Piles - Factor of safety.

BS 8004 recommends a factor of safety of between 2 and 3 for a single pile. The factor of safety is not a fixed constant and depends on the allowable settlement of the pile which is dependent on the pile’s surface and cross-sectional area, the compressibility of the soil, and the reliability of the ground conditions. The factor should be increased when:

(1) The soil is variable, little is known of its behaviour or its resistance is likely to deteriorate with time.
(2) Small amounts of differential settlement are critical.
(3) The piles are installed in groups.

The factor may be decreased when:

(1) As a result of extensive loading tests, the resistance can be confidently predicted.
(2) As a result of extensive local experience, the soil properties are fully known.

A common factor of safety taken in design is 2.5. A properly designed single 500 mm diameter pile driven into noncohesive soil is unlikely to settle more than about 15 mm.

In a load test the settlement is noted for increasing increments of load and a settlement/load graph is plotted. The graph resembles that of the stress/strain graph for many structural materials (see Fig. 14.12). Up to working load there tends to be practically full recovery of settlement on removal of load but beyond that loading there is likely to be a permanent set (as in steel loaded beyond the elastic limit) and at ultimate load there is likely to be no recovery at all.

Fig. 14.12 Load/settlement graph.