Pile Design and Construction Practice, Fifth edition

Piling equipment and methods 133
the ground with limited crowd applied, although for less cohesive soil more thrust will be
necessary to reach the required depth. The auger is rotated out of the hole as concrete is
pumped through the tip to fill the helical profile of the pile, with only minimal soil being
brought to the surface.
3.4.8 Concreting pile shafts under water
Groundwater in pile boreholes can cause serious difficulties when placing concrete in the
shaft. A depth of inflow of only a few centimetres in, say, 5 minutes which has trickled down
behind the lining tubes or has seeped into the pile base can be readily dealt with by baling
or pumping it out and then placing dry concrete to seal the base against any further inflow.
However, larger flows can cause progressive increases in the water content of the concrete,
weakening it, and forming excess laitance.
A strong flow can even wash away the concrete completely. The defective piles shown in
Figure 3.41 were caused by the flow of water under an artesian head from a fissured rock
on which the bored piles were bearing after the boreholes had been drilled through a soft
clay overburden. The lined boreholes were pumped dry of water before the concrete was
placed, but the subsequent ‘make’ of water was sufficiently strong to wash away some of the
cement before the concrete has set. The remedial action in this case was to place dry
concrete in bags at the base of the pile borehole and then to drive precast concrete sections
into the bags.
In all cases of strong inflow the water must be allowed to rise to its normal rest level and
topped up to at least 1.0 m above this level to stabilize the pile base. BS EN 1536 requires
that a tremie pipe be used for concreting in submerged conditions (water or slurry); the
tremie bore must be 6 times the maximum size of the aggregate or 150 mm whichever is the
greater. The maximum outside diameter of the pipe including joints should be less than 0.35
times the pile diameter or inner diameter of the casing. The tremie pipe must be clean and
lowered to the bottom of the pile and lifted slightly to start concrete flow. A flap valve should
be used on the end of the tremie pipe rather than a plug or polyethylene ‘go-devil’. During
concreting, the tremie tip must always be immersed in the concrete; 1.50 m below concrete
surface for piles less than 1200 mm diameter and 2.50 m for piles greater than 1200 mm. If
immersion is lost during concreting, special precautions are required before placement can
continue; for example, steps must be taken to re-immerse the tremie so that any
contamination will be above the final cut-off level. Other limits for the tremie are given for
concreting barrettes.
Although a bottom-opening bucket is sometimes used instead of a tremie pipe for placing
concrete in pile boreholes, the authors as a general rule condemn this practice. This is
because the crane operator handling the bucket cannot tell, by the behaviour of the crane
rope, whether or not he has lowered the bucket to the correct level into the fluid concrete
before he releases the hinged flap. If he releases the bucket flap prematurely, the concrete
will flow out through the water and the cement will be washed out. On the other hand, if
he plunges the bucket too deeply it will disturb the concrete already placed when it is lifted
out. The bottom-dumping bucket method has no advantage over the tremie pipe and the
authors would use it only if a pile were large enough for the lowering and dumping to be
controlled by a diver.
BS EN 1536 provides guidance for piles formed using the technique known as ‘prepacked
concrete’ for underwater concreting, but it is not recommended here in preference to placing