This is because a hammer blow acting on top of the pile causes the tube to expand and push
out the soil at the instant of striking, followed by a contraction of the tube. This frees the
tube from some of the shaft friction as it moves downward under the momentum of the
hammer. The flexure of the pile acting as a long strut also releases the friction at the moment
of impact. However, when using an internal drop hammer, tension is induced in the upper
part of the pile and the diameter contracts, followed by an expansion of the soil and an
increase in friction as the pile moves downwards. Flexure along the piling tube does not
occur when the hammer blow is at the base, and thus there is no reduction in friction from
this cause. Tension caused by driving from the bottom can cause the circumferential cracking
of reinforced concrete and thin-wall steel tubular piles.
Top driving has another advantage in allowing the pile to be driven with an open end, thus
greatly reducing the end-bearing resistance during driving, but the soil plug will have to be
drilled out if the concrete pile is to be cast in place as the tube is withdrawn. The bottom-driven
pile demands a solid plug at the pile base at all stages, but produces a dry open shaft for con-
creting. In easy driving conditions bottom driving will give economy in the required thickness
of the steel and considerable reduction in noise compared with top driving. For example,
Cementation Foundations Skanska installed 508 mm diameter bottom-driven thin wall (6 mm)
steel piles up to 15 m long in Cardiff Bay in preference to thicker-walled, top-driven, cased
piles to reduce disturbance to residents. A 4 tonne drop hammer was used to drive the bottom
plug to found in Mercia Mudstone; concreting was direct from the mixer truck or by skip.
Great care is necessary to avoid bursting of the tube by impact on the concrete when
bottom driving through dense granular soil layers or into weak rocks containing bands of
stronger rock. The concrete forming the plug should have a compacted height of not less
than 2.5 times the pile diameter. In calculating the quantity of concrete required, allowance
should be made for a volume reduction of 20% to 25% of the uncompacted height. The 1:2:4
concrete should be very dry with a water/cement ratio not exceeding 0.25 by weight. A hard
aggregate with a maximum size of 25 mm should be used.
At least 10 initial blows should be given with hammer drops not exceeding 300 mm then
increasing gradually. The maximum height of drop should never exceed the maximum
specified for the final set which is usually between 1.2 and 1.8 m. Driving on a plug should
not exceed a period of 1^1 ⁄ 2 hours. After this time, fresh concrete should be added to a height
of not less than the pile diameter and driving continued for a period of not more than 1^1 ⁄ 2 hours
before a further renewal. For prolonged hard driving it may be necessary to renew the plug
every three-quarters of an hour.
3.3 Equipment for installing bored and cast-in-place piles
3.3.1 Power augers
Power-driven rotary auger drills are suitable for installing bored piles in clay soils. A wide
range of machines is available using drilling buckets, plate and spiral augers, and continuous
flight augers, mounted on trucks, cranes, and crawlers to bore open holes. The range of
diameters and depths possible is considerable, from 300 to over 5000 mm and down to 100 m.
Hydraulic power is generally used to drive either a rotary table, a rotating kelly drive on a
mast or a top-drive rotary head; some tables are mechanically operated through gearing.
Most units have additional pulldown or crowd capability. The soil is removed from spiral
106 Piling equipment and methods