Construction of Diaphragm wall

1. METHOD STATEMENT FOR CEMENT BENTONITE SLURRY WALL FOR
MANDELSHWAR PROJECT
A. Fixing of alignment for Diaphragm Wall:
Alignment of Diaphragm Wall shall be fixed on the ground with the help of total
station as per the relevant drawing. Proper alignment of Diaphragm Wall shall be
maintained by means of guide wall.
B. Construction of guide wall:
Guide wall of 1.20 m depth shall be made of reinforced cement concrete as shown
in the relevant drawing. Distance between internal faces of guide wall shall be
maintained 630mm. Dimensions of guide wall, whether in filled area or in
excavation shall be same as specified in the above-referred drawing. Filled portion
shall be compacted properly and then only excavation activity for guide wall shall
be started.
C. Construction of Diaphragm Wall:
Construction of Diaphragm Wall shall be done by alternate panel method. In this
method primary panels shall be cast first leaving suitable gaps in between.
Secondary panels & closing panel shall then be cast in this gap. The length of panel
may vary from one location to another location depending upon the cut-off level
and the termination depth. The changes shall be intimated in advance. Two stop
end tubes shall be used at the ends of primary/secondary panels to support Cement
Bentonite Slurry and to form suitable joints with the adjacent panel. The shape of
the secondary panel end shall be such as to form proper joint with primary panel.
580 mm dia. MS hollow pipes shall be used as stop end tube. The semicircular
shape shall provide the proper joint between adjacent panels. Length of panels shall
be marked on the guide wall prior to start of trenching work.
C.1. Boring / Trenching:
Boring shall be carried out by wire rope grab operated by means of suitable crane.
The crane shall control the vertical motion of grab. Initially grabbing/ boring shall
be done from guide wall top up to water table level in dry conditions. After
completing the dry boring, the bentonite slurry of minimum density 1.04 gm/cc
shall be pumped to the partly excavated trench and bore shall be filled with
bentonite slurry, and further grabbing shall be continued up to specified depth as
mentioned in the relevant drawing. Continuous supply of bentonite slurry shall be
maintained in bore during entire trenching process to have a constant head above
the ground water level. Sodium based bentonite available at Bhuj/Bhavnagar shall
be used for bentonite slurry. Necessary testing like density, PH value, viscosity shall
be carried out at site until a consistent working pattern is established, taking into

2. account the mixing process, blending of freshly mixed bentonite with previously
used slurry etc. Contaminated bentonite shall be removed from site / bentonite
tank by means of tankers / tractors and shall be disposed off at suitable location
shown by client / consultant. Usually panel width shall vary from 4.0 m to 6.0 m, so
that the entire trenching shall be completed in three cuts as per grab size. Panel
width may vary as per site conditions. During trenching operation adequate care
shall be taken to keep the bore vertical to such an extent so that it meets the
permissible limits specified in the technical specification. To maintain verticality of
trench following measures shall be adopted:
Crane shall be placed on relatively firm and leveled ground and level of track chain
of crane shall be checked by means of sprit level.
During trenching grab shall be rotated by 180 degree after every three to four
grabbing operation.
To check the verticality distance between grab rope and guide wall face shall be
monitored.
After completion of boring up to specified depth, the boring equipment shall be
shifted to another location.
C.2. Fixing of stop end tubes:
Stop end pipes shall be lowered up to the bottom of trench maintaining pre
specified distance marked on the guide wall. The center of stop end tubes shall
match with the panel markings on Guide wall. Shutter release oil shall be applied on
stop end tubes. During placing of stop end pipes, verticality shall be monitored by
means of plumb bob to make the same vertical enough to meet the requirements
specified in the technical specification.
C.3. Cleaning of excavated Trench:
After trenching is completed, for flushing tremmie pipe of 200 mm dia shall be
lowered in the trench keeping a gap of 300 mm to 500 mm from bottom of trench.
Bentonite shall be pumped through the tremmie pipe so as to clean the bottom of
trench. Bentonite level shall be maintained in trench by continuously pouring
Bentonite from the Bentonite tank. Care shall be taken to avoid Bentonite level
from going below bottom of Guide wall. Flushing shall be continued unless
Bentonite density comes 1.15 gm/cc or below 1.15 gm/cc. For checking purpose
the suitable bentonite sampler shall be used for collecting sample from the bottom
of trench.

3. C.4. Forming of Cement-Bentonite Slurry Diaphragm wall:
After Flushing of Excavated trench re-circulation to be started for replacement by
the Cement bentonite slurry with the help of suitable capacity of Vacseal/Electric
Operated Pump s and Venture Mixer using high speed agitators. The Pumping out of
Bentonite Slurry is to be done from the bottom of the trench and the Cement –
Bentonite Slurry is to flow into trench by Gravity. Pumping out of the Slurry from
the excavated Trench to Mixing tank and flow back to Trench till continue and
cement to be added as per specifications. The Circulation of Cement Bentonite
Slurry shall be continued for homogeneous mixing but mixing to be finished before
initial setting time of Cement used and bulking time of Cement Bentonite Slurry.
C.5. Bleeding of Cement-Bentonite Slurry & Top of Trench:
After one day Cement –Bentonite Slurry bleeding occurs which shall cause the top
of the slurry to settle down to be top up by additional Cement Bentonite Slurry
with required density to be pour to raised up to Cut-Off level
C.6. Removal of stop end pipe:
The end tubes shall be taken out gradually with the help of a Crane / Hydraulic
Jacks. Adequate care shall be taken during removal of stop end pipe so that no
damage is caused to the Slurry placed against them.
D. Equipment details for one unit:
Sl.no. Description Type Nos.
1 Grabbing Crane TATA-335 01
2 Bentonite Set up with Pipe Lines Set 01
3 Grab- (Height: 3.5m, Opening: 2.5M, Width: 0.580M, Wire rope 01
Wt.: 5.0MT) opt.
4 Venture Set 01
5 Stop End Pipes(580 mm MS) Set 03
6 Vacseal pump 40HP/30HP 01/01

4. E. Stage wise construction sequence
Marking of Construction of Finalization of panel
Diaphragm wall Guide walls length and marking on
alignment the Guide walls
Boring by means Cleaning of
Making of Bentonite
of Grabbing Excavated Trench
slurry
method
Placing of stop end tubes Forming of Cement- Bleeding of Cement-
Bentonite Slurry Bentonite Slurry & Top
Diaphragm wall of Trench
Removal of stop end Treatment and Protection of Top of
pipe Trench
F. Safety precaution:
Necessary safety precaution is the part of the job. All workers related with the
construction shall be provided with PPE. Periodical checking of cranes, wire shall be
done. After boring, the trench shall be covered with iron mesh till other activity
starts. In the case of collapse of excavated trench, the same shall be back filled
immediately with lean concrete.
G. Quality Assurance Plan:
As per requirement.
H.Storage of construction materials:
1. Cement: Bagged cement shall be stored in a suitable weather tight warehouse
in such a manner as to provide easy access for workers and inspector. Each
consignment shall be stacked separately with a flag showing the date of receipt
and use of cement bags shall be first come first go basis. In any case the no. of
bags in each stack shall not be more than12.Proper record shall be maintained
for procurement and consumption of cement bags.
2. Bentonite: Bentonite shall be stored in dry and covered place near by bentonite
slurry tank.