A good rule of thumb is that the width of the base should be about half the height of the wall. So, if you're building a 4-foot tall wall, aim for a 2-foot wide base.
Rules of thumb commonly used by designers to establish the geometry of the wall include (refer to diagram): Base width = 1/2 to 1/3 of the height of the wall. Base thickness = 1/8 of the height of the wall but not less than 12 inches. Stem thickness = 6 inches + ¼ inch for each foot of wall height.
The footing should be 16in wide (or more depending on you block depth) and 6-8in of compacted crushed gravel. The depth of the trench should also include the height of one full row of blocks which will be buried below the finished grade. So if your blocks are 6in tall, your footing trench should be 12-14″ deep.
Excavation
Preparing the base where the first layer of block will be placed is critical to building a well-placed, stable retaining wall. Dig a trench for the bottom row of blocks. They should be buried about 1 inch deep for every 8 inches of wall height.
Concrete block retaining walls must be stronger than freestanding walls. Insert rebar in the footing when you pour it; this should be done every three blocks or at intervals specified by your local codes.
A typical foundation wall minimum thickness of eight inches applies to walls eight feet or less with no more than seven feet of soil pressing against it. Foundation walls taller than eight feet with more than seven feet of soil pressing against it requires a larger foundation wall 10 to 12 inches in width.
The depth of retaining wall and breast wall below ground level or terrace level shall be at least 500 mm below side drain within soil or highly jointed rock and foundation shall be on natural firm ground.
Standard 20 mpa concrete is suitable for most retaining wall uses and is recommended for most situations. If your particular job is very steep, or the soil heights to be contained over 1.8m, it is advisable to have a structural engineer design the footings and the steel required.
Determining the Width: The Golden Rule
A good rule of thumb is that the width of the base should be about half the height of the wall.
Depth of Footings
Footings should extend to a minimum depth of 12 inches below previously undisturbed soil. Footings must also extend at least 12 inches below the frost line (the depth to which the ground freezes in winter) or be frost-protected.
Masonry-unit footings shall be laid in Type M or S mortar complying with Section 2103.9 and the depth shall not be less than twice the projection beyond the wall, pier or column. The width shall not be less than 8 inches (203 mm) wider than the wall supported thereon.
Under every house is a foundation, and under most foundations are footings. Most of the time we take footings for granted, and usually we can: For typical soils, a common 16- or 20-inch-wide footing can more than handle the relatively light weight of an ordinary house.
Strip Wall Footings are constructed with typical slab depths between 6”-8” (15-20 cm), footing widths of 12”-24” (30-61 cm), and lengths that vary as needed. Strip Wall Footings are commonly paired with column widths of 6”, 8”, 10” (15, 20, 25 cm).
Pour concrete into holes and level. The post holes into which the retaining wall posts will be placed shall be 450mm diameter with minimum 100mm concrete cover below the post. Footing depth is typically equal to the height of the retaining wall.
The footing, or first row of your retaining wall, should be about 4 to 6 inches deep and about 12 inches wide, or twice the width of the block you are using. The blocks should sit about halfway into the soil.
Retaining wall foundation depth should be 300mm if the soil is firm and well-drained, or 450mm if less firm and unstable.
The base of a retaining wall should be set below ground level. The taller a wall is, the further below ground level it should be set. Crucial for supporting the rest of the wall, a good base is made of compacted soil and at least a six inch layer of compacted sand and gravel.
Regardless of the type of masonry material you are using to build the wall, concrete is the material of choice for the footer. A typical footer is twice the width of a wall and equal in depth to the wall's width. The footer should rest on a 6" gravel base set below the frost line.
The minimum footing thickness should not be less than the distance the footing extends outward from the edge of the foundation wall, or 6 inches, whichever is greater.
To determine the size of the footing, a simple formula is used: Structural Load (lbs) / Soil Capacity (psf) = Footing Surface Area Required (sq. ft.)
Using a professional-grade fabric in retaining wall construction is important in ensuring the longevity of the wall.
Where retaining walls are of masonry construction, movement joints must be included. Failure to do so could lead to cracks in the retaining wall that weaken its structural integrity and could eventually lead to its failure.
One major thing to avoid when building your retaining wall is lining the back of your wall with plastic sheeting as this will surely result in the pooling of water behind the wall.