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Module 33/Topic 23
DIAPHRAGM WALLS
Diaphragm walls are relatively thin retention structures incorporated in the soil,
which are constructed in panels, by excavating trenches in the ground, lowering the
reinforcement cage and concreting the trench (Fig.23.1). (The name ‘diaphragm’
arises from the fact of it being thin.) If the soils on the trench sides are likely to cave
in, the trench can be stabilised by drilling mud (Topic 44).
When the construction is complete, excavation of the soil on the inner side starts
and when the same reaches a specific depth, the first line of anchors (prestressed
ground anchors – Topic 22) are installed and fixed to the wall. Excavation proceeds
and is stopped when the next line of anchors become necessary. The number of lines
of anchors depend upon the total depth of excavation and the type of soil met with.
Finally when the work of excavation is complete with anchoring, the wall takes on its
assigned function as a retaining structure.
23.1 Trench cutter systems
Concomitant with the developments in ground anchor technology, and the
increasing number of diaphragm walls anchored by this technique prompted BAUER
(Germany) to look into efficiently mechanising the cutting and excavation of trenches
for constructing the diaphragm walls. This led to the development of dedicated trench
cutter systems and the related technologies of excavation. Although, strictly speaking,
this subject falls within the ambit of mechanical engineering, civil engineers will do well
to acquire a basic familiarity with this field, being the eventual end users of the product,
viz., anchored diaphragm walls.
According to Bauer, no other construction process has changed the face of civil
engineering since the mid 1980s more significantly than the development of
diaphragm walling techniques. The limitations of the ‘grab’ technique, first employed
in this field, were exposed by the ever increasing demands on the depth and watertightness of these walls. The idea of a “cutter” for this work, which involves continuous
excavation, originated in Japan where the first cutter was developed in 1980. As
specialists in the field of foundation engineering, Bauer was drawn into this field in
1984 when they developed their first trench cutter which was deployed successfully in
the construction of a 40 m deep diaphragm cut-off wall in moderately hard sandstone.
In 1987 the versatile city cutter came out which proved to be an instant success in
dealing with restricted inner-city spaces.
The frontiers of this technology were further pushed back when in 1989 cutter
wheels fitted with rock-roller bits were first tried in rock. Such a cutter was delivered
to Japan in 1990 the use of which reached a depth of 80 m. The development of the
‘hose band’ allowed the trench depth to be increased to 150 m, while cutting frame
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steering system incorporating a built-in electronic inclinometer (Kurian, 2013:
Sec.6.7.1) made it possible to limit the cutter’s vertical deviation to 20 mm over a depth
of 100 m! In 1994 a cutter drive with high torque was deployed in the diamond
exploration in the sea bed off the shore of Soth Africa.
23.1.1 The cutter technique
The cutter continuously removes soil from the bottom of the trench, breaks it up
and mixes it with the bentonite slurry in the trench for removal by mud circulation (see
Topic 44). The slurry charged with soil particles is pumped through hose pipes to a
de-sanding plant where it is cleaned of these particles and recycled back into the
trench. These operations are schematically shown in Fig.23.2.
The heart of this system is the trench cutter (partly shown in Fig.23.3) which
consists of a heavy steel frame with two drive gears attached to its bottom end which
rotates in opposite directions round horizontal axes. As they rotate, the soil beneath
is continuously removed, broken up, mixed with the slurry which is continuously
removed by the mud pump. Soil removal rates up to 40 m 3 /h have been achieved.
A specialty of the cutter is its ability to form overlap cutter joints (Fig.23.3). During
excavation of the secondary panel, short lengths of the hardened concrete on the
adjoining primary panels are chipped away by the cutter. Subsequently when the
secondary panel is concreted against the newly formed rough edge of each primary
panel, it forms a water-resistant concrete joint. Fig. 23.4 illustrates the sequence of
construction of diaphragm walls in primary and secondary panels.