Taking on the task of building a dry-stacked stone culvert — that is, a stone culvert that doesn’t use mortar — presents the very satisfying opportunity to build something that’ll be strong enough for you to drive a vehicle over and will look very much like a beautiful dry-stacked stone bridge. There’s no reason to go with bags of sand or cement, blocks, plastic, metal, or anything man-made in your finished culvert. With a little bit of effort and the right stone material, a culvert can become the showcase of your property. After all, it’s the first thing people will see when they pull up to your drive. Why not make it stunning?
Start by digging out and removing the dirt from the location of your future culvert, leaving a 3- to 4-foot wide ditch on either side of where the culvert is to go. The ditch should be deep enough to allow the top of a 1- to 2-foot channel to be about 2 feet below grade. You’ll have to dig at least 11⁄2 feet farther out than the area that crosses under the driveway so the water will run correctly in the direction you require. You’ll then dig a trench 3 inches deep and a foot wider than the width of your culvert opening. Fill the trench level with 3⁄4-inch clear, crushed, sharp aggregate, and then lay flagstone material — or even patio slabs — that are approximately 2 inches thick on top of the aggregate to form a level bed for the watercourse. When you lay the flagstones, you’ll want to butt them close to one another and extend them at least 6 inches past the length of the footprint of your culvert on either side.
To create a small waterfall, you can make your culvert’s openings slightly higher than the ditch it will be flowing into. In this case, you might like to build small stone embankments around the area the water will flow into. Conversely, you can create a small, rounded retaining area in which the water can pool before it goes through the culvert; do this by situating your culvert so it’s slightly higher than the water in the ditch flowing up to it.
Next, lay your form onto the flagstone watercourse. A length of metal or plastic culvert tubing will work well for this purpose. The tubular form should be long enough to extend at least 11⁄2 feet past each opening of the finished culvert. This is so the tube can be supported at the ends. Raise the tube about an inch above the flagstone base by supporting it with a 2-by-4 resting on top of a 2-by-8 running through the tube (see illustration of stone channel in the photo slideshow). These boards should be longer than the tube so you can support the ends of the boards with concrete blocks that are resting on patio slabs placed temporarily at either end of the form. Place two wooden shims between the 2-by-8 board and the concrete blocks. The shims should be placed one on top of the other to form a rectangle that can be adjusted to be taller or shorter. This will allow you to drop the tube down and slide it out, making its removal much easier later in the process.
The stones you’ll need to collect to go over and around the length of the tubular form to create the stone channel don’t necessarily have to be rectangular; they’ll just need to have some length to them and not be too round. You’ll lay similarly sized stones in rows of approximately the same height, called “courses.” Starting from the visible sides, you’ll want to butt the stones tightly up to one another, making sure they all fit together snugly where they touch the tube. Add thin wedges of stone as needed so that each arch stone (called a voussoir) fits snugly into the structure. Place these wedges near the tops of the stones and avoid letting any of them slip down where they might create a pivot point between the voussoirs. For the best visual appearance, you’ll likely need to do some extra shaping to the arch stones that are visible on the outside of either end of the culvert so they fit together especially well.
Then, lay smaller stones on top of the stone tunnel. After you’ve laid these smaller stones, pack a minimum of 6 inches of gravel over the stone tunnel, and place landscape fabric on top of that. The final layer of road-grade aggregate on top of the fabric will bring the culvert up to driveway height. At this point, you can remove the tubular form.
Alternatively, instead of laying flagstone and building a self-supporting stone channel for water to flow through, you can choose to use a permanent man-made pipe as a structural inner part of the culvert. You’ll want to set the outside edge of the plastic or steel culvert back from view, so making templates for the shape of the circular stone openings is a good idea because you won’t be able to build the outside arch on the culvert.
Depending on the size of the opening, you can fashion your temporary form for the two outside arches out of plywood, a plastic bucket, or even a lid from a metal garbage can. The important thing is to get a good continuous contour around the arc of the opening. The dark shadows created by the overhanging stones will help mask the manufactured product used to support the driveway.
Think about the overall design of your culvert wall before starting on your culvert. Is the opening going to be seen from both sides? Will the culvert sides be seen from the road and the driveway or just from the road? If you situate the driveway entrance at an angle to the road, you can create a stonework that you and others can see from both the road and the approach to the road along the driveway.
To build the retaining wall portion of your culvert, begin by laying stones below grade and then placing stones over them in a bonding pattern. You’ll want to keep from creating columns of stone one upon another. Essentially, you’ll be building a dry-stacked stone wall — a system where one stone at a time is laid so that it locks in the two below it and where two stones meet together directly over the middle of the one below. The stones must touch in as many places as possible and be locked in and stopped from moving by shims placed on the inside of your wall. Take care not to have any stones protruding beyond the outside plane of the wall.
The retaining walls will be holding up the driveway, so they need to work. They must have mass and thickness. These walls need to be about 21⁄2 feet thick for a wall 2 feet high. Clear, sharp, 3⁄4-inch crushed gravel can be used to fill in the backs of the walls as you build up the fronts.
There’s some room for interesting designs. You could incorporate an inside curve or an outside curve, perhaps with boulders at the end. Keep in mind that retaining walls that curve in are stronger than walls that curve out. A curved retaining wall that incorporates the actual culvert opening is quite pleasing to look at.
If your retaining wall is any higher than 2 feet and not concave in shape, it should have a batter — that is, it should lean in from bottom to top. A board at the angle of lean you want (called a “batter board”) attached to a level can help you keep the lean, or “batter,” you want to establish. Hold the level vertically against the wall with the board leaning in to see the batter of the wall you’ll need to build to. Generally, a retaining wall 3 units high should be 2 units thick at the base and 1 unit thick at the top (see illustration of retaining wall in the photo slideshow.) This is not only structurally helpful, it’s also generally more visually appealing. Build the insides of the walls with less-useful stones, ones that are awkward in shape or not as nice to look at. You can cap the wall with large, flat stones laid horizontally, or you can set stones on their edge, fitting them together tightly and bookending them with larger boulders at the ends of the walls.
You might even like to build your wall up higher than the grade of your driveway to create a kind of stone railing or curb. This will require losing some width for the driveway itself, but I think it’s worth it. It just feels safer, especially if there’s a significant drop-off into the ditch.
Whichever style of wall you plan to make — straight or curved — the grade will have to eventually slant away from the wall and the driveway down into the ditch. That’s the thing about walls spanning ditches, which is exactly what you’ll be building (with a hole in it, of course): The shape of the retaining walls will always be shaped like an eye. Because the land slopes up and away from the bottom of the ditch, the walls will almost come to a point at the far ends, and therefore the stonework can be thinner at the corners of the “eye.”
Having said all of this, there’s always room for designing as you go. That’s the joy of building with stones minus the mortar. You’ll always be building and designing according to your material. Most modern material requires a fixed plan from which you can’t deviate in height, level, shape, or look. Stone allows for a lot of on-site creativity, which is one of the main reasons I like building with the dry-stacked stone method, and one of the reasons why I think you will too.
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