Pile Design and Construction Practice, Fifth edition

Geometrical dataare concerned with the cross-sectional dimensions of piles. In the case
of precast concrete and manufactured steel sections, the dimensions are required to conform
to manufacturing tolerances as set out in BSEN 1990. These tolerances are insignificant in
relation to the uncertainties involved with soil properties and design methods and can be
ignored. Bored piles in which the concrete is placed in unlined boreholes or driven and cast-
in-place piles where the drive tube is extracted during or after placing the concrete may
undergo reductions in shaft diameter caused by ‘waisting’or ‘necking’as described in
Section 2.4.2.
BSEN 1992-1-1 specifies that the diameters to be used in design calculations should be
in accordance with the tolerances shown in Table 4.9.
Partial factors of unity are used when checking a foundation design for compliance with
serviceability limit-state criteria. The procedure for pile groups is discussed in Chapter 5.
The following sections of Chapter 4 will describe the use of partial factors in obtaining
values for the separate components of base and shaft resistance of driven and bored piles in
clays, sands, and rocks.

4.2 Calculations for piles in fine-grained soils

4.2.1 Driven displacement piles

When a pile is driven into a fine-grained soil (e.g. clays and clayey silts) the soil is displaced
laterally and in an upward direction, initially to an extent equal to the volume of the pile
entering the soil. The clay close to the pile surface is extensively remoulded and high pore-
water pressures are developed. In a soft clay the high pore pressures may take weeks or
months to dissipate. During this time the shaft friction and end-bearing resistance, in so far

Resistance of piles to compressive loads 151

Table 4.8Correlation factors ( ) to derive characteristic

values from dynamic impact tests

n

 2  5  10  15  20

1.60 1.50 1.45 1.42 1.40

1.50 1.35 1.30 1.25 1.25

Notes

n denotes the number of tested piles.

denotes on the mean values of the measured resistances from

dynamic tests.

denotes on the minimum values of the measured resistances from

dynamic tests.

See Table A.11 of EN1997-1: 2004 for qualifications to validity of

above correlation factors.

 6

 5

 6

 5





Table 4.9Design tolerances for diameters of uncased bored

piles

Nominal diameter (dnom) Design diameter (d)

400 mm d dnom20 mm

400 dnom 1000 m d 0.95dnom

dnom1000 mm d dnom50 mm