1.

DIAPHRAGM WALL CONSTRUCTION

1.1 GUIDE WALLS

The purpose of the guide walls is: -

a. To provide a permanent alignment for the clamshell.

b. To prevent the collapsing of soil near the surface.
c. To provide a temporary support for the reinforcing steel cage.
d. To provide a support for suspending the stop-end joint.

The cast-in-site guide walls shall be 1.0m (height) by 0.25m (width). A9 square wire
mesh will be used to reinforce the guide walls and concrete of grade 25 N/mm2 to cast
the guide walls. The distance between the guide walls will be 650mm +/- 25mm

75mm x 50mm timber props will be used to strut the guide walls upon removal of
formwork and will be removed during excavation. The timber props shall be spaced at
a minimum distance of 6m along the guide wall and placed near the top and toe of the
guide walls.

1.2 BENTONITE SLURRY

Bentonite slurry come in dry form in packets and is mixed on the site as and when
required. Bentonite is naturally occurring clay which when added to water, forms
thixotropic slurry. This slurry has the ability to form almost instantaneously a low
permeability membranes or virtually impervious filter cake on the sides of the
excavation i.e. soil / liquid interface. Bentonite is sensitive to impurities such as

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cement, salt and decomposed organic matter. It must be chosen to suit the particular
condition expected and small quantities of corrective additives may be required.

Bentonite slurry will be prepared in the site with a high velocity mixer. The mixing is
by means of a centrifugal pump driven by electric motor running at 1450RPM. The
slurry will be stored in silos (Fig.2.2) and fed into the trench using pump when
required. The hydration time for fresh bentonite slurry shall not be less than 8 hours.
The bentonite slurry will be tested prior to use, during excavation and prior to
concreting of the panel as follows. The following properties of bentonite shall be
tested: -

ITE DESCRIPTI TEST FRESH DURING PRIOR TO

M ON METHOD BENTONI EXCAVATI CONCRETI
& TE IN ON NG
OF TEST APPARAT TANK &
US PRIOR
TO USE

1 Viscosity Marsh cone >31 NA <55

(second)

2 Sand content Sand Screen NA NA <5.5

(Percentage) Set

3 P.H. Value Electric 7-12 7-12 7-12

Meter

4 Density Mud >1.02 NA <1.15

(gms/c.c) Balance

5 Frequency Once a day 1/day on 1 at end of

sample recycling.
coming from
pools or silos

Table Bentonite Properties for Testing

The different types of testing methods as well as the apparatus are shown.

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1.3 EXCAVATION

The trench is excavated using a cable operated mechanical clamshell suspended from
a crawler crane. Where hard stratum is encountered and excavation using clamshell is
not possible, a drop chisel would be used. When using cranes with a heavy load on its
cables such as a clamshell or chisel, care must be taken to ensure that the cables are
able to sustain the heavy load. If the cables are under strength, the cables may snap
under heavy load and this may cause injuries to workers working nearby.

During excavation, the bentonite slurry is continuously fed to the trench and slurry
level is maintained at approximately 500mm below the guide wall. The width of the
trench can be checked using equipment that emits ultrasonic pulses. If the width of the
excavated trench is not of the desired dimension, the frequency of the pulse will
change. If the width is too large, then, the width is left as it is. This is because after
casting the wall, the excess portion can be hacked off. If the width is too narrow, the
structural stability of the wall might be compromised.

Vertically of the diaphragm wall will be checked constantly at every 3m to 5m of

excavation. The verticality of the wall can be checked by using a plumb line as a
guide. The plumb line is connected to the clamshell and crawler crane or alternative
connected to a stand on the guide wall. If the line is not straight, this will mean that
the digging of the trench is also not straight. Steps should be taken to ensure that the
line should be as straight as possible. The excavated material is removed from the site
in dump trucks. A temporary disposal pit may be maintained on the site and used if
necessary.

Hydraulic Grab in Trench Hydraulic Cutter

1.3.1 EXCAVATION SEQUENCE

The diaphragm wall between the guide walls may be excavated in consecutive or
alternating panels. Panels may be excavated as primary, secondary or closing panels.

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Definition of primary, secondary and closing panels are given in. Where a panel has
been cast, the adjacent panel next to the cast panel would be excavated after 8 hours,
starting on the outside bite, away from the stop-end joint. The soil directly next to the
stop-end joint would be only be excavated after 12 hours of concreting of the cast
panel. The length of the panel depends on the soil condition. For example, if the soil
is of good stability, a panel length of 4 to 6 m is quite usual but if the soil is loose and
unstable, the panel length may have to be 2 to 4 m. Similarly, for areas of high
surcharge pressure because of adjacent structures such as in this case, the panel length
also cannot be of too long.

1.4 DESANDING

Once the excavation reaches the required level, it may be necessary to reduce the
concentration of the sand suspended in the slurry. A submersible pump is lowered to
the toe of the excavation and the slurry is pumped to the desander, which separates the
sand from the bentonite slurry. At the same time, fresh bentonite or desanded
bentonite slurry is fed to the trench to maintain the slurry level.

The slurry will be desanded until the sand content of the slurry have achieved the
requirement for concreting of the panel, as indicated in Table 2.1. Bentonite
Properties for Testing. This is to ensure that the viscosity, density, shear strength and
pH value of the slurry does not become excessively diluted or contaminated by soil
particles.

