By Jim Noll, P.E.
| Tuesday, November 01, 2016 | 2339 Views |
Culverts and buried bridge structures rely on a number of practical design and installation criteria to ensure they function structurally, hydraulically and aesthetically. Such structures need to be sized to fit the site hydraulic requirements. Additionally, they need to be installed with proper backfill material, placement and compaction methods. Another key factor is the need for proper end treatment. End treatment brings into play such key issues such as:
1. The need for hydraulic flow protection
for both the culvert end and the side slopes
2. Potential reinforcement
3. Cut-off walls to prevent undercutting
4. Finished appearance
and enhanced aesthetics
End conditions can vary widely based on the project site constraints and conditions. Culverts may simply project from the road embankment. Alternatively, the culvert may have cut ends - either beveled or skewed - to match the alignment of the culvert with the roadway embankment alignment and slope. Culverts with cut ends will usually incorporate some type of reinforced concrete end protection such as slope collars, slope paving or headwalls. Square-ended culverts at sites where attempts have been made to keep the culvert length as short as possible often have reinforced concrete headwalls and wing walls (or similar types of end treatment incorporating alternative retaining wall options) to reinforce the culvert end and intercept the road embankment fill slope. Such headwalls often incorporate aesthetic features such as stone fascia or similar decorative face treatments to provide the desirable appearance and give the culvert a "signature appearance" to a residential or commercial development.
Other culverts, especially those in less developed and rural areas, may incorporate more economical end treatments such as gabions or rock rip-rap slopes. Modular block walls with geogrid or metal reinforcement and tie backs are another type of end treatment often seen utilized in culvert layout, design and installation.
End treatments can be as simple as basic metal culvert end sections which affix to the culvert end and dress up the ends while providing enhanced and more efficient entrance flow conditions and basic protection against flow forces. Such “End Sections’ are commonly available from pipe manufacturers and suppliers.
Example of a metal end treatment.
While it may be ideal to have the culvert cross perpendicularly to the roadway, quite often that isn't possible for a number of practical reasons, including right of way constraints. Culverts that are significantly skewed to the roadway, meaning they cross under the roadway at an angle considerably different than 90 degrees, require added attention and precautions. These might include skew-cut ends combined with concrete headwalls - or a warped fill embankment installed to provide the required balance in loads and support for the culvert ends and the road embankment. Multiple barrel culverts add further complications and must be addressed accordingly. The accompanying photos and illustrations hopefully help to illustrate these practical considerations and features.
|Grade contours can be adjusted (warped) to provide balanced backfill and support on both sides of the structure.
Failure to pay attention to such key factors as maintaining balanced loading and support or providing adequate protection from hydraulic forces that can lead to erosion of fill and undermining of the culvert can have detrimental results. Culverts with cut ends can have compromised structural integrity and performance if adequate end treatment protection is not provided. Culvert installations can see significant flow forces and fluctuating water depths. Unfortunately the type of backfill used in culvert installations and roadway construction is not compatible with significant hydrostatic pressures, saturation and inundation - such as might be seen in typical dam construction. Lack of protective end treatments such as slope paving, headwalls and cut-off walls could significantly compromise the long term performance and stability of the culvert and roadway.
Example of a steel end section for round pipe.
In conclusion, attention to end treatment is a critical factor in the culvert design and installation process. Failure to provide the necessary protection from hydraulic forces or to ensure stable and balanced loading and support for the culvert ends can lead to a number of practical and functional problems. If your objective is to have a quality, stable and functioning culvert for the service life of the project, proper focus on end treatment is key.