7.3 Designing piles to resist driving stresses
It is necessary to check the adequacy of the designed strength of a pile to resist the stresses
caused by the impact of the piling hammer. Much useful data to aid the estimation of driv-
ing stresses came from the research of Glanville et al.(7.1)They embedded stress recorders
in piles to measure the magnitude and velocity of the stress wave induced in the pile by
blows from the hammer. The tests showed that the stress wave travels from the head to the
toe of the pile and is partly reflected from there to return to the head. If the pile is driven
onto a hard rock, the sharp reflection of the wave at the toe can cause a compressive stress
at the point which is twice that at the head, but when long piles are driven into soil of low
resistance, the tensile stress wave is reflected, causing tension to develop in the pile. It can
be shown from simple impact theory that the magnitudeof the stress wave depends mainly
on the height of the drop. This is true for a perfectly elastic pile rebounding from an elastic
material at the toe. In practice there is plastic yielding of the soil beneath the toe, and the
pile penetrates the soil by the amount described as the ‘permanent set’. The weight of the
hammer is then important in governing the length of the stress wave and hence the efficiency
of the blow in maintaining the downward movement of the pile.
The simplest approach to ensuring that driving stresses are within safe limits is to adopt
working stresses under static loading such that heavy driving is not required to achieve the
depth of penetration required for the calculated ultimate bearing capacity. The usual practice
is to assume that the dynamic resistance of a pile to its penetration into the soil is equal to its
ultimate static load-bearing capacity, and then to calculate the ‘permanent set’in terms of
Structural design of piles and pile groups 379Table 7.2Maximum lengths of square section precast concrete piles for given reinforcement
Pile Main Maximum length in metres for pick up at Transverse reinforcement
size (mm) reinforcement
(mm) Head 0.33length 0.2length from Head Body
and toe from head head and toe and toe of pile
300 300 4 20 9.0 13.5 20.5 6 mm at 6 mm at
4 25 11.0 16.5 25.0 40 mm crs 130 mm crs
350 350 4 20 8.5 13.0 19.5 8 mm at 8 mm at
4 25 10.5 16.0 24.0 70 mm crs 175 mm crs
4 32 13.0 20.0 30.0
400 400 4 25 10.0 15.0 22.5 8 mm at 8 mm at
4 32 12.5 19.0 28.0 60 mm crs 200 mm crs
4 40 15.5 23.0 34.5 or
10 mm at
100 mm crs
450 450 4 25 a 9.5 14.5 22.0 8 mm at 8 mm at
4 32 a 12.0 18.0 27.0 60 mm crs 180 mm crs
4 40 a 15.0 22.5 33.5 or or
10 mm at 10 mm at
90 mm crs 225 mm crs
Notes
Piles designed in accordance with BS8110 and BS8004.
Characteristic strength of reinforcement limited to 250 N/mm^2.
Cover to link steel 40 mm.
Characteristic strength of concrete 40 N/mm^2.
a Alternatively, use a larger number of smaller diameter bars.