SOIL STABILIZATION IN CASE OF DIAPHRAGM WALL
Nisha Verma
BEM, FIRST YEAR
SITE CONDITIONS
A diaphragm wall is a
structural concrete wall constructed in a
deep trench excavation, either cast in
situ or using precast concrete
components. Diaphragms walls are
often used on congested sites, close to
existing structures, where there is
restricted headroom, or where
the excavation is of a depth that would
otherwise require the removal of much
greater volumes of soil to provide stable
battered slopes.
In this case, diaphragm walls were
selected as a construction technique for
excavation due to the setback
conditions and presence of nala in
proximity.
Diaphragm wall facilitates certain
construction activities, such as:
a) As a retaining wall
b) As a cut-off provision to support deep
excavation
c) As the final wall for basement or other
underground structure (e.g. tunnel and
shaft)
Explanation on the work sequences
Guide wall – Guide walls are constructed in-situ typically as lighly reinforced concrete elements. Guide walls maintain the
horizontal allignment and wall continuity of a diaphragm wall while the provide support for the upper soils depth during
panel excavation. This temporary support is important as the slurry levels vary during construction and the upper few feet or
one meter of the wall tends to be unstable. Equally important, guide walls help guide the diaphragm wall grabs vertically
and aid in the positioning of the final structure.
Pre-excavation for diaphragm wall installation
Prior to the diaphragm wall grabs starting excavation, the slurry pump must be fully submerged in bentonite slurry. To
achieve this, a small initial excavation by the grab is carried out that is filled with slurry. Occasionally, some pre-excavation
might be required before guide walls are installed to remove certain obstructions.
Trench excavation –Excavation is done using a clamshell or grab suspended by cables to a crane. The grab can easily cut
through soft ground. In case of encountering boulders, a gravity hammer (chisel) will be used to break the rock and then
take the spoil out using the grab.
Excavation support – the sides inside the trench cut can collapse easily. Bentonite slurry is used to protect the sides of soil.
Bentonite is a specially selected fine clay, when added to water, forms an impervious cakelike slurry with very large viscosity.
The slurry will produce a great lateral pressure sufficient enough to retain the vertical soil. it exerts a pressure in excess of the
earth and hydrostatic pressures on the sides of the excavation.
Slurry cleaning and desanding for diaphragm wall construction
Prior to tremieying the concrete, and while the panel is excavated, the supporting slurry fluid must be cleaned so that it's
properties are within acceptable levels (density, sand content, viscocity, PH). Slurry is circulated at regular intervals
throughout the construction period through the regeneration plant. Otherwise, fresh slurry fluid can also be used although
this approach is not the most economical.
Joint constuction methods for diaphragm wall construction
Diaphragm wall joints need to receive special attention do detail. Various joint types are available for diaphragm walls.
Joint selection depends on the excavating equipment as much as contractor preference Joints can be flat, circular, with
steel beams, or special grooved type with water stops. Grooved type joints with water stops are typically preferred while
in the US it is also very common to use steel I beams for water stops. Flat panels and circular joints are generally avoided.
Reinforcement cage lowering and concrete tremieing
Once the bottom of the panel is reached (and cleaned), the reinforcement cage can be lowered into position. The
reinforcement cage is typically suspended from the guide wall panels, and must have enough transverse and diagonal
reinforcement to permit it to be properly lifted and lowered into place. Sufficient space must be left for at least two or
three tremie pipes so that tremieing can take place.
Concrete tremieing refers to the process of replacing the supporting slurry with the permanent concrete with the use of
vertical pipes called tremies. With the tremies, concreting of a diaphragm wall starts from the bottom and the tremies are
lifted progressively as the concrete level rises. During this process the tremies are maintained within the freshly poured
concrete for a minimum depth of 2ft or (0.6m). Overpouring might be required to make sure that all slurry is displaced
from the panel by concrete. Poor tremieing can result in slurry pockets getting entraped within the diaphragm wall
concrete. These pockets can then lead to excessive and costly groundwater leaks or even blowouts.
FORMING THE GUIDE WALL AND USING IT IN THE TRENCHING OPERATION
Clamshell use to excavate the trench to form a
diaphragm wall panel
Bentenite slurry to balance the
soil pressure to protect the
trench from collapse
Guide wall
Trench excavation working in bentonite slurry
with grab alignment control by guide wall
Tongue and groove type stopend to form the joining for two
adjacent diaphragm wall panel
Fixing and placing of reinforcing cage
de-sanding equipment
Debris (sand and gravel) being
filtered from the bontonite slurry
and deposite to the storage tank
Debris
storage tank
De-sanding equipment – the bontonite slurry carrying the soil and gravel re-circulate to this equipment with the
bontonite slurry and the debris being removed. The debris will be collected in the storage tank for disposal later
Diaphragm wall
Diaphragm wall as cut-off for
basement excavation
Diaphragm wall
TECHNICAL DRAWINGS
PLAN
GROOVE JOINT IN REINFORCEMENT
SECTION 1
INSTRUCTIONS SPECIFIC TO SITE
1) Sodium bentonite powder shall be mixed thoroughly with potable water to form a fully dispersed lump-free
homogeneous slurry.
Suitable slurry tanks shall be used for this operation. The temperature of water used and of the slurry used shall not
be less than 5°C.
2) The trench panel shall be kept filled with bentonite slurry of suitable consistency and viscosity during the
excavation period.
3) Guide walls after construction should be suitably propped to maintain specified tolerance
4) Mesh or cage reinforcement hall be used in guide walls.
5) The level of bentonite slurry in the trench shall be 1 m or more, if necessary, higher than the ground water
table.
6) Guide walls get support from adjoining panels and, therefore, their construction should be done continuously.
7) All work shall be carried out with guidelines from IS 9556 and IS-14344