Understanding Foundations: Shallow vs. Deep Foundations and Their Types

The foundation of any structure is perhaps the most critical aspect of construction. Without a strong foundation, even the most intricately designed building will fail to stand the test of time.

Foundations serve to distribute a structure’s weight evenly to prevent settlement, ensure stability, and protect the building from external forces such as earthquakes or wind loads.

What is a Foundation?

In construction, a foundation is the lowest part of a building that comes into direct contact with the ground. It is designed to transfer the load of the structure to the soil or rock beneath it, ensuring that the building remains stable and does not sink or shift.

Foundations are crucial because they bear all the loads that a building exerts, including live loads (people, furniture, etc.) and dead loads (the weight of the structure itself).

Several factors influence the design and selection of a foundation, such as the type of soil, the load-bearing capacity of the ground, environmental conditions, and the type of structure being built. A well-designed foundation also prevents moisture from entering the building, which can cause structural deterioration over time.

The Difference Between Shallow and Deep Foundations

Foundations are broadly categorized into two main types: shallow and deep foundations. The key difference between these two types lies in the depth at which they are constructed.

1. Shallow Foundations

Shallow foundations are placed at a relatively small depth beneath the surface of the ground. These types of foundations are suitable for lighter structures or when the upper layers of soil have sufficient strength to support the load. The depth of shallow foundations typically ranges from just below the ground surface to about 3 meters.

Shallow foundations work by spreading the load of the building over a wide area of soil near the surface. This makes them an ideal choice for structures where the soil close to the surface has enough load-bearing capacity.

Types of Shallow Foundations:

A. Spread Footing: One of the most common types of shallow foundation, spread footing consists of a wide base (or footing) under columns or walls. This helps to distribute the weight of the structure over a larger area. Spread footings can be square, rectangular, or circular in shape.
B. Strip Footing: Used when a load-bearing wall is involved, strip footings run along the length of the wall and distribute the load evenly to the ground beneath. These are common in low-rise buildings and light structures.
C. Mat (Raft) Foundation: A mat or raft foundation is a large slab that covers the entire footprint of a building, distributing the load across a wide area. This type is ideal for buildings where the soil has a low load-bearing capacity or where large areas of the building are likely to experience settlement. Mat foundations are often used in construction on soft soils, such as clay.
D. Combined Footing: This type of foundation is used when two or more columns are close enough that their individual footings would overlap. Instead of constructing separate footings, a combined footing is built to support multiple columns.

Shallow foundations are generally more cost-effective than deep foundations due to the reduced amount of excavation and materials required. However, they are not suitable for all types of structures, especially in cases where the soil near the surface is too weak or compressible to support the load.

2. Deep Foundations

Deep foundations are employed when the surface soil lacks sufficient strength to support the building. In these cases, the foundation must be extended to deeper, more stable soil or rock layers. Deep foundations can extend down to 60 meters or more below the surface.

The primary purpose of deep foundations is to transfer the structural load through weak soil layers to reach stronger soil or rock beneath. These foundations are suitable for heavier structures like high-rise buildings, bridges, or structures built on unstable ground.

Types of Deep Foundations:

A. Pile Foundations: Pile foundations are constructed by driving or drilling long, slender columns (piles) into the ground. These piles can be made of concrete, steel, or wood and are used to transfer the building's load to a deeper, more stable layer of soil or rock. Piles are categorized into two types: friction piles, which transfer loads through friction along their length, and end-bearing piles, which rest on a solid rock or dense soil layer to carry the load.
B. Drilled Shafts (Caissons): Caissons are large-diameter shafts that are drilled into the ground and filled with concrete. These are used in situations where piles are not sufficient, such as in the construction of heavy bridges or large buildings. Unlike piles, caissons are typically used when the load must be transferred directly to bedrock or a strong soil layer deep below the surface.
C. Pier Foundations: Pier foundations involve the construction of large cylindrical columns (or piers) that are extended down to the desired depth. They serve a similar purpose to piles but are generally larger and more widely spaced. Pier foundations are often used for structures built on expansive soils or in seismic zones where additional stability is needed.
D. Well Foundation: Common in the construction of bridges, especially over water bodies, well foundations are deep, hollow cylindrical structures that are sunk into the ground to provide stability for heavy loads.

Deep foundations are typically more expensive and time-consuming to construct than shallow foundations due to the increased depth and materials required. However, they are essential for ensuring the stability and safety of large structures on weak or variable soils.

Choosing the Right Foundation

Selecting the appropriate type of foundation is crucial to the success of any construction project. Several factors must be considered when determining whether a shallow or deep foundation is more suitable for a structure:

A. Soil Conditions: The type and load-bearing capacity of the soil are the most important factors. Shallow foundations are appropriate when the soil close to the surface is strong enough, while deep foundations are necessary when the soil is weak or unstable.
B. Building Load: Larger, heavier structures like skyscrapers or bridges require deep foundations to support their weight, whereas smaller, lighter buildings can often be supported by shallow foundations.
C. Water Table: The presence of groundwater near the surface can complicate foundation design. In areas with a high water table, deep foundations may be required to avoid water-related issues.
D. Environmental Considerations: Some areas may be more prone to earthquakes, landslides, or other natural disasters. In these cases, a more robust deep foundation may be necessary to ensure the building’s long-term stability.

To learn more, watch the following video tutorial.

Video Source: Civil Engineering

Conclusion

Foundations are an integral part of any construction project, as they provide the necessary support for the structure above. Understanding the difference between shallow and deep foundations is essential for engineers and builders to ensure that their projects are both safe and cost-effective.

While shallow foundations are typically sufficient for smaller buildings or projects with strong surface soils, deep foundations are necessary for larger structures or those built on weaker soils. By carefully considering soil conditions, building loads, and environmental factors, engineers can select the most appropriate foundation type for each project.