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Introduction One of the alternatives for the construction of a 5 km tunnel in saturated sandy soil in Valparaiso, Chile was the execution of 2 diaphragm walls connected to floor and ceiling slabs, forming an “impermeable box” in which an electrical train would circulate transporting people and different manufactured products from the interior valley to the coast. 27955
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This project is the motivation to investigate behavior, method of construction and design of diaphragm walls.
Diaphragm walls are structural members constructed by excavating trenches from the surface. They are designed to work as retaining structures for deep excavations and can be installed close to other structures without compromising their alignment or stability. Other applications are to serve as water barriers when an impermeable structure is required and to provide bearing support for structures built on top of them.
This paper presents a brief description of construction methods for diaphragm walls and traditional sequences of construction in the field. As a second topic, four methods of design are described in order to show how analytical and numerical calculations can be applied when designing diaphragm walls. Finally, a case study is used to demonstrate how earth pressures acting as loading patterns on diaphragm walls cannot be represented by a single constant. Additionally, this case study helps to show how the associated deformations in the soil induce deformations on the walls.
2. Method of Construction
A diaphragm wall is a succession of panels joined together to form a continuous retaining and/or bearing structure. It is formed and cast in a slurry trench. According to P. Xantanos, the sequence of construction can be described in the following steps :
1. Excavate a linear trench using one or more equipment passes. As the trench is excavated, suitable bentonite slurry is inserted into the panel to provide stability.
2. On completion of the excavation, insert a round tube to form the panel joint with the adjacent wall unit. The connection on each panel can be done by inserting the round tube or by installing a special steel section. In either case, a water stop joint can be added to avoid water-licking trough the connection.
3. Insert a reinforcement cage (fabricated and assembled on the ground)
4. Pour fresh concrete into the panel using tremie pipes. As the concrete rises, it displaces the slurry by its own gravity with the intent to fill the trench completely. The slurry is pumped into a storage area for reconditioning and reuse, and the tube steel pipe is gradually withdrawn.
The length of a diaphragm wall panel can vary from 1.2 m. to long panels 7.5 m. but as a standard length 3.5 to 4.5 m. is recommended. The depth of the panels is limited by the purposes of the project, according to whether it is needed to provide vertical resistance, lateral stability or seepage control. The reinforcing cage must be assembled at the site and its weight must be considered when choosing the right equipment to lift it and insert it into the trench. The width of the panels depends on the equipment to be used. The most common pieces of equipment are clamshells and rotary drills, so the typical widths range from 40 cm. to 1.2 m. As a standard, 60 cm. width is recommended.
The sequence and arrangement of the panels is an important issue. There are several options for panel sequence but a common one is to excavate a panel while a second is being prepared and a third is being concreted. The sizes and end conditions of the panels can be combined according to the geometry of the project.
An important step that has to be developed before the panel’s construction is to provide two guiding walls on each side of the future trench excavation to help the equipment to maintain the horizontal and vertical alignment.