Pile Design and Construction Practice, Fifth edition

Resistance of piles to compressive loads 219
used the depth H over which the upper-bound force is assumed to act is critical. If the depth
is over-estimated application of the action factor A1 of 1.35 in Table 4.20 will further
exaggerate the drag-down force.
Frank et al. (1.5) give a worked example to compare the application of the three design
approaches in EC7 to the case of a single pile carrying a compression load at the pile
head (structural action) and a drag-down force on the shaft (geotechnical action). The
magnitude and depth to the neutral point of the latter action were determined by an
interaction analysis. The partial factors applied to the actions and ground resistance are
shown in Table 4.20.
M2 is applied as an action factor and noted in the table because the drag-down is usually
calculated by effective stress analysis using a constant, for example, � in equation 4.50,
which is not directly related to the angle of shearing resistance of the soil.
The use of Method (a) requires, as a first step, a settlement analysis to determine the
settlement of the fill and underlying compressible soil. Clause 7.3.2.2(5)P requires the
design value of the ground in a settlement analysis to take account of the weight densities
of the material. However, the partial factors for M1 and M2 sets in Table A4 of EC7 Annex A
are unity (see Table 4.2 in Section 4.1.4 of this book).
When calculating drag-down on the shafts of uncased bored and cast-in-place piles, the
possibility of enlargement of the pile cross-section due to overbreak should be considered
as well as ‘waisting’ in the supporting soil layer. Clause 2.3.4.2 of EN 1992–1:2004(EC2)
does not consider the possibility of enlargement, but Table 4.9 in Section 4.1.4 can be used
as a guide to the required tolerance on pile diameter.
EC7 points out that drag-down and transient loading need not usually be considered to act
simultaneously in load combinations.
4.8.3 Minimizing negative skin friction
The effects of drag-down can be minimized by employing slender piles (e.g. H-sections or
precast concrete piles), but more positive measures may be desirable to reduce the magnitude
of the drag-down forces. In the case of bored piles this can be done by placing in-situ
concrete only in the lower part of the pile within the bearing stratum and using a precast
concrete element surrounded by a bentonite slurry within the fill. Negative skin friction
forces on precast concrete or steel tubular piles can be reduced by coating the portion of the
shaft within the fill with soft bitumen.
Table 4.20 Partial factor sets for a pile axially loaded at the head and subjected
to drag-down on the shaft
Design approach Structural Geotechnical action Resistance to
action Shear strength
parameter
Load compression
DA1, combination1 A1 (1.35) M1 (1.0) A1 (1.35) R1 (1.0)
DA1, combination 2 A2 (1.0) M2 (1.25) a A2 (1.0) R4 (1.3)
Note
a Applied as partial action factor.