Jointed piles
are not pre-stressed, but as each unit is a shorter length, generally
1 to 13 m, although even shorter sections are available from some manufacturers,
handling stresses are much lower, and there is no danger of overstressing
from this cause. The piles are joined by various means: bayonet fittings
or wedges being typical. The joint should be as strong as the pile, and
preferably of the same moment of resistance. The fit of the joints should
be good, as energy is lost through them, and increased hammering is then
required to drive the pile. Angularity produced by the joints can lead
to high bending stresses, and any joints which produce this effect should
be rejected, with a misalignment of 1 to 300 considered a permitted maximum.
The 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. Sizes above
and below this range are however produced. In suitable ground conditions,
piles have been driven to depths of up to 100 m, although the vast majority
of typical applications do not exceed 30 meters.
After driving
the pile to the desired level, it is usual to strip the pile head to expose
the reinforcement, which is then bonded into the pile cap. Pneumatic jackhammers
are the most practical tools for this job. No method should be used which
results in cracking or shattering of the concrete below the level of the
underside of the pile cap.
If the pile
has refused further driving at too high a level it will be necessary to
cut off the surplus length. This can be done by breaking off the corners
to expose the reinforcement, which is cut through by hacksaw or burning.
The pile can then be broken at this point by pulling it over with ropes
attached to the head. The concrete should then be broken away for sufficient
depth below the cut to expose the reinforcement for bonding into the pile
cap.
