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 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
• 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
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.
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.
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.
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
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
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