1.2 Diaphragm walls.pptx

Diaghpragm wall types

1. Diaphragm walls

2. 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 . Diaphragm walls are generally reinforced concrete walls, constructed in the ground using an under-slurry technique/ slurry trench method developed in Europe.

3. This technique involves excavating a narrow trench that is kept full of an engineered fluid of slurry/ Bentonite slurry . ( Bentonite clay (in powder form) and water are combined in a colloidal mixer and clay particles bond to each other and set to form a gel when left to stand for a period of time . ) This way, walls with thickness ranging between 300 mm to 1200 mm can be formed as deep as 45 meters underneath . Inclined rock or soil anchors are used for this purpose. Diaphragm walls are commonly used while constructing structures with deep basements in congested areas, as they can be installed in close proximity to the existing structures.

4. Features of Diaphragm Wall A diaphragm wall forms a rectangular section constructed in-situ under the soil. Hence, this is an underground concrete wall. These walls are constructed panel-by-panel each interlocked to ensure structural stability and water tightness. The diaphragm walls can have a thickness ranging from 60cm to 150cm with a width of 2.0 to 3.5m. The diaphragm walls can be constructed up to a depth of 60m . Walls generally range from 600 to 1500 mm thickness, in wide between 2000 and 3500 mm and can be excavated to depths of 60m or more. The two primary types of diaphragm are flexible and rigid. Flexible diaphragms resist lateral forces depending on the tributary area, irrespective of the flexibility of the members that they are transferring force to . On the other hand, rigid diaphragms transfer load to frames or shear walls depending on their flexibility and their location in the structure.

5. 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.

6. 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.

7. The Process Stage 1: Fixing of Alignment Stage 2: Guide wall Construction Stage 3: Trenching Stage 4: Trench Cleaning Stage 5: Stop ends fixing Stage 6: Reinforcement Cage lowering Stage 7: Placing of Concrete Stage 8: Withdrawal of Stop ends Stage 9: Excavation and Anchoring Stage 10: Stressing and Final Level of Excavation

8. Types of joint design used for diaphragm walls: Stop end pipes: The steel pipes are installed at both panel fronts before concreting. The pipes rest on the excavation bottom and match the circular shape of the panel fronts. The concrete injected takes the shape of the stop end pipes, the latter are filled with bentonite slurry and the panel has concave fronts. Once primary panels have been constructed and the stop end pipes removed, secondary panels are executed. Once secondary panels have been fully excavated, concreting can start. Concrete fills the semicircular joints and provides a very good interlock.

9. Disposable pipes Pipes are in PVC; they are fixed to the cage and lowered to rest on the bottom. During concreting, the whole section is filled by concrete, except for the pipes which remain full of bentonite slurry. The result is a panel with two holes. After excavating the secondary panels, by using a dedicated tool the concrete seam separating the pipe from the secondary panel and the pipes are demolished. As a result, a round indentation is created in the primary panel. When concrete is cast in the secondary panel, it fills the recess and creates a joint with the primary panel.

10. Diaphragm wall is a continuous wall constructed in ground in to facilitate certain construction activities, such as: As a retaining wall As a cut-off provision to support deep excavation As the final wall for basement or other underground structure (e.g. tunnel and shaft) As a separating structure between major underground facilities As a form of foundation (barrette pile – rectangular pile)

11. Diaphragm wall is a reinforced concrete structure constructed in-situ panel by panel. The wall is usually designed to reach very great depth, sometimes up to 50m, mechanical excavating method is thus employed. Typical sequence of work includes: Construct the guide wall Excavation to form the diaphragm wall trench Support the trench cutting using bentonite slurry Inert reinforcement and placing of concrete to form the wall panel

12. 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.

13. Major advantages of diaphragm wall Can be used as permanent structural wall Water retainable Can be installed to deeper depths and for load bearing element Less temporary propping needed Can be applied for top-down construction method Rigid structure so that ground movement induced by basement excavation is less than other flexible retaining wall Vibration and noise generated from installation of diaphragm wall is less than other methods

14. Advantage of Diaphragm wall Can be constructed in a very wide range of soil types and rocks. Diaphragm walls can be constructed to depths of 60 meters (and above) and to widths of 600 to 1500 mm. Diaphragm walls tend to be used for retaining very deep excavations as they can be designed to take very high structural loads. The water tight walls formed can be used as permanent structural walls. Reduced number of joints in the wall which ultimately improves the walls water tightness. Work may be carried out right against existing structures and the line of wall may be adjusted to any shape in plan. Diaphragm wall construction is relatively quiet, and minimum noise and vibration levels make it suitable for construction in urban areas. Can be used as top-down method in order to optimize the sequence of operations to be performed in densely-populated inner city areas.

15. 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.

16. 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.