The status of the nation's infrastructure is such that many of our bridge and culvert structures are in need of serious repair or replacement. Unfortunately, the lack of adequate funding combined with the impact on the public produced by closing such structures to traffic often make total replacement or even significant repairs to these structures impractical. However, there are other options available to restore structural capacity and add years of satisfactory performance to many culvert structures while keeping them open to traffic or at least minimizing the impact on the public.
Often, the extent of repairs needed is relatively minor, perhaps consisting of adding a new wearing surface or invert lining. Even if the existing culvert is determined to be structurally deficient, culvert rehabilitation methods are not necessarily eliminated from consideration. Suitable repairs can be made to restore the culvert to an acceptable level of structural adequacy and integrity. Once this has been accomplished, appropriate rehabilitation methods and procedures can be undertaken to improve the culvert's performance and extend useful service life.
Rehabilitation costs and the time associated with such repairs are often significantly less than what is required for total replacement of these structures. Safety issues must be taken into consideration, and relining or rehabilitation is often a much safer undertaking for construction crews, as well as the travelling public. Keeping roads open, or at least partially open, is often more desirable and safer than costly and time-consuming detours and road closures.
The preservation of resources associated with efforts to salvage existing structures is an important consideration. Extending the service life of these culvert structures is a worthwhile and desirable goal in many instances, freeing up additional dollars, manpower, and available resources for other worthwhile projects.
The first step in any repair, rehabilitation, or replacement project is assessment and evaluation of the structure in question. This starts with proper inspection, along with a key understanding of the features of the culvert type and material. During such an evaluation, it is essential to consider the major structural and geometric characteristics unique to the particular type of culvert product and material under investigation. There are a number of references available (see page PDH6) to assist the engineer or inspector in this process. However, some of the key factors are discussed here.
A critical step in proper evaluation of any culvert structure is determining the type of material used in its manufacture and installation — for example, stone and masonry, reinforced (or unreinforced) concrete, corrugated metal (steel or aluminum), solid-wall steel or iron, or plastic (PVC or HDPE). Each of these culvert material types has a unique set of design and installation standards, along with applicable criteria used for proper inspection and evaluation.
Understanding the nuances and differences relevant to each culvert material type is important to accurately assessing structural adequacy, estimating service life, and determining whether a culvert is a candidate for rehabilitation.
Ideally, there exists a set of plans or other documentation pertinent to the specific culvert that is being evaluated for repair, rehabilitation, or replacement. Previous inspection reports prove invaluable in determining whether a culvert has experienced any changes or deterioration in physical condition, shape geometry, structural performance, or hydraulic capacity since the last time it was inspected. The prior inspection history should include those criteria that are relevant to the particular type of culvert product being evaluated. Not all of these criteria apply to each type of culvert product. Therefore, familiarization with the relevant criteria is important to a thorough and accurate assessment of each culvert.
A proper and thorough culvert inspection should include the following:
- Adhere to applicable OSHA requirements and related agency guidelines relevant to confined space entry and inspector safety precautions. Evaluate the risks associated with factors such as water flow surges, air quality, fall protection, and worker accessibility prior to undertaking culvert inspections.
- Compare inspection data — changes in shape, invert clogging, loss of wall section, perforations, cracking, settlement, joint separation or related problems, pavement distress, et cetera — to that found in previous inspection reports to help indicate any pattern of change or worsening conditions. This aids the inspector and the engineer in determining the proper course of action and assessing the relative urgency for any action required.
- Measure, record, and compare the cross-sectional, geometric shape dimensions of the culvert to shape measurements from previous inspections for any significant change and to establish a pattern of movement or deflection.
- Note and investigate cracks in the culvert wall, spalled or deteriorated wall sections, problems with seams, missing bolts or rivets, and buckling or other visible distortions to the culvert wall for potential effects on structural performance.
