Info

SLS check Yb

^ piles to 1.5x U characteristic load

Preliminary 1.2 load tests

*MF = model factor; ? = value not given in EN 1997-1

YRd = model, Yb = base, ys = shaft, Yt = total, Yst = shaft tension factors tReplacement piles, ^displacement piles

|Partial factors for Set R1 and for accidental design situations are 1.0

Finally, with tests on preliminary piles, verification of both ultimate and serviceability limit states will be corroborated by static load tests - and, again, if found wanting, would lead to re-design of the piles. However, if the results of the static load tests confirm the assumptions made in the calculations, then even lower partial factors can be tolerated than when just working tests are performed.

A common practice in current pile design is ensure the working load is less than the shaft resistance (with Fs = 1.2-1.5) to control settlements where the base performance is uncertain, e.g. in sands. Although the UK NA factors may appear to deal with serviceability implicitly, there will be cases where settlement at characteristic load could still be excessive. Although this can be checked by testing, there will be cases where no preliminary testing is specified. In these cases an assessment of the likely settlement should be carried out.

Figure 13.16 illustrates these ideas by plotting the equivalent global factor of safety F* (defined in Section 13.10) for the three levels of testing discussed above. The diagram also compares the values of F* with those obtained using traditional safety factors for bored piles (taken from Figure 13.15). As can be seen, the National Annex ensures roughly equivalent levels of reliability to traditional practice. Note that, because displacement piles increase the stresses in the ground during installation — and are, in effect, tested during installation — they are inherently more reliable than replacement piles and hence require a lower equivalent factor of safety.

Figure 13.16 illustrates these ideas by plotting the equivalent global factor of safety F* (defined in Section 13.10) for the three levels of testing discussed above. The diagram also compares the values of F* with those obtained using traditional safety factors for bored piles (taken from Figure 13.15). As can be seen, the National Annex ensures roughly equivalent levels of reliability to traditional practice. Note that, because displacement piles increase the stresses in the ground during installation — and are, in effect, tested during installation — they are inherently more reliable than replacement piles and hence require a lower equivalent factor of safety.

Figure 13.16. Comparison between pile design according to UK National Annex to EN 1997-1 and traditional factors of safety for bored piles

13.11.2 Changes to correlation factors

The table below shows the values of the correlation factors recommended in the UK National Annex to BS EN 1997-1. The values have been changed from those given in EN 1997-1 to ensure compatibility with the alterations made to resistance factors, discussed in Section 13.11.1.

No of tests

Static load tests

Ground tests

No of tests

Dynamic impact tests

0 0

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