What is the proper way to install a culvert?

Learn the proper culvert design, size, materials and procedures for safe travel over streams and ditches. Culvert installation will run smoothly if you follow this advice.

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Small streams often crisscross rural land, and even those that remain dry for most of the year may have steep banks that prevent vehicles from crossing. The simplest way to safely drive across such an obstacle, while protecting the stream, is to install a culvert and earth bridge.

Culverts are made from corrugated metal or plastic tubes positioned in a crossing to allow water to pass through without damaging the roadway. The culvert&#;s corrugation provides a greater strength-to-weight ratio than a smooth pipe. This helps support the weight of vehicles, while mimicking the roughness of a natural stream bottom, which slows the speed of flowing water. Follow the culvert installation steps outlined in this article, and you can install a culvert wherever you need it.

Determine Sizing Prior to Culvert Installation

To determine the right culvert size for your situation, you need to consider the dynamics of stream flow and how it changes with the seasons. Peak flow usually is during spring runoff, and it tails off from late summer to early fall. These annual fluctuations can seriously erode exposed soil and cause inadequate crossing structures to fail. As a consequence, culverts should be sized &#; at a minimum &#; to handle the annual peak flow, and installed in a manner that will protect the culvert&#;s strength over time.

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Many methods exist to determine a culvert&#;s correct size. The easiest is one in which field measurements are taken of the stream&#;s cross section when the stream flow is at its lowest, in late summer or early fall. To take the measurements, you will need a tape measure, four stakes, a hammer, string and a notepad.

1. Begin by scouting the intended crossing. Look along the stream banks for the highest water mark. This is where any vegetation meets bare dirt and rock along the river bank in stakes at the high water marks on each bank and tie a string between them. Measure the distance between the stakes and record it as the &#;high water stream width&#; (HWSW).
2. Next, locate the lowest water mark. Typically, it borders the flattest portion of the stream bottom. Establish the other two stakes at this level and measure as you did for the high water marks; record this length as the &#;low water stream width&#; (LWSW).
3. The final measurement is the depth of the stream at high water. This is the distance between the stream bottom and the high-water-line string. If the stream bottom is highly irregular, you will need to take several measurements and average them.

Now use this simple formula to determine the stream&#;s cross-section area (A):

A = (HWSW+LWSW/2) x Depth

A = (5+2/2) x 1

A = 3.5

As shown in this example, if you measured 5 feet as the HWSW, 2 feet for the LWSW and 1 foot for the stream depth at high water, then the stream&#;s cross-section area would be 3.5 square feet.

The culvert&#;s cross-section area should be at least 1.25 times the cross-section area of the stream, if the culvert is for a shallow ditch or seasonal stream. But if the stream is subject to flooding, you should install a larger culvert in order to avoid damage due to water backing up and flowing over the roadway during flooding. If flooding is likely, multiply the stream area by 1.75 (for 25-year floods) or by 3 (for 50- to 100-year flood levels).

In the three examples above, the culvert cross-section areas would be: 3.5 x 1.25 = 4.375; 3.5 x 1.75 = 6.125; or 3.5 x 3 = 10.5.

Determining Length Prior to Culvert Installation

Culvert lengths depend on how wide you want your roadway, as well as the diameter of the culvert and the depth of the dirt fill placed on top of the culvert to make the earthen bridge. Culverts should be set perpendicular (90 degrees) to the road&#;s centerline. (Culverts not set in at 90 degrees will require a greater length.)

To calculate the length (L) of the culvert you&#;ll need, add up the width of the roadbed (RB), the width of the two side slopes (SW), plus 1-foot extensions (E) at either end of the culvert:

L = RB + SW + 2E

The side width (SW) will depend upon the diameter of the culvert (CD), the depth of the road fill (RF) and the slope of the sides (S):

SW = (CD + RF) x S x 2

For example, if you want a 10-foot-wide roadbed and you have a 24-inch-diameter culvert, 1 foot of earthen fill on top of the culvert, a standard 1.5:1 side slope (1 1/2 feet wide for every 1 foot in height) and 1-foot extensions on either side, the calculation is:

SW = 2-foot culvert diameter + 1-foot fill = 3 feet x 1.5 slope = 4.5 x 2 = 9

L = 10-foot roadbed + 9 feet of side width + 2 feet of culvert extensions

This example computes to a culvert 21 feet long.

If you&#;re concerned about the accuracy of your math, contact an Extension agent, state forester or professional consultant to have the figures double-checked before you buy the culvert. Also, check with state and federal agencies for regulations concerning activities within stream zones. Although installing a culvert is permissible under most regulations, you may be required to follow certain guidelines, including what time of year you install the culvert (usually late summer or early fall) or how to dam the water flow, if necessary, while you execute the installation.

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Culvert Installation Steps

Culverts can be purchased at farm or building supply stores. Sometimes, highway departments, the Forest Service or Bureau of Land Management have old culverts in usable condition that can be bought for less than a new one.

