Chapter 8: Using Stone to Build Bridges and Dams
Building bridges to get people, animals, and vehicles across a span of water is one of the oldest ways in which people used stone. Using stones to build bridges started with a single stone set in a small stream to step on to avoid getting wet and evolved to the massive stonework involved in bridges, such as the original London Bridge that Peter de Colechurch built, which was the first bridge to span the Thames River. A 150-year-old stone bridge built at the Manassas Civil War Battlefield in Virginia carried soldiers and artillery cannons across the river, and it employed massive arches and buttresses for strength. These provide just a few examples of the long, historic use of stone to form bridges and dams.
A Brief History of Bridge Building
The early Romans built some of the most impressive stone-constructed bridges, and many have still survived, such as the Pons Fabricius, now called the Ponte dei Quattro Capi, in Rome. The Romans built this arched bridge in 62 B.C., and it has two semicircular arches that the Romans are credited with inventing, each spanning a massive 78 feet. They built this bridge with an upper arch built between the two arches to allow the release of water during the possible event of a flood, thereby maintaining the integrity of the bridge itself. Arches are also called compression structures because they rely on compression of two sides on a keystone, which is a large center stone put in place to keep the two arched sides from collapsing.
The Romans’ efficient bridge building meant that future generations did not have to build any bridges for hundreds of years. When people started to move more within their countries and began developing societies, it became necessary to build more bridges. Sometime during the 12th century, Catholic priests and hired professionals commissioned the building of bridges. In France, a new order of priests who called themselves the Fréres du Pont designed and built bridges, the most famous of which was the Pont d’ Avignon. This bridge was built in 1177 and spanned the Rhône River. This bridge had 22 stone arches, the longest being 115 feet from one end to the other.
You do not have to build a bridge spanning rivers or lakes. This section will teach you how to build a simple stone bridge you can use over water or over any place on your landscape.
Building a basic stone bridge
A simple stone bridge is a slab of stone with smaller stones supporting it built over a culvert or stream. The support stones need to be embedded into the soil on both ends of the culvert, which is a natural ditch that fills with water when heavy rains occur. The support stones form a level base for the slab to sit upon.
Materials you will need (this is for a small bridge, 36 inches by 36 inches):
Tools you will need:
Note: Lifting a large stone slab of this nature can be too much for one person. If possible, get a couple of people to help you set this slab, or make use of a wooden tripod with a ratchet hoist — instructions on making one on following page.
Instructions:
1. On either side of the culvert or stream, dig two level trenches parallel to each other using your pick and shovel.
2. Lay the stones into this trench, making sure the leveled stone will be no more than 3 inches above the ground. Use the level to make sure the stones are sitting balanced.
3. Lay the big stone slab across the culvert or stream, resting either end on each stone “step” you have built from the instruction in step one.
4. Build the soil up around the stone slab and up over the steps, adding gravel if necessary, to create a ridge of soil connecting the slab to the surface of the earth.
Making a Ratchet Hoist
If you cannot find help to lift the stone slab, you can set it in place by making a simple ratchet hoist, which allows you to lift a heavy object by moving it with pulleys, hooks, and straps using the principles of leverage.
Materials you will need:
Tools you will need:
Instructions:
1. Anchor the three pieces of lumber in a tripod fashion using the electric drill and anchor bolts to attach them together.
2. Set the tripod in place over the stream, making sure the legs are securely sitting on either side of the stream.
3. Screw in the large hook, which will hold the ratchet hoist.
4. Attach the ratchet hoist with a large S-hook.
5. Attach the moving straps to the ratchet hoist.
6. Place the moving straps securely around the large stone slab.
7. Hoist the slab in place over the culvert or stream.
Building a basic arched bridge
Arched bridges are more complex and require more materials compared to a basic stone bridge, but they are more visually appealing. Ideally, an arched bridge is built over a larger span of water. Three basic styles of arched openings for bridges include:
You can shape true arches into semicircles, ellipses, and catenaries (very pronounced arches not used for bridges but for interior and exterior design and sculpture, such as the St. Louis Arch). Rim-type arches are very slight arches commonly found on fireplaces and window openings.
Building a mortared, semicircular arched stone bridge approximately 36 inches wide and 5 feet long will require the materials, tools, and instructions listed below. This project should take no more than a week to complete.
Materials you will need (this will make a 5 foot long, semi-circular arched bridge 36 inches tall x 36 inches wide):
For the mortar:
For the concrete footing:
Tools you will need:
Instructions:
1. Bridges are built on soft soil surrounding streams and culverts and need a footing. The larger and more complex the bridge, the bigger and more stable a foundation needs to be. Dig two trenches, also called footing ditches, 24 inches wide and 48 inches long on either side of the water source, such as a river, stream, pool, etc., to below the frost line, approximately 21 inches deep. You should make the trenches parallel to one another. Note: You can find your frost line by checking with your local building inspector.
2. Mix enough cement to fill both ditches to within 18 inches of the top. The basic recipe combines two shovels of cement, four of sand, and six of gravel in your wheelbarrow or mortar pan, adding water as necessary to make the cement a thick concrete that has the consistency of pancake batter. For more information on mixing cement, see Chapter 3.
3. Cut 12 lengths of rebar, unless they are already cut, 36 inches long, and set two into the cement of each ditch 8 inches apart from each other. Lay additional rebar — at least three pieces cut to 24 inches long — across the longer pieces. You can wire these together using any construction-grade wire, overlapping one wire over the other encasing the rebar, but once you have poured the cement, the structure will be encased. Most stonemasons recommend wiring the rebar to provide additional support for the bridge.
4. Place three pieces of upright rebar into the cement in the middle of the trench, and pound them into the earth, leaving each piece of rebar to project approximately 6 to 8 inches above the cement.
