The foundations of a building provide support, they hold up a building and make sure that it doesn’t sink.
A building structure is typically broken down into two main categories, above grade – the superstructure and below grade, the substructure. The general misconception is that the structural element which transmits the load to the soil (such as a footing) is the foundation, however, the substructure (or anything below grade) is otherwise known as the foundation and this forms the base for any structure. Generally, it covers about 25-35% of the total construction cost.
What is the objective of a foundation?
A foundation serves the following purposes:
- To distribute the total load coming from the structure on a larger area.
- To support the superstructures.
- To give enough stability to the structures against various disturbing forces, such as wind and rain.
- To prepare a level surface for above grade construction work.
Not all foundations are created equally, and each has its own use cases. In this section we’re going to run through each of the types, when you should use them and the pros and cons of each.
This article will break down as follows:
Table Of Contents
Let’s get started:
What are the two different types of building foundations?
In general building foundations can be classified into one of two main categories:
- Shallow Foundations
- Deep Foundations
A Shallow Foundation is defined as being a foundation where it’s depth is equal to or less than it’s width. Think the footings on your house, or on a shed.
A Deep Foundation on the other hand is a type of foundation where the height of it is larger than it’s width. An example of deep foundations would be piles or caissons.
With the definitions out of the way, let’s now dig into the different types of each of these foundations.
What are the most common types of shallow building foundations?
The four most common types of shallow building foundations include:
- Spread footing
- Combined footing
- Strap Footing
- Mat Foundation or Raft Foundation
Spread / Pad Footing
Spread footings are those which spread the super-imposed load of wall or column over a larger area. These are typically found on houses and low rise structures. Spread footings can support either columns or walls.
There are various different types of spread footings including:
- Single footing for column: In which the loaded area of the column has been spread to the large size through a single spread. The base is generally made of concrete.
- Stepped footing for column: This type of footing is provided for heavily loaded columns which require greater spread with steps. The base is generally made of concrete.
- Sloped footing for column: In this type of footing, the concrete base does not have uniform thickness but is made sloped.
- Wall footing without step: It consists of a concrete base without any steps, including the masonry wall.
- Stepped footing for wall: It consists of a masonry wall having a stepped footing with a concrete base.
Grillage Foundation
A grillage foundation is a is a special type of isolated footing generally provided for heavily loaded steel columns and used in locations where the bearing capacity of the soil is poor.
The load of the steel column is distributed over a very large area by means of two or more tiers of steel beams and joists which can be consolidated using concrete.
Each layer is laid at a right angle to the layer below it. Below is a figure illustrating a grillage foundation:
Combined Footing
A spread footing that supports two or more columns is termed as combined footing.
The combined footing may be of the following kinds:
- Rectangular combined footing: The combined footings will be provided in rectangular shape if columns carry equal loads. The design of rectangular combined footing should be done in such a way that the center of gravity of the column coincides with the centroid of the footing area.
- Trapezoidal combined footing: If columns carry unequal loads, the footing is of trapezoidal shape.
- Combined column-wall footing: It may be required to provide a combined footing for column and wall. Such combined footings are shown in fig.
Strap Footing
If an independent footing of two columns is connected by a beam, it is called a strap footing. A strap footing may be used where the distance between the columns is so great that trapezoidal footing becomes quite narrow. The strap does not remain in contact with soil and does not transfer any pressure to the soil.
Raft Foundation
A raft foundation is a combined footing that covers the entire area beneath a structure and supports all the walls and columns.
They are used in areas where the soil masses contain compressible lenses, or the soil is sufficiently erratic so that differential settlement would be difficult to control.
Raft foundations may be divided into three types based on their design and construction:
- Solid slab system
- Beam slab system
- Cellular system
All three types are basically the same, consisting of a large, generally unbroken area of slab covering the whole or large part of the structure.
What are the most common types of deep foundations?
Unlike shallow foundations, deep foundations distribute the load of superstructure into the ground vertically rather than laterally. The three most common types of deep building foundations include:
- Pile foundation
- Pier foundation
- Caissons or Well foundation
Pile Foundation
Pile foundation is a type of foundation in which the loads are taken to a low level by means of vertical members, which may be timber, concrete, or steel.
Pile foundation may be adopted when no firm bearing strata is available and the loading is uneven. The piles are driven down, consolidating the ground and supporting the structure through friction.
Piles may be of the following types:
- Sheet Piles – sheet piles are used for the retaining walls such as dock walls and other marine works. They help to protect against erosion and help to retain the sides of foundation trenches.
- End bearing piles: These types of piles are used to transfer load through water or soft soil to a suitable bearing material.
- Friction pile: Friction piles are used to transfer loads to a depth of friction load-carrying material by means of skin friction along the length of piles.
