Civil engineering structures that impose load on the ground can be grouped into two types; mass structures and framed structures.
Mass structures made of soil, rock or mass concrete include such constructions as embankments, dams, mining waste tips and breakwaters. These are in direct contact with the ground and the magnitude and distribution of stress imposed upon the ground is related to the size and shape of the constructed mass, and the density of the material from which it is composed.
Framed constructions are buildings containing hollow spaces for dwellings, storage or industrial use. The weight of the upper parts of the structure is transmitted to the ground via the load bearing members of the structure. These load bearing members are composed of steel, concrete, bricks and wood, whose strength is generally much greater than that of the ground immediately beneath them. Contact between these load bearing elements and the ground is made via a foundation, which generally distributes the load of the structure so that the stresses on the ground are not excessive. There are four main types of foundation (Fig. 1).
In an ordinary house or modest size building, external and internal walls carry the building load. The foundation under the wall is usually a long strip of either mass or reinforced concrete, which is somewhat wider than the wall it supports. It is very much longer than it is wide. Foundations are usually placed at some depth, normally greater than a metre below surface, to avoid swelling or shrinking associated with the ground freezing or drying.
Larger buildings may be built around a framework of load bearing columns connected together by horizontal beams. Foundations at the foot of these columns are square or rectangular ‘pads’ of either reinforced or massive concrete. The pads are designed so as to distribute the load from the column to the base of the pad without overstressing the ground on which the pad rests.
If the foundation loads are high and the foundation ground weak, the area of pad or strip footings required to give a bearable foundation stress may be so great that the footings coalesce to form a single plate, extending under the whole area of the structure; i.e. a raft. The practicalities of foundation construction are such that if the combined area of pad and strip footings exceeds about one third of the plan area of the building, then it may be easier and more economic to construct a raft foundation. Raft foundations are almost always of reinforced concrete, perhaps stiffened by cross beams.
Piles are essentially columns of reinforced concrete, steel or timber, usually between about 0.3 and 1.5 m diameter which are inserted into the ground by a variety of methods and whose resistance to the load they carry is provided by friction on the sides of the column and reaction with the ground at their base. There are many different types of pile but they are mostly either ‘driven’ or ‘cast-in situ’.
Driven piles may be square or round in cross-section and are generally from 0.3 to 0.5 m diameter. They are mostly made of reinforced concrete, but may sometimes be of timber or steel. Steel piles are often constructed of H or I section steel beams. These piles are driven into the ground using mechanical hammers until they encounter a strong layer that they cannot penetrate and/or until sufficient side friction is built up to prevent further penetration. Often piles are driven until a limit is reached defined by the number of blows required for a given increment of penetration. Driven piles displace and compact the ground into which they are driven.
If a foundation is composed of many closely spaced equal length piles and piling begins on the foundation periphery, it is not uncommon for the piles at the centre of the foundation to encounter difficult driving conditions because they are driven into ground already compacted by the earlier piles.
Cast in situ piles are formed in place. A hole is first bored down to a bearing stratum and the sides of the borehole supported by casing. A cage of reinforcing steel is placed in the hole and concrete poured into the hole through a pipe going down to the bottom of the hole. As the hole becomes filled with fluid concrete both injection pipe and casing are pulled to the surface, leaving a reinforced concrete column in the ground. The holes are made by percussion or auger drilling machines; diameters range from about 0.4 to 1.5 m. ‘Shell’ piles combine driving and casting-in-place. A tube with a toe cap is driven into the ground using a hammer inside the tube and the tube is then filled with concrete.
The carrying capacity of both driven and cast in place piles is built up of ‘end-bearing’ capacity (at the toe of the pile) and ‘skin friction’ (on the sides of the pile). The proportion of pile load carried in end-bearing and in skin friction varies greatly, depending on ground conditions and the type of pile.
Other Foundation Types
The types of foundation discussed above are those most commonly used; others exist which are essentially variations of the above e.g. basements, caissons, and stone columns.