Pile Design and Construction Practice, Fifth edition

Types of pile 61
Transverse reinforcement is also specified in BS EN 1536. Un-reinforced bored piles can
be considered as Clause 12 of EC2-1-1, subject to serviceability and durability requirements,
but BS EN 1536 requires minimum longitudinal reinforcement of four 12 mm diameter bars,
unless the design demonstrates otherwise.
Over 1100 large diameter bored piles were installed at Canary Wharf by Bachy
Soletanche in London Docklands ranging from 900 to 1500 mm and to depths of 30 m
through terrace gravels, Lambeth clays, sands and gravels, and Thanet sands. It was possible
to bore the piles without the aid of drilling fluids due to the low water table in the Thanet
beds. Once the piles had reached the required depth using temporary casing, the shaft was
filled with bentonite slurry to minimize the risk of pile collapse during concreting operations.
The reinforcement cage was inserted to which were attached tubes-à-manchette for pile base
grouting two days after concreting.
When using bentonite or other drilling fluids to support the sides of boreholes or
diaphragm walls, the bond of the reinforcement to the concrete may be affected. Research
by Jones and Holt (2.17) comparing the bond stresses in reinforcement placed under bentonite
and polymer fluids indicated that it is acceptable to use the BS 8110 values of ultimate bond
stress provided that the cover to the bar is at least twice its diameter when using deformed
bars under bentonite. The results for the polymers investigated showed that the code bond
stresses could be reduced by a divisor of 1.4. EC2-1-1 Clause 4 includes for a minimum
cover factor dependent on bond requirements and Section 8 gives a reduction factor of 0.7
to apply to the ultimate bond stress where ‘good’ bond conditions do not exist – compatible
with the Jones and Holt data for polymers. It also covers laps between bars using the reduced
bond stress as appropriate. BS EN 1536 states that only ribbed bars shall be used for main
reinforcement where a stabilizing fluid, bentonite or polymer, is used.
It is easier to remove drill cuttings from polymer stabilizing fluids for reuse compared
with bentonite slurries. They are also better suited for drilling large diameter piles and shafts
where the hole has to be stabilized for up to 36 hours of drilling time. The filter cake
formation on the sides of the hole is limited and the sides do not soften to the same extent
as with bentonite slurry support.
Barrettes can be an alternative to large diameter bored and cast-in-place piles where in
addition to vertical loads, high lateral loads, or bending moments have to be resisted. They
are constructed using diaphragm wall techniques to form short discrete lengths of rectangular
wall, and interconnected Ell-, Tee-shapes and cruciforms to suit the loading conditions in a
wide variety of soils and rock to considerable depths. The ‘Hydrofraise’ reverse circulation
rig (see Section 3.3.6) is particularly well adapted to form barrettes, as verticality is accurately
controlled and the time for construction is reduced compared with grab rigs thereby avoiding
the potential for the excavation to collapse. Barrettes are usually only economical when the
rig is mobilized for the construction of other basement walls.
Continuous flight auger or auger-injected piles, generally known as CFA piles, are
installed by drilling with a rotary continuous-flight auger to the required depth. In stable
ground above the water table the auger can be removed and a high slump concrete pumped
through a flexible pressure hose that has been fed down to the bottom of the unlined hole.
This type of pile is referred to as cast-in-place. In unstable or water-bearing soils a flight
auger is used with a hollow stem temporarily closed at the bottom by a plug. After reaching
the final level a high slump concrete is pumped down the hollow stem and, once sufficient
pressure has built up, the auger is withdrawn at a controlled rate, removing the soil and forming
a shaft of fluid concrete extending to ground level (Figure 2.31). Thus the walls of the borehole