Types of Foundation in Building Construction

In the era of tall and supertall buildings, foundation designs present tall challenges to engineers. Besides mammoth weight and wind pressure that exert multiple types of loading on the foundation, the geotechnical nature of the land might sound the alarm bell if not addressed properly. Let’s look at the various types of foundations in building construction and how they are arming design engineers to deal with these increasingly complex issues. 

Substantial vertical load, cyclic loadings by the wind, and geotechnical causes are the major concerns in the foundation design of skyscrapers. Conventional methods are not always dependable because they require extrapolation beyond experience. Engineers are increasingly applying newer technologies and ideas to the foundation designs.

Key considerations to selecting the right type of foundation in building construction

The following are some of the variables that could affect the kind of foundation chosen to support a tall building:

• Nature of land
• Type of structure
• Magnitude of load (weight of the building)
• Settlement of load
• Access to equipment
• Effects on adjoining or nearby structural foundations
• Local construction methods 
• Budget

What design issues are to be considered ?

Generally, the following questions will need to be addressed when designing foundations for high-rises: 

1. What should be the highest strength of the foundation under vertical, lateral, and moment loading combinations?
2. The impact of cyclical wind, earthquakes, and wave loadings (where applicable) on foundation capacity and movements 
3. Settlement of the total loading
4. Differential settlements, both within the high-rise footprint and between high and low-rise areas (podiums, etc.) 
5. The potential effects of ground movements on the adjacent areas on the foundation system (e.g., from pile cap excavations or adjacent facilities) 
6. The dynamic response of the structure foundation system to wind-induced and wave-induced forces (if appropriate). 
7. Potential earthquake effects on the foundation, including the response of the structure-foundation system to earthquake excitations and the chances of liquefaction in the soil surrounding or supporting the foundation 
8. Structural design of the foundation system to address the load-sharing between the different components, such as the piles and the supporting raft, etc., and the load distribution within the piles. 

Different types of foundation options for high-rises

Fundamentally, there are two types of foundations in building construction, namely

Shallow foundations and deep foundations

Shallow foundations are best suited when the soil close to the surface has sufficient strength and rigidity to support the structure’s loads. The depth of a shallow foundation is less than the width. This type of foundation is most commonly used in residential and light commercial buildings, as well as in some low rise industrial structures. 

A deep foundation, also known as a pile foundation or drilled shaft foundation, is a type of foundation that is suitable for applications where the upper soil layers are weak or compressed. This type of foundation transfers the load of the building to the deeper part of the earth, where the soil is stable. It is a popular choice for high-rise buildings, bridges, and sea structures.

The list below comprises the commonly used foundation types for high-rises:

• Raft or Mat foundations
• Compensated Raft Foundations
• Piled Foundations
• Piled Raft Foundations
• Compensated Piled-Raft Foundations

Raft or Mat foundations

Raft (mat) foundations may be suitable for buildings of medium height. Raft or mat foundations are large, continuous, rectangular, or circular concrete slabs that carry the load of your superstructure and distribute it over the entire area beneath your building.

This type of foundation is considered one of the most shallow foundations and helps to control differential settlement. Mat foundations spread out under your building’s footprint and reduce contact pressure in comparison to traditional strip or trench foundations. 

Mat foundations are suitable for low-bearing capacity soils, where spread footing covers approximately 70% of your structure, high structure loads, soft pockets/cavities in your soil (unknown extent of raft), and high compressible soil extending deep down.

Ideal applications for Raft or Mat foundations

Mat foundations are commonly used for commercial buildings as the loads are usually high for such requirements. Mat foundations are often used for high-rises with basements. 

Mat foundations are also used in low-bearing capacity soil. So the mat allows the load of the building to be evenly distributed. Mat foundations can also be used to help reduce differential settlement in buildings. Even where the soil layer is unsteady, a raft or mat foundation may be suitable. For example, the soil layer may suffer from movement in a mining area. If this is the case, a strip foundation would cover over 70% of the area under the building.

Compensated Raft Foundation

Towering buildings often have one or several basements to accommodate car parking, as well as commercial and retail spaces. In these cases, the raft is constructed by digging out the soil before building the foundation and superstructure. 

The compensated raft foundation is an example of a deep foundation and is ideal when the construction site soil is compressible, soft clay, or loose sand. The depth of the raft is decided by the rule that the weight of the raft slab plus the weight of the structure to be supported should be equal to the weight of the excavated soil.  

Since the underlying ground is stressed by the excavation, the net rise in ground stress caused by the structure will be reduced. Therefore, the settlement of the foundation and the differential settlement will also be reduced. The resultant foundation is called a ‘compensated’ or buoyancy raft. This can be very useful when building on soft clay/loose sand, as settlements can be significantly lower than if the foundation was at or close to the ground surface.

Piled Foundations

Pile foundations are deep foundations. They are constructed of long, slender columnar parts that are normally composed of steel, reinforced concrete, or, in rare cases, timber. A foundation is classified as ‘piled’ when its depth exceeds three times its breadth. 

A pile foundation is applied where the ground conditions on a site do not support a shallow raft or mat foundation system, particularly for high-rise buildings in which significant vertical and lateral loads can play a role on the foundation. In these cases, the building loads must be supported on piles (single piles) or groups of piles (groups of single or multiple piles), usually located under columns and load-bearing walls. 

A pile foundation for high-rise structures often consists of a large number of piles. The challenge in designing a pile foundation system is to capture the effects of group interaction. 

It is well known that the settlement due to a pile group for the equivalent average load level can vary significantly from a single pile settlement. The ultimate load that a group can support may not be the same as the maximum load that each pile can carry within the group. Therefore, pile group efficiency must be taken into account.

Piled Raft Foundation

Piled raft foundations can be used for large structures and in situations where the soil is unsuitable to prevent excessive settlement. Piled raft foundations are becoming increasingly popular for high-rise buildings. 

The number, position, reinforcement, and length of piles are determined during the design process to ensure the stability of the structure and provide an economical solution. The raft and piles act together to ensure that the required settlement does not exceed it. 

Usually, the piles provide the majority of the stiffness, and the raft provides the additional capacity at the final loading point. If there are one or several ineffective piles, then the raft can allow for some degree of load to be redistributed to other piles to reduce the impact of the pile’s weakness on the foundation’s overall performance. 

There can be an Unconnected Piled Raft Foundation (UCPRF), where the piles are not connected to the raft but are separated from the raft by a structural fill ‘cushion’ (e.g. a compacted mixture of sand-gravel or compacted soil). This can provide a more efficient and economical solution.

Compensated Piled Raft Foundations

A compensated piled raft foundation is normally used when the soil does not have sufficient bearing capacity, so piles are required to carry some of the load. However, many foundation designers are hesitant to consider piled raft foundations for soft clays because: 

• Soft clay often has little bearing capacity and stiffness to build the raft. So the piles have to carry most of the load. 
• If the soft clay becomes prone to settling (e.g., from reclamation filling, or dewatering), the soil may move away from the raft’s base, again causing most of the load to be carried by the piles.

For compensated pile rafts, the excavation and pile installation processes must be tailored to each individual instance. In some constructions with shallow excavations, the piles can be erected from the ground level before the excavation. In other cases, if a larger depth is required, some or total excavation is completed first, followed by the installation of the piles.