where T is the measured torque and K is a constant that depends on the vane's dimensions and shape (D and H being the diameter , i.e. the width of two blades, and height). The equation given above for K assumes a uniform distribution of shear strength across the ends and perimeter of the cylinder. For many vanes, H/D = 2 and so K = 3.66D3. The theory on which this equation is based clearly does not depend on any factor of safety and hence it can be used equally well within the framework of Eurocode 7 as it was in traditional practice.

Theories may have universal application but are often limited to particular situations and their users should understand and work within those limitations. Normally, theories have wider application than empirically-derived formulae, which only apply to the data sets on which they are based.

There are reasonably robust theories to explain observations made under laboratory conditions, but theories become less reliable as the number of unknowns increases.

5.2.4 Empiricism

Following the introduction of Eurocode 7, existing empirical rules appear to have one of three fates: some may continue to be used 'as-is', some may need to be re-cast, and others will fall into disuse.

Many existing empirical rules were developed before the widespread use of partial factors in geotechnical design. Some of these rules implicitly include a factor-of-safety, usually to avoid excessive movement of the structure or to prevent the mobilization of high soil stresses. An example of this type of empirically derived correlation is the relationship provided by Terzaghi and Peck5 for allowable bearing pressure and SPT blow count in sands, shown in Figure 5.5. Because the Structural Eurocodes ensure reliability through the application of partial factors, rules such as this cannot be used within the framework of Eurocode 7, unless they are suitably modified.

Water table below depth 2B

Water table below depth 2B

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