You will need a backhoe or excavator and a soil compactor when installing all but the smallest culverts. Excavate the streambed 6 inches deep to make space for the fill on which the culvert will sit. As you excavate, follow the existing streambed gradient (slope), which typically will run from 2 percent to 6 percent. A slope of less than 2 percent can result in excess buildup of sediment and debris inside the culvert, so be careful to get this right. If the culvert is set too high on either end, water can work its way underneath the tube, causing the bridge to wash out; if the culvert is set too deep, it can plug with debris.

After the excavation, spread a 6-inch-deep bed of rock-free soil or sand over the bottom and compact it with a mechanical soil compactor or a blunt-ended, long-handled compacting tool. When compacted, the fill will be about 4 inches deep, allowing the culvert to be set slightly below the stream grade so the water will drop a bit as it enters. Now, place and center your culvert, being careful to minimize any abrasions to the pipe&#;s protective exterior coating.

Next, fill around the culvert with 6-inch layers of dirt. Compact each layer before you add another and be especially sure the fill material adjacent to the culvert is rock-free. Continue adding fill over the culvert to a depth of at least 1 foot; this will protect the culvert from failure due to the weight of road traffic.

Finally, seed the bare fill dirt slopes with native grass and apply mulch. Also place flat stones or broken pieces of concrete in the streambed at each end of the culvert to protect the fill material from erosion &#; or you may opt to pour concrete protection. This is called &#;armoring&#; and will be especially useful when the water flow increases in the spring.

Sizing and installing your own culvert can be a rewarding experience in self-sufficiency. Take your time and build it correctly, and it will provide many years of service.

Culvert Size Calculation

Kurt Swearingen is a forest engineer and consultant who lives in Taos, N.M. He regularly assists country landowners with improving stream crossings and protecting water resources.

Introduction

In the dynamic world of construction, installing culvert pipes is a crucial task that bridges the gap between engineering efficiency and environmental stewardship. These pipes, typically used for directing water under a road, railway, or similar obstacle, play a pivotal role in managing water flow and preventing erosion. This guide aims to demystify the process of installing culvert pipes for professionals in the construction industry. We&#;ll cover the types of pipes you can use, installation techniques, and other essential tips, all in an easy-to-understand, conversational manner.

Understanding Culvert Pipes

Types of Pipes

• Corrugated Metal Pipes (CMP): Known for their strength and durability, CMPs are great for areas with high traffic loads. They are, however, prone to corrosion in certain environments.
• Plastic Pipes: These include High-Density Polyethylene (HDPE) and Polyvinyl Chloride (PVC) pipes. They are lightweight, resistant to corrosion, and easy to install, making them suitable for smaller applications with lower traffic loads.
• Concrete Pipes: Highly durable and resistant to high traffic loads and environmental factors. They are heavier and require more manpower or machinery for installation.

Choosing the right type of pipe depends on factors like soil type, water flow, traffic load, and environmental considerations.

Installation Process

Step 1: Site Assessment
Begin with a thorough site assessment. Check soil type, water flow patterns, and environmental impact. This step determines the pipe material and size best suited for your project.

Step 2: Design and Planning
Design the culvert considering the water flow capacity, road width, and soil characteristics. Ensure compliance with local regulations and environmental guidelines.

Step 3: Excavation
Excavate the site to the required depth and width. It&#;s crucial to maintain a stable slope to prevent collapses. Remember, safety first!

Step 4: Laying the Foundation
A solid foundation is key. For most culverts, a bed of compacted gravel provides adequate support. Ensure the foundation is level to avoid future structural issues.

Step 5: Installing the Pipe
Place the pipe carefully on the foundation. For longer pipes, joining sections might be necessary. Ensure proper alignment and secure the joints as required by the pipe type.

Step 6: Backfilling
Backfilling is a critical step. Use appropriate materials and compact them in layers to prevent future settling or pipe deformation. Be cautious about the material you use; sharp stones can damage certain types of pipes.

Step 7: Finishing Touches
Once the pipe is installed and backfilled, add the finishing touches. This might include re-establishing vegetation, installing headwalls, or adding riprap for erosion control.

Best Practices and Tips

• Follow Manufacturer&#;s Instructions: Each pipe type has specific requirements. Adhering to these ensures longevity and functionality.
• Consider Environmental Impact: Installing culvert pipes can affect local ecosystems. Strive for solutions that minimize environmental disruption.
• Regular Inspection and Maintenance: Regular checks can prevent major issues down the line. Look for signs of blockage, corrosion, or structural damage.
• Safety: Always prioritize safety. Use the right tools and machinery, and follow best practices to ensure the safety of your team.

Conclusion

Installing culvert pipes is a task that blends technical precision with environmental consideration. By choosing the right type of pipe, following a meticulous installation process, and adhering to best practices, you can ensure that your culvert installation is successful and sustainable. Remember, every project is unique, so adapt these guidelines to suit your specific needs. With careful planning and execution, your culvert installation will contribute positively to both the infrastructure and the environment.

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