The concept of the separate evaluation of shaft friction and base resistance forms the basis
of all ‘static’calculations of pile bearing capacity. The basic equation is
Qp QbQsWp (4.1)where Qpis the ultimate resistance of the pile, Qbis the ultimate resistance of the base, Qsis
the ultimate resistance of the shaft, and Wpis the weight of the pile. The components Qsand
Qbof the failure load Qpare shown at the final loading stage in Figure 4.2. Usually Wpis
small in relation to Qpand this term is generally ignored. However, it is necessary to provide
for Wpin such situations as piles in marine structures in deep water where a considerable
length of shaft extends above sea bed.
Permissible stress methods are adopted in BS8004. The actual dead load of a structure and
the most unfavourable combination of imposed loads are assumed to be applied to the
ground. The foundation is assumed to be safe if the permissible stress on the soil or rock is
not exceeded, taking into account the likely variable strength or stiffness properties of the
ground and the effect of a varying groundwater level. In the case of piled foundations,
uncertainty in the reliability of the calculation method is also taken into account. It is
generally accepted that current methods cannot predict failure loads to a greater accuracy
than plus or minus 60% of the value determined from a full-scale loading test taken to
failure. Hence, the safety factors used to obtain the allowable load on a single pile from the
calculated ultimate load are correspondingly high.
In BS8004 a safety factor between 2 and 3 is generally adopted. Experience of a large
number of loading tests on piles of diameter up to 600 mm taken to failure, both in sands
142 Resistance of piles to compressive loads
Load on pile headLoads carried in
end bearingPile toe levelPoint DPoint BPoint ADepth below pile headFigure 4.2Load transfer from head of pile to shaft at points A, B and D on load/settlement curve
in Figure 4.1.