settlement that may be expected. If there is no such experience available, then it may be
necessary to undertake loading tests on full-scale piles. This is very costly for large piles and
a more economical procedure is to estimate values from the results of loading tests made on
circular plates at the bottom of the pile boreholes, or in trial shafts.
However, loading tests on piles are more helpful when designing ‘ductile piles’
(Section 5.2.1). Instrumentation can be provided to determine the relative proportions of
load carried in friction on the shaft and transmitted to the base and hence to determine the
degree of settlement needed to mobilize peak friction (e.g. at a pile head settlement of about
10 mm in Figure 4.27), and to determine whether or not the lower ‘long strain’value of shaft
friction is operating when load distribution between piles in a group takes place.
Burland et al.(4.38)plotted the settlement of test plates divided by the plate diameter (i/B)
against the plate bearing pressure divided by the ultimate bearing capacity for the soil
beneath the plate (i.e. q/qf) and obtained a curve of the type shown in Figure 4.28. If the
safety factor on the end-bearing load is greater than 3, the expression for this curve is
i/B (4.36)When plate bearing tests are made to failure, the curve can be plotted and, provided that the
base safety factor is greater than 3, the settlement of the pile base ican be obtained for any
desired value of B.
The procedure used to estimate the settlement of a circular pile is as follows:
(1) Obtain qffrom the failure load given by the plate bearing test
(2) Check qfagainst the value obtained by multiplying the shearing strength by the appro-
priate bearing capacity factor Nc, i.e. qfshould equal Nc cub
(3) Knowing qf, calculate the end-bearing resistance Qbof the pile from Qb Abqf
Kqqf194 Resistance of piles to compressive loads
0.2 1.0q/qf
0.4 0.6 0.800r/iB (%)2 3 4 5 6 7 8Figure 4.28Elastic settlements of bored piles in London clay at Moorfields (after Burland et al.(4.38)).