Types of Foundation Systems and the Cost Implications of Building

Foundation Systems in Building

There are two classifications of foundations in building: shallow foundations and deep foundations. These categorizations refer to the depth of soil in which the foundation is formed. A shallow foundation can be constructed in as little as a one-foot depth, whereas a deep foundations is formed at a depth of 10-300 feet. As such, a shallow foundation is used for projects that are small or lighter-weight buildings, and deep foundations for larger or hillside developments, or those on poor soil.

Shallow Foundations

Shallow foundations are usually located less than six feet below the lowest finished floor of a structure. These systems are used when the soil close to the ground surface has sufficient bearing capacity and underlying weaker strata do not result in undue settlement. These are the most commonly used foundation systems for small residential and wood structures. As a result, there are several types of shallow foundations in construction. They are often referred to as spread footings because they distribute large loads over a greater volume of soil.

Deep Foundations

In cases where shallow foundations are not possible, deep foundations are necessary. Deep foundations are structural elements that are used to transfer loads from weak and compressible soils to a stronger layer, usually located at a significant depth below the ground. These foundations can also instead utilize the friction of the soil adjacent to it for support. The load of the structure above is transferred to these elements using surface level concrete members, such as grade beams or pile caps. Deep foundations are recommended for large design loads (4 stories+), and where poor soil is present at a shallow depth.

Types of Shallow Foundations

Shallow foundations, or footings, are an important part of the foundation of construction, especially where soil is problematic. Footings are the structural elements that transfer loads of soil from columns, walls or lateral loads from earth retaining structures. Footings must be designed to prevent and minimize settlement of the foundation, and provide safety from overturning and sliding. The size of footings will depend on the type and magnitude of the structure. It is important to use professionals in the construction and placement of footings in order to ensure the foundations is properly supported and structured.

Isolated spread or pad footings are one of the most common types of foundations used in construction. These are used under individual columns or other load points, each with their own footing. The footings can be a square or rectangular pad of concrete, and the size is calculated by the load on the column and safe bearing capacity of the soil.

Combined footings are used to support two or more columns that are close to each other in situations where their foundations would otherwise overlap. The term ‘combined’ comes from a combination of isolated footings, however the structural design of the foundation is different. The shape of a combined footing is typically rectangular, and is only necessary when load points are close together.

Strip or continuous footings are used under lines that are loaded throughout. This most commonly occurs under bearing walls or shearwalls, and are typically shaped like an “L” or upside-down “T”. These types of footings can also support individual columns that occur along these lines, but if the load is large there may be additional width at these points.

Mat foundations are required when there are many different loads occurring over an area, causing a number of individual foundations to overlap. It is common to use this type of foundation when building basements, as the basement floor slab will act as the foundation. These are also seen in areas with poor soil, to ensure that settlement does not crack the concrete floor. Mat foundations spread across the entire area of the building in order to support heavy structural loads from columns and walls, and tend to be deeper than a typical concrete floor slab (12”+ as opposed to 4-5”). The weight of the structure is then spread evenly across the soil below. This type of foundation is usually cheaper and easier to implement than many individual footings, especially when load points are not finalized above.

Types of Deep Foundations

Pile foundations are a type of deep foundation, structured out of concrete or steel, as a slender column or cylinder. A pile foundation is used to transfer heavy loads from a structure to a hard rock strata deep below the ground. They are designed to support the structure and transfer the load at a desired depth usually three times more than its breadth [6]. Pile foundations are used for large structures and where shallow soil cannot resist settlement or uplift. Pile foundations can be further classified as follows:

  • Sheet Piles: used to provide lateral support
  • Load Bearing Piles: Used to transfer the vertical loads from the structure to the soil
  • End Bearing Piles: The bottom end of the pile rests on a layer of strong soil or rock. The pile is located at a transition layer of weak and strong soil.
  • Friction Piles: Transfers the load of a building to the soil by the frictional force between the surface of the pile and its surrounding soil.

Drilled shafts, also known as caissons, are another type of deep foundation with a high-capacity, cast-in-place element formed using auger. Drill shafts not only provide structural support but earth retention as well. A drill is used to create a hole of the required diameter and depth. If necessary casing or drilling fluid is used at this point if the hole needs additional support to stay open. Then full-length reinforcing steel is lowered into the hole and afterwards filled with concrete. The finished foundation can resist loads from the structure through shaft resistance, toe resistance or a combination of both. Drill shafts are able to transfer column loads larger than pile foundations can.

Cost implications of shallow and deep foundations

The foundation of a structure can be considered one of the most important parts of a structure, since it is the base on which everything will be built. There are many markers that will dictate the type of foundation needed for a structure. The following are important factors to consider when estimating foundation costs:

  • Soil tests, drainage and moisture: Before any design or construction begins, it is recommended to hire a professional geotechnical engineer to test the soil on site. This will ensure the right technique is chosen based on the content of the soil. Once poured, foundation slabs and crawl spaces will need to be sealed to protect against water and moisture.
  • Depth: Hillsides often require deeper foundation to avoid the added load contributing to a landslide. Colder climates and wet soil may need to have deeper foundations to protect against freeze and thaw damage. The deeper the foundation, the higher the overall cost
  • Type: Concrete slab foundations can range from $4,500 to $21,000 depending on the project. - Monolithic slab foundations are cheaper, as only a single concrete is poured. - Shallow foundations are in the mid-range of pricing as builders need to dig holes and fill them with concrete, as well as connect these to the structure above. - Retaining wall and deep foundations are amongst the highest pricing of foundations, because more digging, equipment and materials are required to construct these, and they tend to be more complex.
  • Building codes, permits and local fees: It is important to consider how local rules will impact the price of a project. Project size and area will influence pricing. Adding elements and obtaining necessary permits to follow local building code can also add expenses to a project.

The best way to protect your home is to hire a licensed and credible builder to install or fix your foundation. This will ensure the safety and security of your home or building structure. Here at Design Everest we can help you find an experienced builder for your property. Contact us today at 877-704-5687.


Sources:
[1] https://www.newhomesource.com/guide/articles/solid-foundation
[2] https://theconstructor.org/geotechnical/shallow-foundations-types/5308/
[3] http://www.understandconstruction.com/types-of-foundations.html
[4] https://theconstructor.org/geotechnical/types-of-deep-foundation/7252/
[5] https://www.homeadvisor.com/cost/foundations/
[6] https://civiltoday.com/geotechnical-engineering/foundation-engineering/deep-foundation/176-pile-foundation-definition-types