- Compaction pile: unlike the piles noted above, compaction piles do not typically support the structure directly, rather are used to compact loose granular soils, thus increasing their bearing capacity.
Pier Foundation
A pier foundation consists of one or many cylindrical columns to support and transfer large superimposed load to the firm strata below. These can include caissons, masonry piers, concrete piers, timber piers or steel piers.
Generally these piers consist of a dug hole down to more stable soil or rock, then the hole is filled with concrete, stone and or rebar. A pile cap is then traditionally installed on the top of the piers to help spread the weight across multiple piers.
Generally, pier foundations are shallower in depth than pile foundation.
Caissons / Well Foundation
Well foundation or caisson are box-like structures which are sunk from the surface of either land or water to the desired depth.
They are much larger than the pier foundation or drilled caissons. Caisson foundations are used for major foundation works like:
- Bridge piers
- Docks
- Large waterfront structures
Factors to consider when selecting a foundation type for your building.
While foundation selection can be a complex process, there are a number of factors that you should consider, those include:
Economic – how important is the building and what is the expected life cycle of the structure? As an example, is this a commercial space that will be torn down in 50 years or is this a stadium that needs to stand for 100 years?
Code Related – what do the building codes in your area say about foundations? Typically buildings are required to meet minimum life expectancy and design requirements that are outlined in your building code.
Size of the Superstructure – how big is your building and what will the loads be that are imposed on your foundation from your above grade structure.
Building Use – different uses will have lower or higher loads depending on their end user requirements. As an example, an office building might have higher loading requirements than a residential building.
Water level – a high water table can impact your foundation selection and make specific types of building foundations non-viable. Understand where the water is relative to your building foundation will help you in your foundation selection.
Existing Soils – the type of soil your building will be founded on has a huge impact on the foundation selection.
The subject of soil and how it impacts your foundation selection requires a deeper dive, so let’s get into it.
Selection of Foundation Based on Different Types of Soil
Depending upon your soil condition, a different foundation type is recommended. Below we’ve summarized general foundation types based on which soil is present on your site.
Foundation recommendations for hard soils (rocks, hard sound chalk, sand and gravel, sand and gravel with little clay content, and dense silty sand) include strip foundation, pad foundations and raft foundations.
The following factors should be taken into account when finalized your foundation selection:
- Minimum depth of 500 mm shall be used for the foundation if the area is prone to frost so as to protect the foundation.
- Strip or trench base shall be kept above groundwater table if possible.
- Sand slopes possibly eroded by surface water, so protect foundation by perimeter drainage.
- Weathered rock needs to be given proper attention.
Foundation recommendations for Uniform Firm and Stiff Clay include reinforced pile in locations of any trees or vegetation that was removed, strip foundations, raft foundations.
The following factors should be taken into account when finalized your foundation selection:
- Piles shall be tied properly into suspended reinforced concrete slabs or ground beams.
- Sufficient length of slab shall be provided to withstand clay heave force.
- Special pile design may be needed for clay slopes greater than 1 in 10 because creep possibly occurs. In few cases, reinforced trench fill could be employed.
- Minimum depth to the underside of the foundation shall be 900 mm.
- When strip foundation is constructed in desiccated clay in dry soil, then the foundation shall be loaded with the building prior to the return of rains.
- Specially designed trench fills or ground beams may be required if the foundation location is in close proximity to to trees.
Foundation recommendations for Soft Soil (soft clay, soft silty clay, soft sandy clay, and soft silty sand) include wide strip footings, raft foundations, pile to rock or firmer soil below, pier and beam foundations.
The following factors should be taken into account when finalized your foundation selection in soft soil:
- Wide strip footing is used when bearing capacity is sufficient and predicted settlement is acceptable.
- Strip footing shall be reinforced based on the thickness and projection beyond wall face.
- Frequently, Sub-soil can be improved using vibro treatment, and it would be an economical solution if employed in conjunction with strip or raft foundation.
Foundation recommendations for peat include concrete piles extending to softer soil below, pad and beam foundations, raft foundations.
The following factors should be taken into account when finalized your foundation selection in peat:
- Pile types include bored cast in place with temporary casing, driven cast in place, and driven precast concrete.
- Allow for peat consolidation drag on piles.
- Special high grade and protection is likely to be required in aggressive peats.
- If peat layer is shallow over firm layer of soil, dig it out and replace it with compacted fill. For this, use raft or reinforced wide spread foundation dependent on anticipated settlement.
- Frequently, sub-soil can be improved using vibro treatment, and it would be an economical solution if employed in conjunction with strip or raft foundation.
Foundations are the structure that we build our homes, our places of work and our lives on. Getting them right is imperative. For further reading and other helpful resources on this subject, check out the below links: