Types of Foundations

This article is aimed at giving detailed knowledge on the types of foundations and their applications.

What is a foundation in construction?

Foundation in construction, therefore, is the substructure of a building or any civil structure that connects the entire structure directly with the ground on which the building stands, through which loads both live and dead are safely transferred to the soil underneath. Foundations in general, are classified into two categories:

1. Shallow foundation: This category of foundation transfers structural loads to shallow soil stratum between the depth 0.6m to 1.2m, and this class of foundation does not support multi-story buildings, examples of such foundations are: Strip or wall foundation, individual or isolated foundation, raft or mat foundation, combine footing, and cantilever or strap foundation.
2. Deep foundation: Unlike the shallow foundation, the deep foundation transfers structural loads to a deeper soil stratum to support large and high-rising structures, they can be as deep as 20m to 65m, even more, depending on the size, the examples of deep foundations are as follows: caisson foundation, pile foundation, and pier foundation.

Structural foundations are meant to provide structural stability, create a level ground for the development of the structure, even distribute structural loads, reduction of load intensity within a safe bearable capacity of the soil, to resist or prevent soil movement effect on building structures. Foundations are the most important and critical parts of every building or civil structure, a good foundation for any building will always guarantee its structural stability of that building.

Due to the different types of soil and their varying bearing capacities, foundation or structural engineers are forced to make choices of the types of foundations that will be suitable for a particular building, bearing in mind the load and the size of the structure and also the soil profile and the bearing capacity of the soil on which the structure will stand. This article is aimed at giving detailed knowledge of the different types of foundations and their advantages and disadvantages.

Types of foundations

The different types of foundations and their advantages and their disadvantages include the following:

1. Strip or wall foundation
2. Pad foundation
3. Raft or mat foundation
4. Combine footing
5.  Strap footing
6. Caisson foundation
7. Pile foundation
8. Pier foundation

Strip foundation

A strip or wall foundation is a continuous slab strip that runs through the entire length of a strip to distribute structural or nonstructural load-bearing walls to support the entire structure. Strip foundations are more suitable where the structural loads are bored by the entire walls rather than the individual or isolated footings and columns. In most cases of strip foundations, the strip widths are usually twice or thrice the width of the load-bearing walls.

Strip or wall foundations are usually constructed with plain concrete, reinforced concrete, stones, or bricks depending on the nature of the soil, the building size, and the economic viability of the available materials on the construction site. In a case of a block wall, the foundation is made of a few numbers of brick courses, and the last course is made of plain concrete or reinforced concrete of twice or thrice the width of the wall and of a thickness not less than 300mm which constitute the strip footing. These types of foundations are economical when the load magnitude transmitted to the soil is within a bearable soil bearing capacity and the structure is placed on stable ground.

Advantages of strip foundation

• Strip or wall foundations are technically simple to design and construct.
• The walls in the strip foundation are used as basement walls to support the structure.
• Their construction does not require heavy equipment which makes them relatively inexpensive and affordable
• Strip foundations are usually suitable for both small and large building apartments.
• They have a very minimal and bearable differential settlement.
• They can withstand reasonable load intercity and they are reliable and durable.

Disadvantages of Strip foundation

• Their construction requires a lot of manual labor and requires a lot of earth filling between the structural partitions
• The construction of strip foundations consumes a lot of materials.
• They are not suitable for unstable ground or soils with low bearing capacity.
• They are most prone to dampness and require intense waterproofing.

Pad foundation

A Pad foundation is an extended part of the foundation which consists of gravel, crushed stone, or soil. A Pad foundation is used to distribute building load over a larger area than that covered by piles, allowing the pad to support a substantial portion of the total dead and live loads.

Pad foundations can be constructed using treated timber beams instead of steel pile casings, especially in areas where good quality timber with a long life expectancy is available. Construction material may be changed according to the use of pad foundation i.e if it has to sustain a corrosive environment then construction material can be changed into non-corrosive materials like concrete etc.

