^'depending on designer uncertainties' ts = pile head settlement

^commonly used when base capacity is uncertain16

Figure 13.15 compares the equivalent global factor of safety F* obtained from the various recommendations given above, for different amounts of shaft capacity (Qs) expressed as a percentage of the total ultimate capacity (Qult). The left-hand end of the horizontal scale represents mainly end-bearing piles and the right-hand end mainly friction piles.

For each curve, the allowable capacity Qa is calculated as the lower of the values obtained from the application of a single factor F and the application of twin factors Fs and Fb described above. The equivalent global factor is then the ratio of the ultimate capacity to the allowable (i.e. F* = Qult/Qa).


Figure 13.15. Equivalent global factors of safety recommended in the literature

The following table summarizes guidance issued by the London District Surveyors' Association (LDSA)17 for bored piles in London Clay. The LDSA's advice is to decrease the factor of safety F as more testing is undertaken. This guidance is currently being revised to bring it into line with Eurocode 7.

Adequate Pile load tests LDSA recommended F for a

Preliminary Working F a

CRPt 2.25 2.25

tConstant rate of penetration test, ^Maintained load test

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