there is a greater risk, especially with raking piles, of the hammer losing its alignment with
the pile during driving than in the case of piling frames which support and guide the hammer
independently of the pile. For this reason the method of supporting the hammer on the pile
in conjunction with trestle guides is usually confined to steel piles where there is less risk
of damage to the pile head by eccentric blows. When driving long steel raking piles in guides
it is necessary to check that the driving stresses combined with the bending stress caused by
the weight of the hammer on the pile are within allowable limits.
Pile guides which are adjustable in position and direction to within very close limits
are manufactured in Germany. Their principal use is for mounting on jack-up barges for
marine piling operations. A travelling carriage or gantry is cantilevered from the side of the
barge or spans between rail tracks on either side of the barge ‘moon-pool’. The travelling
gear is powered by electric motor and final positioning is by hydraulic rams. Hydraulically
operated pile clamps or gates are mounted on the travelling carriage at two levels and are
moved transversely by electric motor, again with final adjustment by hydraulic rams allowing
the piles to be guided either vertically or to raking positions. Guides provided by hydraulic
clamps on a guide frame fixed to the side of a piling barge are shown in Figure 3.7.
Trestle guides can be usefully employed for rows of piles that are driven at close centres
simultaneously. The trestle shown in Figure 3.8 was designed by George Wimpey and Co.
for the retaining wall foundations of Harland and Wolff’s shipbuilding dock at Belfast(3.1).
Piling equipment and methods 79Figure 3.7Installing a 4 m dia monopile foundation for North Hoyle offshore wind farm with pile
top rig and specially designed leader leg pile frame (courtesy Seacore Ltd).