Pile Foundations - Elements of Soil Mechanics - page 303

a significant downward movement of the pile is required which cannot occur in this example because of
the end resistance of the gravel. As a result of this phenomenon, it is common practice to apply a differ-
ent factor of safety to the shaft resistance than that applied to the end bearing resistance. Typically a
factor of safety of around 1.5 is applied to shaft resistance, and a factor of safety between 2.5 and 3.0 is
applied to the end bearing resistance.

Returning to Example 10.3, and adopting F b = 3, F s = 1.5, the allowable load now
becomes:

1000
3

210
1 5

+ = 473 kN

.

10.5.4 Negative skin friction, or downdrag

If a soil settles or consolidates around a pile then the pile will tend to support the soil and there can be
a considerable increase in the load on the pile. This effect is known as downdrag and is quantified as the
additional shear stress applied to the surface of a pile by the soil as it settles.
The main causes for downdrag are:

(i) bearing piles driven into recently placed fill, which then begins to settle;
(ii) fill placed around the piles after driving which causes consolidation in the soil below;
(iii) consolidation due to a reduction in the pore water pressure in the soil;
(iv) consolidation due to pile installation (of particular concern in sensitive and normally consolidated
clays).

If negative friction effects are likely to occur then the piles must be designed to carry the additional
load. In extreme cases the value of negative skin friction can equal the positive skin friction. However, the
maximum value of negative skin friction cannot act over the entire embedded length of the pile, and it
is found to be virtually zero at the top of the pile and reaches the maximum value at its base.

10.6 Designing pile foundations to Eurocode 7

The principles of Eurocode 7, as described in Chapter 5, apply to the design of pile foundations and the
design of pile foundations is covered in Section 7 of Eurocode 7 Part 1.
There are 11 limit states listed that should be considered, though only those limit states most relevant
to the particular situation would normally be considered in the design. These include the loss of overall
stability, bearing resistance failure of the pile, uplift of the pile and structural failure of the pile. In this
chapter we will look only at checking against ground resistance failure through the compressive loading
of the pile.
Pile design methods acceptable to Eurocode 7 are in the main based on the results of static pile load
tests, and the design calculations should be validated against the test results. When considering the
compressive ground resistance limit state the task is to demonstrate that the design axial compression
load on a pile or pile group, F c;d , is less than or equal to the design compressive ground resistance, R c;d ,
against the pile or pile group. In the case of pile groups, R c;d is taken as the lesser value of the design
ground resistance of an individual pile and that of the whole group.
In keeping with the rules of Eurocode 7, the design value of the compressive resistance of the ground
is obtained by dividing the characteristic value by a partial factor of safety. The characteristic value is