A solid foundation is the backbone of any residential or commercial structure. The foundation is a crucial component of the building, ensuring the structure remains stable and safe.

Different types of building foundations have unique features, benefits, and limitations that must be considered while designing a building.

This article will discuss the different types of foundations used in residential buildings, their applications, pros, and cons.

Shallow Foundations vs. Deep Foundations

Shallow foundations are typically used when the soil is strong enough to support the weight of the building. Shallow foundations usually sit on top of the soil, and they are not buried too deep. These types of foundations are commonly used for residential buildings, light commercial buildings, and smaller structures.

In contrast, deep foundations (usually piles) are used when the soil is weak or unstable, and the building's weight needs to be supported by deeper, stronger soils or rocks. Deep foundations can be costly and are usually used for larger and more complex structures, such as skyscrapers, bridges, and large commercial buildings.

Common Types of Foundations in Residential Structures

Concrete Slab Foundations

A concrete slab foundation (also known as slab on ground) is a type of foundation that consists of a single layer of reinforced concrete poured directly on top of the soil.

It is suitable for buildings on level ground or with minimal slope and is commonly used for smaller, lighter structures such as single-family homes, units, or townhouses.

The design of a concrete slab foundation involves calculating the load of the building and the soil-bearing capacity to determine the thickness and strength of the slab. Slabs on ground are often designed to have additional concrete cover to prevent moisture ingress from reaching the reinforcement and degrading the integrity of the foundation. The soil is then leveled and compacted, and a layer of crushed stone or gravel is laid on top to help with drainage. A layer of plastic sheeting is then placed on top of the stone to prevent moisture from seeping into the slab. The concrete is then poured on top of the plastic sheeting and reinforced with steel bars or mesh to increase its strength and durability.

Figure 1: Concrete slab on ground (Reference)

The benefits of concrete slab foundations include their simplicity of design, ease of construction, and cost-effectiveness for smaller, lighter structures. However, they do not have sufficient capacity for commercial buildings and there is potential for cracking and settling if the soil is not properly compacted, and their lack of access to plumbing and electrical systems, given the building services are cast into or beneath the slab.

Concrete slab foundations are one of the most cost-effective and easy to construct but may not be suitable for larger, heavier structures and can be susceptible to cracking and settling if the soil below the slab differentially settles due to not being properly compacted. Unlike spread footings, concrete slab foundations are not suitable for homes built in areas with unstable soil.

Figure 2: Construction of a slab on ground foundation (Reference)

Raft Foundation (Mat Foundation)

A raft foundation, also known as a mat foundation, is a continuous slab that spans the entire footprint of a building.

It is suitable for buildings constructed on unstable soil or with high water tables. This type of foundation is designed to distribute the weight of the building evenly across the soil, providing a stable platform for the structure.

Raft foundations have internal and edge beams that are thickened sections to support vertical loads like columns or load-bearing walls. In commercial buildings, these thickened sections are changed to deeper piles.

Figure 3: Raft foundation diagram (Reference)

The design of a raft foundation involves calculating the load of the building and the soil-bearing capacity to determine the thickness of the slab. A reinforced concrete slab is then poured on top of a layer of crushed stone or gravel, which helps to improve drainage and prevent moisture from seeping into the building.

The benefits of raft foundations include their ability to provide a stable base for buildings even on weak soils, their ease of construction, and their cost-effectiveness compared to other foundation types. However, limitations include the need for a careful soil analysis to ensure the foundation can adequately support the weight of the building, and there is the potential for cracking and settling due to soil movement.

Figure 4: Raft foundation construction (Reference)

Raft foundations are suitable for poor soil conditions and can distribute loads evenly but may be susceptible to uplift and difficulty accessing plumbing and electrical systems, which are often cast into or below the raft in conduits. Raft foundations distribute loads evenly across the entire foundation, making them suitable for larger residential buildings. Raft foundations require more excavation and concrete than spread footings.

Spread Footing

A spread footing is a type of foundation that spreads the weight of the building across a wider area of soil that can be isolated (several separate footings) or continuous (a single-footing row).

This type of foundation is suitable for smaller, lighter structures such as single-family homes and smaller apartment buildings. Like the raft foundation, the design of a spread footing involves calculating the load of the building and the soil-bearing capacity to determine the size and depth of the footing. The footing is then excavated to the appropriate depth, reinforcement is formed, and a concrete base is poured into the excavation.

Figure 5: Types of spread footings (Reference)

The benefits of spread footings include their simplicity of design, ease of construction, and cost-effectiveness for smaller, lighter structures.

However, limitations include their insufficient capacity for larger buildings and their potential for settling and cracking if the soil is not properly compacted. Spread footings are generally the most cost-effective option and are suitable for most soil types. Spread footings are typically the easiest footing to repair or replace.

Figure 6: Reinforcement detailing for a spread footing ([Reference](Reinforcement Detailing for a Spread Footing))

Pier Foundations

A pier foundation is a type of foundation system that consists of a series of columns known as piers, which are typically made of concrete or steel, and are installed deep into the ground to support the weight of a building or structure. Each pier is placed at strategic locations under the load-bearing walls or columns of the building, and the weight of the structure and loads applied are transferred to the piers.

