Pile Design and Construction Practice, Fifth edition

58 Types of pile
Rigs are similar to the high torque, instrumented CFA pile units, but the power required to
install screw piles can be 20% greater than that required for equivalent CFA piles; additional
pull-down is usually necessary. As only a small amount of material is removed as the auger
is initially inserted, the screw pile is particularly useful for foundations in contaminated
ground. The method of concreting is either by injection through the auger tip during rotation
out of the hole, which can improve shaft friction, or by tremie depending on the system.
Design of displacement screw piles should be based on a detailed knowledge of the
ground using pressuremeter tests, CPTs and SPTs, and pile test data in the particular soil.
Care is required in selecting the effective diameter of the helical shaft for determination of
shaft friction and end-bearing capacity. Bustamante and Gianeselli (2.14) have provided a
useful simplified method of predetermining the carrying capacity of helical shaft piles
based on a series of tests and recommend that a design diameter of 0.9 times the outside
diameter of the auger flange should be used for calculating both base and shaft resistance
for ‘thin’ flanges. For thick flanges (say 40 mm deep 75 mm wide), the outside diameter of
the helix is appropriate. Depending on the ground conditions and the size of the helical
flanges formed, savings of 30% in concrete volume compared with the equivalent bored pile
are claimed. Typical pile dimensions are 500 mm outside auger diameter and 350 mm shaft diameter
and lengths of 30 m are possible. The technique is best suited to silty sands and sandy
gravels with SPT N-values between 10 and 30; above N � 50 there is likely to be refusal with
currently available rigs, and unacceptable heave and shearing will occur in most clays.
An enlarged pile base can be formed with straight shafted displacement piles, such as the
Penpile by Pennine Vibropile Ltd, in granular soils and weak chalk to improve end-bearing
capacity.
Guidance on installation of displacement screw piles in BS EN 12699 is limited, but
comprehensive trials of different types of pile at Limelette (2.15) in Belgium during 2000 and
2002 in stiff dense sand, together with earlier trials in stiff clay, have produced significant
data on design, installation and performance of screw piles (including references to EC7
design procedures and CPT testing). Two main conclusions were that the bearing capacity is
of similar magnitude as that for full displacement piles, and the prediction of bearing capacity
was in good agreement with load tests, irrespective of the method used.
Solid steel shaft helical screw piles are used mainly in the USA for foundations in
collapsible and expansive soils as described by Black and Pack (2.16) . The central shaft up to
57 mm 2 has one or more circular steel plates up to 350 mm diameter, shaped into a single
helix, welded to it. As the shaft is rotated into the soil, the leading edge of the helix bites
into the soil, transferring the rotational force into axial thrust; extension shafts with plates
are added as needed. Reduced drag-down and ultimate load-carrying capacities of up to
890 kN in compression or tension are claimed in these soil conditions. In addition to selecting
soil parameters, care is needed during design and installation to consider the effects of
groundwater around the shaft, corrosion and buckling. (See also Section 9.2.2 for use in
underpinning.)
2.4 Replacement piles
2.4.1 General
Replacement piles are installed by first removing the soil by a drilling process and then
constructing the pile by placing concrete or some other structural element in the drilled hole.