Types of pad foundation:

There are mainly three types of pad foundations as follows:

1. Timber Pad Foundation
2. Timber Frame Pad Foundation
3. Concrete Pad Foundation

Pad foundation is used where soil conditions are not stable or suitable for piles to be found. They can be used in the critical building areas as follows:

• electric substations, electrical control rooms and generating plants, etc.

• commercial buildings with high importance i.e. shopping complexes, office towers, hotels, etc.

• houses having more number of floors above ground level which require bigger foundation area than piles foundation can offer etc. In this case, pad foundations allow a building to have highly expansive floor plans on the upper levels of a structure.

Advantages of pad foundation

Pad foundations are becoming increasingly popular as a low-cost option for residential housing, the advantages of pad foundation design are many and include:

• Fire protection
• Space efficiency
• Lower cost, and more.

Disadvantages of Pad Foundation

While many construction experts agree that pad foundations may be an acceptable alternative for low-rise buildings such as single-storey homes, two-storey homes, or mobile homes, they are not appropriate for large buildings such as malls, department stores, or larger commercial buildings. There are several disadvantages associated with pad foundations including:

•  Pad foundations cannot support heavy loads – Since the strength of a foundation is measured by the weight it can hold securely against shifting soil and other factors, pad foundations will never be able to support large commercial buildings. This disadvantage is compounded when there are many people in one building because more weight must be supported.
• Pad foundations consist of only four walls – Slabs that rest on footings at each end of the building tend to flex under pressure (such as an increase in occupants) resulting in cracking on interior walls and ceilings. If extra precautions aren’t taken during construction to allow for this flexing, such as installing support columns or walls to add rigidity, cracking will occur.
• Pad foundations are not suitable in areas with high water tables – Areas that have a high water table pose special problems when it comes to foundation design because the weight of the building pushes down causing the surrounding soil to become saturated and collapse around and under the foundation. This can cause serious structural damage.
• Pad foundations are more susceptible to uneven settlement – When buildings settle unevenly, they shift which can cause cracks throughout all interior walls. The shifting usually occurs on one side of the building creating an extremely unsafe environment for occupants who do not know that they are being subjected to potentially dangerous shifts.

Mat foundation

Mat foundation is a type of footing that does not require excavation. It is called mat foundation because of its “mat”-like shape that can be seen during construction, and the literal translation from Japanese to English means “floor base”.

Mat foundations are used when building structures on soft soils such as clay or sand. A mat foundation consists of precast concrete blocks which hold up an elevated structure above ground while transferring the load (weight) to the ground beneath it. This reduces the pressure on top of the soil, preventing it from collapsing and allowing for larger structures to be built without settling over time due to their own weight.

Types of mat foundations

The three main types of mat foundations are:

1. Continuous strip: Continuous strip mat foundations consist of several rows of closely spaced, tapered, precast concrete blocks. The joints between them are filled with mortar and the structure is built on top of this “mat” once it has sufficiently cured and reached project level height. These mat foundations can be used for all load-bearing structures such as buildings, parking garages, etc.
2. Moment frame: The moment frame mat foundation consists of a grid of steel beams that span in two directions (usually four) perpendicular to each other. The beams sit on top of the rebar reinforcement within the precast blocks which support them from below. Once construction is complete, this type of mat foundation acts as a rigid floor system and does not allow deflection or movement.
3. Hollow-core slab (also known as drop-in): The hollow core slab (drop-in) mat foundation is a variation of the moment frame mat foundation, but with one or more hollow concrete cores. These precast concrete blocks are usually placed in two rows and accommodate several beams which span between them. The cores take up space that would otherwise be left empty, and they function as formwork for the cast-in-place floors within. This type of mat foundation is mostly found in parking garages and similar structures that require less weight-bearing capacity than regular buildings.

Mat foundations can also be built on top of other mat foundations to create multi-level structures such as parking garages, residential high rises, etc. In this case, steel beams will run through all levels to allow deflection under live loads. These floors are connected to the columns above and below them with post-tensioned cables. This provides a self-centering effect that prevents the mat foundation from “jumping” around when exposed to lateral forces, even if there is no structural beam at that level.

