DIAPHRAGM WALL CONSTRUCTION METHOD

This construction method has been prepared in order to determine the methods for selecting, preparing, applying and controlling methods needed for impervious reinforced concrete curtain wall construction so that deep excavations could be carried out through the soil which has high water levels and cannot hold itself together.
The “DIN EN 1538 Execution of Special Geotechnical Work-Diaphragm Walls” standard will be complied with during the application.

PREPARING THE WORK SITE

The construction site and roads should be kept smooth so that the machines and personnel could work efficiently and the targeted daily construction amount and quality could be achieved.
It will be smoothed and hardened so that the drilling machine, crawler crane, concrete mixer, concrete pump and various other heavy duty machines can work without sinking more than 10 cm. Fillings will be made with suitable materials which will not make the drilling work more difficult.
Dryness of the platform will be ensured by setting up a suitable surface draining system in the working site.
Destruction of the working site will be prevented by continuously removing drilling materials and underground water.

SOIL INFORMATION

Application projects and construction methods will be determined according to soil reports.

ENVIRONMENTAL INFORMATION

Concrete and steel obstacles and infrastructural canals making underground and aboveground drilling more difficult will be removed by getting government agencies make official examinations.

PLACEMENT OF WALL PANELS AND CONSTRUCTION TOLERANCES

Construction site supervisor will mark locations of wall panels one by one.
Deviations in the plane, vertical and inclination of wall panels may not exceed tolerances.
Construction sequence of wall panels will be such that it will move the previously constructed anchorages at minimum in horizontal and vertical direction.

DRILLING WORKS

Diaphragm wall constructions are completed by casting them in panels.

Guide Wall Construction

This wall, which will be built for ensuring that the excavation could be made in horizontal and vertical directions without deviations and for guiding the excavation equipment, will be constructed along the route determined in accordance with the approved project line as a protection against possible wreckages and collapses possible to occur due to bentonite level changes and excavator bucket movements.
Guide wall fronts have to be vertical and smooth all the times. Construction of the diaphragm wall will be performed within the guide wall. The guide wall provides the following benefits:

Perfect smoothness of the diaphragm wall application and construction alignment,
Sensitive check of reference points and elevations,
Initial reservoir for bentonite,
Supportive function for stop-end pipes and reinforcements,
Supportive function against collapsing along the working platform.

Excavation of the guide walls will be performed by an available back-hoe, its alignment will be perfectly fitting with the diaphragm wall, it will be made at least 5 cm wider than the curtain width to enable comfortable work in the diaphragm equipment and will be at least 1 m deep.

Panel Excavation

The excavation may be carried out by constructing 2.5, 4.8 or 6 m panels depending on soil stability conditions. The diaphragm wall excavation will start after necessary markings are made and bentonite slurry is filled between the guide walls. The bentonite slurry will ensure that the well remains vertical and is not wrecked. Speed and amount of the bentonite filling in the well will continue in perfect accordance with the excavation. It will not be dropped below the guide wall elevation. Whether or not the equipment preserves its verticalness will be checked during the excavation. When partially hard soil is encountered, chisels will be used to overcome the obstacle. Hydromill/cutter, which is a different machine, is used to construct a diaphragm wall in a persistent rock environment. Well excavation is finished with the drilling which goes down to the well bottom and the well full of bentonite is prepared for the reinforcement installation by taking the necessary safety precautions.

PREPARING THE REINFORCING CAGE AND LOWERING IT TO THE WELLS

The cage which has been prepared in site in advance according to the excavated diaphragm wall, to the project and to the current bearing capacities will be lowered into the well with the help of the crawler service crane in the system and suspended by suspension hooks. Sufficient working spaces will be left over the cage in order not to obstruct the free motion of the tremie pipe to be used during the pouring of concrete. The below points will be taken into consideration about reinforcing cage:

Manufacturing and tensile strength experiment certificate will be obtained from the manufacturing company of the reinforcing bars.
The bars will be transformed into a cage along with the necessary in-cage supports by using necessary templates near their panel locations so that no falling apart is caused during the lifting.
Steel fixing area and drilling field should be kept clean so that the reinforcing cage does not get dirty. The assembled reinforcement is preferably kept as a stock on wood bricks.
Binding wire will be used for adding cages exceeding 12 m, and the connection will be made by welding or, if required, by a fitting like a clemens if the cage is too heavy or long.
The reinforcing cage prepared will be equipped with concrete spacers and the cage will be slowly lifted, carried and descended into wells using the service crane so that it does not fall apart.

The reinforcement descended into the wells will not be allowed to escape in any way.

INSTALLING THE STOP-END PIPES

Stop-end pipes will be added down to the bottom in order to ensure the rigidity between panels and increase the water-tightness level in the diaphragm wall curtain. After the concreting, the concrete, as it passes to the wall socket, will be pulled upwards by special-vibration pumps (extractors). Stop-end pipes need to be pulled within at least 4 to 6 hours after the concreting.

