Figure 13.3. Single pile subject to vertical compression

The total characteristic compressive resistance Rck is given by:

Rck = Rsk + Rbk where Rsk is the characteristic shaft resistance and Rbk the characteristic base resistance, which should include an allowance for the total overburden pressure at the base of the pile, ovb. [en 1997-1 §]

The design resistance Rcd is given by:

Rsk R

Ys Yb Yt Yt where ys and Yb are partial factors on the shaft and base resistances, respectively, and Yt is a partial factor on the total characteristic resistance Rtk. The first equation is normally used when designing piles by calculation (i.e. on the basis of ground test results) and the second when the shaft and base components cannot be determined separately (for example, when designing piles using static load or dynamic impact tests). [en 1997-1 §]

The pile's self-weight W is often omitted from traditional pile capacity calculations and the beneficial effect of the overburden pressure at the pile base ovb is ignored, since they are numerically similar in value:

where Ab is the area of the pile base. Eurocode 7 allows this simplification to be used if the terms 'cancel approximately'. [en 1997-1 §]

13.4.1 Downdrag

As well as carrying loads from the structure, piles can be subject to actions that arise from movement of the ground in which they are installed. This phenomenon is known as 'downdrag' when it involves the ground consolidating, thus bringing additional downwards force onto the pile shaft. Ground movements in other directions (e.g. upwards or horizontal) can induce heaving, stretching, or other displacement of the pile.

Eurocode 7 Part 1 requires such displacements to be treated in one of two ways: either as an indirect action in a soil-structure interaction analysis; or as an equivalent direct action, calculated separately as an upper bound value.

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