How to Build a Stone Culvert

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A dry-stacked wall made with large stones secures this 9-foot-diameter (2.7-m) stone culvert at its discharge end.
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"The Complete Guide to Stonescaping," by David Reed, presents the basics of stonescaping in easy-to-understand terms for projects that will be functional and beautiful.

Learn techniques for building functional and beautiful stoneworks that will last for ages with The Complete Guide to Stonescaping (Lark Crafts, 2013). With easy to follow instructions and hundreds of breathtaking photos David Reed shares stone projects to turn any yard or garden into a retreat. In the following excerpt, Reed outlines how to build a stone culvert under a driveway that crosses a creek or stream. 

Reworking a Creek Culvert

Many property owners with driveways that cross over creeks or streams already have culvert pipes in place. In this project, you’ll learn how to build a stone culvert with dry-stacked walls around the intake end of one of these pipes.

The 4-foot-diameter (1.2-m) culvert shown in the photo above allows a creek to run beneath my driveway. When I first moved to the property, the outflow end of the culvert — a concrete-block wall faced with stone — was still in good shape. The dry-stacked stonework at the inflow end, however, had collapsed into piles of stone on both sides of the pipe’s opening, and the slope from the driveway down to the top of the pipe was caving in. The simplest and most practical solution was to stabilize the bank by dismantling the old stonework and restacking the stone walls.

One tip before you start a similar project: Unless the creek or ditch is dry, there’s no way to avoid getting wet as you build the stone walls of a culvert! You’ll be standing right in the bed of the creek or ditch as you work. To minimize sogginess, wear a pair of tall rubber boots. Making a few walkways by placing boards just above water level will also help.

Dismantling the Old Stonework

I started work when the creek was low. First, I completely dismantled the old stonework on the right-hand side of the inlet end. In order to redirect some of the creek water away from my work area and off to the left, I removed some of the largest stones and placed them in the creek itself.

Some of the original stones were suitable for restacking. The others, irregular in shape, were good backfill material, so I saved them to thicken the new wall and add to its ballast. To supplement the useable recycled stones, I handpicked about 1/2 ton (508 kg) of stone at a stone yard.

Stacking Flared Wing Walls

During heavy spring rains, the volume of this creek increases dramatically. Sometimes, the water level reaches the bottom edges of the capstones on the right-hand side of the culvert, in front of the hemlock tree. I therefore decided to build dramatically flared new wing walls — ones that would funnel this rush of water into the pipe. Because the water flow is sometimes extremely heavy, I also knew I’d have to use very large stones.

I dry-stacked the right-hand wall first. I started by wedging stones between the outside surface of the pipe’s opening and the soil bank to make sure that water wouldn’t flow behind the pipe. Then, to clear a site for the first course of the wall, I removed a few stones that jutted out from the creek bed. Next, I laid out the first course of the wall by placing some of my largest stones directly into the creek.

As I stacked this wing wall, I set the first stone in each course to overlap the lip of the pipe. I also rounded the wall dramatically in order to make sure that water flowing out of a smaller creek on the right would hit stones rather than soil as it entered the larger creek. To backfill the wall, I combined the irregularly shaped culls that I’d saved with crushed limestone that I had on hand and small stones gathered from the creek. The creek was also a good source for shims, wedges, and the smaller stones across the top of the pipe.

As I continued to stack courses, I had to remove the lower branches of the hemlock tree in order to provide working space. I took care to leave enough soil and space for the tree’s future growth.

After capping off the section of the wing wall in front of the tree, I cut a shelf in the bank behind the tree and stacked a couple of courses of that wall. Then I shifted my attention to the wall on the left-hand side of the creek.

First, I had to move the water-diverting stones that I’d set in the creek. I placed these at the opposite side to divert water from my new work area at the left-hand side of the culvert. Then I stacked the new wing wall, using my largest stones in the bottom course and backfilling as before.

The final stage of this project was stacking the wall just beneath the road, which I did in the usual fashion.

Working with New Culvert Pipe

If you have a small creek on your property and would like to install a culvert pipe with a pathway over it, try the following project. If, however, your creek is large and you want to build a driveway over the culvert, I strongly recommend that you consult with a professional. You can certainly dry-stack the stones at the ends of a large culvert, but setting the pipe, gravel, and road bond will require the use of heavy equipment. For smaller projects, you’ll only need a pick-up truck to haul the pipe, gravel, and stones.

