The land surrounding your home may be hilly, making it difficult to find a level spot to erect a structure. Often the solution is to create a terrace by building a retaining wall.
Earth and water exert considerable pressure behind a retaining wall, so you must make your structure strong and provide adequate drainage. Begin by checking your local regulations on drainage (any change in the contour of your land will affect the flow of water) and any other restrictions.
Special concrete wall blocks (page 174) create the look of a stone wall in a fraction of the time and are just as strong. Depending on their design, they can be raised to a height of 24 to 48 inches without any anchoring. Each row of blocks is linked to the one above with matching grooves and ledges. The front face of the blocks is usually wider than the back, making it easy to create circular and serpentine walls.
For a simple retaining wall up to 36 inches high, plant a row of vertical timbers in the ground and spike the timbers together with bars of reinforcing steel. “Found” materials such as old pier pilings or telephone poles make strong and economical choices for building supplies.
The terraces created by these walls are suitable for light-duty demands. Walls supporting terraces for heavier loads, especially dynamic loads like vehicular traffic, need extra anchoring and should be referred to a structural engineer. Also, walls higher than 36 inches often require a building permit and are best left to a professional.
Protect your eyes with goggles when drilling.
Cutting back the slope
Working upward from the base of the slope, cut away the earth along the path of the wall with a pick and shovel. Pitch the excavated soil behind the site of the wall to create a plateau, leaving a drop slightly less than the planned height of the wall.
Setting a level base
Drive stakes to mark the ends of the wall. Stretch a mason’s line between the stakes and level the line with a line level. With a shovel and rake, level the soil to a uniform distance under the string all along its length.
A look behind the wall
Modular block construction eliminates the threat from frost heaves by preventing the buildup of water behind the wall. The mortarless joints allow water to weep through the wall face. The base of the wall consists of 4 inches of compacted and leveled coarse sand. In soils that drain poorly, a 4-inch perforated drain pipe is installed at the level of the first course. Each course is slightly offset from the one beneath it by alignment lips on the blocks. The wall is backfilled with ¾ inch clean drainage aggregate and finished with 3 inches of topsoil. If the soil is very fine, a filter fabric (available at a landscaping supplier) is placed behind the aggregate to keep the backfill clean. Modular block manufacturers recommend heights to which their walls can be built without reinforcement. Always refer to the manufacturer’s height guidelines.
Laying the first course
Dig a shallow trench along the excavated terrace 12 to 18 inches wide and 4 inches deep. If the soil is very fine, spread a layer of filter fabric on the side of the trench, as shown on page 174. Cut the fabric long enough to fold over the aggregate once the top course of blocks is in place. Spread a 4-inch layer of coarse sand in the trench, then compact and level it. Arrange the first course of blocks on the base, digging a shallow trench in the sand to accommodate the alignment lips; align the back face of the blocks with the mason’s line you set up when excavating the trench (page 173). Level each block front to back and side to side (left), making sure adjacent units are flush; a rubber mallet can help to position the blocks. To cut a block for the ends and corners, first score it with a mason’s chisel and maul. Continue striking with the chisel on all four sides until the block splits.
Backfilling each row
To improve drainage from behind the wall, install a 4-inch perforated plastic drain pipe at the level of the first row of stones, as shown on page 174. Remove the alignment stakes and mason’s line, then position the next row of blocks over the first so the alignment lip overhangs the back edge. Overlap the blocks so the joints of the two rows are staggered. Backfill behind each course with ¾-inch drainage aggregate as you proceed (right). Continue adding rows of stones followed by backfill to reach the necessary height.
Securing the top course
Before you lay each block of the last course, apply two beads of concrete adhesive to the blocks in the previous row (right). Fold any filter fabric over the aggregate and fill in the last 2 or 3 inches behind the wall with topsoil.
Digging the trenches
Excavate the wall site (page 173). Dig a trench under the string to a depth equal to the wall height plus 2 inches. Make the trench 4 inches wider than the posts to be used for the wall. Create a flat, even bearing surface on the downhill face of the trench with a square-edged spade (photograph). Starting at one end, use a posthole digger to make a hole for a single timber twice the depth of the trench (left).
Pinning the wall together
Set the corner post in its hole, add gravel to fill the hole, and pack the gravel down with a tamping bar. Dig a shallow trench for four shorter posts next to the first one, set the second post in the trench, and, with a ⅜-inch ship-auger bit, drill two holes, one above grade (left) and one below, through one timber and into the next. If the wood is hard you may need to rent a ½-inch drill, as shown. Drive ⅜-inch reinforcing bars into the holes. Repeat for each short post, staggering the heights of the holes so the bars do not intersect. Add gravel to hold the posts in place. When you have laid and pinned four timbers, make another deep hole with the posthole digger; repeat the procedure for every fifth timber. Fill in the trench with gravel as you go, tamping it down every 4 inches, then fill in the rest of the space between the wall and the retained soil with the soil that was removed.