How to Slipform a Rock Wall

Excavate the area for the footing. The footing should be wider than the planned wall, with at least enough extra space on either side to accommodate the slip forms for the first layer, and be as deep as the wall will be wide. It should extend below the frost line to protect against heave. Make sure that the bottom of the footing trench is level. Walls higher than about 12 feet may require additional support.

Prepare and pour the footing. A wooden frame may be necessary for the footing, depending on the soil type. Place rebar that extends the length of the wall every 6 inches or less. Place lengths of rebar oriented vertically every 2 feet. Mix a suitable concrete using a power mixer and a formula such as three parts sand, three parts gravel and one part cement, adding as much water as needed to reach the proper texture. Pour the footing and smooth the top using a wooden board.

Build slip forms. Screw 2-foot-tall pieces of 1/2-inch plywood onto the inside of a frame made using 2x4s. Coat the inside of the slip forms with a commercial form oil or another suitable lubricant to keep the concrete from bonding to the wood.

Set the slip forms on the footing, making sure they are aligned correctly. Nail the forms for each side together and brace them, if needed, using scraps of timber. Drill holes near the top of the forms through the studs in the frames. Run a section of wire through each hole and its partner hole on the facing form and pull the wires taut.

Pour 1 to 2 inches of concrete into the forms. The concrete should have a texture that allows it to fill in around rocks placed into the forms, but does not allow it to leak through joints.

Place rocks into the concrete. Put the biggest rocks in first, then fill in around them with smaller rocks. Avoid any temptation to put large, flat rocks near the edge to improve the face of the wall, as these are more likely to fall off.

Pour a layer of concrete on top of these rocks and place more rocks. Repeat this until the top of the first form layer is reached. Stagger rock placement so that large rocks are placed over spaces between rocks below them. Use gloved hands to reach into the wall and vibrate it regularly to make sure the concrete is filling in spaces between rocks. Set some rocks in the top layer of each form that extend into the next layer, but make sure they are at least 1/4 inch in from where the form for the next layer will be to prevent bulging.

Pull the forms from the first level a day or two after the pour. Clean globs of concrete off the forms and scrape or chip concrete off of the rocks. Be careful when working around the rocks, as the concrete is not yet fully cured.

Move the forms up the wall. Prop the slip forms up using timber as stilts, either setting the forms on top of them or screwing them in. Clamp the forms tightly against the wall using the wire ties. If needed, drill additional holes near the bottom of the form and tie additional wires through the bottom to maintain form shape.

Continue pouring concrete, placing rocks and moving the forms up the wall until the desired height is reached.