Civil Engineering Reference
In-Depth Information
4 Checking of pile positions prior to placing concrete in cast-in-place piles is highly
desirable where the piles are heavily reinforced. If an error is then detected, steps
may still be taken to cater for extra bending stresses through the provision of
reinforcing steel.
In general tie beams between pile heads to minimize lateral deflections are only
necessary where the upper ground is soft and the piles are slender, and if tolerance
effects have not been taken into account adequately at the pile design stage.
For large-diameter piles or for most conventional piles where the upper ground is
firm or dense, calculation indicates and experience shows that tie beams are generally
unnecessary. The fixing of pile heads into a stiff cap reduces moments to about half
of what they would be in a free-headed case.
Reinforcement may also be required in driven piles to cater for uplift forces where
piles are driven into soils that heave as a result of volume displacement. This includes
all clays and most other soils occurring in moderately dense to dense states.
The above considerations demonstrate how tolerances can have significance in rela-
tion to the selection of pile type on any particular contract, and how they influence
subsequent structural work at ground level and the amount of the steel reinforcement
that is necessary.
10.4 Constructional consideration of pile use
10.4.1 Site access
The problems encountered in gaining access to sites are numerous, and many invi-
tations to tender for piling work are accompanied by the sensible requirement that
before tendering the specialist contractor must visit the site and be satisfied as to the
means of getting in and out and that there is adequate working space.
Naturally if a small and inaccessible site is considered, perhaps in the basement of
an existing building, a machine that can work in those conditions will be required, and
this often means the selection of bored piling equipment rather than driven, because
small boring machines are more readily available. Some driven steel or iron piles are
constructed in this way or under such conditions, for example in Sweden, but the
system does not appear to be used much elsewhere.
When working in enclosed areas it has to be remembered that if standard diesel
engines are used the exhaust fumes must be ducted away to open air. Otherwise the
equipment for such work can be powered either by remotely provided compressed air,
electricity or hydraulic pumps.
Whatever the size of a piling site, the physical dimensions of entrances and of the
slopes and stability of access ramps require consideration. Ramps should not in general
slope more steeply than about 1:10. It is rarely sensible to have to undertake a partial
dismantling job on a piling machine just to get it into a site, but there are times when
such action becomes necessary. Occasionally piling equipment is picked up bodily and
lowered into deep basements by heavy cranes.
Several questions must be asked in relation to the disposition of piles within a site:
1
Is there enough space for all the equipment needed? This may include extra
craneage, various materials that must be stored in readiness for use, and items
