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The UK National Annex to BS EN 1997-12 attempts to deal with this anomaly by noting:

The value of the partial factor for soil parameters should be taken as the reciprocal of the specified value if such a reciprocal value produces a more onerous effect ...

The curve labelled 'characteristic/0.8' on Figure 7.7 shows the effect of entering y9 = 0.8 (= 1/1.25) in the previous equation for 9d,inf. However, there is a serious problem with this approach, as illustrated in Figure 7.8 and explained below.

Figure 7.8 shows a hypothetical normal (aka Gaussian) probability density function for the soil's angle of shearing resistance 9, assuming a mean value = 30° and standard deviation o9 = 3° (giving a coefficient of variation ô9 = o/^ = 0.1).

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Figure 7.8 shows a hypothetical normal (aka Gaussian) probability density function for the soil's angle of shearing resistance 9, assuming a mean value = 30° and standard deviation o9 = 3° (giving a coefficient of variation ô9 = o/^ = 0.1).

Angle of shearing resistance (deg)

Angle of shearing resistance (deg)

Figure 7.8. Difference between 'inferior' and 'superior' design values of strength

The inferior characteristic angle of shearing resistance 9kinf may be estimated from:

where k is a statistical coefficient assumed here to be 1.645. (See Chapter 5 for a full discussion of this equation and the choice of a suitable value for k.)

The inferior design angle of shearing resistance 9d/inf is then given by:

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