Civil Engineering Reference
In-Depth Information
If the bearing stratum is at a greater depth, say normally down to 40 m, then a
segmental jointed precast pile has much to commend it. A balance has to be struck
between the segment length, joint cost, and driving time in order to make best use of
the system.
Where the depth of pile required becomes much greater, or where harder driving
can be expected on the way to a deeper founding stratum, driven steel 'H' sections
may prove economic. They can easily be raked and have high tensile as well as com-
pressive bearing capacity. In work in ports and harbours steel box piles are frequently
used, since they are compatible with sheet pile driving and have similar durability.
The 'H' pile, because of its low displacement volume, can be used to minimize heave
problems which might otherwise result from the use of larger-section piles, but to be
used efficiently they need to be stressed to high levels.
It is of interest to note that engineers are now beginning to think in environmental
terms in particular with a view to the recovery and/or re-use of piles which have served
one life and which may now be extracted and either re-used or re-smelted. In this sense
steel piles have an advantage in that they are easy to withdraw in most circumstances.
By comparison with most concrete piles, which are difficult and costly to extract, it
may be that H piles and tubes will find new uses in city centre areas but this may take
a long time to happen. The recovery of the reinforced concrete piles and indeed of
rendering the ground in which they were originally formed re-usable, is unlikely to
be far from simple but of course the value of city sites may still be a powerful factor
influencing such considerations as will the costs of trying to obtain re-insurance on
piles installed by others under a different contract.
Tubular piles are used mainly for port and harbour works. They may be driven
open-ended or closed-ended depending on whether there is a suitable stratum from
which a closed-ended system can take benefit. They have good characteristics with
regard to multi-directional lateral loading conditions. Open-ended tubes can plug
during driving subject to their diameter and the specific ground conditions but great
care is required if it is decided to clean out upper soil from the plug and replace
it with concrete. If, for example, a clay is grabbed out and then underlying sand
is allowed to blow into the tube, even in the slightest degree, it can change the
bearing capacity of the pile dramatically so that considerable re-driving will be
necessary.
Pile plug inducers are occasionally used, offering a partially plugged section that
will readily drive through clays but are sufficient to cause a full blocking mechanism
when the inducer enters medium dense to dense sand. The reason such devices are not
well known is probably because they are appropriate for some conditions; but not all.
For example, clays may plug open ended piles relatively easily; closed ended piles will
stop generally close to the top of a cohesionless stratum, but piles with plug inducers
have the objective of trying to secure a pre-set penetration. Such inducers are generally
aimed at stopping the pile in a medium dense to dense cohesionless soil but at the same
time giving a total penetration which may, for example, mainly be aimed at resisting
lateral load.
Intrinsically steel is an expensive material and therefore steel piles are mainly used
for the special purposes as described.
Permanently lined cast-in-place piles formed in steel tubes are invariablymore expen-
sive than piles of similar size without such linings, but piles formed within concrete