Advantages of mat foundations

The following are some advantages of mat foundations:

•  No need to excavate underneath
•    Less expensive than traditional foundation methods
• Aesthetically pleasing since it does not require any type of excavation or load-bearing walls to be built during the construction.

Disadvantages of mat foundations

The following are some disadvantages of mat foundations:

• Must be reinforced adequately in order for it to support the weight without settlement
• Higher risk of damage due to construction crew working on top
• Limited load-bearing capacity if built on soft soils such as clay or sand
• Requires a heavier steel framework, which adds to the overall cost and time required for the construction process.

Mat foundations are a very efficient and economical way of building structures that require the least amount of resources possible. They can be used for different types of buildings and other similar structures with equal efficiency. This type of foundation does not require any type of excavation underneath which makes it environmentally friendly and aesthetically pleasing during the construction process. Considering its uses and application mat foundation is advantageous over traditional methods in many ways, however, there is always room for improvement.

Combined footing

Combined footings refer to two or more footings that share a common concrete pad or footing. There are several combined footing design and construction methods, combined footings can be used in any combination and number, as required by the building layout and structural requirements. Any type of foundation is achievable with combined footings. It can also be constructed at different stages of the build process because separate components are not needed for construction. A single pour provides all necessary load transfer between components.

Advantages of combined footings

Advantages of using combined footings include

• Saves time and money when building a foundation for a structure
• Reducing surface area provides the opportunity to create more usable space within your structure by utilizing space normally occupied by footings for storage or other purposes
• Fewer penetrations into the ground
• Multiple uses for one structure (such as storage space under a platform)
• Improved durability due to less settlement
• Reduces expenses due to simplification of the foundation design process.

Combined footings may be used in any application where a standard footing can be utilized. Combined footings have been used successfully in residential homes, commercial buildings, sports arenas, and industrial platform construction projects.

Strap footing

Strap footing is a type of foundation to support the vertical load of beams or walls. They are also called cantilever footings, cantilever piers, and cantilever shores.

They are usually used in situations where there is no other suitable supporting ground besides the soil to bear weights. To member heavy loads, strap footings are composed of bars arranged in regular spaces like many strips (straps) that are connected together firmly in pairs and then installed into excavated holes. The weight of the structure will act downwards to force both ends of each bar into the soil below it. This helps prevent them from tipping away from their lowest position while maintaining their strong resistance against lateral earth pressure caused by soil filling around them during construction works.

Types strap footings

There are two types of strap footings:

1. At-grade and
2. Below grade

Strap footing design basically depends on the depth of the foundation, local soil conditions, and the loads that it will carry.

Caisson foundation

The term “caisson” originates from the French word meaning lock or chamber. Caisson foundation is a type of composite-action caisson that is usually built by driving or jetting, though it can also be cast in place. It comprises a taperable concrete box with an internal tubular steel column within the low-stability, nonliquefiable soils at depths typically between 15 to 50 meters. They are used for the construction of structures that require deep footings on soft or loose soil with high compressibility, including road embankments, runways, railway beds, and wharves.

A  typical caisson foundation system consists of two components: one has an upper part comprising longitudinal steel piles, lateral steel piles around the circumference, and a cylindrical-shaped caisson that provides a connection between these two.

A standard caisson is about 8 feet in diameter and 30 to 40 feet long, though there are slightly larger units available for taller structures. For some projects, the caisson is prefabricated onshore and lifted into place with a crane vessel or derrick barge. In other cases, it may be built on-site by driving or jetting from within the construction site using earth-moving equipment fitted with hydraulic extensions which can drive the unit forward under its own weight as it sinks deep enough to reach a suitable bearing stratum.

In general, a caisson foundation has many advantages over other types of foundations such as a pier, spread footing, and pile foundation. By using a caisson type of foundation, the structure will be saved from soil settlement and vibrations caused by earthquakes. There is no need for piling, which can cause large excavation and disturbance on adjacent buildings during construction.

