Traditional design

Traditional design based on geotechnical calculations is often verified by static pile load tests. Wherever possible preliminary pile load tests are carried out to failure, thereby establishing with some certainty the ultimate capacity of the piles in the particular ground conditions at the site.

'A factor of safety of 2.0 is often deemed sufficient when test piles have been loaded to failure. However ... 2.5 is recommended where only proof loads are applied to working piles.'10

When no pile tests are carried out, it is usual to apply a larger factor of safety normally no greater than 3.O.11

There are two common methods of introducing safety factors into calculations of the 'allowable' (or 'safe working') load Qa. Either a single factor F is applied to the pile's ultimate capacity Qult; or two factors Fs and Fb are applied separately to the shaft and base capacities Qs,ult and Qb,ult, i.e.:

Qa is often taken as the lower of the values calculated by the two methods.

Various researchers and local and national authorities have recommended values for these factors of safety - for bored and continuous flight auger (CFA) piles - and some of these recommendations are summarized below.

Reference Factor of safety for bored/CFA piles
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