Canada Builds


Foundations will generally come in one of three forms. Poured Concrete, Concrete block, or ICF (Insulated concrete forms). Start first with the subgrade conditions and determine the bearing capacity of the soil. If no values are known, use 75 KPa (1500 psf) to start the calculation. Generally, with a soils test, you may find that the soils will bear 150 KPa (3000 psf) or higher. The higher the bearing capacity of the soil, the smaller the size of the footings.



For renovations, the footings are considered when lowering floors with two techniques to deal with the existing footings. Underpinning or bench footings. In a design sense before calculations are made, realize that underpinning will create more lowered floor area than bench footings. On the other hand, ench footings will cost less than underpinning.



The section detail above shows a typical underpinning treatment to lower the basement floor slab of an existing building. The length of the underpinning sections are a maximum of 4'-0" (1219 mm).
If the footings are made of brick, then the maximum length of the sections needs to be reduced to around 2'-0" or 3'-0" depending on the 'firmness' of the brick. The 4 foot maximum length will generally be OK for poured concrete. Less for brick.




The plan above shows how the underpinning is indicated on a drawing. Note the numbered stages 1, 2, 3, 1, 2, 3. This means that all of the underpinning sections marked 1 are excavated and poured first. After waiting about 48 hours, the sections marked 2 are excavated and poured. Note in the plan detail below that the length of the underpinning stages is 3'-0" and not the 4'-0" maximum. This is because the foundations are comprised of brick masonry.

Plan detail showing stages of underpinning for foundation wall.