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.
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.
Even for a 2-foot retaining wall, there are benefits to adding a footing: Enhanced Stability: A footing provides additional stability, reducing the risk of settling or tilting over time. Improved Drainage: Footings can include drainage features, preventing water buildup behind the wall that could weaken it.
Soil saturation is the most common external factor that causes retaining wall failure, but there are many ways to prevent it.
You can look up the recommended footing size, based on the size and type of house and the bearing capacity of the soil. As you can see, heavy houses on weak soil need footings 2 feet wide or more. But the lightest buildings on the strongest soil require footings as narrow as 7 or 8 inches.
Width of the base slab can be determined by considering equlibrium of various forces at the base. Base width varies from 0.4 H to 0.6H. For the Preliminary analysis, thickness of the base slab can be taken H/10 to H/15. H is the total ht of the retaining wall.
Retaining Wall Backfill Specification
The base of the wall should be installed at a minimum width of 6″ before the face of the wall, the depth of the wall block, and another 12″ behind the wall block.
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.
You should also know the size and length of your steel posts, calculating how deep they need to be installed. Generally speaking, steel post depth should equal the wall's height plus 100mm for cover on steel. So if your wall is 3.0 metres high, then your steel posts should be 3.1 metres deep.
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.
To determine the size of the footing, a simple formula is used: Structural Load (lbs) / Soil Capacity (psf) = Footing Surface Area Required (sq. ft.)
The foundation thickness shall not be less than the following: For spread footings under load bearing walls twelve (12) inches. For spread footings under columns twelve (12) inches. For mat foundations, twelve (12) inches under concrete walls, twelve (12) inches under columns and six (6) inches elsewhere.
Requirements for Safe Design of Retaining Walls
Factor of safety against sliding, forsliding=μRV/RH where μ is the coefficient of friction between the base of the wall and the soil (μ=tanδ).
A rule that states there will be no tension in an unreinforced wall if the resultant force lies within the middle-third of the wall.... ... Access to the complete content on Oxford Reference requires a subscription or purchase.
The basic principle for the design of the Retaining Wall is that the reinforced concrete stem and footing flexure and shear design strength must e at least equal to the factored moment and shears determined from the analysis. The wall stem is designed as a cantilever, fixed at the footing.
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.
5 feet to 6x6 feet footing size. Similarly, a 2-storeyed one requires 5x5 feet and single storied will need 4x4 feet footing size. In case if the soil type is not provided for the design, it is essential to mention the footing sizes.
Your first and most important task is to lay the wall's foundation. Mark out the line and position of the wall and then dig out the foundations. Dig a trench (approx. 200mm deep and 300mm wide) where the wall will be.
Sometimes retaining walls are designed as cantilevers. These typically include reinforcement and rely on the combined strength of the concrete and reinforcement to resist bending at the base of the wall. In that case, plan on waiting seven days before placing the full depth of backfill.
Discover how the right retaining wall can reduce soil erosion, improve water drainage, and transform your property. Depending on the material, soil, and groundwater, a permanent retaining wall can last anywhere from 25 to 100 years.