Culvert pipe is cylindrical in shape and made of rigid plastic or heavy-gauge galvanized steel. Diameters can range from 1 to 10 feet (30 to 305 cm). To create a 6-foot-wide (183 cm) path over a small creek, a pipe 10 feet long and 3 feet (91 cm) or smaller in diameter will be sufficient. To find a local supplier, just look in your telephone book, under “Pipes” or “Culverts.”

Have the pipe cut to length before you pick it up or before it’s delivered. Most pipe sellers will deliver, but if you don’t want to pay for this service, a small truck can usually transport short sections. You can join these with a connector once they’re at your site. Elbow sections that allow pipes to be joined at 90- and 45-degree angles are also available.

Determining the diameter of the pipe you’ll need is tricky, as there’s no way to gauge exactly how much water will flow through the pipe at any given time. One way to estimate the diameter is to observe the volume of water in the creek after a heavy rainfall. Take note of the water’s width and depth at the spot where you want to set the culvert; your culvert pipe must be able to cope with that volume of water.

Order a pipe the diameter of which is slightly larger than the diameter you think you need; you never know when the hundred-year flood will hit! If your creek is small and the culvert site is close to its head­waters (where the creek begins), the volume of water may not change drastically from season to season or day to day. If, on the other hand, the site is below a number of tributaries that feed into your creek, or if much new development is taking place upstream, the water can rise dramatically.

Setting the Pipe in Place

Work on this project during a dry season, when the water level in the creek or stream is low. To divert any water flow, refer to the tips presented at the beginning under “Reworking a Creek Culvert.”

As you visualize the finished project, keep in mind that the bottom lip of the culvert pipe should sit at or slightly below the bottom of the creek bed. The pipe should slope slightly downward from its intake end to its discharge end.

Start by clearing out any rocks that protrude from the creek bed; set them aside to use in the walls at each end of the pipe. The creek bed must be free of any material that will keep the pipe from sitting evenly along the bottom. Use 3/4-inch (2-cm) gravel to fill any holes left by the rocks you remove.

Next, spread a shallow gravel bed down the center of the creek, sloping it downward toward the discharge end and shaping it slightly to match the contour of the bottom of the pipe. The depression in the gravel will ensure good contact between the bed and pipe. Ideally, the lower lip of the pipe should rest at or slightly below the bottom of the creek at the intake end, so don’t make the bed too deep.

To position the pipe on the gravel bed, simply roll it into the creek and adjust it by hand. If you’re working with two sections of pipe, band them together with a metal collar after placing them on the gravel bed. (Large diameter pipes must be set by heavy equipment.)

After the pipe is in place, stabilize it by placing a few stones along both sides, 1 foot (30 cm) in from each end. Then, toward the center of the pipe, start pouring and tamping layers of gravel in the spaces between the creek banks and the pipe. Your goal is to create a tightly tamped bed that buries most of the pipe and fills the area above it from one side of the creek to the other. Leave enough space at the ends of the pipe to dry-stack walls around the pipe’s openings.

Building the Wing and Head Walls

At the inlet end, you’ll build angled wing walls that flare out so they’ll funnel water into the pipe. The wall at the discharge end will be perpendicular to the pipe rather than flaring outward. Use the same instructions from before to dry-stack the inlet-end walls first; then stack the wall at the outlet end. As you stack the stones, you’ll probably have to add extra gravel behind them to fill the space between their backs and the gravel you poured earlier.

Stack both walls until they’re tall enough to ensure that your pathway will be at the height you desire. To stabilize them, add capstones. To make a pathway over the gravel above the culvert, just add a layer of soil, pea gravel, small crushed stone, bark mulch, wood chips, or tamped road bond.

When the Creek Floods

Some friends of mine live in a beautiful valley with a large creek flowing through it. Shortly after moving in, they noticed that during heavy rains, the 4-foot-diameter (1.2-m) culvert beneath their driveway couldn’t handle the volume of water in this creek. Excess water flooded the driveway and eroded its edges.

When the culvert was built, it was more than adequate in size, but substantial development upstream had significantly increased the water volume. New rooftops, paved driveways, and other hard surfaces had almost doubled the runoff into the creek. To remedy this situation, my friends widened the creek at their bridge and installed a second, 4-foot-diameter culvert next to the first.

For more projects from The Complete Guide to Stonescaping: Read DIY Stone Benches.

Reprinted with permission from The Complete Guide to Stonescaping: Dry-Stacking, Mortaring, Paving & Gardenscaping by David Reed and published by Lark Crafts, 2013.

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