Any footing must be at least 12″ below the frost line of the area it's to be built on or it will heave up when the ground freezes and thaws over and over. Same goes for fence posts.
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).
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).
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)
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.
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.
According to an example cited on TATA Tiscon site, if a 3-storeyed building rests on sandy soil, it will require minimum 5.5x5. 5 feet to 6x6 feet footing size. Similarly, a 2-storeyed one requires 5x5 feet and single storied will need 4x4 feet footing size.
Depth of footing above bottom reinforcement shall not be less than 150 mm for footings on soil, nor less than 300 mm for footings on piles.
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.
❖ Footings are required to extend below the ground surface a minimum of 12 inches (305 mm). This is considered a minimum depth to protect the footing from movement of the soil caused by freezing and thawing in mild climate areas.
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).
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.
Footings need to be 600mm wide x 1m deep minimum,in some cases depending on ground quality/type,proximity of large trees footings are required to be up to 2m deep.
As per Cl. 7.2, IS:1904, All foundations shall extend to a depth of at least 50 cm below the natural ground level. The minimum depth of footing on sandy soil is 0.8 - 1 m, for rocky soil, it is 0.05 - 0.5 m and for clayey soil, it is 0.9 - 1.6 m.
D = V Times 1/2 T D = Depth (in meters) T= Time (in seconds) V = 1507 m/s (speed of sound in water) Calculate the depth foe each of the times given below, using the formula above.
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.
In summary, retaining walls do need a footing to ensure they are structurally sound and have proper drainage to prevent cracking or collapse. Without a footing, the ground beneath the retaining wall can shift, which can cause the wall to shift as well, leading to cracking or collapse.
Excavation
Dig a trench for the bottom row of blocks. They should be buried about 1 inch deep for every 8 inches of wall height. This provides strength and stability to your retaining wall. Make sure that your trench is level and compacted.
Footing must be dug down into undisturbed soil and to a minimum depth of 12-in below the finished grade.
Even the best contractors occasionally have to build on a sloppy footing. Ian Geisler, a noted ICF expert who has worked on hundreds of projects across the continent, says that realistically a perfect footing is anything within ¼” of level. “If it's within ½”, then it's workable,” he writes.