Step 1 : Guide walls construction
Construction of two parallel guide walls, made of low reinforced concrete casted on the working platform. These narrow guide walls have the following functions:
- outline and precisely define the structure to be built
- guide the, excavation tool,
- provide an extra storage of support fluid
- keep the reinforcement steel cages in place before and after concreting.
Concrete guide walls can be poured flush to existing structures and foundations. This allows to build diaphragm walls very close to nearby constructions.
Step 2 : Excavation phase
Excavation phase is made of one or several overlapping bites forming a panel, with the help of a support fluid. The properties of this fluid and the length of the excavated panel are adapted to site conditions and presence of structures in the vicinity to guarantee an optimal trench stability at all times.
While excavated, the soil is progressively replaced by this support fluid, called “drilling slurry”. Usually, a specific type of clay (bentonite) is used. Bentonite forms a local cut-off barrier on the sides of the excavation. This barrier keeps the excavation open by applying a hydrostatic pressure that counteracts with earth pressure which tends to collapse the sides of panel.
Depending on specific ground conditions, bentonite can be replaced by biodegradable polymers. Other options are possible.
While excavating, the properties of the support fluid are regularly monitored, and corrected when necessary.
Step 3 : Equipment and concreting phase
Once excavation of a panel has been completed, the support fluid is recycled to limit the size and quantity of soil particles in suspension within the slurry. Then a reinforcement cage is placed in the panel. This cage can be pre-equipped with further elements, such as reservation pipes for ground anchors.
Once the reinforcement cage has been placed, plunge columns are put in place through the cage till the bottom of the excavated panel.
Concrete is then poured from platform level. The plunge columns allow to deliver concrete progressively from the bottom of excavation to the top. During this operation, the concrete encapsulates the steel cage and pushes out the support fluid which is pumped away and recycled.
Water tightness between two consecutive panels can be ensured:
- Either by overlapping into the concrete of the adjacent panel (only possible when using an Hydrofraise® equipment
- Or by using a temporary metallic joint CWS® on both sides of the panel. When removed, this joint leaves a waterstop rubber joint inside concrete. Second half of this joint will be sealed in concrete when pouring the adjacent panel.
Step 4 : Excavation (& strutting) phase
Depending on design, a concrete capping beam can be casted to bound the top of panels together or prepare an interface with a further construction stage.
Once finished, earthworks can begin within the diaphragm wall perimeter. During these works, and depending of the finishing degree required, the concrete surface of the D-Wall can be trimmed.
The excavation phase can proceed in several phases. Depending on the support of excavation design, intermediate operations can be carried out, such as: construction of waling beams or strutting system, installation and pre-tensioning of ground anchors, construction of concrete slab and more.
The diaphragm wall thicknesses are usually: 0.50m; 0.60m; 0.80m; 1.00m; 1.20m; 1.50m; 1.80m.
Other specific thicknesses are possible, depending on countries.