Last edited 06 Nov 2020

Principles of foundations


[edit] Introduction

A foundation is the part of a building below ground level that transfers loads from the construction above it to load-bearing ground or rock strata below it. Installing foundations normally requires excavating the ground to the required depth. The foundation depth is the distance from the ground level to the lowest point of the foundation construction.

[edit] Key factors for choosing the type of foundation

The key things to consider are:

[edit] Types of foundations

There are two main types of foundation:

The base of a deep foundation is typically 3m or more below ground. They are typically more expensive to construct, partly because of the greater volumes of spoil that need to be excavated, and require higher levels of skill, technology and expertise.

Shallow foundations are laid closer to ground level and tend to be easier and cheaper to construct. They may require less design and construction input when compared to deep foundations..

[edit] Shallow foundations

[edit] Spread footing

A spread footing (also called a strip or wall footing) distributes loads from walls or columns over a larger area. Normally, strip footings are constructed with plain or reinforced concrete materials. Care should be taken when excavating footings as space should be allowed for the installation of formwork and to leave enough working space inside the trench for relevant tradespeople such as bricklayers, carpenters and steel fixers.

Pad foundations are a type of spread footing that carries column loads from the superstructure to the ground. Square or rectangular in plan, the pad footing can be formed with a two-way reinforced concrete slab.

[edit] Isolated footing

Refers to a footing or footings that act alone in resolving the applied stresses. Isolated footings are designed and constructed as a single unit, so applied bending moments, shear and axial stresses are coveyed into the ground by a single structure. They may also be termed mono-struct footings.

[edit] Strap footing

A strap footing involves two simple isolated footings linked together by an underground beam. The two footings can be square or rectangular in plan with the beam centrally laid between them. Strap footings have the advantage of improving the foundation's capacity to carry high axial loads and bending moments at congested sites where it is not possible to lay large footings.

[edit] Sloped footing

A sloped footing has sloping (or canted or bevelled) upper surfaces which have the advantage of increasing the footing's resistance against shear and crushing forces. The column is centrally placed on the top surface of the footing; the corners of the footing slope up to the corners of the column at the defined angle as specified by the engineer. The footing's strength depends on the class of concrete, the size of rebar, the bar bending layout and the number of bars fixed.

[edit] Stepped/pedestal footing

Stepped (or pedestal) footings have steps with the base being larger than the upper sections. They have the advantage of being stronger against punching shear. However, they are more expensive than common spread footings due to the greater labour and materials involved in their construction.

[edit] Combined footing

A combined footing runs between two or more columns and obviates the need to have multiple isolated footings placed at each column location. Combined footings help to resolve applied stresses throughout the footing and hence have greater stability and can carry significantly more stresses into the ground.

[edit] Cantilevered footing

A cantilevered footing comprises two columns whose pads are connected by a ground beam sometimes called a strap beam. One of the columns will be eccentrically placed on its pad due to the proximity of an existing wall or some other immoveable feature. This means the pad and column can be constructed without impinging on the neighbouring foundation. Cantilevered footings require careful design consideration, from the type of load to be applied to the strength of materials to be used.

[edit] Raft foundations

Raft foundations (sometimes referred to as raft footings or mat foundations) are formed by reinforced concrete slabs of uniform thickness (typically 150mm to 300mm) that cover a wide area, often the entire footprint of a building. They spread the load imposed by a number of columns or walls over the area of foundation, and can be considered to ‘float’ on the ground as a raft floats on water.

[edit] Deep foundations

Deep foundations are used where the topsoil has low bearing capacity or the frost line is deep. They may are also used where equipment in the building in question is sensitive to settlement, eg in hospitals or research facilities.

[edit] Pile foundations

Pile foundations are deep foundations. They are principally used to transfer the loads from superstructures through weak, compressible strata or water into stronger, more compact, less compressible and stiffer soil or rock at depth, increasing the effective size of a foundation and resisting horizontal loads. They are typically used for large structures, and in situations where soil is not suitable to prevent excessive settlement.

[edit] Drilled bell piers

This is a type of deep foundation where the the bottom of the pier is belled (has a base that extends beyond the shaft of the pier).

[edit] Caissons

A caisson is a box-like structure commonly used in civil engineering projects where work is being carried out in areas submerged in water, such as the piers of a bridgethat spans over a river. It is created by sinking an enclosed excavation into a river or sea; the water and soil are evacuated to allow construction work to proceed within. Caisons are made to act as both temporary and permanent structures, abd remain part of the foundations after the works are completed.

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