General
Since their
introduction some fifty years ago in North America, continuous flight
auger (CFA) piles, also known as augercast piles, have become increasingly
popular, as they can be considerably cheaper than alternative pile types.
With proper planning and design, efficient equipment and experienced personnel,
high production rates can be achieved.
Piles are
available in sizes 0.3, 0.45, 0.6, and 0.75 m diameter, and exceptionally
0.9 m diameter. They can be constructed to depths in excess of 30 meters.
CFA Equipment
The equipment
normally comprises of:
- a base
machine with a fixed, hanging or swinging lead
- an auger
drive unit (usually hydraulic) with sufficient power to advance the
continuous flight auger to the required depth with minimum decompression
of the surrounding soil
- continuous
flight augers of the required length and diameter (small or large stem),
equipped with auger cutting heads suitable for the material to be penetrated
Installation
process
CFA piles
are formed by drilling a continuous flight auger into the ground. The
sides of the hole are supported at all times by the soil-filled auger,
eliminating the need for temporary casing or bentonite slurry. Upon reaching
the required depth, sand-cement grout or concrete is pumped down the hollow
stem as the auger is steadily withdrawn. Reinforcement is placed immediately
after withdrawal of the auger.
The bearing
capacity and settlement behavior of CFA piles is to a large extent influenced
by the equipment used and the experience of the operator. The significance
of these two aspects is often underestimate or overlooked at the design
stage, but plays an important role for the performance of CFA piles.
Casting
of pile base and shaft
When the
auger has reached the required depth, the temporary plug (usually of wood
or cork), which prevents soil from entering the hollow stem, has to be
ejected. This is accomplished by slightly lifting the auger and injection
of the concrete. During this phase soil decompression is almost unavoidable
and it is common practice to reinsert the rotating auger while concrete
is injected. This method minimizes to some degree decompression at the
bottom of the pile but the quantitative effect is difficult to assess.
When the
grout head is established, extraction is commenced at a rate, consistent
with grout supply. Positive rotation of the auger is necessary to retain
the drilling spoil and to ensure that the grout fills the entire pile
cross section. The grout pressure must be sufficiently high and the auger
must not be extracted too fast. Otherwise, drilling spoil can enter into
the freshly placed grout, resulting in a soil-contaminated pile shaft.
Placing
of reinforcement
In the case
of a small stem diameter, which is most commonly used, the reinforcement
must be installed after the auger has been withdrawn and while the grout
or concrete is still fluid. The placement of a long reinforcement cage
into an uncased hole can pose considerable practical problems and contaminate
the concrete shaft with eroded soil. Therefore, it is important that the
cage is stiff and properly welded In order to overcome the problem of
proper placement of the reinforcement, augers with a larger stem are used
and the reinforcement is installed inside before extraction of the auger.
Advantages
The CFA pile
is essentially a non-displacement type pile. Hence, there is limited risk
of damage to adjacent foundations or underground utilities from ground
displacement or densification of loose sands, as can occur with displacement
piles. Another advantage is that CFA piles can be installed with little
vibrations or noise. Should problems occur during pile construction, it
is relatively simple to re--drill and install the pile at the same location,
thereby eliminating the need to redesign the pile group or the pile caps.
An important
feature of present day CFA piles is the use of a reliable flow meter to
monitor and record the whole process of construction of this type of pile.
The items recorded are usually penetration / uplift per revolution, auger
depth, concrete supply per increment of auger uplift during placing, and
injection pressure at the auger head.
Limitations
The most
severe disadvantage is poor and/or inconsistent quality and load carrying
capacity. The advantages of the CFA pile are many times outweighed by
the limitations, which are inherent to this pile type. Initially, the
method of CFA pile construction was crude and did not always ensure a
pile of high structural and geotechnical quality. Major advances have
been made with respect to the design and execution of CFA piles. However,
the most severe limitation of the CFA technique is still its sensitivity
to operator performance, which can lead to a pile of poor or inconsistent
quality and reduced load carrying capacity. Thus, it is vitally important
that experienced personnel install the piles. Great attention must be
given to every phase of the field installation procedure, including the
drilling of the hole, the casting of the shaft, the extraction of the
auger and the placement of the reinforcement.
In contrast
to driven piles, the drilling resistance usually does not provide a direct
indication of the soil stiffness and the strength of the bearing stratum,
except when a very dense soil layer or rock is encountered. In spite of
its apparent simplicity, the installation process of a conventional auger
pile involves the use of several pieces of equipment, which need to be
operated in a specialized manner.
Soil decompression
during augering
The installation
procedure of CFA piles influences the stress conditions in the ground.
The magnitude of the pile friction is determined by the lateral earth
pressure after completion of the pile shaft. Decompression of the surrounding
soil can adversely affect the capacity of the pile, and is likely to vary
with ground conditions, equipment and operator.
The term
soil "decompression" refers to the reduction in soil stiffness and strength
brought about by the pumping action of the continuous flight auger. Increasing
the diameter of the hollow stem can to some extent reduce the risk of
decompression. However the larger the hollow stem, the greater the power
needed to advance the auger. Decompression results in an increased grout
volume, which can be between 15 and 60 % in excess of the theoretical
pile volume. The cost of additional, unexpected concrete consumption can
have severe economical consequences for a piling project.
