Piling equipment and methods 101Precast concrete pile6 layers of 25 mm (1")
hardwood strips
as packingHelmetLifting lugsM.S. plate
Plastics
HardwoodDollyFigure 3.20Dolly and helmet for precast concrete pile.
over the pile head. More recently, a thick cushion block of softwood, further softened by
soaking, has been used for each pile to avoid damage when driving prestressed concrete piles.
Driving caps are used for the heads of steel piles but their function is more to protect the
hammer from damage than to protect the pile. The undersides of the caps for driving box or
H-section piles have projecting lugs to receive the head of the pile. Those for driving steel
tubular piles (Figure 3.21) have multiple projections that are designed to fit piles over a
range of diameters. They include jaws to engage the mating hammers.
Plastic dollies of the Micarta type have a long life when driving steel piles to a deep
penetration into weak rocks or soils containing cemented layers. However, for economy
contractors often cushion the pile heads with scrap wire rope in the form of coils or in short
pieces laid cross-wise in two layers. These are replaced frequently as resilience is lost after
a period of sustained driving. If dollies have to be changed while driving a pile, it should be
noted that the blow count could change significantly.
3.1.9 Jetting piles
Water jets can be used to displace granular soils from beneath the toe of a pile. The pile
then sinks down into the hole formed by the jetting, so achieving penetration without the use
of a hammer. Jetting is a useful means of achieving deep penetration into a sandy soil in
conditions where driving a pile over the full penetration depth could severely damage it.
Jetting is ineffective in firm to stiff clays, however, and when used in granular soils
containing large gravel and cobbles the large particles cannot be lifted by the wash water.
Nevertheless, the sand and smaller gravel are washed out and penetration over a limited
depth can be achieved by a combination of jetting and hammering. Air can be used for
jetting instead of water, and bentonite slurry can be also used if the resulting reduced shaft
friction is acceptable.