Foundations are an important aspect of building construction and can be broadly classified into two categories: shallow foundations and deep foundations. Shallow foundations, also known as spread footings, are typically used for structures that are relatively light and where the soil is strong enough to support the weight of the building. Some examples of shallow foundations include strip footings, pad footings, and raft foundations.
On the other hand, deep foundations are used when the soil is weak or unstable and cannot support the weight of the building. These foundations extend deep into the ground, providing more support and stability. Some examples of deep foundations include pile foundations, drilled pier foundations, and caisson foundations.
It is important to carefully consider the suitability of each type of foundation before making a selection for any construction project. Factors such as soil conditions, building weight, and the depth of bedrock must be taken into account when determining the appropriate type of foundation. Failure to choose the correct foundation can result in structural instability and potentially dangerous situations. Therefore, it is crucial to consult with a qualified engineer to ensure that the foundation chosen is appropriate for the specific project requirements.
Types of Foundation and their Uses
Foundations are an essential part of any construction project, as they provide a stable base for the structure. There are two main types of foundations: shallow and deep. Shallow foundations are used when the soil is stable and can support the weight of the building. Individual or isolated footing, combined footing, strip foundation, and raft or mat foundation are some of the common types of shallow foundations used in construction.
Individual or isolated footings are used to support columns or walls and distribute the load over the soil. Combined footings are used when two or more columns are close to each other and can share a single footing. Strip foundations are used when the load is distributed over a wider area, such as for load-bearing walls. Raft or mat foundations are used when the load is distributed over a large area and the soil is weak.
Deep foundations are used when the soil is unstable or unable to support the weight of the structure. Pile foundations are used to transfer the load of the building to a deeper, more stable layer of soil or rock. Drilled shafts or caissons are similar to pile foundations, but they are wider and can support heavier loads. These types of foundations are commonly used for tall buildings, bridges, and other large structures.
Types of Shallow Foundations
1. Individual Footing or Isolated Footing
An individual footing, also known as an isolated footing, is a common type of foundation used in building construction. It is designed to support a single column and is typically square or rectangular in shape. This type of footing is used when the loads from the structure are carried by the columns.
The size of the individual footing is calculated based on the load on the column and the safe bearing capacity of the soil. For example, if a column has a vertical load of 200 kN and the safe bearing capacity of the soil is 100 kN/m2, then the required footing area will be 2m2.
If the foundation experiences moments due to eccentricity of loads or horizontal forces, a rectangular isolated footing is selected. For a square footing, the length and width of the footing will be 1.414 m x 1.414 m. Overall, individual footings are an important and commonly used foundation type in building construction.
2. Combined Footing
A combined footing is a type of footing that is utilized when two or more columns are located in close proximity to one another, and their isolated footings overlap. This type of footing is essentially a combination of isolated footings, although its structural design is distinct from that of isolated footings.
The shape of a combined footing is rectangular, and it is commonly employed when the loads from a structure are being carried by the columns. The rectangular shape is designed to distribute the load from the columns to the soil in a more even and uniform manner. This helps to prevent uneven settlement, which can lead to structural problems over time.
Overall, a combined footing is a practical solution for situations where multiple columns are situated close together, and their loads need to be distributed uniformly to the soil. It is a vital component of the foundation system for many buildings, as it helps to ensure the stability and longevity of the structure.
3. Spread footings or Strip footings and Wall footings
Spread footings refer to a type of foundation whose base is wider than the conventional load-bearing wall foundations. This broader base is designed to distribute the weight of the building structure across a larger area, resulting in improved stability. By spreading the load over a wider surface area, spread footings can effectively bear the weight of the building and provide enhanced structural support.
Spread footings
Spread footings and wall footings are utilized for supporting individual columns, walls, and bridge piers when the underlying soil layer is located within 3 meters (10 feet) from the ground surface. It is imperative that the bearing capacity of the soil is adequate to uphold the weight of the structure over the base area. However, caution must be exercised in avoiding the use of spread footings and wall footings in areas where the soil is susceptible to ground flow of water above the bearing layer. Such scenarios can lead to scour or liquefaction of the soil, rendering it unsuitable for supporting the structure effectively.