Advantages of caisson foundation

• Constructed in soft soils with higher compressibility and low strength
• Easier to construct than many other types of foundations due to the lower depth required
• No need to pile up when constructing the building [causing less disturbance]
• Maximum bearing capacity can be attained [in some cases] compared to other types of foundations

Disadvantages of caisson foundation

On the other hand, there are also disadvantages to constructing a building using a caisson type of foundation.

• This type of foundation is more likely to have a higher cost compared to other types.
• Another disadvantage of a caisson foundation is that it will be difficult to remove the concrete caissons after the project has been completed since they are large and heavy objects.
• Also, this type of foundation is only suitable for smaller buildings that are shorter than 30 meters.

Pile foundation

Pile foundation is also known as piles. Piles are excellent deep foundation solutions for buildings in areas where soil conditions are not geotechnically stable enough to support the building without piles.

Piles are long, slender structural elements that when driven into the ground can transfer an enormous amount of load onto the soil or rock below without displacing the materials. They do this by transferring their load through friction generated between the piles and the surrounding soil or rock.

The pile foundation was introduced several decades ago before it gained popularity, especially in modern engineering projects such as skyscraper building foundations. It has since been used in numerous major projects worldwide. As a matter of fact, piles have become one of the popular deep foundations for modern high-rise buildings.

Despite piles being a popular deep foundation solution, many people still do not have any idea regarding what piles are and their applications.

Types of piles

The types of piles are determined by the material used to make piles. Piles can be constructed from either steel or wood or a combination of both. The two most common piles are:

1. Cast-in-place piles: Cast-in-place piles are installed using heavy machinery without removing the soil at the location where they will be placed. They do not require any earthmoving equipment, which means less noise and is a better option in busy locations such as highways, railways, etc. These piles can also be installed underwater if necessary since no excavation work is needed when installing them. Also, they have a good capacity for resisting lateral forces that may exist due to construction activities and can be very efficient piles in terms of economy.
2. Driven piles: Driven piles are usually made from steel or wood after being driven into the ground using heavy vibrating equipment called pile drivers. They are quite easy to install because all you need to do is drill a hole into the ground and you can install piles at any depth underwater.

Driven piles are usually considered to be less economical than cast-in-place piles because of their susceptibility to damage during installation. Also, piles that are installed using pile drivers do not provide much capacity for resisting lateral forces as well as uplift forces even if they are made from steel. So piles must always be connected to other structural members like beams or walls either directly or through shear studs (which is a special type of connection device) in order to make them effective against lateral forces.

Pier foundation

A Pier foundation is a type of foundation that utilizes a pillar or column made of materials such as wood, steel, or reinforced concrete to support a structure. This pillar is then placed into the ground usually with the use of pilings.

Pier foundations are used when a building can’t utilize solid ground for support, i.e., over water, swampy land, and soft soil. Pier foundations are also used in areas where there might be unstable soil or erosion problems since they don’t have deep footings like other foundations do.

Typically pier foundation consists of a simple footer in which the “piers” are attached in some way to transfer forces from above down through to the ground below it.  A pier foundation may consist of a footing or pad placed on firm ground, an isolated pier extending from the footer, a pile placed to transfer forces through soft soil and or bedrock.

Types of pier foundations

There are two types of pier foundation designs:

1. Braced piers:  Braced piers have both ends resting on firm ground which will often require deep footings, as well as piles, driven into the bedrock below the building site. The main advantage of this design is that it provides good resistance against horizontal forces incurred during earthquakes.
2. Eccentric loaded piers: Eccentrically loaded piers have one end attached to a footing placed horizontally across the firm ground and the other end suspending over a void area such as water or air. This type has less lateral force than a braced pier because the soil below the pier can be removed in order to lower building stress.

The loading on a pier foundation is substantially larger than that of a spread footing or mat foundation and must be designed accordingly. In addition, there is usually no need for a cap beam if the pier is embedded into concrete which acts as its own cap beam. However, it can accumulate groundwater at its base which creates uplift pressure, especially in very loose soils such as silt or sandy soils. The water from surrounding land sometimes enters a pier basement through weep holes causing damage to walls and may even undermine foundations. An appreciable amount of load transfer takes place through friction between the wet clay around piers and their vertical surfaces.