Pile Design and Construction Practice, Fifth edition

Piling equipment and methods 135
Piles drilled by tripod rigs are relatively small in diameter, requiring extra care when placing
the concrete as this is more likely to jam in the casing tubes when they are lifted. Curtis (3.26)
suggests checking the concrete level by hanging a float on top of the concrete and comparing
its measurement from the top of the tube with the amount of tube extracted. He also suggests
that the position of the reinforcing cage should be checked by a ‘tell-tale’ wire and indicator.
Problems can occur when placing concrete in raking piles. Internal ramming is impossible
as the rammer catches on the reinforcing cage. A high slump concrete is necessary with special
precautions being taken to prevent the reinforcement being lifted with the lining tubes.
3.4.11 The installation of raking piles
BS EN 1536 states that pile bores, whether drilled or driven, should be cased throughout their
length if the rake is flatter than 1 horizontal to 15 vertical unless it can be shown that an
uncased pile bore will be stable. Similarly, stabilizing fluids should not be used if the rake
is flatter than 1 in 15 unless precautions are taken when inserting casing and concreting.
The advantages of raking piles in resisting lateral loads are noted in Chapters 6 and 8.
However, the installation of such piles may result in considerable practical difficulties, and
they should not be employed without first considering the method of installation and the
ground conditions. If the soil strata are such that the piles can be driven to the full penetration
depth without the need to drill out a soil plug or to use jetting to aid driving, then it should
be feasible to adopt raking piles up to a maximum rake of 1 to 2. However, the efficiency of
the hammer is reduced due to the friction of the ram in the guides. It may therefore be
necessary to use a more powerful hammer than that required for driving vertical piles to the
same penetration depth.
The vertical load caused by the pile and hammer on the leaders of the piling frame must
be taken into consideration. Also when driving piles by guides without the use of leaders the
bending stresses caused by the weight of the hammer on the upper end of the pile must be
added to the driving stresses and a check should be made to ensure that the combined
stresses are within allowable limits.
The principal difficulties arise when it is necessary to drill ahead of an open-ended pile
to clear boulders or other obstructions, using the methods described in Section 3.3.5. When
the drill penetrates below the shoe of the pile tube it tends to drop by gravity and it is then
likely to foul the shoe as it is pulled out to resume further driving. Similarly, under-reaming
tools are liable to be jammed as they are withdrawn. The risks of fouling the drilling tool are
less if the angle of rake is small (say 1 in 10 or less) and the drill string is adequately
centralized within the piling tube. However, the drill must not be allowed to penetrate deeply
below the toe of the pile. This results in frequent alternations of drilling and driving with
consequent delays as the hammer is taken off to enter the drill, followed by delays in entering
and coupling up the drill string, and then removing it before replacing the hammer.
Difficulties also arise when installing driven and cast-in-place piles by means of an internal
drop hammer, due to the friction of the hammer on the inside face of the driving tube.
Installers of these piles state that a rake not flatter than 1 in 3.7 is possible.
Power augers can drill for pile boreholes at angles of rake of up to 1 in 3 but when casing
is necessary to support the pile borehole the same difficulties arise with the jamming of the
bucket or auger beneath the toe of the easing. As shown in Figure 3.2 the self-erecting leader
rigs are capable of drilling open holes at rakes up to 1 in 1, but where casing has to be
drilled, rakes flatter than 1 in 3 are difficult to manage.