114 Piling equipment and methods
24 m deep at a 1 in 4 rake, but difficulties can occur when extracting the casing during
concreting. The German Bade oscillators are used for piles up to 2500 mm in diameter.
Drilling and installing casing simultaneously (‘duplex’ drilling) through cobbles,
boulders and rubble using special casing shoes and casing under-reamers attached to top
drive, down-the-hole compressed air hammers has advanced significantly. For example,
Numa Hammers of USA manufacture a range of drills capable of installing casing up to
1219 mm diameter to 15 m deep using a rotary percussive under-reamer which can be
retracted to allow concreting of the pile as the casing is withdrawn (Figure 3.32).
3.3.3 Continuous flight auger drilling rigs
A typical continuous flight auger rig is shown in Figure 3.33. Drilling output with the rigs
in Table 3.6 is greater than achievable for standard bored piles as the pile is installed in one
continuous pass, hence the mast must have an adequate stroke for the auger under the rotary
head. A kelly may be inserted through the rotary head to increase depth on some rigs. Most
CFArigs have crowd capability to assist in penetrating harder formations, and augers should
be designed to suit the high torques available. Possible diameters range from 500 to 1400 mm
to a maximum depth of 34 m.
Displacement auger piling is carried out with rigs similar to the high torque CFA
equipment, but the diameter is limited to less than 600 mm by the shape of the displacement
tool; maximum depth is around 28 m.
3.3.4 Drilling with a kelly
The kelly is a square or circular drill rod which is driven by keying into a rotary table either
fixed to the rig near the ground surface, to the crane attachment or by a moveable drive head on
the mast. The full range of drilling tools, plate and bucket augers, drag bits, compound rotary
Figure 3.31Soilmec casing oscilator.