Civil Engineering Reference
In-Depth Information
3.4.5.1 Boring and concreting in water-bearing ground
In a stratigraphical succession in which water-bearing gravel overlies stiff clay, the
temporary casing is used to support the borehole through the gravel, and is then
driven a short distance into the clay in order to seal off the water. Boring then con-
tinues under dry conditions. This may not always be possible, and the base of the
pile may be in a granular deposit below the water table. The tremie method of con-
creting is then employed. This is a practical method of forming a pile shaft provided
certain precautions are taken. It is essential that a total collapse slump mix is used
(i.e. greater than 175mm - see also CEN EN 15336 for European Standards), and
that the tremie pipe is always well below the water/concrete interface during con-
creting. The tremie pipe and hopper connections should be watertight and in clean
condition to permit free flow of the concrete. A minimum internal diameter for
the tremie pipe of 150mm is suggested for use with concrete having a maximum
aggregate size of 20mm, increasing for larger aggregates. A vermiculite or expanded
polystyrene plug should be used in the tube between the water and first batch of
concrete, and immediately before placing the tremie pipe in the pile bore, a check
on sedimentation should be made, and any significant sediment should be air-lifted
out. The procedure is illustrated in Figure 3.24. Where the pile cut-off level is above
ground, concrete should overflow from the pile head on completion. The upper part
of the pile is composed of weak concrete from an excess of water and may also
be contaminated from boring detritus. Where there is a deep cut-off, the concrete
level should be raised to allow for around 0.5 to 1.0m for trimming off the weak
concrete.
1
Reinforcement and
Tremie tube inserted in
pile shaft at the start
of the concreting
operation
2
3
4
5
Concreting started
with water and thin
slurry being displaced
Water discharging as
concreting proceeds
Withdrawing
temporary casing
Completed pile
Casing
Floating Plug
Tr e m i e Tu b e
Temporary Casing
Pile Reinforcement
Slurry
Note.
Any slurry should be removed
by pump or air-lift prior to
concreting, especially if
end-bearing is significant
Figure 3.24
Placing concrete by the tremie method.
