Advice for culvert design and installation
The Wisconsin LTAP center has created an excellent 12-page bulletin titled Culverts—Proper Use and Installation. To give you a taste of the content, we reprint a few excerpts below. Our thanks to Wisconsin for giving us the permission to do so.
Installation
A critical consideration in designing a culvert installation is whether the culvert is rigid or a flexible pipe. Concrete pipe is considered a rigid pipe; corrugated metal pipe (CMP) and plastic (HDPE) pipe are considered flexible. Understanding the characteristics of these two types of culverts and how they resist loads is important because they require different backfilling and bedding treatments.
It is also important to determine if the soil at the site will make an adequate foundation for the new pipe. Sometimes this is hard to determine before excavating. Have a good source of granular material readily available in case you have to undercut and remove poor soil.
Another key item in planning is to excavate a large enough area so that you can work along side of the new pipe and do proper compaction, whether by hand or with equipment. Installing culverts in dry conditions is less expensive, and it is easier to achieve good soil compaction. For dry run and intermittent flow streams, it is desirable to schedule culvert replacements for dry conditions. Permanent streams will require diversion or pumping. Maintain adequate downstream flow with at least 25 percent but preferably the entire flow volume.
Installation is of prime importance when completing road construction, rehabilitation, or replacement. If culverts are installed early in construction and heavy equipment is allowed to cross over them, place a 3-foot cushion over the top of the culvert to minimize live impact loading.
Maintenance
Any drainage system is doomed to failure if it is not properly maintained. These failures can range from scoured stream banks or stream bottoms to such large failures as road washouts and damaged property adjacent to the stream. Large culverts should be inspected every two years. Maintenance should include periodic inspection to see that:
- The inside of the pipe is free from obstructions.
- Both the inlet and the outlet ends are free of debris and beaver dams.
- Embankment soils are free of erosion.
- The endwalls or riprap are in place, and undercutting has not washed away soil from below and around the culvert, creating a perched or elevated culvert end.
- There is no misalignment or joint failure of the pipe.
- Culvert materials are not corroded or deteriorated.
- There are no pavement cracks or settling on the surface above the culvert.
Misalignment and joint failure can show up as soft spots in relatively shallow fills and as piping, or water movement outside the pipe, on the outside of the pipe's discharge end.
Remove large brush, weed growth, and any other materials from the upstream end of the culvert. This is to prevent their reducing flow by getting in and becoming lodged in or blocking the pipe. Repair ends and correct erosion problems.
For additional reading:
Erosion Control Handbook for Local Roads (2003-08)
This manual will assist counties, townships, and local units
of government by providing guidelines and methods for effective
erosion control practices on low-volume roads. You can download
a PDF from the publications
page.
Best Practices Handbook on Roadside Vegetation Management (2000-19)
This manual strives to provide assistance to local agencies in
roadside vegetation management operations by sharing information
obtained from the many years of experience of those working
in the field, and by highlighting new technology that is improving
operations. You can order a print copy or download a PDF from
the publications
page.
Improving the Design of Roadside Ditches to Decrease Transportation-Related
Surface Water Pollution (Mn/DOT 2004-11)
Improving the design of roadside ditches can significantly reduce
the levels of pollutants in stormwater runoff. This report describes
research conducted at the MnROAD research facility and funded
by the LRRB and Mn/DOT. Results suggest that short vegetative
strips and swales, if properly designed, can produce significantly
cleaner stormwater runoff. Download
the report (1.1 MB PDF).
The Effect of Novel Soil Amendments on Roadside Establishment
of Cover Crop and Native Prairie Plant Species (Mn/DOT
2004-41)
Roadside slopes are notoriously difficult environments for re-establishing
native vegetation. This report examines the effects of several
different soil treatments and cover enhancements on the establishment
of cover crops and native species. Download
the report (350 KB PDF).