- Note and investigate changes to the road surface, guardrail, shoulders, and adjacent side fill embankments, along with any changes to the end slopes associated with the culvert site. Changes that could affect the integrity of the roadway should, in the interest of public safety, be considered in such an inspection.
- Since the primary purpose of a culvert is to convey flow, an inspection must consider the hydraulic performance and efficiency of the culvert. Any buildup of debris that would affect the ability of the culvert to carry flow should be noted and addressed. Undermining of the pipe invert at either the inlet or outlet ends should be noted, as should the loss of fill along the end slopes. Either situation can lead to loss of fill and support for both the culvert and the roadway above and may require attention in the form of cut-off walls, slope collars, or paving or other suitable modifications to the end protection.
- Inspect joint integrity and alignment. Open joints that allow for significant exfiltration of water or infiltration of backfill should be noted and marked for possible repairs. Misaligned sections of pipe affect the efficiency and rate of flow, as well as provide the means for infiltration/exfiltration to occur.
- Note and evaluate abrasion loss, effects of corrosion, perforations, and similar damage to the culvert material itself or to the coatings for the potential effects on long-term service life and structural performance. Record the presence of abrasive materials — such as rocks, gravel, sand, and other debris — representing a potentially abrasive bed load situation. Note that corrosion may appear to be much worse than is actually the case because of the voluminous nature of rust products associated with corroded steel. Often, after using an abrasive pad to remove such apparent rusting, it is discovered that the "rust" is actually staining of the culvert wall, and the metal and coating underneath is performing as intended. Conduct core sampling to verify the extent of actual metal loss. Any perforations should be noted for size, location, and frequency in order to quantify their severity.
It is recommended that all inspection personnel be adequately trained in the key aspects of proper culvert inspection and that they become familiar with the nuances and unique design features of the various culvert materials and types used in their area of responsibility. Such inspections can be, and often are, undertaken by agency personnel, but the need for proper knowledge in the functional hydraulic and structural design aspects of the particular culvert product type, along with familiarity of the site-specific installation conditions, can not be overemphasized.
There are also firms that specialize in such inspection services and that are capable of evaluating culverts based on the proper set of criteria applicable to that particular type of culvert. These firms should be familiar with the proper design procedures, standard installation practices, and relevant factors influencing evaluation and assessment of existing culverts for the type of culvert being investigated.
Repair/Rehabilitation versus replacement
Once a culvert has been identified as needing attention, the decision whether that particular structure should be totally replaced versus undertaking a repair and rehabilitation program needs to be made. Factors affecting this decision include the extent of damage and distress the culvert has experienced; site conditions; the effect on the travelling public for replacement work as opposed to any rehabilitation efforts; the relative costs of either approach; availability of funding; safety issues applicable to each type of undertaking; and estimation of the remaining service life of the existing culvert, whether the repairs are made or not. Local safety codes and regulations, combined with any state or federal guidelines, stipulations, or requirements, are all factors influencing the scope of work involved.
Often, the work associated with repair or rehabilitation of a culvert is significantly less expensive, less time consuming, and less disruptive to the travelling public than what is involved with total replacement of that culvert. Eliminating costly and time-consuming detours, combined with the inherent safety advantages to keeping the roadway open, may be key advantages to a repair-versus-replacement decision. Conserving material resources and preserving budget dollars for other infrastructure needs in the local community are other benefits associated with keeping the existing culvert in service and simply repairing or relining it. The cost and time associated with engineering services for a new culvert versus those associated with a replacement or reline undertaking are yet other factors to consider.
If a culvert is deemed to be structurally, geometrically, and hydraulically functional, needing only some relatively minor repairs to ensure continued satisfactory performance and a reasonable service life, rehabilitation is a logical approach.
