Shallow Foundations

What is a Shallow Foundation?

Shallow foundations must be protected from freezing since water in the soil around the foundation can freeze and expand which damages the foundation. These foundations should be built below the frost line (level in the ground above which freezing occurs). If they cannot be built below this line, they must be protected by insulation (e.g. a little heat from the building will permeate into the soil and prevent freezing) [4].

Spread Footings / Individual Footings

This type of foundation is shallow (usually no more than 3 ft deep). It can support one column or multiple columns (called “joined footing”). It’s normally used for small loads and the top soil layers are not weak (e.g. flag poles, single story buildings). “To construct these types of footings, the required depth is excavated, reinforcing steel bars are placed according to design, framework is placed according to design dimensions, and concrete is poured. After the concrete reaches its maximum compressive strength, the excavation is filled with soil, and the soil is compacted.” [5].

Strip Footings

This type of footing is commonly found in load-bearing masonry construction. It acts as a long strip that supports the weight of an entire wall. These are used when the building loads are carried by entire walls instead of isolated columns [4].

Mat Footings

Mat foundations, also known as raft foundation, are used when the soil has low bearing capacity. It has multiple columns supported on one large foundation. Due to having a large foundation, when the loads are distributed, it reduces the stress exerted on the low-bearing capacity soil. Mat foundations are normally used in situations where piles cannot be constructed and spread footings are not practical (e.g. if the top layers of the soil are weak and you’re constructing a low-rise, multistory building) [5]

Additional Reading

Deep Foundations

Foundation Building Code

Shallow Foundations

Underpinning

References

Contributors