DECREASING / CLEANING THE BENTONITE DENSITY

The reinforcement is placed into the well after the diaphragm wall drilling is carried out up to the panel length and depth. Meanwhile, in order to purify the condensed bentonite on the floor, it is drawn out by a “floor pump” or “double development set + airlift” while clean bentonite is added into the well from above and the continuity of stability is followed. This renewal operation is maintained until the density of the bentonite in the well complies with the test values.

CONCRETING THE PANELS

After positive bentonite test results are obtained, the diaphragm wall concrete casting will be performed using tremie pipes and the below points will be taken into consideration:

The concrete will be supplied from those companies which can provide the necessary experiment reports and the necessary service in the demanded time, amount and continuity.
As soon as the drilling operation is completed, the reinforcement will be placed and the concrete casting will be started in the same day as soon as possible.

If immediate concreting is not possible and if a heave occurs in the panel bottom and thus the bottom pushes the reinforcing cage upwards, the reinforcement is removed, inside of the well is swept again and a cleaning is performed within the panel and then the concrete casting is carried out. The bottom may be drilled as much as the heave depth in the case of a heaving risk.

After the reinforcing cage is descended into the wells, the tremie pipe with a of 25 to 35 cm diameter, depending on the panel width (sometimes 2 lines of tremie pipe is descended based on the panel width) will be descended into the well by the service crane, and concrete disintegration will be prevented by performing the concreting with the help of pipes.

The concreting pipe (tremie) will meet the below conditions.

Pipe walls will be clean and resistant against bending and twisting.
Joints will be water-tight and thick-threaded sleeves will be used.
The tremie pipe will be pulled 30 to 40 cm upwards in the well bottom before the concreting starts.

When working with the concreting pipe, the concrete in the well-head should be high slumped (with a minimum value of 16 to 20 cm in concretes containing admixtures) and the time for starting the settlement should be at least 3 hours.
Concreting will be carried out continuously by a concrete pump or direct pouring. Pouring speed should be at least 15m3/hour. As the concrete which is poured with less speed would cause accumulation in the pipe, care should be taken so that the speed is kept steady. In the case that double tremies are used, rising of the concrete in the well in the same levels should be ensured.
In order to prevent the underground water or bentonite slurry is mixed with the concrete, the concreting will start after the tremie pipe reaches the bottom of the well and after it is drawn upwards 30 to 40 centimeters first and it will remain in at least 2 m of concrete.
As the concrete rises in the well, the tremie pipe will be shortened progressively and the concreting will be maintained until clean concrete comes from the mouth of the well.
One tremie pipe will maximally be used at panels width of 2.5 m. The concrete will be poured from each tremie pipe equally and no interruption will be made.
After the concreting is finished, the concrete, while it is fresh, will be leveled with the upper wall elevation.

CONCRETE CUBE SAMPLES FOR PANELS

Sampling and experimenting will be carried out in accordance with standards if nothing otherwise is stated in the contract or technical specifications.

MATERIALS

Cement: Portland Cement with a 28 days compressive strength of 325 Kg/cm2 will be used in the construction.
Water: Water to be used in the injection mix should be purified of residues and foreign solid matters and it should not damage the cement.
Bentonite: Bentonite density, which will ensure the panel stability, is going to be determined depending on the bottom conditions and underground / artesian water.

MACHINES AND EQUIPMENT

When the diaphragm wall equipment is brought to the site, it will be checked before the construction and after the machinery is mounted and facility is setup whether the drilling machine and the bentonite facility are operational.

Drilling Machine: A hydraulic drill rig which can perform drills of 600 to 1500 mm width and descend to the depth shown in its project. This excavator will be mounted on the crawler crane.
Crawler Crane: A crawler service crane will be used in concrete casting and material carriage.
Stop-End Pipes and Extractor: Stop-end pipes which can descend down to the depth with a diameter of 600 to 1500 mm as stipulated in the project.
Bentonite Facility: A bentonite facility will be established in a suitable part of the site. There will be tanks and fittings to keep bentonite sufficient for the excavation, a mixer to make bentonite slurry and a de-sander for sieving the bentonite recovered from the excavation.

TESTS AND CHECKS

Before concreting each diaphragm wall panel, the bentonite in the well will be checked (viscosity (max 45 sec), density (max 1.30tons/m3) and sand (max 5%) rate).
When the construction is completed, horizontal displacements of the diaphragm wall panels will be measured during the excavation. (max 0.5% of the depth)
Four samples from each 100 m3 or from daily concrete cast and breaking results will be reported.

MEASUREMENTS AND REPORTS

A record, in accordance with the project standards, will be kept for each diaphragm wall panel in the diaphragm wall construction. This record will contain:

Excavation date and times,
Concreting date, concrete start and end time, amount (m3) of concrete cast,
Excavation depth,
Tested bentonite values,
Panel number,
Panel top elevation,
Panel width,
Panel bottom elevation, and
Reinforcement weight.