Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter 9 FOUNDATION ENGINEERING
ultimate load capacity of a pile group
sum of ultimate load capacities of individual piles in the group
(9.29)
A number of empirical formulae have been proposed, generally relating the group efficiency
factor to the number and spacing of piles. However, most of these formulae give no more than
arbitrary factors in an attempt to limit the potential pile group settlement. A comparison of a
range of formulae made by Chellis (1911) shows a considerable variation in the values of η for a
given pile group configuration.
There is a lack of sound theoretical basic on the rationale and field data in support of the
proposed empirical formulae (Fleming & Thorburn 1983). The use of these formulae to calculate
group efficiency factors is therefore not recommended
A more rational approach in assessing pile group capacities is to consider the capacity of both the
individual piles (with allowance for pile-soil-pile interaction effects) and the capacity of the
group as a block or a row and determine which failure mode is more critical. There must be an
adequate factor of safety against the most critical mode of failure.
The degree of pile-soil-pile interaction, which affects pile group capacities, is influenced by the
method of pile installation, mechanism of load transfer and nature of the founding materials. The
group efficiency factor may be assessed on the basis of observations made in instrumented model
and field tests as described below. Generally, group interaction does not need to be considered
where the spacing is in excess of about eight pile diameters (CGS, 1992).
March 2009 9-47