exposed Rebar
There are a few tools and programs that have been developed by DOTs and private industry to aid the engineer in performing load ratings. The National Corrugated Steel Pipe Association (NCSPA) Data Sheet 19 (References 6b with link) serves as the basis for load rating buried flexible corrugated metal pipe. Many DOTs, including Ohio, have modified this data sheet to include the LRFR method. A spreadsheetdriven solution is available from the Ohio Department of Transportation (References 9).
BRASS-CULVERT™ (References 10) is another tool for analyzing reinforced concrete box culverts and uses all three load rating methods. It is available from the Wyoming DOT, which developed this program with the FHWA, North Carolina DOT, New York DOT and the University of Wyoming. BRASS-CULVERT analyzes and designs one- to four-barrel reinforced concrete rigid or flexible box culverts, with or without bottom slabs. End skews can be defined. Wall and slab thicknesses may be specified or set by the program.
Photo 6: Spalling and Exposed Rebar
Photo 7: Corrosion at Field Cut Tie In
Culvert load rating tools for HDPE and PVC plastic pipe, precast concrete arches and boxes, reinforced concrete pipe and fiberglass reinforced pipe may be available through local manufacturers. Additional resources also are available though trade organizations, such as:
Photo 8: Minor Invert Corrosion
Photo 9: Typical Culvert Posting
Next step action items
There are many available options to consider when deciding if a culvert should be replaced, repaired, rehabilitated or assigned a diminished load rating. The structural capacity and hydraulic functionality of the culvert will be governing factors when it comes to making decisions as to the proper course of action for deficient or deteriorated culverts. Other key factors include the safety and convenience of the traveling public and adjacent property owners; traffic volume and vehicle loading details; economic considerations and budgetary constraints; and the feasibility of various repair versus replacement options.
Often, there will be multiple courses of action deemed as appropriate and these may be combined for a period of time as satisfactory, temporary action plans until funds are available for a more permanent solution. Partial rehabilitation or repair, temporary bracing when appropriate, invert paving, etc. — perhaps combined with a posted load reduction — could be employed as methods to keep a culvert in service and functional until more permanent measures can be undertaken. A thorough assessment should provide an indication of whether minor and "cosmetic" repairs would suffice, as opposed to a more elaborate repair or rehabilitation operation. Transportation officials must consider the safety and conveniences of the travelling public, assess the feasibility of various repair methods, take into account the effects of any posted load reductions and evaluate the economics and practicality of various rehabilitation methods compared to total replacement. Often relining of a deteriorated culvert is chosen as the most logical and appropriate means of ensuring continual performance with minimal disruption. (Photos 10, 11 and 12)
It's an unfortunate reality that culverts are not always installed properly and/or the site conditions may have changed over the years. For example, cover height may be below current recommended minimums for the anticipated live loads the culvert is expected to handle. Careful and controlled removal of the existing pavement section and installing a reinforced concrete load relief slab over the culvert perhaps combined with higher strength fill such as a concrete slurry or a controlled low strength mortar fill for the upper section of the select fill zone may be an option to consider when dealing with situations involving less than the required minimum cover.
Photo 10: CMP Reline of Concrete Box Culvert
Culverts represent a significant portion of the highway infrastructure. There is a percentage of installed culverts that are operating at diminished functionality from the perspective of structural performance and hydraulic efficiency. As such, it is highly recommended these culverts be properly inspected and maintained by state DOTs and other roadway and public transportation departments, agencies and officials to ensure satisfactory performance and safety. Unfortunately, typical bridge inspection procedures and guidelines may not be adequate for many culverts. Often there are not sufficient personnel to carry out such detailed inspections and evaluations.
The references and methods discussed in this article and listed in the References section provide a starting point for those who evaluate in-service culverts to develop a logical culvert inspection and maintenance program. Options are available when it comes to deciding if a culvert should be replaced, repaired/rehabilitated or assigned a diminished load rating. Key factors influence the choices made regarding the feasibility of various repair versus replacement options.
Using outside firms that specialize in these services may prove to be the most logical and cost-effective approach to a comprehensive culvert inspection and load rating program.
Photo 11: Structural Plate Reline of Masonry Culvert
Photo 12: Structural Plate Reline of Stone Culvert
Jim Noll, P.E.,is director of engineering services for Contech Engineered Solutions. He has 36 years of experience in the corrugated metal pipe industry and is an active member of various technical organizations including ASCE, AREMA, NCSPA and ASTM.
Mitchell T. Hardert, P.E.,is the chief engineer for CBC Engineers & Associates, Ltd. He has 17 years of experience designing, installing, evaluating and load rating buried corrugated metal pipe structures across the US.
Joseph A. Dennis, Jr.,is director of business development for CBC Engineers & Associates, Ltd. He has 24 years of experience designing, manufacturing, installing, evaluating, and load rating buried corrugated metal pipe structures across the US.
American Association of State Highway and Transportation Officials (AASHTO):
- 1. American Iron and Steel Institute (AISI), Handbook of Steel Drainage & Highway Construction Products, 1994.
- American Railway Engineering and Maintenance-of-Way Association (AREMA), Manual for Railway Engineering, Vol. 1, Chapter 1, Section 4.19, "Culvert Inspection," 2012.
- American Association of State Highway and Transportation Officials (AASHTO):
- AASHTO LRFD Bridge Design Specifications, Sixth Edition, 2012.
- AASHTO Manual for Bridge Evaluation, 2nd Edition, with 2011 Interim, 2011.
- David Cowherd and V.G. Perlea of Bowser-Morner Associates, An Evaluation of Corrugated Metal Pipe Arches, 1988.
- Federal Highway Administration (FHWA):
- Culvert Repair Practices Manual Volumes 1 and 2, Report Nos. FHWA-RD-94-096/FHWA-RD-95-089, May 1995.
- Culvert Inspection Manual, Report No. FHWA-IP-86-2, July 1986.
- National Bridge Inventory (NBI) database, April 2005 - current.
- National Corrugated Steel Pipe Association (NCSPA):
- Corrugated Steel Pipe Design Manual, 2008.
- Load Rating and Structural Evaluation of In-Service, Corrugated Steel Structures, Design Data Sheet No. 19, 1995.
- Stormwater/Culvert Rehabilitation Methods, Design Data Sheet No. 16, 1991.
- Jim Noll, P.E. and Bob Frascella, P.E. for Contech Engineered Solutions, "Rehabilitation and Relining of Culverts," CE News, September 2008.
- Jim Noll, P.E. and Matt Westrich, P.E. for Contech Engineered Solutions, "Practical Factors and Considerations Related to Culvert Inspection," CE News, July 2010. < www.ContechES.com/PDH>
- Ohio Department of Transportation (ODOT), Bridge Management System LRFR spreadhseets, 2012.
- Wyoming Department of Transportation (WYDOT), Analysis, Design and Rating of Reinforced Concrete Box Culverts, BRASS-CULVERT™ Version 2.3.4, June 2012.