| Westpile Shell Pile
The West
Shell piling system uses short cylindrical concrete shells, which are
combined to form a continuous concrete shaft. After installation of the
concrete shells, the concrete pile is provided with a cast-in-situ concrete
core.
Pile sections
are driven, using a crawler mounted pile driving rig and can be installed
at a rake of up to 1:3 both towards and away from the machine. Specially
constructed rigs allow Shell Piles to be driven in headroom conditions
as low as 6 m.
Cylindrical
precast shells are threaded onto a central steel mandrel and rest on a
precast concrete driving shoe. The complete assembly is driven to a depth
corresponding to the length of the mandrel, using a purpose-designed driving
head and drop hammer. The driving head allows the full weight of the hammer
to strike the mandrel and the pile shoe, while a cushioned blow is transmitted
to the shells. The intensity of blows, delivered to the shells, can be
adapted to variations in driving resistance by adjusting the drive head.
When the
first mandrel section has been driven into the ground, additional concrete
shells are threaded onto a second extension mandrel, which is attached
to the top of the bottom mandrel. Then, the driving operation is repeated.
Additional mandrels and shells can be added until the required depth is
reached.
Once driven
to the required depth, the steel mandrels are withdrawn, leaving a fully
cased hollow concrete pile shaft. A suitable reinforcement cage is installed
at the center of the pile. Thereafter, the hollow core is filled with
concrete to form the completed concrete Shell Pile.
Care must
be taken when driving this type of pile through ground, containing large
boulders or onto steeply sloping bedrock. In such conditions the pile
shaft may be deflected, which causes bending of the shaft. Then, it may
be difficulty to withdraw the mandrel and to place additional shells.
If the shells are not properly jointed, a too large part of the pile load
is transmitted to the in-situ concrete core, which has a comparatively
small diameter and limited load capacity. Also, if the piles are driven
in groups, ground heave may cause separation of the shells, unless precaution
is taken, such as installing the piles in a predetermined order, or by
using pre-boring.
The use of
short pile shells has the advantage that the excess length of a completed
pile is never more than one shell length. It also enables the core to
be concreted to an accurate level - usually a maximum of 0,3 m above cut-off
level. As a result, trimming a Shell Pile to cut off level is a rapid,
low-cost process, compared with the often hidden costs associated with
trimming conventional precast piles. In general, the depth of core concrete
to be removed, averages 0,15 m. Exposed reinforcement bars are subsequently
connected to the pile cap or ground beam reinforcement.
Execution
sequence of a Westpile Shell Pile
- The pile
shells are threaded on a steel mandrel, the diameter of which is slightly
smaller than the internal diameter of the shell elements. A steel band,
acting as a sleeve, is installed at each shell joint. The internal face
of each joint band is coated with mastic to prevent water penetration.
Finally, a pile shoe is fitted to the lower end of the bottom shell.
- The whole
assembly is then driven into the ground, extending the pile length by
addition of steel mandrel sections and concrete shells, until the required
depth is reached.
- Withdrawal
of the mandrel leaves a stable, water tight, hollow concrete column,
at the base of which is an integral pile shoe
- Steel
reinforcement in the form of a prefabricated cage, is placed in the
hollow core, which is then filled with concrete and compacted. This
concrete core, together with the precast shells, forms the final solid
load bearing pile. As the core is cast-in-situ, there is no risk of
damage to the concrete strength, which in the case of ordinary driven
piles may be affected by high driving stresses.
- Completed
pile, trimmed to cut-off level
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