By M F Randolph,University of Cambridge
This paper was first published in GE’s May 1983 issue
The manner in which piles are used as part of a foundation varies with the ground conditions and with the requirements of the foundation. The simplest use of piles is as columns, carrying the load of a structure, through soft overlying soil, down to a competent bearing stratum. For deeper deposits of compressible soil, friction piles are used as a means of reinforcing (strengthening and stiffening) the soil. In general, the main reason for such piles is to reduce the settlement of the structure. In some cases, where permissible settlements are relatively large, piles loaded up to their ultimate capacity may be used to reduce the net load on the main raft foundation. The phrase ‘settlement reducers’ has been applied to such piles (after Burland et al, 1977) although the description ‘load reducers’ perhaps reflects more the manner in which they are effective,
Current practice in the design of piled foundations tends to concentrate on the overall stability of the foundation. The pile dimensions are chosen to ensure that the full load of the structure may be carried safely by the piles. However, within this design process, there are many questions which may be better answered by considering the settlement characteristics of the piles. For example, the decision to make piles a few metres longer in order to reach a stiffer stratum may seem attractive from a bearing capacity point of view. On the other hand, this decision would seem illogical if the piles were so compressible that the lowest parts of the piles remain effectively unstressed during their working life. Similarly, while the friction capacity of a pile is broadly a function of the surface area of the pile, the settlement is affected to a much greater extent by the relative slenderness. Thus, for a given pile surface area, settlement considerations may indicate the most suitable diameter of pile—though the economics and practicalities of pile installation will be important factors in such a decision.
The design of a pile to carry a given load involves two separate aspects. The first of these is the choice of relevant soil parameters, such as skin friction, shear modulus, etc., from the site investigation data. The second aspect is the choice of the pile type and dimensions.
This paper is concerned with the latter part of the design process.