Design by calculation
Design by calculation involves the use of equations that relate soil and rock parameters to shaft friction and end bearing. Unfortunately, available calculation models do not reliably predict the ultimate capacity of piles, owing to the complex interaction between pile type, construction processes, workmanship, and group effects. Consequently, relatively large factors of safety (in the range 2.03.0) are applied in such calculations.
The partial factors given in Eurocode 7 for pile resistance are numerically much lower than have traditionally been used in the design of pile foundations, i.e. 1.01.6 on base resistance and 1.01.3 on shaft (see Section 13.7.1). These factors were chosen for the purpose of design by testing, in which additional 'correlation factors'  numerically between 1.0 and 1.6  are used to ensure the reliability of the foundation (see Section 13.9). For this reason, Eurocode 7 Part 1 allows a 'model factor' YRd > 1 to be applied in design by calculation to 'correct' the resistance factors:
the values of the partial factors yh and ys recommended in Annex A may need to he corrected hy a model factor larger than [1.0]. The value of the model factor may he set hy the National annex. [en 19971 ยง7.6.2.3(8) note]
Some guides9 on Eurocode 7 suggest combining YRd with Yb and ys to produce a single set of 'enhanced' resistance factors, but we do not recommend this. Instead, we prefer to include the model factor in the formulation of the pile's characteristic base and shaft resistances (Rbk and Rsk), as follows:
YRd YRd where the symbols are as defined in Section 13.3.2. The design resistance in compression is then given by:
Ys Yb YRd xYs YRdx Yb
Note that Figure 13.9 to Figure 13.11 omit the box for 'Design material properties Xd' that appears in the flowcharts in Chapter 6. The reason for this is to maintain a distinction between characteristic values, which have not had partial factors applied to them, and design values, which have. Since all the factors in Set M1 are unity, we consider the material strengths that go into the calculation model to be characteristic rather than design values.
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