4. Raft or Mat Foundations
Raft or mat foundations are a type of foundation that is designed to cover the entire area of a building, providing support for heavy structural loads generated by columns and walls. These foundations are characterized by their broad coverage, extending across the entire footprint of the building. They are specifically engineered to distribute the weight of the structure evenly across the soil, ensuring that the load is adequately supported and preventing excessive settlement or subsidence. Raft or mat foundations are commonly used in buildings where the soil conditions are weak or unstable, or where there are heavy loads that need to be supported, such as in high-rise buildings or industrial structures. By spreading the load over a larger area, these foundations help to minimize the risk of differential settlement and ensure the stability and safety of the building.
Raft or Mat Foundation
Mat foundation, also known as combined footing, is used for supporting columns and walls in structures with high loads to prevent differential settlement of individual footings. It is designed as a single, large foundation that encompasses all load-bearing elements of the structure. This type of foundation is ideal for structures built on expansive soils with low bearing capacity, where spread footings or wall footings may not be suitable. Raft foundation, which covers at least half of the area of the structure with individual footings and wall footings, is considered economical in such cases.
However, mat foundations should not be used in areas where the groundwater table is above the bearing surface of the soil, as it may result in scour and liquefaction. Proper consideration of groundwater conditions is crucial in the design and construction of mat foundations to ensure their effectiveness and stability.
Types of Deep Foundation
5. Pile Foundations
Pile foundation is a specialized form of deep foundation that serves the purpose of transferring substantial loads from a structure to a solid rock layer located deep below the ground surface. It is commonly employed in situations where the load-bearing capacity of the soil near the surface is insufficient to support the weight of the structure. Pile foundations are designed to transmit the weight of the structure to a more stable and stronger rock stratum, which is located at a greater depth. By utilizing piles, which are long columns made of materials such as steel, concrete, or timber, the foundation is able to distribute the weight of the structure across a larger area, effectively reducing the risk of settlement or structural failure. Pile foundations are commonly used in the construction of high-rise buildings, bridges, and other structures that require significant load-bearing capacity and stability.
Pile Foundation
Pile foundations are essential for transferring heavy loads from structures to hard soil strata that lie below ground level, where shallow foundations like spread footings and mat footings are not suitable. These types of foundations are particularly used to counteract uplift caused by lateral loads such as earthquake and wind forces. Typically, pile foundations are chosen for soils where the conditions near the ground surface are not suitable for supporting heavy loads. The depth of the hard rock strata can range from 5m to 50m (15 feet to 150 feet) below the ground surface. Pile foundations resist the loads from the structure through both skin friction and end bearing. In addition, they help to prevent differential settlement of foundations, ensuring stability and durability of the structure.
6. Drilled Shafts or Caisson Foundation
Drilled shafts, commonly known as caissons, are a type of deep foundation that can handle high capacity loads. Similar to pile foundations, they resist loads from the structure through shaft resistance, toe resistance, or a combination of both. The construction of drilled shafts or caissons involves the use of an auger, making them cast-in-situ foundations.
Fig: Drilled Shafts or Caisson Foundation (Source: Hayward Baker)
Drilled shafts, also known as caisson foundations, are a preferred choice when it comes to transferring heavy column loads that exceed the capacity of pile foundations. They are particularly effective when the hard strata below ground level is located within a depth range of 10m to 100m (25 feet to 300 feet).
However, it’s important to note that drilled shafts or caisson foundations may not be suitable in certain ground conditions. For instance, they may not be recommended in areas with deep deposits of soft clays and loose, water-bearing granular soils. Additionally, if the soil contains caving formations that are difficult to stabilize, or if it is comprised of boulders, drilled shafts may not be a feasible option. Furthermore, if an artesian aquifer exists in the soil, drilled shafts may not be suitable for the foundation construction. Careful consideration of the soil conditions and geotechnical characteristics of the site is crucial in determining the appropriateness of using drilled shafts as a foundation solution.
Summary:
What are broad classifications of foundation?
Foundations are a crucial component of any building structure. They serve as the base upon which the entire building rests and provide stability and support to the structure. Foundations are generally classified into two broad categories – shallow and deep foundations.
