Pile Design and Construction Practice, Fifth edition

102 Piling equipment and methods

For jetting piles in clean granular soils a central jetting pipe is the most effective method,

as this helps to prevent the pile from deviating off line. A 25 to 50 mm nozzle should be used

with a 50 to 75 mm pipe (Figure 3.22). The quantity of water required for jetting a pile of

250 to 350 mm in size ranges from 15 to 60 l/s for fine sands through to sandy gravels.

A pressure at the pump of at least 5 bars is required. The central jetting pipe is connected to

the pump by carrying it through the side of the pile near its head. This allows the pile to be

driven down to a ‘set’on to rock or some other bearing stratum immediately after shutting

down the jetting pump.

A central jetting pipe is liable to blockage when driving through sandy soils layered with

clays, and the blockage cannot be cleared without pulling out the pile. A blockage can result

in pipe bursting if high jetting pressures are used. An independent jetting pipe worked down

outside the pile can be used instead of a central pipe, but the time spent in rigging the pipe

and extracting it can cause such delays to pile driving as to be hardly worth the trouble

involved. Open-ended steel tubular piles and box piles can be jetted by an independent pipe

worked down the centre of the pile, and H-piles can be similarly jetted by a pipe operated

between the flanges. Large-diameter tubular piles can have a ring of peripheral jetting pipes,

but the resulting pile fabrication costs are high. Gerwick(3.11)has described the system for

jetting 4 m diameter tubular steel piles for a marine terminal at Cook Inlet, Alaska. Sixteen

100 mm pipes were installed around the inner periphery of the pile. The nozzles were cut

away at each side to direct the flow to the pile tip. Gerwick recommends that jetting nozzles

Lifting lugs

Jaws to mate with

hammer and

leaders

Recess for

cushion block

Steps to enlarge

steel tubular piles

Figure 3.21Vulcan driving cap for steel tubular pile.