86 Piling equipment and methods
from rebound forces and shock loads and will largely eliminate a tension wave in the pile (see
Section 7.3). The MHU hammers are designed to operate either as free-riding units mounted on
the pile with a slack lifting line, or to reduce weight on the guides they can be suspended from
the floating crane with a heave compensator to maintain constant tension in the lifting line.
Slender hammers can operate inside the pile or with a follower attached to the pile.
Double-acting(or differential-acting) hammers are steam- or air-operated both on the
upstroke and downstroke, and are designed to impart a rapid succession (up to 300 blows
per minute) of small-stroke blows to the pile. The double-acting hammer exhausts the steam
or air on both the up- and down-strokes. In the case of the differential acting hammer,
however, the cylinder is under equal pressure above and below the piston and is exhausted
only on the upward stroke. The downward force is a combination of the weight of the ram
and the difference in total force above and below the piston, the force being less below the
piston because of the area occupied by the piston rod. These hammers are most effective in
granular soils where they keep the ground ‘live’and shake the pile into the ground, but they
Figure 3.14BSP CX110 hydraulic piling hammer on Hitachi crane-mounted leader (courtesy BSP
International Foundations Ltd).