Removable concrete forms can create a vertical concrete retaining wall with almost any shape desired. Large walls and special situations require design by a professional engineer. The typical installation procedure for a simple retaining wall less than 4 feet tall on good soil is described in this article. It is advisable to get local building department approval for the design, however.
Line out the foundation trench on the ground. Excavate the trench to about 3 feet wide and to the desired depth, generally 12 inches. Check that the soil's bearing capacity matches the design specifications, generally 2000 psf, and the trench depth is appropriate for the frost depth.
Install steel reinforcement bars per the construction drawings using cover blocks and spacer chairs to position them. Typically the bars are #4, but consult the construction drawings. The bottom layer of rebar usually is 3 inches above the ground, supported by rebar chairs or concrete bricks. It includes five lines of bars running parallel to the length of the wall. One line lies along the centerline of the footing and the others are spaced 8 inches on center from the adjacent bars. The lower layer also includes one bar perpendicular to the length of the wall, lying across the bars running parallel, spaced every 18 inches on center.
Install dowels so their bottom legs lap one perpendicular bar in the lower layer and extend toward the open (non-earth) side of the wall. The top leg extends directly upward. The vertical leg should be of such length as to extend 1 inch short of the planned height of the final wall. If bars do not extend this far, each bar can be provided with a lapped bar. Install horizontal stem bars, with one line of rebar across the dowels every 12 inches up the wall.
The top layer of rebar sits on chairs so that it is 6 inches above the bottom layer. It consists of three lines of bar parallel to the length of the footing, approximately above the three lines in the bottom layer, which is closest to the earth side of the wall. Add a perpendicular bar extending across the three parallel lines every 18 inches on center.
After rebar inspection, pour the concrete, typically a minimum of 3000 psi, into the footing trench. When casting, create a 2x4 keyway in the footing to the open (non-earth) side of the top leg of the dowels, separated from the dowels by 1 to 1½ inches.
After the footing cures, install the wall formwork. Set forms for an 8-inch wall thickness, centered over the keyway. Install spacers to ensure that the top leg of each dowel is held at least 1 inch from the forms. Pour concrete of specified strength in the forms and properly vibrate it to ensure compaction of the concrete.
After the wall is complete, install drainage stone and tile on top of the earth side of the footing along the wall. Fill the stone to a depth of at least 12 inches and to a width of at least 12 inches. Extend the drainage tile to daylight with the proper slope for efficient drainage. Cover the stone with proper filter fabric and backfill.
— Raj Jalla, PE, is president of Consulting Engineers Corp. (www.engineer-cec.com), a nationally recognized expert in the design of retaining walls. Pieter VanderWerf is president of Building Works Inc. (www.buildingworks.com), a consulting firm that helps companies introduce new construction products.