Civil Engineering Reference
In-Depth Information
founded in rock, it is advisable to penetrate the rock by 2-3 pile diameters to create a
rock socket, where at least the working load is carried by side friction.
The frictional resistance of a pile depends on the condition of the walls of the bore,
which in turn depends principally on how the pile was made. If the pile passes through
clays into fi rmer strata, it is important that the tool used for excavation does not
smear the walls at depth with a layer of soft clay. Also, if the clays are being relied on
for part of the frictional strength, the design should take into account that the boring
may rework the surface layer, reducing its strength, while if the bore is left open for
too long, the clay surface may deteriorate. If the pile is being bored under water or
bentonite, some boring tools that are insuffi ciently vented may cause suction as they
are being drawn out, loosening or even collapsing the side walls.
If the pile is bored through sand or gravel it will usually be under bentonite. The
bentonite limits the loss of drilling water through the soil by building up an impermeable
layer, or fi lter cake, on the pile wall. Thus the concrete of the pile walls will not be
in direct contact with the ground, and the frictional load capacity of the pile will be
defi ned by the lower of the internal friction of the bentonite fi lter cake or of the soil;
it is usually the former that governs.
When a pile is loaded, some settlement takes place as the load transfers to the soil.
As friction is a stiffer action than end bearing, initially the load is all carried in friction.
When the limiting friction is reached continuing settlement brings the end bearing
into play. Consequently it is often considered good practice to design bored piles such
that they carry their working load with a small factor of safety in friction alone. For
instance, the design could be based on friction alone carrying 1.5 × working load,
while the required factor of safety of 2.5 or 3 is provided by the combined action of
friction and end bearing. Clearly, the detailed defi nition of these factors of safety needs
to be established for each case.
c) Pile sizing
A foundation consisting of numerous bored piles beneath a pile cap suffers from the
economical disadvantage described for vertical driven piles, namely that only the
corner piles work at their maximum load, and the average load in the piles is thus
signifi cantly less than their rated load. Corner piles work hardest, followed by edge
piles, with piles in interior rows carrying the least load. The design logic that follows
is to attempt to limit the number of piles to a maximum of four.
If a single pile is used, it only needs to be designed to carry the actual vertical load.
The horizontal loads and moments in both directions are carried in bending in the pile.
(Considerations of fl exibility of the pier/pile or the amount of bending reinforcement
required may require the diameter of the pile to be increased beyond that required for
vertical load.) If two piles are used side by side, longitudinal forces and moments are
carried in bending, while transverse loads and moments are carried in push/pull and in
portal action, increasing the load on the piles and requiring greater pile cross-sectional
area. If three or more piles are used, moments in both directions will be carried in push
pull. If more than four piles are used, non-corner piles will be under-used.
It should be noted that the vertical load capacity of piles is usually assessed at the
SLS, while the bending strength of piles is assessed as for any other reinforced concrete
structure at the ULS. The initial sizing of bored piles may be based on a compressive
stress at the SLS of 5 MPa (generally a 25 per cent over-stress may be appropriate for
