Diaphragm Wall: Construction and Design - PowerPoint Presentation

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

.Slide3
Slide4

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.