Civil Engineering Reference
In-Depth Information
Figure 3.22
Tone RRC-U rope-suspended rotary drill for variable-diameter boreholes/pile shafts.
(Photograph courtesy of Tone, Japan and Geo Publications Ltd., Brentwood.)
driving the casing with a drop hammer. Alternatively, the casing can be advanced with
an hydraulic casing vibrator suspended from the attendant crane. The process can be
made more efficient in some instances by adding bentonite powder to the granular soil,
and 'mudding-in'. Water may also need to be added. The effect is to produce a column
of loosened soil within a stable bore through which the casing can be installed to depths
of about 10m. A casing vibrator can increase the depth of insertion to around 25m.
Casagrande hydraulic casing oscillators provide an alternative means of installing cas-
ing (Figure 3.23) and can be of use in freeing-up long lengths of casing at the start of
extraction.
3.4.5 Boring and concreting
Concreting in 'dry' bores is usually a straightforward procedure. On completion of
the borehole, the reinforcement is placed with suitable spacers to locate it centrally
in the bore. During concreting, the casing is withdrawn, always maintaining a head
