Deep foundations are the go-to for heavy structures or in situations where the upper soil layers are too weak or inconsistent to bear the structural load. In such cases, the foundation needs to bypass these layers and transfer the load to deeper, more competent strata.
First off: Why you need accurate footings calculations
If you make your foundations too shallow or too deep, there is a risk to the integrity of the build, and a danger that any reinforcement, be it mesh panels or rebar rods, is positioned incorrectly, leading to reduced tensile strength and load-bearing capacity.
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).
Cons of Deep Foundations
Deep foundations may face damage from corrosion caused by water, rocks, or natural forces, impacting the long-term integrity of the structure. A strong foundation in larger buildings requires thorough preparation, research, and planning, making these projects complex.
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.
Deep foundations are essential when the bearing capacities of surface soils are inadequate to support loads imposed. They are typically are utilized for large structures, bridges, tall buildings, and in areas with challenging soil conditions like marshes or those with liquefaction potential during an earthquake.
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.
Deep foundations are commonly used when building superstructures such as a skyscraper, apartment buildings, or shopping centers. They are commonly used in situations where the soil is soft or weak and not able to support the weight. These foundations help to provide a deeper and more stable base for any building.
The location of the foundation as well as the weather greatly affect your foundation walls' strength. Inward movement in your basement walls may also happen depending on your soil and the amount of moisture it contains. The moister it is, the heavier it is, causing your foundation to shift.
Raft footing: Raft footing provides common footing to multiple columns, ensuring uniform weight distribution when the column load is excessive or the load-bearing capacity of the soil is low.
For every 2 feet of adjustment to the width of the house, add or subtract 2 inches of footing width and 1 inch of footing thickness (but not less than 6 inches thick).
Required Footing Sizes
In most cases, a house should have a minimum footing of 12 inches wide by 12 inches deep, with foundations that are 8 to 10 inches thick. This should be enough to support house weight and resist lateral soil and water pressure.
Footings are structural supports that are used in addition to a foundation. They help transfer the load's weight from the foundation into the soil. Footers are typically in direct contact with the ground, while the foundation is in contact with the footing.
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.
Use compaction whenever the soil is disturbed during construction or when it's used for fill. Compact disturbed soil beneath footings, slabs, basement floors, patios, garage floors, driveways, concrete steps and sidewalks. If a sand, gravel, or crushed stone base is used beneath a slab, compact it too.
Footings function by dispersing the weight on the post over a wider area, thereby reducing the number of pounds per square foot (psf) at any point on the soil surface. By increasing the surface area with a footing, more load can be supported by the same post without settling.
Soft, low density soils and/or improperly compacted soil beneath a home is the leading cause of foundation failure.
Building Foundation Materials
Poured concrete walls are the most durable for use below ground. They resist breaking and cracking and keep out water and earth better than concrete blocks.
Deep foundations are generally more expensive compared to shallow foundations. The additional materials, labor, and equipment required for excavation and installation contribute to higher construction costs. Extensive site investigation and engineering design are also necessary, further adding to the overall expenses.
Foundation Depth: When dealing with shallow foundations, a rule of thumb suggests that the depth of the foundation should be equal to or greater than its width. This practice helps mitigate excessive settlement and ensures adequate stability.
Deep foundations are a type of foundation that is used to transfer building loads further down the earth to utilize stable soil. This process is utilized when the existing soil is not stable enough to handle a foundation. Deep foundations are essential for safety and maintaining the integrity of a building.
Acceptable house leveling percentages can vary depending on factors such as the type of foundation, the severity of the settlement, and structural engineering standards. Generally, acceptable house leveling percentages fall within the range of 0.1% to 1%.
According to the rule of practice, the minimum allowable rebar diameter in a footing is Ø12 and the maximum acceptable spacing between the reinforcement bars is 150 mm. In certain occasions, the footing may have a reinforcement wire mesh both at its upper surface (see following figures).