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Diaphragm walls
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- 2. INTRODUCTION Diaphragm wallsare underground structural elements commonly used for: i. Retention Systems ii. Permanent Foundation Walls It is an in-situ reinforced concrete structure that is constructed panel by panel. The wall is usually designed to reach very great depth, sometimes up to 50m.
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- 4. Diaphragm wallsof 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.
- 5. TYPICAL SEQUENCE OFWORK INCLUDES a) Construct the guide wall. b) Excavation to form the diaphragm wall trench. c) Support the trench cutting using bentonite slurry. d) Inert reinforcement and placing of concrete to form the wall panel.
- 6. FURTHER EXPLANATION ONTHE WORK SEQUENCES Guide wall – Guide wall is two parallel concrete beams constructed along the side of the wall Guide walls maintain the horizontal alignment and wall continuity of a diaphragm wall while they provide support for the upper soils depth during panel excavation.
- 8. Trench excavation– In normal soil condition excavation is done using a clamshell or grab suspended by cables to a crane. In case of encountering boulders, a gravity hammer (chisel) will be used to break the rock and then take the spoil out using the grab. 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.
- 11. Excavation support– The sides inside the trench cut can collapse easily. Bentonite slurry is used to protect the sides of soil. Reinforcement – Reinforcement is inserted in the form of a steel cage, but may be required to lap a few sections in order to reach the required length.
- 13. Concreting – Placing of concrete is done using tremie pipes to avoid the segregation of concrete. As concrete being poured down, bentonite will be displaced due to its lower density than concrete. Bentonite is then collected and reused
- 15. JOINING FOR THEDIAPHRAGM WALL PANEL Diaphragm wall cannot be constructed continually for a very long. The wall is usually constructed in alternative section. Two stop end tubes will be placed at the ends of the excavated trench before concreting. The tubes are withdrawn at the same time of concreting so that a semi-circular end section is formed.
- 16. Wall sectionsare formed alternatively leaving an intermediate section in between. The in-between sections are built similarly afterward but without the end tube. At the end a continual diaphragm wall is constructed with the panel sections tightly joined by the semi-circular groove.
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- 18. APPLICATIONS As permanentand 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.
- 22. BENEFITS OF DIAPHRAGMWALLS 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.
- 23. DISADVANTAGES They arerelatively costly. They are also unsuited to strong soils conditions where penetration is slow and difficult due to the use of the slurry trench method.
- 24. REFERENCES Fuchsberger, M.“ Some Practical Aspects of Diaphragm Wall Construction”. Thomas D. Richards, Jr. P.E. Nicholson Construction Company, Cuddy, Pennsylvania. “Diaphragm Walls”.
Editor's Notes
- #8 Guide walls are constructed in-situ typically as reinforced concrete elements. This temporary support is important as the slurry levels vary during construction and the upper few feet or one meter of the wall tends to be unstable. Equally important, guide walls help guide the diaphragm wall grabs vertically and aid in the positioning of the final structure.