Pile Design and Construction Practice, Fifth edition

62 Types of pile

are continually supported by the spiral flights and the soil within them, and by the concrete.
Reinforcing steel can be pushed into the fluid concrete to a depth of about 12 m.
Exceptionally, reinforcing cages up to 17 m long were pushed down into the 30 m long piles
for the foundations of the approach viaducts to the Dartford Bridge. Vibrators may be used
to assist penetration. The shaft diameters range from the minipile sections (about 100 mm
in which sand–cement grout may be injected in place of concrete) up to 1.5 m exceptionally.
Concrete is usually mixed with a plasticizer to improve its ‘pumpability’, and an expanding
agent is used in grout to counter the shrinkage while it is setting and hardening. Pile capacities
up to 7 500 kN are possible depending on ground conditions and pile dimensions.
In granular soils a hollow-stem auger can be used in conjunction with wing drill bits to
mix the soil in place with a cement grout pumped down the stem. This process is well developed
for encapsulating contaminants in landfill.
The CFApile has considerable advantages over the conventional bored pile in water-bearing
and unstable soils. Temporary casing is not needed, and the problems of concreting under-
water are avoided. The drilling operations are quiet and vibrations are very low making the
method suitable for urban locations. However, in spite of these considerable advantages the
CFApile depends for its integrity and load-bearing capacity, as much as any other in-situ type
of pile, on strict control of workmanship. This is particularly required where a high proportion
of the load is carried in end-bearing. Because it is not possible to check the stratification and
quality of the soil during installation as with conventional bored piles, considerable research
and development has been undertaken by piling companies into the use of computerized
instrumentation to monitor the process and ensure the quality and integrity of CFApiles. For
example, a computer screen is positioned in the drilling rig cab in front of the operator which
continuously displays the boring and concreting parameters. During the boring operation the
depth of auger, torque applied, speed of rotation, and penetration rate are displayed. During
concreting a continuous record of concrete pumping pressure and flow rate is shown, and on

Hollow stem of

continuous flight

auger

Soil debris

Sand–cement grout

pumped down

hollow stem

Figure 2.31Pumping grout to form an auger-injected pile.