The 7 Types of Culverts

Culvert filled with water

Culverts are highly efficient structures that move water and people underneath a road.  However, there isn’t a one-size-fits-all culvert type that meets all design considerations.

To that end, let’s explore the different types of culverts that can be considered for a site:

  1. Corrugated Steel Pipe (CSP)
  2. Structural Plate CSP
  3. Open bottom CSP
  4. Concrete Pipe
  5. Concrete Boxes – Precast
  6. Concrete Boxes – Cast in Place
  7. Polymer (plastic) Pipe

Corrugated Steel Pipe

This is almost always the cheapest type of culvert. They consist of sheet metal rolled into corrugated pieces and fastened together through a lockseam.  They can be ordered as one whole piece or as multiple pieces that can be installed on site with a coupler. Galvanized coatings can help resist corrosion, or in extreme cases a polymer coating can be used. Because of the lightweight nature of the sheet metal the structural strength of the culvert is in the backfill, therefore the backfill must be meticulously controlled.  For bridge sized culverts we would recommend density testing during compaction.

Construction of culvert

If you can use CSP there is usually no other reason to investigate anything else. But if any of the following situations are present you might need to try something else:

  1. The cover over the culvert is too high or too low.
    In low cover situations, vehicular traffic (live load) is the failure mechanism for the pipe.  In high cover situations, the weight of the fill above the pipe (dead load) is the failure mechanism. If you install a CSP with less than about 3 feet (1 m) or greater than 30 feet (10 m) of cover you should consider a stronger structure.
  2. The corrosion potential of the site is high.
    CSP’s are generally supplied with a galvanized coating. But if this is not enough, your options are to use a polymer coating or switch to a different type of pipe – concrete or polymer (plastic). Both of these options are significantly more expensive.
  3. There is too much flow for a CSP.
    CSP’s get built up to a size of about 12 feet (3600 mm), because after that the pipe is too big to move on the highway and too flimsy to install easily. If you need a bigger size you must go to a structural plate CSP, which is also made out of corrugated steel but consists of many pieces that are bolted together on site.  Structural plate CSP’s can be built with greater thicknesses as well.

Structural Plate CSP

Structural plate Corrugated Steel Pipe is similar to CSP except it is built in pieces and bolted together. Clearly this is more expensive than building a single piece of pipe, and thus you would not normally use structural plate unless there is a compelling reason to do so. As described above, the reasons for using structural plate are:

  1. Stronger pipe
    Structural plate can be built thicker than regular CSP, therefore the strength of the pipe itself is higher.
  2. Corrosion protection
    The thicker pipe material takes longer to corrode through.
  3. Bigger culvert sizes
    Structural plate theoretically has no maximum size but they are generally built to a maximum size of 750 inches (18.75 m) diameter.

The ASTM specification which most plants adhere to (ASTM A761) specifies the following parameters:

  • Sizes range from 60 inches (1.5 m) to 750 inches (18.75 m).
  • Thicknesses range from 0.099 inches (2.51 mm) to 0.358 inches (9.09 m).
  • Three sizes of corrugation profile (length x depth): 6″ x 2″, 15″ x 5.5″, or 16″ x 6″.

The field bolting that must be performed to connect the pieces together is generally a very tricky proposition, thus if a contractor is chosen that is not familiar with how to do it a significant increase in time and cost could be experienced. The structure must be kept perfectly round during the bolting operation (it tends to sag into an oval shape), either by tying the sidewalls of the structure together with a rope at certain intervals or by building a small wooden structure that holds the pipe in its round position. Failure to keep the pipe round will make it difficult to complete the bolting of the full pipe circumference.

Structural plate CSP can be made virtually any size and shape, and the shape options are virtually limitless:

  • Horizontally ellipsed
  • Vertically ellipsed
  • Pipe arch (flatter bottom)

Open bottom CSP

When environmental or site conditions dictate, an open bottom corrugated steel pipe could be the answer. These structures are like structural plate CSP but they don’t have an invert, that is, only the top half of the culvert is present. They need to be supported on a foundation such as cast-in-place concrete, precast concrete, or steel. The foundation should be below the streambed level to ensure it doesn’t get exposed to erosion and scour. Rock riprap or other erosion control should be considered a high priority.

Etherington Creek open bottom culvertThe foundation is built first. Cast in place concrete footings are built with bolted connections built in to the concrete which can attach to the corrugated plate structure. Precast or steel footings are built with the connections in place. Once the foundations are constructed the corrugated steel plate is bolted to the foundation. One “ring” is built first, to stabilize the structure, then the others are built adjacent to it.

The open bottom CSP is still a “flexible” structure, that is, its strength comes from its backfill. For that reason the compaction is a top concern during construction. However, most manufacturers can design I-shaped “ribs” on each corrugation which serve to convert the structure from flexible to rigid. Alternatively a concrete slab can be poured above the structure which achieves the same outcome. Both of these options come with a price but result in open bottom CSP’s being a viable alternative to a bridge at almost any site.

Open bottom CSP’s can be very aesthetic. The headwall can be any type of retaining wall, and the most basic corrugated steel headwall is quite aesthetic.

Concrete Pipe

Concrete pipe is not a flexible structure, meaning that it must provide the full structural strength of the culvert within the pipe walls, without the assistance of the surrounding backfill.

Concrete pipes are significantly more expensive than corrugated steel (about 50% premium) but they will have about 50% more life (75 years instead of 50 years).

This type of culvert is generally used most for sewers in the city, because it’s expensive and disruptive to excavate it therefore the longest possible life span is the biggest consideration.

Box Culvert – Precast

BF 74525Box shapes normally match to the stream shape better than round culverts.  Hence, they have better hydraulic characteristics which can be important when streamflow velocities are high or erosion control methods can be avoided.

Precast box culverts, like precast round culverts, are built in the plant therefore they have higher strength than cast-in-place concrete.  This is because the strength of concrete is inversely proportional to its viscosity (flowability).  That is, if it is strong it is difficult to work with – it must be vibrated in the field.  In the plant the forms can be vibrated and the concrete is easily accessible for vibrating.

Polymer (Plastic) Pipe

Polymer pipe is generally used in smaller sewer pipe applications (less than about 6 feet diameter) because it doesn’t have the strength of steel or concrete.  However, it has the added benefit of longevity in that it doesn’t corrode.  The price is similar to corrugated steel but it can last 100 years instead of 50 years.

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