Q:
When building a 3 ft wall, does the base have to be 24 inches wide and 12 inches deep, or can it be slightly less wide and less deep?  How many inches of wall rock are required, as opposed to recommended? What risks are there by not meeting the standard?
A:
If this wall is being designed by a local engineer, always follow their recommendations. Otherwise, a gravity wall (a wall without geogrid reinforcement) should have a minimum base of at least 4 inches deep by 18 inches wide. Once you add geogrid to the wall, it falls into another wall category and the base should be a minimum of 6 inches deep by 24 inches wide. The AB Landscape Walls Guide is a great place for this information.
The base helps prevent excessive settlement by spreading the wall weight over a greater area on the native soil below the wall structure. The base material also allows for some water movement to aid in relieving some of the water pressure that could buildup behind the wall. A third key benefit of the base material is creating a leveling pad where the Allan Block units can be placed. The base row is the most important part of the wall and starting with a level, compacted base will make the wall construction much easier.
