Pile Design and Construction Practice, Fifth edition

Types of pile 51

from 250 to 1500 kN (subject to ground conditions). This employs an internal drop hammer
acting on a plug of gravel at the bottom of the drive tube. The drive tube (248 to 600 mm
diameter with a wall thickness of 32 mm) is carried down with the plug until the required
toe level is reached when the tube is restrained from further penetration by rope tackle. Then
the gravel plug and batches of dry concrete are hammered out to form a bulb or enlarged
base to the pile. The reinforcing cage is then inserted, followed by placing a semi-dry
(no-slump) concrete in batches as the drive tube is pulled out in stages. After each stage of
withdrawal the concrete is compacted by the internal hammer (Figure 2.27). The operations
of driving by internal hammer and concreting in stages are slower than the top driving
method described above. Hence, these techniques are used only when there are economic
advantages, for example, when the enlarged base adds appreciably to the bearing capacity
of the pile.

Lifting ropes

Hammer Reinforcing cage

Driving tube

(a) (b) (c)

(d)

Gravel plug

Concrete base

Figure 2.27Stages in installing an open-ended Franki pile (a) Driving piling tube (b) Placing concrete
in piling tube (c) Compacting concrete in shaft (d) Completed pile.