DIAPHRAGM WALLS

DIAPHRAGM WALLS Diaphragm walls are underground structural elements commonly used for retention systems and permanent foundation walls. Diaphragm walls provide a water tight barrier and are constructed with a minimum back slope subsidence. They are formed from reinforced concrete and are constructed as normal cast-in-place walls with support, which become part of the main structure. They can also be used as deep groundwater barriers.

SLURRY TRENCH METHOD The slurry trench method involves the excavation of alternating panels along the proposed wall, using bentonite slurry to prevent the sides of the excavation from collapsing. The slurry trench technique was developed in Europe and has been used in the United States since the 1940's. The technique involves excavating a narrow trench that is kept full of an engineered fluid or slurry. The slurry exerts hydraulic pressure against the trench walls and acts as shoring to prevent collapse. Slurry trench excavations can be constructed in all types of soil, even below the ground water table.

BASEMENT TOP-DOWN CONSTRUCTION USING DIAPHRAGM WALLS

PROCEDURE The panel dimensions 50 to 100 cm thick and up to 7m height, extending to the excavation bottom. The installation starts with the construction of shallow concrete or steel guide walls. The excavation is then made using special equipment, such as the thin-grab clamshell. Bentonite slurry is then pumped into the trench to provide temporary support and a prefabricated reinforcing cage is lowered in. The bentonite slurry is then replaced by concrete and the sequence proceeds onto the next panel.

GRAB USED FOR EXCAVATION

DIAPHRAGM WALLS Diaphragm walls of shallow depths are often left unsupported since they are classed as semi rigid structures. However for deeper excavations support is required to restrict lateral deflections. Diaphragm walls are ideal for soft clays and loose sands below the water table where there is a need to control lateral movements.

REINFORCEMENT

DIAPHRAGM WALL REINFORCEMENT & CONCRETING

FINISHED WALL AFTER EXCAVATION

APPLICATIONS As permanent and temporary foundation walls for deep basements. In earth retention schemes for highway and tunnel projects. As permanent walls for deep shafts for tunnel access. As permanent cut-off walls through the core of earth dams. In congested areas for retention systems and permanent foundation walls. Deep ground water barriers through and under dams.

BENEFITS OF DIAPHRAGM WALLS Can be installed through virtually all soil conditions, to any plan geometry and to considerable depths. Can be constructed ahead of time and independent of other site activities. Can be constructed in relatively low headroom and in areas of restricted access walls can be quickly formed several hundred feet deep and through rock, with good control over geometry and continuity.

DISADVANTAGES They are relatively costly. They are also unsuited to strong soils conditions where penetration is slow and difficult due to the use of the slurry trench method.