The simplest
form of hammer is the drophammer, which in many countries was the most
frequently used method for the installation of concrete piles. They are
mounted on a leader on mechanical or hydraulic piling rigs. Drop hammers
usually consist of a solid mass of forged steel from 1000 to 5000 kg,
fitted with a lifting eye and lugs for sliding in the leaders. The drops
range usually from 0,2 - 2 m. Since the peak stress at the pile head can
be greatly increased if the hammer strikes the pile eccentrically, a long
narrow hammer is preferable, as then the blow will be more axial and has
better impact characteristics.
Some increase
in hammer drop is usually required for raking piles, as the driving energy
is reduced by approximately 10% for a 3:1 vertical rake. Distress at the
pile head is more likely when a too light hammer is used, which may require
an excessive drop, than using a too heavy hammer.
Modern drop
hammers can be operated with accuracy. However, simple (early types of)
drop hammers have the disadvantage that it is not easy to control the
height of drop with accuracy, causing the operator to use too great a
drop when driving becomes difficult. This in turn may greatly increase
the risk of damage to the pile. Drop hammers have useful applications
where noise abatement procedures are necessary, as they can be adapted
to operate within a sound-proofed box.
During driving,
the ground conditions will dictate the way in which the compressive stresses
produced at the pile head by the hammer blow are reflected from the pile
tip. Under easy driving conditions, the compressive stress is reflected
from the toe as a tensile wave. When the pile length is somewhat greater
than half the length of the stress wave, there is a likelihood of tensile
stresses appearing towards the top of a pile. These can be damaging to
precast concrete piles, and when driving such piles in soft ground the
hammer drop should be reduced, especially if using a light hammer to drive
a long pile.
When heavy
driving conditions are encountered, the initial compressive wave will
be reflected from the pile toe as a compressive wave, but depending on
the dynamics of pile/hammer contact, this compression wave can be further
reflected as a tension wave at the pile head if the hammer is not in contact
with the pile head at the time the returning wave arrives. Tensile forces
can therefore occur under heavy driving conditions, which can be particularly
damaging to concrete piles. Large compressive stresses occur at the pile
toe and head when obstructions or dense strata are encountered, which
may be sufficient to cause steel piles to deform or, if driving is protracted,
fatigue can occur with similar results.
