What is the cheapest type of culvert?

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Culverts are efficient structures designed to move water and traffic under a road. However, there isn't a universal culvert type that fulfills every design requirement.

In this article, we will examine the different types of culverts that can be suitable for a project:

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

Corrugated Steel Pipe

CSP is typically the most affordable type of culvert. These pipes consist of sheet metal formed into corrugated panels and are joined using lockseams. They can be ordered as a single piece or multiple pieces that can be assembled on-site with couplers. Galvanized coatings provide corrosion resistance, while polymer coatings may be necessary in extreme environments. Due to their lightweight design, the structural strength of CSP largely relies on the backfill, which must be carefully compacted. For larger culverts, density testing during compaction is recommended.

If CSP is a viable option, other alternatives are usually unnecessary. However, consider these situations where a different culvert type may be needed:

1. Improper Cover Height.
   You may encounter issues with too high or too low cover conditions. Low cover can lead to vehicular traffic causing structural failure, while high cover can result from excessive dead load above the pipe. For CSP installations with less than 3 feet (1 m) or more than 30 feet (10 m) of cover, a stronger structure should be considered.
2. High Corrosion Potential.
   CSPs are typically galvanized but may require polymer coating or alternative materials like concrete or plastic if corrosion risks are high. These alternatives are markedly more expensive.
3. Excessive Flow Conditions.
   CSPs can be manufactured up to approximately 12 feet (3.7 m) in diameter. Larger sizes require structural plate CSP, which consists of multiple bolted sections on-site and allows for increased thicknesses.

Structural Plate CSP

Structural plate CSP is similar to regular CSP but consists of individual pieces fastened together. This method is generally more costly and typically employed only when necessary. Here are several reasons that justify using structural plate:

1. Enhanced Strength.
   Structural plates can be constructed with thicker materials, enhancing the pipe's overall strength.
2. Enhanced Corrosion Protection.
   Thicker material prolongs the lifespan before corrosion occurs.
3. Larger Sizes Available.
   While theoretically size limitations don't exist, these culverts are generally produced with diameters up to 750 inches (18.75 m).

The ASTM specification most manufacturers follow (ASTM A761) includes the following parameters:

• Sizes from 60 inches (1.5 m) to 750 inches (18.75 m).
• Thicknesses ranging from 0.099 inches (2.51 mm) to 0.358 inches (9.09 m).
• Three corrugation profiles: 6" x 2", 15" x 5.5", and 16" x 6".

Bolting the pieces for structural plate installations can be challenging. Contractors unfamiliar with the process may face delays and increased costs. Maintaining a perfectly round shape throughout assembly is crucial to ensure proper connection.

Structural plate CSP can be customized in size and shape, with options including:

• Horizontally Ellipsed
• Vertically Ellipsed
• Pipe Arch (flatter bottom)

Open Bottom CSP

An open-bottom corrugated steel pipe may be the solution in certain environmental or site conditions. These structures resemble structural plate CSP but lack an invert, requiring support from a solid foundation, such as concrete. The foundation must rest below the streambed to avoid erosion. Implementing erosion control methods, like rock riprap, is vital.

To learn more about Culvert Steel Pipe, feel free to reach out.

Initially, the foundation is constructed, with bolted connections embedded in cast-in-place concrete or precast foundations. Once the foundation is in place, the corrugated steel plates are fastened. This process begins with one ring for stabilization, followed by adjacent connections.

Open-bottom CSP remains a flexible structure, with strength derived from backfill. Therefore, compaction is critical during construction. Some manufacturers design I-shaped ribs on the corrugations, enhancing rigidity, while a concrete slab on top can achieve similar results. Although these enhancements add costs, they can render open-bottom CSP a practical alternative to bridges in various locations.

Additionally, open-bottom CSP can be visually appealing, allowing for diverse headwall designs and retaining walls.

Concrete Pipe

Concrete pipes are non-flexible; they must provide structural strength solely through their walls. These pipes are generally 50% more expensive than corrugated steel, but they also last approximately 50% longer, averaging 75 years. Concrete pipes are predominantly used in urban areas for sewer applications due to their durability and lifespan.

Precast Box Culverts

Box culverts often match stream shapes better than round designs, offering improved hydraulic efficiency, especially during high-velocity flows. Precast box culverts are manufactured in plants, giving them superior strength compared to cast-in-place options. Proper vibration techniques during precasting produce higher-quality concrete.

Polymer (Plastic) Pipe

Polymer pipes are typically employed for smaller sewer applications (under 6 feet in diameter) due to lower strength compared to steel or concrete. However, they boast longevity since they do not corrode. Pricing is similar to corrugated steel, but polymer pipes can last up to 100 years compared to 50 for CSP.