Jointed concrete
piles are usually of square section, but other forms, such as hexagonal
sections are also produced. Typical capacities range from 700 to 2500
kN with pile cross-sections usually in the range 250 x 250 to 400 x 400
mm. Jointed piles are usually not prestressed, and as each unit is a shorter
length (generally up to 15 m), handling stresses are much lower, and there
is less danger of over-stressing. In suitable ground conditions, piles
have been driven to depths of up to 100 m, although the vast majority
does not exceed 30 m.
Precast concrete
piles can have various types of splices, such as bayonet joints (Hercules
joint) or wedge joints (Stabilator joint). The joint must be as strong
as the pile, and preferably have the same moment of resistance. The quality
of the joints must be high, as otherwise during driving, a significant
amount of energy can be lost. Misalignment in joints should be avoided,
1 to 300 considered a permitted maximum. Angular deviations in joints
can lead to high bending stresses. The quality of joints shall be checked
and joints of poor quality should be rejected.
Hexagonal
section piles "Herkules" type have six main tensile reinforcement bars.
An advantage of a hexagonal pile cross-section is a slightly better distribution
of stress along the pile section during driving, and possibly less susceptibility
to damage from torsional stresses.
Transverse
reinforcement in the form of 5 mm diameter wire "spiral" controls longitudinal
cracking, sometimes with additional bars near the pile ends. The joints
are normally formed from a steel plate with integral starter bars projecting
into the pile.
A fixture
in the casting formwork sets the plate square to the pile, but strict
quality control of the squareness of the plate and concrete strength are
required to produce successful joints. The concrete cover should not be
excessive but provide protection from corrosion, as a thick cover can
lead to spalling. Accurate positioning of an adequate number of spacers
is therefore an important aspect of casting the pile units.
Hard steel
points (fixed or attachable driving shoes) can be used at the toe of jointed
piles for protection when penetrating soils containing boulders, or in
weak rock.
