116 Piling equipment and methods
3.3.5 Reverse-circulation drilling rigs
Reverse-circulation drilling rigs operate on the principle of the airlift pump. Compressed air
is injected near the base of the centrally placed discharge pipe. The rising column of air and
water lifts the soil which has been loosened by rotating cutters, and the casing tubes are also
rotated to keep them freely moving in the soil as they sink down while the boring advances.
The reverse-circulation rig manufactured by Alfred Wirth and Co. of Germany is shown in
Figure 3.34. The casing tubes and airlift riser pipe are rotated together or separately by means
of a hydraulic rotary table or power swivel. Dual airlift drill pipes, maximum bore 300 mm,
either flange jointed or flush, inject air through the annulus between the inner and outer tubes.
The riser pipe is maintained centrally in the casing by one or more stabilizers, and the soil
boring is effected by rock roller bits on a cutter head. The diameters of the latter range from
0.76 to 8.0 m. The injected air-flow and pressure and the point of injection all affect the
efficiency of cuttings removal; air injection rate is up to 130 m^3 per minute at a pressure of
12 bar, requiring large air compressors. For offshore work the hole will be kept full of
seawater, but on land drilling mud is used to convey the cuttings necessitating the use of mud
tanks and cleaners (see Section 3.3.8). Also on land the reverse circulation system with mud
may maintain a stable hole without the use of casing for cast-in-place piles. The more
powerful self-erecting crawler rigs in Table 3.6 can be rigged for reverse circulation for holes up
to 3300 mm diameter to 100 m depth. The Calweld reverse circulation rigs are manufactured
to drill without reaming to diameters of up to 2.1 m and to depths up to 230 m.
Piletop rigssuch as the Wirth PBA range (Figure 3.35) and the ‘Teredo’units built and oper-
ated by Seacore Limited (Figure 3.36) using powerful top-drive swivels are more versatile than
large rotary tables. The Wirth 1238 rig has a maximum power swivel torque of 380 kNm forFigure 3.33Installation of CFApiles in chalk with crane handling reinforcement cage (courtesy
Cementation Foundations Skanska).