5. Pour an additional 3 inches of cement on top of the existing footing. Let this cement cure and dry for about an hour.
6. Set a 2-by-15-inch board in an upright position approximately 12 inches away from the back wall of each footing ditch. Brace the board tightly by using the stakes pounded into the soil at each end of the boards. You need this form to create a back “step,” which will include the rebar that projected from the top.
7. Splash a little water on the hardened cement, and pour the remaining cement over the rebar but not over the board form. This will create an L-shaped section. Keep these sections wet, let them cure for two days, and then remove the boards. Let the cement cure and harden an additional two days.
8. Lay out and cut two arch forms from the plywood. You should cut these into a curve 12 inches tall and 48 inches wide.
9. Set these arches on either edge of the concrete footing, and nail a 1-by-4 board every 4 inches across both arches, connecting the arches. Use wooden shims under the arch to wedge it in place on the concrete footing.
10. Mix your mortar, and begin laying the stone up against the vertical wall of the L-shaped foundation. You should use stones as flat on all ends as possible, and you should use as much mortar as necessary on the underside, but be aware that the outside of the stone will be visible so try to keep those mortar joints tidy. When you lay the second row of stones, overlap the existing joints.
11. When you reach the center, you will need wider keystones, also called arch stones. These will provide the strength that will keep the arch in place.
12. You can build up around the arch in a horizontal fashion to provide a rail for the walkway. You can do this before or after you remove the wooden forms from the arch. Either way, you must keep the structure damp for at least two days to allow the mortar to cure, and then allow it to dry.
13. Build up the soil around either side of the waterway around the stones to ease the transition from the flat earth to the stones.
Building a Dam
If you are damming a stream or creek that flows onto public lands or other property, you may need to obtain a dam permit. Small dams that create a minor impact are not usually subject to regulation nor is any stream or creek that is totally man made and on your own property. However, anything that affects landowners downstream from your residence needs a permit.
Here are instructions on how to obtain a permit:
1. You need to get a copy of the U.S. Geological Survey (USGS) quadrangle map for your land or creek site — providing the dam on your creek or stream will affect other lands, especially wetlands, as in the Florida Everglades or Carolina marshes, where damming creeks and rivers is strictly prohibited at any level. You can get these maps off the USGS website, www.usgs.gov, for a minimal fee.
2. Check the maps for any wetland or other sensitive ecological areas on the property you are planning the project. You can research these areas by accessing the U.S. Environmental Protection Agency (EPA) at www.epa.gov. Go to your individual state profiles for specific laws regulating the damming of waterways. If your dam affects waterways on federal land, you will also need to contact the district office of the Army Corps of Engineers to determine if you will need a federal permit, especially if your dam affects navigable waterways.
3. You will need to make sure you have accurate and up-to-date plans of your proposed project and clear definitions of how and when you will complete the dam. Depending on the scope of your project and the affect on other areas, an inspector with the state agency may need to visit the proposed dam site. Make sure you keep all paperwork together with reference to the project and get everything in writing.
Building a simple stone dam
Why should you build a dam? Well, by definition, dams are blockages created to retain water in a specified location. Floodgates and levees are built to manage water flow. Natural dams are created when sticks and debris from a stream or river wash down the river, creating a blockage but still allowing water to seep through. Beavers build dams to create lodges for housing and to retain water into small pools that allow them to trap fish and other underwater wildlife on which they feed.
Building a dam from stone in a flowing river or stream would create a pool of water in your landscape. If you have already created small water gardens or streams, you may want to build yet another water feature such as a dam that will terrace your water down into another pool. Or, perhaps you have a stream and want to create a living pool as an accent in your landscape — the perfect reason for building a dam — and both the stream and dam will be healthy and vibrant water features. Adding a natural pool by damming a creek or stream and diverting water to the pool will draw birds and other wildlife to your pool.
Building a dam uses all the same techniques as building a retaining wall. However, most dams do not require mortar, and for this wall, you will only use a dry stacking technique. For more information on building a retaining wall, see Chapter 6.
Materials you will need:
Tools you will need:
Instructions:
You will need to divert the water from the stream off to another area of your landscape to build the dam. You can do this by creating a small stream by taking your shovel to the bank of the stream and digging a ditch for the spillover away from the existing streambed. If your small stream flows only during heavy rains or when snow melts, you may possibly build the dam without having to divert the water so plan on building your dam during the dry season.
1. Measure the width of the creek or stream. This will help you determine how many pieces of pressure-treated lumber you will need to effectively delay or stop the water flow. For example, for a 5-foot wide riverbed, you will need to have your 2-by-10 boards cut into 5-foot pieces.
2. Determine the height of your dam. For a 3-foot tall dam that uses 2-by-10 boards, you will need to divide 10 inches into 36 inches, which means you will need 3.6, or four, boards. Keep in mind that if you want the water to flow into a pond or pool, you may want to build the dam slightly lower than the river at its highest point so water will flow into the pond.
3. Using a sledgehammer, drive two two-by-four boards into each side of the creek, at least 1 foot deep and at least 4 to 6 inches apart into the creek bed. You should drive these into the creek bed in the opposite direction of the flow of the water. These will act as a brace for the stones that you will place in the dam.
4. Place your lumber boards in between the braces. Stack your stones as tight as you can on either side of the lumber wall. For more information on dry stacking, see Chapter 6.
You may be able to use scrap lumber from construction sites as the lumber you need will only act as an inner brace for the rock. The water will begin to settle the stones, and earth and debris will build up on the flow side of the stream, creating impaction for the stones. Impaction means the stones will become more fixed as the soil begins to act as an adherent.
Once you have constructed your dam, you may want to work on additional stone projects for your home. The next chapter will tell you how to create fireplaces, entry gates, and mailboxes.