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Ground Stability, Foundations and Substructures 161
reuse. The disposal of bentonite at the end of the operation is expensive as the mixture is
treated as a contaminated material. The diaphragm wall is constructed in alternate panels
usually around 5 m long. The excavators are usually guided by shallow concrete beams that
are cast so that the beam faces form the desired position of the wall. Diaphragm walls have
been constructed to depths of 120 m; however, there are some practical difficulties when
attempting to splice and link the reinforcement cages over such depths.
The joints between each section can be cast using steel tubes or interlocking junctions
to reduce the ingress of water through the joints (Figure 3.76b and c). The hydrofraise
machine is used to cut an interlocking surface into the previously cast segment of wall
(Figure 3.76d and Figure 3.78). Interlocking precast concrete sections can also be used.
Once the precast concrete sections are in place, grout is used to fill any remaining joints.
Clamshell excavates soil
through bentonite slurry
Grab length 2 and 2.8 m
Grab width 600, 800, 1000,
1200 and 1500 mm
Guide wall
Segments
already cast
Figure 3.77 Diaphragm walls – clamshell rig (adapted from Bachy Soletanche; http://
www.bacsol.co.uk).