Civil Engineering Reference
In-Depth Information
with Dutch Cone penetration records; this in turn allows the results to be associated
effectively with well conducted static load tests. Using the TIMESET, CEMSOLVE,
CEMSET suite of programs for static load test analysis the system can be highly
effective as a control system.
Likewise, in ideal conditions and where very little temporary casing is needed to
stabilize soft soils near the ground surface, rotary boring of small diameter piles can
be very rapid.
However, driving piles through relatively stiff cohesive strata, or boring piles to
substantial depths through soft and unstable strata where long temporary casings
are needed, leads to reduced output and therefore increased costs. Fortunately the
conditions that slow down bored piling are often those in which driven piling thrives
and vice versa, so that economic choice becomes fairly obvious.
In the case of large bored piles, there is a slowing down of the processes as the
diameter increases, but the use of long temporary casings is probably the factor of
greatest influence in production. These casings may be installed relatively quickly if
they are short, and a 'mudding in' process, which involves creating a slurry column of
soil in bentonite suspension, may be used to aid insertion down to a depth of about
10 m. To place much longer casings through cohesionless soils, a large vibrator may
be attached to the casing and by this method casings of the order of 25 m have been
inserted. However, the effectiveness of such vibrators diminishes rapidly in the presence
of clay soils and their use for situations where interbedded layers of cohesive and
cohesionless soils occur is strictly limited. Therefore, if large bored piles are required to
go to great depths in unstable soils, bentonite or polymer suspensions remain the most
practical way to sustain an open bore hole throughout the process of pile formation. It
is clear that the more elaborate the method becomes, the more plant will be required
and in general the slower the process will become.
The continuous-flight auger method (CFA), with grout or concrete injection through
the stem of the auger as it is withdrawn, represents a neat way of using bored piles to
avoid casing problems and in circumstances where, but for environmental considera-
tions, a driven pile would be used. It provides a rapid installation rate, perhaps just a
little slower than for a comparable solution using driven piles.
The use of on-board computers to monitor the construction of continuous-flight
auger (Augercast) piles is now common among major specialist contractors in the
United Kingdom following their introduction by Cementation Skanska. Full records
of construction can be printed on-site and analyzed off-site by a central computer
using a program designed to scan for such items as under-supply of concrete in
relation to auger lift and other possible untoward events. It is believed that this pro-
cedure can aid considerably in the production of reliable piles. More recently it has
proved possible to carry out the concreting of piles automatically and this is gen-
eral practice on Cementation contracts. Further, automatic pile excavation is also
possible under well defined ground conditions in such a way that rotations of the
auger are minimized and risk of draw-in of surrounding soils is minimized (Fleming,
1995
discussion). An additional development in this field has been the wider use
of automatic auger cleaners for safety reasons, though this is still an area of further
development.
Rotary displacement, or screw, piles have been available in a limited way for
many years, the Atlas system of Franki in Belgium being a good example. In the last
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