Rehabilitation and relining methods
Typical rehabilitation or repair measures undertaken with culverts include the following:
Install a reinforced concrete invert to repair or replace a deteriorated invert in a corrugated metal culvert pipe — There are situations where the damage or deterioration to the culvert is limited to the invert. The remainder of the culvert is in good shape and in satisfactory structural condition. The culvert may be salvaged by adding a new invert through installation of a reinforced concrete invert pavement section (see Figure 1). The culvert must be of sufficient size to permit worker access. Flow will need to be temporarily diverted during installation of the concrete invert. Loose debris will need to be removed from the bottom of the culvert and the surface of the existing invert will need to be cleaned.
Figure 1: In-place installation of a concrete invert
A layer of steel reinforcement or wire mesh is then placed in the invert and secured to the original culvert bottom. Concrete is then placed in the invert to a thickness typically ranging from 2 inches to 4 inches over the corrugation crests. The surface of the concrete is troweled smooth to provide a new invert approximately matching the geometry of the original culvert invert. An approved sealant or curing compound is usually applied to the surface of the concrete after finishing and the edges of the invert pavement can be sealed with a mastic or an asphalt emulsion.
Slipline or install a new internal pipe inside the existing culvert — This approach can be used for any type of existing culvert and normally involves installing sections of new pipe that will be of a size that passes through the tightest obstructions and shape change locations yet provides the maximum flow capacity possible. Additionally, the reline pipe wall profile is often smooth to maximize flow capacity. Products available include smooth interior wall corrugated metal pipe such as spiral rib or double-wall pipe; plastic pipe, such as PVC or HDPE solid-wall or profile-wall pipes; fiberglass pipe; and reinforced concrete pipe.
At times, liner plate or structural plate sections are used to reline culverts. Liner plate and structural plate structures are constructed in a bolted plate fashion to furnish the erected-in-place finished shape product. This allows the individual plates to be brought into position and erected in place to form the liner pipe structure and can be advantageous for larger culverts where alternative means of relining would be difficult or impractical.
Other options include expandable reline pipe that is pulled into place and expanded into proper shape. Attention needs to be given to hydraulic performance and capacity along with consideration of required structural capacity and long-term load-carrying capabilities of the reline pipe. The void between the reline pipe and the host pipe is normally filled with a grout, slurry cement, or similar flowable fill material. Care must be taken to ensure uniform, balanced, and controlled placement of this filler material to avoid displacement and distortion to the reline pipe. The liner pipe is normally temporarily blocked, braced, or somehow supported and secured in position during the grouting operations to reduce the possibility of displacement or distortion during placement of the grout filler (see Figure 2).
Figure 2: Typical section of a corrugated steel pipe fabricated for sliplining
Spot patch and repair — Localized repairs can be made to the culvert wall and to the coatings using spot patching. Such repairs can be used to address spalling concrete culvert wall sections and to repair damaged coatings and wall sections in metal pipe culverts. The section of the culvert needs to be cleaned, repaired, and then coated or painted. Choose repainting or recoating materials based on their compatibility with the culvert product type and on their acceptance for use as an approved repair material. Damages that are structural in nature should be investigated for proper repairs that restore structural integrity to the culvert wall. Repairs to the coatings for metal culverts must use approved methods and materials that are compatible with the original coating material.
Repair and modification to culvert end treatment (applicable to all culvert types) — This may take the form of a reinforced concrete cut-off wall combined with slope collars or slope paving to restore integrity to the fill slopes at the culvert ends. Other slope protection products and methods include gabion walls, reinforced modular block walls, reinforced soil masses, and grouted rip-rap.
Apply internal bands or similar repairs to problem joints — Joint problems occur in all types of culverts, involving misaligned or separated pipe ends, and can often be addressed through the use of internal bands combined with gaskets and sealing materials that help restore uniformity of flow across the joints and seal the area against significant infiltration or exfiltration. Such bands can be pulled or moved into place and then expanded out against the pipe section to form a reasonable seal. This can be enhanced using gaskets and suitable filler or sealer products (see Figure 3). Misaligned and separated joints in concrete pipe culverts can be improved through an injection grouting process.
