01 July 1997
(a) Fixed Jets. Fixed jets are jets which are a permanent part of the pile. Precast jets in concrete
piles and concrete sheet piling may be used to avoid off-center and/or unsymmetrical jetting and the problem
of keeping proper alignment. This type of pile is costly but may be desirable where conditions do not permit
use of a movable jet.
(b) Movable Jets. These are attached to the pile to allow their removal after pile installation. Two
jets symmetrically located give the most rapid penetration and best control of the pile path.
(2) Pipes. The diameter of the pipe is essential to allow the required water flow. The diameter of the
pipe should be no less than 2 inches and can vary up to 4 inches. Nozzle diameters should be from 3/4 inch
to 1-1/2 inches.
(3) Hose. The hose should be approximately 1 inch larger in diameter than the jet pipe but no less
than 3 inches in diameter. It should have a protective jacket of canvas, cotton, or steel wire mesh. Hose
length should be as short as possible to minimize friction losses.
(4) Pumps. To jet pile properly a large flow of water is required. This is accomplished by using a jet
pump, whose flow should be no less than 250 gallons/minute and can range up to 1,057 gallons/minute.
Water pressures should generally vary from 100 to 200 psi for most soils. However, in gravels the pressures
should be set at 100 to 150 psi, and 40 to 60 psi in sands. They should be equipped with only bronze fittings.
The prime mover for the pump should have adequate torque and horsepower to pressurize the water, and all
fittings, hoses, and orifices should be properly sized to accommodate the flow and output the water jet at the
(5) Methods and Limitations of Jetting. Jetting can be performed using various methods and, as with
any other techniques, has its limitations.
(a) Water Jetting. This is a method designed to discharge a water jet at the pile tip, with both
volume of water and pressure sufficient to allow the discharge to come up around the pile.
(b) Spade or Multiple Jetting. This is a method for assisting the driving or sheet piling.
(c) Air Jetting. This method is practical for shallow depths and for probing or friction reduction but
not for deep penetration.
(d) Combined Air and Water Jetting. This method is useful when a double water jet and heavy
driving cannot secure the desired penetration.
(e) Limitations. Jetting applications are limited in clay soils where the jets may become plugged,
in cohesive soils generally where jetting is not useful or practical, in fine grained and poorly grained soils where
jetting may loosen the soil around the pile already driven, and in locations where there is considerable
groundwater and the material disturbed by the jets cannot escape.
b. Underwater Driving. Most pile driving for coastal and river structures can be driven from the surface;
however, in some cases it is advantageous to drive piling underwater. This eliminates the use of pile followers
which add weight to the system. Although some air/steam hammers have been and are used for underwater
driving, the best type of hammer to use underwater is the hydraulic hammer. Such a hydraulic hammer is
shown in figure 3-35. The hydraulic power pack is generally on the barge deck and the hoses extend into the
water. In some cases, large hose reels are used to store the hoses. With the largest hammers, the power
pack is mounted on the hammer and driven with an electric motor, the cables extending to the generator on
the deck. Hydraulic vibratory hammers can also be used underwater. The technique for using such a