Shallow foundations are typically used when the soil is stable and can support the weight of the building. These types of foundations are relatively easy to construct and are used for smaller buildings such as houses, small commercial buildings, and low-rise structures. Shallow foundations are generally less expensive than deep foundations, and they do not require as much excavation.
Deep foundations, on the other hand, are used when the soil is unstable or unable to support the weight of the building. These types of foundations are constructed deeper into the ground to reach more stable soil layers or bedrock. Deep foundations are typically used for larger buildings such as skyscrapers, bridges, and tall commercial buildings. The construction of deep foundations is generally more complex and expensive than shallow foundations due to the required excavation and specialized equipment needed for their construction.
What are Types of Shallow Foundation?
Shallow foundations are classified into several types, including individual footing (also known as isolated footing), combined footing, strip foundation, and raft or mat foundation. These types of foundations are commonly used in building construction to provide a stable base for the structure.
Individual footing, or isolated footing, is a type of shallow foundation that is used to support individual columns or pillars. It consists of a single concrete block that is placed under each column and transfers the weight of the column to the soil.
Combined footing is another type of shallow foundation that is used to support two or more columns that are located close to each other. It consists of a concrete block that spans the distance between the columns and transfers the weight of the columns to the soil.
Strip foundation, also known as a continuous footing, is a type of shallow foundation that is used to support a linear structure such as a wall. It consists of a continuous strip of concrete that runs along the length of the wall and transfers the weight of the wall to the soil.
Finally, a raft or mat foundation is a type of shallow foundation that is used to support an entire building. It consists of a large, flat slab of concrete that is placed over the entire building area and transfers the weight of the building to the soil.
What are Types of Deep Foundation?
There are two main types of deep foundations used in construction projects: pile foundations and drilled shafts or caissons. Pile foundations involve driving long, slender columns or piles into the ground to support a structure’s weight. These piles can be made of various materials such as concrete, steel, or timber and are designed to transfer the load from the structure to the soil or rock below.
On the other hand, drilled shafts or caissons are deep, vertical excavations in the ground that are filled with reinforced concrete. These foundations are created by drilling a hole into the soil or rock and then pouring concrete into the hole. The concrete hardens, providing a strong foundation for the structure above.
Both pile foundations and drilled shafts or caissons are used when the soil near the surface is unable to support the weight of a structure or when a deeper foundation is required to resist lateral forces such as wind or earthquakes. The choice of foundation type depends on various factors such as soil conditions, site constraints, and the weight of the structure.
What are difference between pile foundation and drilled shafts?
Drilled shafts are a type of foundation that shares similarities with pile foundations. However, they are cast-in-situ and have a higher capacity to bear loads. These foundations can transfer larger column loads compared to pile foundations. They are typically used in situations where hard strata are located at a depth of 10m to 100m (25 feet to 300 feet) below the ground level.
What are difference between isolated and combined footing?
When two or more columns are situated closely together and their individual footings overlap, a combined footing is often constructed. Although it is a combination of isolated footings, the design of the combined footing varies from that of the individual footings. This is because the combined footing must support the weight of multiple columns and distribute the load evenly across the soil. Therefore, the structural design of the combined footing must take into account the proximity and arrangement of the columns, as well as the soil properties and load distribution. By considering these factors, an effective combined footing can be designed and constructed to ensure the stability and longevity of the columns.
When is Raft or Mat Foundations used?
Raft or mat foundation is typically utilized in constructing buildings with columns and walls that are subjected to high loads. The primary function of these types of foundations is to prevent differential settlement of individual footings. Raft foundations are designed to function as a combined footing for all load-bearing elements of the structure.
Differential settlement occurs when one part of the foundation settles at a faster rate than another part, leading to unevenness and structural damage. Raft foundations help prevent this by spreading the load from the building uniformly over a larger area, thereby reducing the risk of differential settlement. In essence, rafts can be thought of as a single, continuous foundation that covers the entire area beneath the building.
These types of foundations are ideal for buildings constructed on weak or compressible soil, as they can distribute the weight of the structure more evenly, reducing the risk of foundation failure. Additionally, rafts can be designed to accommodate irregularly shaped buildings, making them a versatile option for a wide range of construction projects. Overall, the use of raft or mat foundations can greatly enhance the structural stability and durability of a building, ensuring its longevity and safety.