Pile Design and Construction Practice, Fifth edition

56 Types of pile
improved axial uplift resistance. After top driving is completed the tapered shell pile is filled
with concrete. Ultimate bearing capacities up to 4000 kN and lateral resistance to 200 kN
have been determined in pile tests.
The West shell pile is no longer available in the UK, and this type of short precast
cylindrical concrete shell has generally gone out of favour with the development of
improved precast pile joints and CFA piling techniques.
2.3.4 Working stresses on driven and cast-in-place piles
It can be seen from the above brief descriptions that driven and cast-in-place piles encompass
a wide variety of shapes, combinations of materials and installation methods. A common
feature of nearly all types is an interior filling of concrete placed in-situ, which forms the
main load-carrying component of the pile. Whether or not any load is allowed to be carried
by the steel shell depends on its thickness and on the possibilities of corrosion or tearing of
the shell. As noted in Section 2.3.2, BS 8004 limits the working stress in the concrete to 25%
of the characteristic cube strength at 28 days with a minimum cement content of 300 kg/m3 .
While the required strength classes in BS EN 12699 are apparently higher than BS 8004, the
working stresses are lower than for precast concrete piles to take account of possible
deficiencies in workmanship during placing the concrete, or reductions in section of the pile
shaft due to ‘waisting’ or buckling of the shells. When semi-dry concrete is tamped during
installation the concrete class should be at least C25/30 with a minimum cement content of
350 kg/m3 .
Where steel tubes or sections are used as part of the load carrying capability or reinforcement
of the pile, BS EN 12699 requires Eurocode 4 (EC4) BS EN 1994: 2004 Design
of Composite Steel and Concrete Structures Part 1-1 General rules to be applied.
Steel shell piles (‘pipe piles’) are more widely used in the USA than elsewhere and most
of the American codes require the shells to be at least 2.5 mm thick before they can be
permitted to carry a proportion of the load. Frequently a wall thickness of 3 mm is required.
2.3.5 Rotary displacement auger piles
Displacement auger piles and screw piles are drilled piles, but the soil is displaced and
compacted as the auger head is rotated into the ground to form the stable pile shaft, with
little soil being removed from the hole. The methods were mainly developed in the 1960s in
Belgium from continuous flight auger (CFA) techniques (see Section 2.4.2) and are now
widely available. The original system is the cast-in-place ‘Atlas’ pile in which the special
single flight auger head is screwed and jacked into the ground on a thick-walled steel tube
using a specially designed rotary high torque rig. The helical shape of the pile shaft
produced by screwing in the auger flange is maintained as the auger is back-screwed to form
a stable hole into which the reinforcement cage is placed prior to concreting. Other proprietary
displacement piles such as the ‘ScrewSol’ pile by Bachy Soletanche (Figure 2.29a and b)
which produces a helical flanged pile shaft in weak soils and the ‘Spire’ system by SEFI in
France for a straight shaft also use specially shaped augers on the end of the drill tube to
compact the soil and inject concrete. A helical ‘threaded’ shaft is also produced by Bachy
Soletanche with a reduced pitch and shorter flange for use in London Clay. The amount of
reinforcement which can be inserted is limited. The use of the thin-flanged hollow stem CFA
augers in short lengths to form the shaft helix has not been successful.