Foundations

A foundation is used to support a building or structure and transmits loads directly to the underlying soil or rock. It must satisfy two fundamental requirements. Firstly, it must provide an adequate factor of safety against failure of the supporting strata. Secondly, any resulting settlement, and in particular differential settlement, should not be detrimental or interfere with the function of the structure.

Foundations can be split into two main types, shallow foundations and deep foundations:

Shallow foundation

Shallow foundations are constructed relatively close to the ground level. Shallow foundations can only be used where the soil at that level is capable of adequately supporting the load. They are founded at a minimum depth in the UK to resist damage from frost, and in cohesive soils, for example clays, the presence of nearby trees can significantly affect the depth of the foundation.

Shallow foundations can be sub-divided into the following main types;
• Strip foundations (footings) – a linear foundation which generally support walls.
• Pad foundations – a discrete square or rectangular foundation supporting columns or piers.
• Raft foundations – a large single foundation supporting a whole structure.

Shallow foundations are generally constructed using in-situ concrete but some substructure elements can alternatively be constructed in precast concrete in part or whole to improve speed of construction on site.

Deep foundations (excluding basement construction)

A deep foundation is used to transfer loads from a structure above ground through upper weak strata of soil to a more competent one at depth, beyond which shallow foundations become both impractical and uneconomic. The most common form of deep foundation is provided by using piles which can be categorised as either replacement or displacement.

Replacement piles

The primary systems are Contiguous Flight Auger (CFA) Piles and Rotary Bored Piles. Piles can be installed with diameters between 300mm and 3.0m.

Single large diameter piles have been used on projects to support individual columns rather than the traditional method of using a group of smaller piles which tend to have more complex pilecaps and restraint systems. The single piles are designed assuming no restraint from the substructure and have a simple pilecap thus increasing speed of construction.

Geo-thermal piles incorporate flexible plastic pipes within the pile reinforcement cages. The completed concrete piles provide a conduit for the energy derived from the ground and are used in heating and/or cooling of the structure.

Displacement piles

These include precast concrete driven piles, steel (top or bottom) driven piles and jack-down steel piles. The steel cased systems are filled with concrete following installation. Precast concrete driven piles are available in section sizes up to 350mm diameter or 400mm square.

Pilecaps

At the head of the pile or pile group there is a pilecap which provides a connection between the pile and the ground slab and/or superstructure elements (columns or walls).

Pilecaps incorporating a multiple pile system comprising three or more piles require no additional restraint. A pilecap for a single pile will require restraint in orthogonal directions provided by either a suitable connection with the ground slab or by ground beams. Similarly a pilecap incorporating two piles will require restraint in only the weaker direction again by providing either a suitable connection with the ground slab or by ground beams.

Consideration should also be given to using piling in ground which would ordinarily be favourable to a shallow foundation solution. There are many reasons for this, but they include reduced excavation and soil disposal particularly in contaminated ground and this alternative solution may also offer an increase in the speed of construction.

Design and specification

Foundations should be designed using the Eurocodes, particularly BS EN 1997-1 and specifically for the design of concrete structures BS EN1992-1-1.

Concrete should be specified using BS 8500-1. Further guidance can be obtained using BRE Special Digest SD1 where concrete foundations are cast or placed within aggressive ground conditions.