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The Method Statement provides the description about the construction method and the
construction sequence of the permanent diaphragm wall including the main equipment which
is anticipated to execute the works at the Metro Project, Bucharest Romania.
This technical information describes the general principles of the Diaphragm methodology.
Prepared for tender purposes only, this technical information may differ from a final method
statement for execution purposes.
2. Technical information
Diaphragm walls are wall panels which are constructed by excavating the existing soil and
replacing it with construction material.
1. Working process
These walls are constructed in two distinct construction stages. This means, that during each
stage different materials are used.
During the first stage, in which the soil material is excavated, a support slurry (first phase) is
required to stabilise the trench. Bentonite slurry is generally used as support slurry; in certain
applications a polymer slurry may be used as an alternative.
During the second construction stage, the support slurry is replaced by the permanent wall
construction material (second phase).
As a rule, e.g. for deep excavations, the slurry of the first construction stage is replaced by
structural concrete.
In a number of special applications, such as encapsulations of contaminated sites, a wall
construction material which will resist particularly aggressive contaminants is used as an
alternative to concrete. This material may be cement free.
The main difference between the grab and the cutter technique is the sequence of operations.
Whilst the cutter is inserted into the trench only once and excavation is continuous, the grab is
inserted several times and the excavation process is repetitive. This means that the grab is
inserted into the trench, soil material is excavated and lifted out of the trench by the grab and
dumped on the surface. The grab is then re-inserted into the trench and the process repeated
until the design depth has been attained.
As this process prevents extensive mixing of the excavated soil material with the support
slurry, regeneration/desanding of the support slurry is not carried out on a continuous basis,
but is only required to be carried out once immediately prior to placing the concrete.
2. Excavation Equipment
The BAUER hydraulic grab type DHG is a hydraulic diaphragm wall grab unit which is rope
suspended and operates hydraulically. The closing forces are activated by a cylinder which is
installed vertically inside of the base body.
High closing forces in combination with the possibility of multiple grabbing at the trench
bottom result in high excavation rates even within dense or very hard soil strata. Even weak
rock or rock-like material can be excavated without excesive chiseling works.
It is also very effective in layers where coarse gravel or cobbles are encountered.
- Liebherr HS
guidance to ensure
873 the
correct alignment of the cut-
- hydraulic main winch
off wall.
attached to the mast
- protection against
- modular mast head
unstability of the top soil
system
zone of the trench caused by
- hose drum
fluctuating system for
cement-bentonite
hydraulic hoses (operated
levels and agitation of the
hydraulically)
slurry during excavation.
- windingfor
support system for
the trench
inclinometer control cable
stability at the top zone,
affected by the vertical
surcharge weight of the
crane/grab unit and other
heavy jobsite traffic adjacent
to the trench.
3. Construction Sequence
Guide Walls will be constructed in prior to the cut-off wall construction to provide:
The reinforced concrete guide wall will be constructed cast-in-situ using scaffolding at the
inside . It is imperative, that prior to starting the works, to locate and relocate all existing
utility lines. It is assumed, that the utilities are deviated by others, and all ducts, pipes, sewage
lines or other channels, thr ough which a loss of drilling fluids may occur, are obstucted in
advance by others
The excavation proceeds usually by primary - secondary panels sequencing. Depending on the
actual soil conditions and the slurry mix design it may be also possible to excavate continouus
panels beside eachother.
3.3. Excavation of starter panels
The main equipment for excavation of panels is the hydraulic grab DHG mounted on the Liebherr
HS873 or similar.
The maximum length of one bite is varying between 2.8 m and 3.2m depending of geometry
of the grab being employed. Whenever the soil conditions and the geometry of the wall
allows, it is intended to excavate one large primary panel consisting of 3 consecutive single
“bites” (e.g. left bite 2.8 to 3.2 m - right bite 2.8 to 3.2 m - center bite 1.0 to 1.6 m = total
primary panel length 6.6 to 8.0m). The trench width of all panels will be 800 mm.
