DHARM656 GEOTECHNICAL ENGINEERING
The advantage of power hammers is that the blows follow in rapid succession, keeping
the pile in continuous motion and reducing the effect of impact, thus minimising the damage
to the pile head.
Diesel hammer
This works on the internal combustion of diesel oil. Energy is provided both for raising the
hammer and for downward stroke. This type is self-contained, economical, and simple. The
energy delivered per blow is relatively high, considering the weight of the hammer, as it is
developed by a high-velocity blow. The disadvantage is that the energy per hammer blow
varies with the resistance offered by the pile and is difficult to evaluate. Thus, the diesel
hammer is best adopted to conditions where controlled energy is not critical.
A double-acting hammer activated by hydraulic pressure is somewhat faster and lighter
compared to equivalent steam hammers because the operating pressure is much higher. The
compact hydraulic pump system is easier to move than the bulky air compressor or steam
generator.
A heavy single-acting hammer may be more effective sometimes than a light double-
acting hammer, since the hammer may bounce back due to high velocity, in soils of high resist-
ance to penetration.
Vibratory hammer
The driving unit vibrates at high frequency and thus, the driving is quick and quiet. A variable
speed oscillator is used for the purpose of creating resonance conditions. This allows easy
penetration of the pile with a relatively small driving effort. This method is popular in the
U.S.S.R.
Most pile hammers require the use of driving heads, helmets, or ‘pile caps’ that distribute
the force of the blow more evenly over the pile head. A ‘cushion’, consisting of a pad of resilient
material such as wood, fibre, or plastic, is interposed between the pile head and pile cap, as the
top portion of the pile cap.
Piles are ordinarily driven to a resistance measured by the number of blows required for
the last 1 cm of penetration depending upon the material and weight of the pile.
16 .5 Pile Capacity
The ultimate bearing capacity of a pile is the maximum load which it can carry without failure
or excessive settlement of the ground. The allowable load on a pile is the load which can be
imposed upon it with an adequate margin of safety; it may be the ultimate load divided by a
suitable factor of safety, or the load at which the settlement reaches the allowable value.
The bearing capacity of a pile depends primarily on the type of soil through which and/
or on which it rests, and on the method of installation. It also depends upon the cross-section
and length of the pile.
The pile shaft is a structural column that is fixed at the point and usually restrained at
the top. The elastic stability of piles, or their resistance against buckling, has been investigated