Pile Design and Construction Practice, Fifth edition

Piling equipment and methods 103

should terminate about 150 mm above the pile tip. He gives the following typical requirements
for jetting large diameter piles:

Jet pipe diameter — 40 mm

Pressure — 20 bar (at pump)

Volume — 13 l/s per jet pipe

The large volume of water used in jetting can cause problems by undermining the pil-
ing rig or adjacent foundations as it escapes towards the surface. It can also cause a loss
of shaft friction in adjacent piles in a group. Where shaft friction must be developed in a
granular soil the jetting should be stopped when the pile has reached a level of about 1 m
above the final penetration depth, the remaining penetration then being achieved by
hammering the pile down. The jetting method is best suited to piles taken down through
a granular overburden to end-bearing on rock or some other material resistant to erosion
by wash water.
Water jetting is also used in conjunction with press-in and vibratory piling techniques
to assist penetration of sheet piles in dense granular soil. A lance is fitted inside the pile
pan and both are driven simultaneously into the ground. On reaching the required depth
the lance is removed for reuse. Low injection rates are used at high pressure (5 l/s at
150 bar).

Standard

elbow bend

Detachable

screwed nozzle

Figure 3.22Centrally placed jetting pipe.