Desanding Unit

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1.5 STOP-END JOINTS

To secure a watertight connection between the panels, the stop-end joints with rubber
waterstop are used. The joint is used to form an interlocking shape in the first concrete
pour, providing a concrete face with rubber waterstop for the pouring of the adjacent
panel.

The stop-end joint is installed to the depth of 2m below the final excavation level. It
will be inserted prior to installing the reinforcement steel cage and extracted after
concreting, while excavating the adjacent panel next to the stop-end joint.

Stop-end removal is totally independent from concrete placement operations; this

allows better site efficiency, organization and planning. The regular shape stop-ends
use in this case allows the installation of water stops. The stop-end is left in place at
the end of a panel while the next panel is being excavated; it protects the concrete of
the previous panel. Therefore, the geometry, cleanliness and the quality of the joint
can excellent.

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1.6 REINFORCEMENT STEEL CAGE

Reinforcing steel can be placed only in the form of prefabricated cages, which are
suspended in the slurry filled trench. Reinforcement steel cage will be fabricated on
site in a suitable flat surface. The steel cage will be adequately stiffened to allow
lifting without any permanent distortions. Generally, for a panel length which exceeds
3m, the steel cage will be fabricated with two openings for tremie pipe. The steel cage
will be fabricated in 12m lengths and shifted to the excavated trench for assembling to
the required length.

The reinforcement will have a minimum cover of 90mm each on excavation face and
soil face. At the toe of the diaphragm wall, a cover of 300mm will be given from the
bottom of the reinforcement to the required excavation toe level.

Reinforcement cages are then lowered into the trench.

A crawler crane will be used for lifting the steel cage and assembling the steel cage in
the trench. U-clips will be used to secure connections between the lower steel cage
and upper steel cage. Spacers will be installed to the steel cage during lowering of the
steel cage into the trench at 5m vertical spacing.

When the full sections have been assembled, the steel cage would be suspended on
the guide wall using temporary steel beams. Due to the heavy weight of the cage,
there are difficulties in hoisting up the cage vertically. If the U-clips are not secured
properly, then the cage may just fall apart and hurt the workers working nearby.

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The picture below shows workers handling the steel reinforcement cages and
carefully aligning it to the correct position. If the depth is too deep, 2 steel cages may
have to be join together to lengthen the steel reinforcements cages. Care must be
taken to ensure that they are properly connected together in order to achieve
maximum structural strength and stability.

Illustrations.

1. Excavation of bite 1 and 2 using rope

suspended grab bucket.

2. Excavation of third bite

completes panel.

3. Stop end removed form

adjacent concreted panel, and
placed against new soil end.

4. Steel reinforcement cage lowered

into stabilized excavation.

5. Concrete placed by tremie pipe from

the base of the panel. Bentonite
displaced upwards.

6. Finished Wall Panel.

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1.7 CONCRETING

The placing of the concrete is carried out by the “tremie” method. Concrete placing is
a very important and perhaps the most difficult stage of the technique. A tremie pipe
is used to feed the concrete by gravity to the bottom of the trench. It is kept immersed
in the wet concrete and is raised progressively with the level of concrete. This is
important because the diaphragm wall will form part of the structural framework.
Thus the quality of the wall must be good to ensure structural stability, hence the
pouring of concrete must be stable and in pace with the level rising accordingly. That
is the reason for raising the tremie pipe progressively with the level of concrete.

A 250mm or 200mm diameter tremie pipe is lowered into the bottom of the excavation and
a hopper is attached at the top. For the initial pour, the tremie pipe is lifted about 150mm
from the bottom of the excavation to allow the smooth flow of concrete at the bottom of
the pipe. To ensure separation of the concrete and the stabilizing fluid during initial placing
of the concrete, a layer of vermiculite will be placed in the tremie pipe before discharging
the concrete. The concrete is then fed to the hopper from a ready mix truck and the level of
the concrete is monitored to ensure at least 3m of tremie pipe is within the concrete during
the whole process of concreting.

The concrete shall have a slump of 150 to 225mm and a maximum coarse aggregate of size
of 20mm. Concrete shall be cast to 300mm above the required cut-off level. One of the
reason for doing so is because water will be normally displacing upwards and accumulate on
the top of the concrete, thus the concrete formed here is of lower quality as it contains
more water. To ensure that the concrete quality is not affected, concrete will be poured to a
higher level.

During concreting, the displaced bentonite slurry is continuously pumped from the trench to
the desanding unit from which the desanded bentonite is pumped back to the silos for reuse
or disposed if contaminated. A continuous supply of concrete should be ensured. Otherwise
interruptions in the concreting may result in cold joints. This means that there is no proper
bond between the partially set concrete and the fresh concrete.

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LATERAL SUPPORT DURING EXCAVATION

Excavation works would take place once after the diaphragm has been
constructed and has gained sufficient strength. As excavation works progress,
horizontal struts and bracings are installed to provide lateral resistance. This is to
relief the stress loads exerted on the diaphragm walls by the surrounding soil.

In the picture above, the bracings are supported by king posts where required. Such
supports are joined by welding and bolting. Heavy struts are also used in certain areas
(picture below – flying shores)

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