Diaphragm wall

A diaphragm wall (slurry wall) is a reinforced cast-in-place continuous concrete wall. The construction includes the excavation of a trench under the protection of a support suspension (i.e. a thixotropic slurry such as bentonite mud for example) designed to temporarily stabilize the trench wall.  After excavation with a mechanical or hydraulic grab, the support slurry is replaced by concrete and steel reinforcement is placed.

Construction sequence

Installation process in accordance with the requirements of NBN EN1538:1999

1. In advance construction of temporary guide walls and excavation of (the first) lamella with grab – continuous filling of the trench with the thixotropic stabilizing fluid.

Excavation of the first rectangular panel, followed by the excavation of the second one (alternating method).

In order to avoid instability of the trench, the level of the bentonite slurry in the excavation trench should at any moment be maintained at least 2 m above the highest water level in the full height trench, especially in the case of an artesian aquifer.

2. Panel excavation in progress: excavation of the central panel or “merlon” located between the two previously excavated panels.

3. Desanding or replacement of the bentonite suspension.

After reaching the required installation depth the lamella (= combination of several single panels) is equipped: insertion of reinforcing cage and removable flat steel joint elements with PVC joint tapes, called “water stop”, at both lamella fronts  (see detail of plasticized PVC water stop or water bar).

4. Concreting of the lamella by the tremie pipe method (one or more tremie pipes, depending on the size of the grab) and at the same time, recycling of the bentonite slurry

5. Removal of the temporary formwork or joint elements after concreting – the water stop membranes remain.

Construction chart

Fields of application

• The diaphragm wall can have a water retaining function (impervious wall) and / or a soil retaining function. Eventually also a foundation function for heavy vertical loads (structural wall).

• Diaphragm wall panels are constructed as continuous walls or are designable as individual structural elements to carry vertical loads (barrettes).

• Designed to withstand very high vertical loads

• Support of deep excavation pits in the direct vicinity of existing structures

• Projects for tunnels and car parks in urban areas, quay walls

Environmental impact

• Vibration-free installation

• Low noise pollution: engine noise only

• Removal of soil and disposal of bentonite

• The groundwater level must be located at least 2 meters below the working level.

• Wide working platform for the bentonite mixing process and large enough to create a material stockpile area: a minimum of 300 m² is necessary.

• Workspace surface: at least 17 m wide (minimal 15 m from the axis of the wall and 2 m in front of the wall)

• Removal of debris, demolition of old foundations and backfill with stabilized sand

• A stable, dry and flat work platform is required

Technical specifications

• Thickness of the wall from 0.5 m up to 1.5 m (0.50 m, 0.60 m, 0.80 m, 1 m, 1.20 m and 1.50 m)

• Standard depth of the wall up to 35 m, exceptionally up to 55 m and more

• The height of the retaining wall can be in excess of 20 .

• The installation of guide walls to stabilize the soil in the upper diaphragm level and guide the grab is necessary.

• The width of the trench is minimum one time up to 2.5 times the clam shell size width.

• Normally, a lamella is excavated in 3 successive sections: panel 1, panel 2 and the central panel or ‘merlon’, where the first two correspond with the clam shell width of the grab: 2.85 up to 3.30 m. The last section (central part or merlon) has a width of at least the thickness of the wall.

• The different trenches are connected together by means of joints so as to form a continuous enclosure inside the excavation (one single unit).

• Upon completion of the wall, provision of a uniform distribution beam is to be made (connection with the raft / slab and the structure)

Related projects for this technique

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