Concrete masonry units (CMUs) are available in many sizes and are an inefficient means of building permanent, low-maintenance structures. The fundamental design of a concrete block retaining wall relies on a poured concrete foundation, referred to as a "footing," that defines a level base for the concrete block wall structure above. The basic procedure for installing a straightforward concrete block retaining wall can be adapted to a wall of any length and height.
Install a poured concrete footing for the retaining wall. According to the Dixieland ProBuild website, the footing should be a minimum of 12 inches wide and 8 inches deep, and continuously level for the entire length of the retaining wall. The footing concrete should be allowed to cure for a minimum of 48 hours prior to beginning construction of the concrete wall. Dixieland ProBuild also recommends the base of the footing be placed on undisturbed soil.
Clean the top surface of the footing thoroughly using a broom to remove any dirt.
Establish the wall line for the concrete block by inserting wood or steel stakes at each end of the retaining wall location using a sledgehammer, positioned approximately 3 feet beyond the ends of the wall. Be sure the stakes are vertical and driven in deep enough to be held firmly.
Attach a piece of nylon string to both stakes as a guideline for the wall. Adjust the stakes, as necessary, to align the string with the desired outside face of the retaining wall. Be certain the string is kept taught and unobstructed. This string line will be used as a vertical guideline for the concrete block placement.
Level down from the string guideline set in Step 4 using a level and make a mark plumb to the guideline on the top surface of the retaining wall footing at both ends of the wall. Use a chalk line to mark a line connecting the two marks. This establishes a wall line for the first layer, referred to as a "course," a concrete block.
Mix a quantity of premixed mortar, also referred to as "grout," in a mortar mixing box or wheelbarrow according to the manufacturer's directions and recommendations for the length of the first course of concrete blocks. The consistency of the mortar should be moist and similar to the consistency of peanut butter.
Begin at either end of the retaining wall and lay down a line of mortar along the wall side of the chalk line snapped in Step 5 approximately 1 1/2 inches wide, 3/4 inch thick and 2 feet long using a trowel. Lay a second identical parallel mortar line 8 inches from the first.
Slowly lower the concrete block onto the mortar lines and level the block using a torpedo level. Gently tap the top of the block with the handle of the trowel or a rubber mallet as needed to level, making certain to keep a minimum 3/8 inch mortar joint beneath the block. Also make certain the end of the block aligns with the desired end of the retaining wall and the outside edge aligns with the chalk line. Use the trowel to scoop up the excess mortar that squeezed out from beneath the block and return it to the mix to reuse.
Lay down two lines of mortar along the wall side of the chalk line for the next block, again approximately 1 1/2 inches wide, 3/4 inch thick, 2 feet long and 8 inches apart using a trowel. Set the next block, lowering it gently onto the mortar and making certain the block is levelled and the adjacent edges are tight.
Repeat Step 9 for each concrete block for the rest of the first course of the wall. When the course is complete, its a good idea to recheck each block for alignment with the wall guideline and that each has held its level.
Return to the starting point of the first course of concrete block to begin the second course of block. Spread two lines of mortar along the top edge of the first block, again approximately 3/4 inch thick, 2 feet long, using a trowel.
Lower the starting concrete block into place, then gently tap the top of the block with the handle of the trowel or a rubber mallet as needed to level. Note: Start the second course (and all alternating courses) of concrete block with an 8-by-8-by 8 "half" block to offset the vertical joints by 8 inches from the preceding course.
Repeat Steps 10, 11 and 12 to complete the concrete block installation for the retaining wall.
Wait a minimum of 24 hours prior to filling concrete block's cells with concrete to allow time for the mortar joints to cure.
Fill the retaining wall's concrete blocks with standard concrete, if applicable, to maximise the wall strength. For a finishing touch, the filled retaining wall can be capped with 2-by-8 by-16 concrete cap blocks secured with mortar.
Allow the completed retaining wall to cure for a minimum of 48 hours prior to backfilling with soil or rock.
For ease of installation, try to plan the dimensions of your retaining wall to coincide with the standard 8- and 16-inch block sizes to minimise the need for cutting blocks. For retaining walls more than 32 inches high, it's recommended you consult an engineer for advice concerning footing size and other structural considerations. Although not a requirement in many areas, you might consider adding some horizontal bars of 1/2-inch reinforcing steel at 24-inch increments for added strength in the retaining wall.
Local, state and national building codes have specific requirements regarding footing dimensions and concrete block retaining walls, and you should contact your local building department to verify compliance of your retaining wall plans.