Figure 3: Use of an internal, expanding-type coupling band is recommended to
connect the sections if there is insufficient clearance on the outside of the liner pipe.
It also can be used to improve a misaligned joint.
Apply a shotcrete or gunnite lining — Such a lining system is applied pneumatically, using compressed air to force mixtures of cement plaster or concrete onto the surface of the culvert wall in a controlled and uniform manner. Such linings are typically in the 2-inch to 4-inch thickness range and provide a dense lining resistant to weathering and flow forces. Reinforcement can be added to improve the strength and durability of such a lining.
Sandblast and repaint or recoat — This can be done to remove rusting, corrosion, or similar coating damage and restore integrity to the wall coating in situations where the damage is more superficial in nature. Suitable coating materials are available that are compatible and accepted for use as repair materials for the various culvert pipe product types.
stabilize the fill surrounding a culvert or fill isolated voids in the backfill envelope — This can often be handled with a controlled injection or pressure grouting operation. Take care to prevent excessive pressure buildup and to conduct such operations in a controlled and balanced fashion to avoid distorting or overstressing of the culvert.
Consider a number of practical design issues before deciding on the best course of action when attempting to repair or reline an existing culvert. The possible reduction in flow capacity because of loss of cross-sectional area is one factor. This can perhaps be mitigated by use of a more hydraulically efficient pavement treatment or reline product, compared with the original culvert wall or invert surface profile. Incorporation of certain end treatments can also improve the efficiency and enhance the flow capacity of certain existing culverts, while also providing increased stability and improved aesthetics to the culvert site in question. All such repairs must be carried out in a manner that maintains the integrity of the existing culvert and ensures the safety of workers and the travelling public throughout all phases of the operation. Follow applicable local agency and OSHA guidelines and regulations during all phases of culvert inspection, rehabilitation, repair, and replacement.
Summary and conclusions
Culverts represent a significant portion of our infrastructure and need to be regularly inspected for functionality and to ensure proper shape, structural adequacy, and hydraulic efficiency and performance. Such inspections may result in the need to rehabilitate, repair, or replace certain culverts. The costs associated with culvert replacement and repair efforts, along with the disruptions caused by these procedures, can be significant and prohibitive. Alternatives to total replacement often exist and are justified by the relative cost savings and improvements to traffic flow patterns and safety. There are significant benefits resulting from extending the service life of culverts while maintaining acceptable levels of structural and hydraulic performance.
Engineers and agencies charged with the task of inspecting culverts and identifying problems must understand the unique aspects of culvert design, installation, and evaluation as they pertain to the various culvert products encountered. They also must be familiar with the various methods used to repair and rehabilitate these culverts, or contract with firms that specialize in such work.
Jim Noll, P.E., is director of engineering services for CONTECH. He has more than 30 years experience in the corrugated metal pipe industry and is an active member of various technical organizations, including ASCE, AREMA and ASTM.
Matt Westrich, P.E., is a civil engineer for CONTECH. He has 11 years experience in the civil/ structural engineering field.
- Handbook of Steel Drainage & Highway Construction Products, American Iron and Steel Institute, 1994
- Rehabilitation Methods, Design Data Sheet 16, National Corrugated Steel Pipe Association, 1991
- Load Rating and Structural Evaluation of In-service Corrugated Steel Structures, Design Data Sheet 19, National Corrugated Steel Pipe Association, 1995
- An Evaluation of Flexible Metal Pipes, D.C. Cowherd and V.G. Perlea
- Culvert Inspection Manual, Report No. FHWA-IP-86-2, U.S. Department of Transportation, Federal Highway Administration, July 1986
- American Railway Engineering and Maintenance-of-Way Association Manual, Chapter 1, Part 4, Section 17, Culvert Inspection
- Culvert Repair Practices Manual Volumes 1 and 2, Report Nos. FHWA-RD-94-096/FHWA-RD-95-089, U.S. DOT, FHWA, May 1995