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158 Barry’s Advanced Construction of Buildings
Photograph 3.18 Diaphragm walls (courtesy of D. Highfield).
Hard/hard secant piles
With hard/hard secant piles, both male and female piles are cast with full strength concrete
and both are fully reinforced. The female piles are cast first and a high-torque-cutting casing
is used to drill through the female pile. The reinforcement in the female pile is positioned
so that the rig does not cut through it when boring the male pile. The depth of overlap is
usually about 25 mm; considerable care is required to ensure that this is maintained along
the full length of the pile. I-section beams can also be added to the pile to further increase
the lateral strength of the wall (Figure 3.75e).
Diaphragm walls
Diaphragm walls are formed by excavating a segmented trench to form a continuous wall
(Figure 3.76). While the deep trench is being excavated, it is filled with bentonite slurry
(supporting fluid). The slurry fills the trench and exerts pressure on the sides of the excavation,
thus preventing the excavation walls collapsing. The excavation is carried out using a
clamshell grab (Figure 3.77 and Photograph 3.20), which digs the material out through the
bentonite slurry. The width of the wall is determined by the width of the grab. The diaphragm
wall is cast in the ground. Using a tremie tube about 200 mm diameter, the concrete
is fed into the trench and placed in position. As the concrete settles at the base of the
excavation, the bentonite slurry is displaced, drawn out of the excavation and cleaned for