How Deep Should a Footing Be for a 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.
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.
Fill trench with 1 1/2″ processed gravel or 3/4″ crusher run base material and firmly compact in 2-3″ layers until base is 6″ thick. Place and level the first course of wall units on the base. Place perforated pipe behind the wall for drainage and backfill with 3/4″ crushed stone.
Footing thickness - 8 to 12 inches. Footing depth - varies based on frost line and soil strength (some footings can be shallow, while other must be deep)
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.
A minimum of 12”inches deep for all footings and the width will be determined by the amount of stories, 12” inches for a single story, 15” inches for a two story and 23” inches for a three story. This is based on a soil bearing capacity of 1500(psf).
Granular soils are better to build with than clay soils. Sand and gravel will compact better, drain better, and often will need less reinforcement.
Absolutely! Even a 2-foot retaining wall needs an effective drainage system. Without it, water can build up behind the wall, leading to pressure that might cause the wall to lean or even collapse. All walls need systems to manage water effectively, preventing damage over time.
Yes, every retaining wall requires a footing, but the type of footing it requires will depend on the material used, the design, as well as the height and weight 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.
The minimum depth of footings below the surface of undisturbed soil, compacted fill material or controlled low strength material (CLSM) shall be 12 inches (305 mm).
Soil saturation is the most common external factor that causes retaining wall failure, but there are many ways to prevent it.
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.
Typically, retaining walls up to four feet in height can be constructed based on the manufacturer's guidelines without additional engineering input. These recommendations ensure that the wall can support itself and the forces exerted by the soil behind it.
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.
Proper drainage is critical to prevent water buildup behind the wall, which can lead to pressure and eventually cause the wall to fail. For effective drainage, the choice of gravel is paramount. Fractured rock is often the preferred choice for the drainage layer of retaining walls.
The size of the wall will largely determine the size of the footing. For concrete, a poured concrete footing is the best. Block or brick walls should be a minimum of twice as wide as the planned wall. The footing of your retaining wall should be as thick as the wall's planned width.
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.
Base Installation
The depth of the trench should be one inch for every one foot of wall height.
In reinforced and plain concrete footings, the thickness at the edge shall be not less than 150 mm for footings on soils.
The footing depth, on the other hand, is typically determined by the soil type and the extent to which you must dig before striking undisturbed soil. Still, we recommend that the depth of the footing extends to a minimum of 12 inches after you've hit undisturbed soil.
Thickness shall be at least 8” thick. However, if the projection (P) is > 6”, the thickness shall be equal to P. Note: Where minimum footing width is 12-inches, a single Wythe of solid or fully grouted 12-inch concrete masonry units is permitted to be used.