A correctly constructed footing is the beginning of a strong concrete block wall. Typically, this poured concrete foundation must be twice the width of the wall and installed at least 12 inches deep or as deep as the frost line in your area, as determined by your local building codes.
In average soil conditions you can pour the concrete footing to the required dimensions, in the bottom of a trench on firm/tamped soil. Since you can expand the trench wide enough to provide a work area, without much additional labor, construction of the wall can began on top of the footing, below ground level.
In harder to dig, firmer soils, it's less labor intensive to dig the trench the required depth and just wide enough to accommodate the width of the footing, then fill it up to ground level with concrete. The wall is then constructed above ground since there's no room to work in the trench.
Before concrete can be poured for the footings, lengths of steel reinforcement bars (often referred to as rebar) must be laid in the trench to reinforce the block wall foundation. Local building codes determine the correct diameter of the steel bars to be used.
It's important to reinforce masonry walls more than 4 feet high, both vertically and horizontally, to avoid any shifting or collapsing. By inserting steel reinforcement rods through the hollow cores of the stacked concrete blocks, spaced every 4 feet, the wall is strengthened vertically. Laying continuous lengths of truss-shaped joint reinforcement over the tops of the blocks, every second course, strengthens the wall horizontally. The cores must then be filled with grout to lock the steel rods and blocks together while laying the courses.
Mortar is the glue which holds block masonry walls together and must meet standard codes. Type S mortar is classified for use in structural applications and required by most local building codes. Prepackaged bags are available, but less economical for large jobs where mixing your own formulation may be desired. Consult your local building codes for the correct formula of Portland cement, lime, sand and water for your mortar.
The CMUs you use to construct your wall must comply with the American Society for Testing and Materials (ASTM) C90 standards to be approved by most local building inspectors. Stretcher blocks are a common CMU used to make up the bulk of a block wall and have standard dimensions of 8-by-8-by-16 inches. Shorter half blocks measuring 8-by-8-by-8 are used as needed to avoid cutting the longer blocks, to complete the wall construction.
The grout used to filled the hollow cores of your stretcher blocks is a formulation of mortar thinned with water and must follow ASTM guidelines.