Civil Engineering Reference
In-Depth Information
The effect of the mass of the hammer is to alter the length of the stress wave produced
in the pile. The length
L
w
of the stress wave is given approximately by the relationship
3
W
h
W
p
/
L
w
=
L
where
W
h
denotes the total weight of the hammer and
W
p
/
L
denotes the weight of
the pile per unit length.
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 pre-cast 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. Depending on the dynamics
of pile/hammer contact, this compressive 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 also occur under heavy
driving conditions, which can be particularly damaging to concrete piles and may
lead to a reduction in overall concrete modulus as a series of fine transverse cracks
can be generated. 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.
Variants of the simple winch-raised drop hammer include rams raised by steam
or compressed air or as is most usual today, hydraulically. Hammers raised by fluid
pressure may be free falling from the top of the stroke (single-acting hammers), but
by applying fluid pressure on the down-stroke, double-acting or 'differential-acting'
hammers will apply more impact energy for a given hammer weight. Double-acting
air hammers are not generally suitable for driving concrete piles. However, large
single-acting hammers have been developed with maximum net hammer weights
approaching 300 tonnes, producing rated energies of up to 2.5MNm using steam
pressure (Vulcan 6300). By using a fluid cushion, the impact of the hammer is con-
trolled in the larger pile hammers of the Hydroblok type, and a sustained push rather
than a sharp peak of stress is transmitted to the pile at each blow. This type of hammer
blow is better for driving through soils where the tip resistance of the pile is low as
in clays.
Hydraulic hammers such as those manufactured by BSP, Junttan and Menck tend
to be more efficient and quieter in operation than other types, and may be preferred
to simple drop hammers and diesel hammers in many applications. The rated energies
(as ram weight
stroke) are as high as 3MNm. They are versatile and may be used
on a wide variety of bearing piles, including raking piles and usually with only slight
modification, may be used under water. They are also suitable for pile extraction.
A typical hydraulic hammer is shown in Figure 3.13.
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