Foundations are generally divided into two categories: ‘deep shallow’ and ‘shallow’ foundations. Any good foundation must support the structure above it, act as a barrier to water and soil moisture, and keep out groundwater. Here’s what you need to know about how shallow foundations are constructed, how they are used, and what they are used for.
Shallow foundations are also known as spread footings or open footings. A few advantages of using a shallow foundation include cost reduction as no piling is required, reduction of soil settlement with compressive soils, and convenience for foundations that have depth that is less or equal to the foundation width. A shallow foundation transfers building loads to the earth near to the surface. This type of foundation is mainly used for small, light buildings.
Shallow foundations are constructed at a depth of as little as three feet. Shallow foundations (or open footings) are made by excavating the earth to the bottom of the footing. At this time the footing will be visible, hence the term “open.” The goal of this type of footing is that each individual footing will take the concentrated load of the of the column and spreads it around. This results in the weight on the soil will not exceed the bearing capacity. There are different types of shallow footings, which include: individual footings, strip footings, and raft foundations.
Individual footings are the most commonly used. This type of footing is used when the load of the building is carried by columns. Each column will have its own footing, which will be a square or rectangle pad of concrete. The column will sit on the footing. The individual footings are connected by a horizontal beam at ground level or slightly below ground level.
These beams are called “plinth beams.” Strip footings are used for buildings in which the walls carry most of the load of the building. The strip footing will be an entire strip that can support the load of wall. Strip footings can withstand greater weights and are a good choice for buildings made of brick, stone, or concrete. Raft (or mat) foundations are mainly used in buildings where there will be a basement.
The floor of the basement will act as the foundation for the entire building. This foundation is referred to as a raft since the building is like the vessel that floats on a sea of soil. The raft foundation is also used where soil conditions are poor. Raft foundations are easy and inexpensive to construct as they do not require deep excavations. A concrete raft foundation will often include a steel reinforcement to prevent cracking.
Shallow foundations are a good choice for buildings that don’t have a lot of weight to them. They are a good choice for when the bearing capacity of the soil is high at a shallow depth. They are also cost-effective and can help to reduce settlement.