(or a mixture of water, bentonite, cement and air) can be used to aid
the penetration of a pile into a dense sand or sandy gravel stratum. Jetting
is often less effective in firm to stiff clays or soils containing coarse
gravel, cobbles, or boulders. Jetting can assist pile installation in
several ways. The jetting pressure may loosen (erode) the soil at the
tip of the pile. In addition, the flow of the jetting fluid can reduce
the shaft friction along the pile. However, the effect of the jetting
procedure on the bearing capacity of the pile and sheet pile must be taken
It is also
possible to use grout as a jetting fluid, which during driving reduces
(mainly) the shaft resistance. After pile installation is completed, the
hardening grout may increase the bearing capacity of the pile.
also be used to reduce ground vibrations during driving in vibration-sensitive
the jetting tube is attached to the shaft of the pile or sheet pile. If
jetting is required to aid penetration of an occasional pile which "hangs-up"
in driving, a separate jet pipe is used. This jetting pipe is then moved
up and down close to the side of the pile. An angled jet may be used to
ensure that the wash water flows to the pile point. In difficult conditions,
two or more jet pipes may be used for a single pile. Tube or box piles
driven open-ended can be jetted from within the pile, and steel H-section
piles can be jetted by sinking the jet pipe down the space between the
web and flanges.
quantity of water is essential for jetting. Suitable quantities of water
for jetting a 250-350 mm pile are:
of at least 5 bar (and in many cases significantly higher pressure) is
required. It is sometimes a difficult problem to dispose of the large
quantities of water and sand flowing at ground level from around the piles,
and great care is needed when jetting near existing foundations or near
piles driven to depths shallower than the jetting levels. The escaping
water may undermine the pile frame, causing it to collapse. Jetting through
sands may be impossible if the sandy strata are overlain by clays, which
prevent escape of the jetting water.