By Umer farooq GNDEC Ludhiana Diaphragm Wall Diaphragm walls are concrete or reinforced concrete walls constructed in slurrysupported open trenches below existing ground Concrete ID: 751731
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Slide1
Diaphragm Wall: Construction and Design
By
Umer
farooq
GNDEC, LudhianaSlide2
Diaphragm Wall
Diaphragm
walls are concrete or reinforced concrete
walls constructed
in slurry-supported, open trenches below
existing ground.
Concrete
is placed using the
Tremie
installation method
or by
installing pre-cast concrete panels (known as a
pre-cast diaphragm
wall). Diaphragm walls can be constructed to depths
of 150
meters and to widths
of
0.5
to 1.50
meters.
Diaphragm
wall construction methods are relatively quiet and
cause little
or no vibration. Therefore, they are especially suitable for
civil engineering
projects in densely-populated inner city
areas.
Due
to their ability to keep deformation low and provide low
water permeability
, diaphragm walls are also used to retain
excavation pits
in the direct vicinity of existing structures
.Slide3Slide4
IS 14344 :
1996
DESIGN AND CONSTRUCTI-ON
OF DIAPHRAGMS
FOR
UNDER-SEEPAGE CONTROL
- CODE OF PRACTICE
Which code to use?Slide5
TYPES OF DIAPHRAGM WALL
Depending on the use of
construction materials
there are the following types
of diaphragm
walls:
a
) Rigid type
1
) Reinforced cement concrete
.
b) Flexible type
1
) Plastic concrete,
2
) Cement
bentonite
slurry trench, and
3
) Earth backfilled slurry trench.Slide6
Depending on the function the following kinds of diaphragm walls are used:
Structural walls
Load Bearing Elements
Cutoff walls
Structural Diaphragm walls:
they are used as retaining walls for the perimeter walls of deep basements and underground parking facilities, subways, underpasses, etc
Load bearing walls:
they are used in place of drilled piers in foundation of tall buildings, bridge piers, etc
Cutoff walls:
in hydraulic structures diaphragm walls are used as impermeable cutoffs to prevent seepage below earth dams, weirs, and
leeves
Slide7
cut & cover tunnelSlide8
Cutoff WallSlide9
Undeground
Water tanks and water stationsSlide10
Retaining WallSlide11
SELECTION OF TYPE OF
DIAPHRAGM WALL
Selection of type of diaphragm depends
upon a number of factors such
as
:
Site conditions
Heterogeneity/
perviousness
of subsurface data
Geological features
Depth of overburden features
Anticipated stress and deformations due to embankment construction and reservoir loading conditions
Availability of construction materials
Techno-economic considerationsSlide12
Materials used for the construction of Diaphragm wall
Ordinary Portland Cement
Aggregate: Course aggregate of size 20mm
Sand: Well graded sand consisting of 50% coarse sand
Water: Clean water free from impurities
Admixtures: if required chemical admixtures shall be used as per IS 456:1978
Reinforcement: Mild Steel bars
Bentonite
:
bentonite
used shall conform to IS 12584:1989
Clay: Clay shall conform to IS1498:1970
Concrete Mix: For
plastic concrete diaphragm wall the
water
cement ratio shall not be greater than 0.5.
Slide13
1. The excavation is carried out using a heavy self guided mechanical grabsuspended from a large crawler crane.
2. The diaphragm walls were excavated and constructed in discrete panels of
between 2.8m and 7.0m lengths, with a depth reaching 30m.
3. As the excavation proceeds, support fluid was added into the excavation to
maintain the stability of the surrounding ground and to prevent a collapse.
This fluid is called “
Bentonite
”, which is a poser made of a special type of
soluble clay and is mixed at the mixing plant with potable water.
4. A heavy chisel may be used if an obstruction of hard strata is encountered, to break up the obstruction for removal by the grab.
5. When the excavation is completed, a submersible pump connected to
tremie
pipes will be lowered into the panel excavation down to the toe level. This pumped the fluid down to the toe level and then from the bottom of the
excavation back to a descending unit, in order to separate the
bentonite
from
the suspended particles contained in it. At the same time, fresh fluid will be
added to the top of the excavation to maintain the stability of the ground.
General Procedure of Construction Slide14
Design Considerations
Utmost consideration shall be given,
while designing
the diaphragm wall, so as to achieve
:
perfect
embedment at both the ends,
to avoid/minimize
possibilities
of cracking both
within and surrounding the
diaphragm wall
,
and
imperviousness or water tightness
.
The most
important consideration in the design of
a diaphragm
wall is to form an impervious
wall having
flexibility to avoid cracking
.
The
best location for a diaphragm wall
within a
structure is where the loads
are reasonably balanced
on both sides of the wall
.
Location
of
diaphragm wall
is often
influenced by
site
conditions. Location
shall,
therefore, be
decided after careful study of
site requirements
and localised features.Slide15
GUIDELINES FOR STRUCTURAL DESIGN
Structural Analysis:
Rigid type of diaphragm wall is to be
analyzed either
by the method of beam on elastic
foundation or
by
finite element
method (FEM
).
Method of
beam
on elastic
foundation:
The diaphragm wall shall be considered in
plane strain
state and as such, unit length of
diaphragm wall
with entire depth as the span is to be
considered as
a beam resting on elastic soil media
on the downstream
face. Loads are to be
considered acting
upon the upstream face of the
diaphragm wall
. Analysis is to be carried out using
appropriate equations
for bending moments and shear
stresses for
a beam resting on elastic foundation with
assumed end
conditions.Slide16
Finite
element
method (FEM
):
Finite element analysis takes into account
soil structure
interaction. Finite element analysis
shall be
carried out
as sequential
construction analysis. Sequential
construction analysis
is, however, preferred as it
takes into
account the elastic modulus of soil
changing with
different stress
Levels
during construction.
The finite element
analysis shall
incorporate interface elements
along the contact boundary
of the diaphragm
wall and surrounding
soil mass. Elimination of interface elements
results in
faulty stress and
displacement computations, due to
stress transfer
through common nodes.Slide17
Flexible types of Diaphragm wall
Structural
Analysis:
Plastic concrete diaphragm wall or cement
bentonite
slurry trench diaphragm wall or earth backfilled slurry trench diaphragm wall are relatively flexible and capable to deform under stresses in surrounding soil
. Hence development is not a design problem. These diaphragm wall shall, therefore be designed to undergo deformations compatible with those in the surrounding soil without development of cracks.Slide18
Requirements of Slurry
Bentonite
slurry
is made
by passing dry powder through water jet. A conical hopper is used with bottom nozzle through which water is pumped under pressure. The
bentonite
powder is poured directly from top and when it falls down the hopper, it gets agitated in the water. After getting circulated, the mixed
bentonite
thus falls in the tank.