The level of the bentonite slurry within the excavated trench will be continously checked
to ensure stability of the open trench.
After completion of excavation of two adjacent primary panels, an intermediate secondary panel
will be excavated. The distance of adjacent primary panels are set up to leave a clearance of 1.4 to
2.5 m for the excavation of a secondary panel
After completion of excavation of two adjacent starter panels, an intermediate consecutive panel
will be excavated.
The above dimensions are to be considered as preliminary values and have to be adapted to the to
the certain project requirements and the employed equipment.
The same grab with a jaw width of 2.8 to 3.2m will be used to excavate the secondary panel,
grabing into the hardened slurry of the adjacent primaries. The bentonite slurry will completely fill
the grooves within the exposed faces of the primary panels during the excavation.
After the panel excavation, will be removed the stop end element from adjoin panels and will
be lowered the trench with water bar.
5. Reinforcement
After the panel excavation, the ready made reinforcement cage, complete with stiffeners and
spacers will be lowered into the trench in segments.
6. Concreting
Concrete will be supplied to the trench locations by concrete trucks at a rate sufficient to
achieve a pouring rate of about 40 m3/hour per tremie. The number of tremie pipes will be
determined primarily based on the length of the individual panel to be concreted. For panels
up to 4 m length one tremie, for 4 to 8 m two tremies will be used. The distance between
tremie tubes and to the edge of the trench shall not exceed 3 m, according to DIN 4126. When
conreting starts, the bottom outlet of the tremie tube shall not be higher than appr. 50cm
above the toe level of the trench.
3. Concreting will be carried out to a level appr. 0,7 m above the bottom of guide wall.
4. The remaining depth to the top of guide wall, will be backfilled with sand to avoid safety
hazards.
5. Tremie System
1. Bentonite Slurry
1.1. Mixture
The site set-up for the mixing plant consists mainly of bentonite and cement treatment and
storage facilities.
The bentonite slurry storage containments is provided in containers. For the dimensioning of the
bentonite slurry storage basins the permeability have to be taken into account. As an additional
precaution in case of very high permeability it is recommended to have a sand stock readily
available close to the excavated trench, which can be placed into the trench in case of sudden fluid
losses of critical extent.
- MAT Mixing System
- Silos for bentonite
powder
- Swelling tank
- Storage tank for fresh
bentonite
- Storage facilities for
bentonite
In case of massive local fluid losses which cannot be controlled by adding additional cement-
bentonite slurry or dry materials, it may be required to refill the trench with sand or lean
concrete and reexcavate after initial hardening.
The main components for the bentonite slurry handling are listed below:
The basins and mixing plants will be connected with a fixed pipe / pump system to allow for a
continuous operations.
Before installation of the guide wall the position shall be checked by survey and rechecked
again after construction before start of any excavation.
During excavation the excavated soil shall be recorded.
To ensure the required verticality the built in inclinometers of the hydraulic grab can be used.
The properties of the bentonite and the cement-bentonit slurry will be checked at regular
intervals at the trench to ensure consistent quality of the slurry. For checking the quality of the
slurry the following equipment will be used:
In certain intervals samples of the bentonite-cement slurry shall be taklen for laboraty
- Marsh funnel
- Fann Viscometer (or
similar)
- Standard filter press
- Mud balance
- Sand content measuring
kit
testing.
The main quality data like excavated soil and measured slurry parameters will be recorded
and submitted with a report.
The following main equipment will be provided by BAUER for the trench excavation and
for handling the cement-bentonite slurry. Small tools and equipment are not listed.
DHG Hydraulic Grab 1 Unit
Liebherr HS 873 base mashine 1 No
Bentonite mixer MAT Mixing Unit 1 No
Slurry Pump 1 No
Pipeline system for bentonite slurry supply 1 Unit