Figure 7: Pier & Capping Beam Diagram (Reference)

Pier foundations are often used in areas where the soil is too weak to support the weight of a traditional foundation, or where the ground is prone to shifting or settling. Piers are deeper than other residential foundations and can be used to reach deeper stronger soils that have higher bearing pressures and can carry more load. The piers are typically placed at regular intervals around the perimeter of the building, or in a grid pattern across the entire area of the foundation. The piers can be designed to be either precast and driven into the ground or bored and constructed in situ, depending on the soil conditions and the load requirements of the structure.

One advantage of pier foundations is that they can be designed to accommodate variations in soil conditions or building loads, making them a versatile foundation option.

When piers are precast and driven, construction time can be greatly reduced, making them a popular choice for smaller buildings and residential homes. Pier foundations typically require large machinery to bore a deep hole or drive a precast unit into the soil which usually need a large working area and temporary works design done for bearing pads for the heavy equipment.

Figure 8: Example of pier foundations in a residential building (Reference)

Basement Foundations

A basement foundation is a type of foundation that provides living or storage space below the main level of a building. It is suitable for buildings on sloping sites or in areas with limited land space. A basement foundation can be constructed using either a concrete or masonry retaining wall system.

The design of a basement foundation involves calculating the load of the building and the soil-bearing capacity to determine the depth and size of the foundation walls. The design of a basement foundation is more detailed than other residential foundation designs, as active and passive pressure analysis of the soil must be performed to design the walls and ensure they have sufficient capacity to retain the lateral load of the adjacent soil.

Figure 9: Basement foundation diagram (Reference)

Basement foundations require significant excavation to create room for the basement itself. The walls are then constructed using either poured concrete or masonry units, which are reinforced with steel bars or mesh to increase their strength and durability.

The benefits of basement foundations include their ability to provide additional living or storage space, which provides the potential to increase property value, and their resistance to weather and natural disasters. However, limitations include the potential for water infiltration and moisture issues, the need for proper ventilation and insulation, and the higher construction costs associated with excavating and building below ground level.

They can also be susceptible to water infiltration, mold, and difficulty accessing plumbing and electrical systems due to the depth that building services are typically installed. Basement foundations also require the installation of drainage systems and waterproofing to prevent water damage.

Figure 10: Basement foundation construction (Reference)

Crawl Space Foundations

A crawl space foundation is a type of foundation that provides a shallow space between the ground and the main level of a building. It is suitable for buildings on level ground or with minimal slope. A crawl space foundation can be constructed using either a concrete or masonry wall system.

The design of a crawl space foundation involves calculating the load of the building and the soil-bearing capacity to determine the depth and size of the foundation walls.

Figure 11: Crawl space foundation diagram (Reference)

Like basement foundations, the design of a crawl space foundation requires active and passive pressure analysis of the soil that must be performed to design the walls and ensure they have sufficient capacity to retain the lateral load of the adjacent soil.

However, the depth of excavation for a crawl space foundation is much shallower than a basement foundation which means that the walls are not required to have a significant load capacity such as basement foundations. The internal load-bearing members in a crawl space foundation can be piers or strip footings.

The benefits of crawl space foundations include their ability to provide access to plumbing and electrical systems, their potential to improve indoor air quality by providing ventilation, and their lower construction costs compared to basement foundations. However, limitations include the potential for moisture and pest issues in the crawl space, the need for proper ventilation and insulation, and the potential for settling and cracking if the soil is not properly compacted.

Unlike other types of foundations, crawl space foundations require the installation of a ventilation system to prevent the buildup of moisture and humidity. Another difference is that crawl space foundations can be more expensive than concrete slab foundations due to the additional excavation and installation of a ventilation system.

Figure 12: Crawl space foundation with piers (Reference)

Conclusion

While providing an accurate cost estimate for each type of foundation is difficult without specific project information, there are some general cost ranges that can be used as a guide.

It is important to note that these costs can vary greatly depending on factors such as the location of the project, the availability and cost of materials, and the complexity of the design.

Design and construction methodologies can also vary between the different types of foundations.

For example, spread footings typically require excavation and the pouring of separate concrete footings, while raft foundations require the pouring of a single, continuous concrete slab. Basement foundations require excavation and the construction of walls and a floor, while crawl space foundations require the construction of walls and a raised floor.

The design and construction procedures for each type of foundation should be considered along with the budget and the specific needs of the building and the site.

Foundation crack, sinking, or leak repairs can range from 200 t o 200 to 200to7000 and sometimes require significant temporary works designs to prop the building.

Most of the structural issues for each foundation can be overcome by proper soil analysis by a qualified engineer, as well as careful construction quality procedures in accordance with local building standards to ensure the longevity of the foundations.

ClearCalcs has a streamlined design process for Concrete Spread/Pad Footings, wall footings Concrete Strip/Wall Footings (continuous spread footing), Concrete Pier Footings (IBC 2021), and Cantilever Retaining Walls that can greatly assist in the design of basement foundations and crawl space foundations according to the ASCE 7 standards with references to the American Concrete Institute for the US as well as AS 3600 for Australia.