Building a garden wall may look like a daunting task, but like so many do-it-yourself projects, it is just a matter of taking the time to do careful work. A garden wall could be made of breeze blocks, precast concrete pavers or natural stone, all of which require a poured concrete foundation to be stable. For a wooden garden wall, posts set individually in concrete would do the job. The most important criteria for pouring a concrete foundation for a garden wall are that it be deep enough, wide enough and level.
Pound small wooden stakes into the ground to indicate where the wall will begin and end. Run a piece of twine to connect the two stakes and use the twine as a guide to spray a line of landscaper's paint on the ground. Remove the stakes and twine.
Measure the pavers, blocks or stone that you will use to build the wall. Dig a trench twice as deep and twice as wide as the measurements, using the line spray painted on the ground as your guide. For example, if you are building the wall of 20 by 20 by 40 cm (8 by 8 by 16 inch) breeze blocks, you would dig a trench 40 cm (16 inches) wide and 40 cm (16 inches) deep.
Make sure the sides of the trench are vertical by squaring them off using a square spade. Use a piece of 5 by 10 cm (2 by 4 inch) timber to tamp down the bottom of the trench.
Build a form (a frame) around the trench using 5 by 10 cm (2 by 4 inch) boards held in place by wooden stakes pounded into the ground. Level the form.
Mix a batch concrete in a wheelbarrow, following the manufacturer's directions. Use a hoe to mix the concrete with water. Tip the wheelbarrow to pour the concrete into the trench, overfilling it a little. Draw a piece of 5 by 10 cm (2 by 4 inch) timber across the top of the form to level the foundation.
Continue mixing and pouring concrete until the foundation is complete. Allow it to cure before you construct the wall on the footing. Follow the manufacturer's directions on how long the foundation should cure.