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Figure 1 shows a typical sequence of construction of diaphragm walls. Two guide walls have been constructed before a clamshell or a rotary drill starts digging the trench. At the same time, the slurry is pumped into the trench from a slurry plant next to the excavation site. When the excavation is finished, the reinforcing cage and the tremie pipes are inserted until the bottom of the excavation in order to proceed pumping concrete. Figure 2 presents the clamshell digging the trench while the slurry is retaining the walls of the trench.
Fig. 1: Sequence of construction of diaphragm walls.
3. Methods of Design
Different methods of design have been developed for diaphragm walls. They depend on the geometry of the excavation and the conditions of the soil in the surrounding area. One of the most important considerations when designing is the mathematical representation of the ground response during the excavation associated with the construction of the diaphragm wall. The ground response is influenced by several factors that can be described as followed:
• Dimensions of the excavation
• Soil properties
• Groundwater control
• Time element and sequence of construction steps
• Type of ground support and its bracing
• Presence of structures and utilities
• Transient surcharge loads.
These factors have to be completely studied in order to provide enough information about the site conditions. With a good understanding of the factors shown above, one or more approaches for design can be chosen to best represent the interaction between the soil and the structure, in this case, the diaphragm wall. It is necessary to indicate that, when excavating, the larger the decrease in total stress the greater the movements in the surrounding soil, and the risk of a total loss of shear resistance increases to the point of total failure. Even when an appropriate method of design and the results from structural analysis show “reasonable” values, it is always convenient to consider monitoring the construction of the diaphragm walls and retrofitting the model with real data.
Four different approaches can be considered in the design of diaphragm walls depending upon the bracing system provided for the excavation:
• One line of bracing:
- Fixed earth support
- Free earth support
• Multi lines of bracing:
- Springs support method
- Finite elements methods
Fixed earth support method: In this analytical approach, diaphragm wall panels are considered as beams fixed at the bottom and simply supported at the bracing point and at the zero bending moment point (Blum’s equivalent beam method). Figure 3 shows a section of a diaphragm wall loaded with active and passive pressures on each side of the excavation. The embedment of the wall can be calculated with the following expression:
Fig. 3: Fixed earth support method.
Where tN : distance from the bottom of excavation to zero moment point
n : factor of safety
g : soil weight
Kph : coefficient of horizontal passive pressure
Kah : coefficient of horizontal active pressure
B0 : horizontal reaction assuming the portion of the wall from the point of zero moment to the
end of the embedment.
Maximum moments are calculated at the point of zero shear. For multilayered soils, the point of zero moment can be estimated only approximately.
Free end support method: This method considers the panels as beams as well. The beam is rigid and without a support at the bottom (free end). It follows that such a beam can rotate about the bracing level. Figure 4 shows the panel and its loading. Since the wall is rigid, passive pressure is developed in front of it and active pressure behind it. Embedment dimensions are calculated through equilibrium of active and passive pressure. Considering the moment diagram of the beam, a straight line is drawn from the point of support to its positive portion as shown in figure 5. Next, the embedment length can be found using the relation: