“Diaphragm walls” are underground structural members made of reinforced concrete, concrete or generally cement-based materials. They have structural and/or water retention and/or protective functions and are suitable for both temporary and permanent applications.
Application Areas Of Diaphragm Walls
- Walls are often subjected to horizontal and/or vertical static loads, for example as ground support in open excavations and shafts. It is accepted that the deformation of these walls is low.
- They can be used as foundation elements exposed to vertical and/or horizontal static loads. E.g; bridge deep foundations or for transmitting large pulling forces.
- They are used as waterproof walls in open excavations.
- They are used as impermeability curtain. E.g; such as underground waterproofing in dam construction projects, core waterproofing for earth-fill dams or pollution control, and containment wall in industrial facilities.
Diaphragm Walls Advantages And Disadvantages
The advantages of diaphragm walls are as follows:
- It can be built on many types of ground and rocks.
- It can be built to a depth of 60 meters (and above) and a width of 600 to 1500 mm.
- They are used to support deep excavations as they can be designed to take very large structural loads.
- The resulting waterproof walls can be used as permanent structural walls.
- The reduced number of joints in the wall increases the watertightness of the walls.
- Its manufacture is extremely quiet, with minimal noise and vibration levels making it suitable for construction in urban areas.
- The disadvantages of diaphragm wallsare;
- It requires large equipment, long construction time and high cost.
Diaphragm Wall Construction Steps
In diaphragm wall fabrication , a rectangular tool is often used to lift the ground, thus creating a rectangular excavation. The panels have the dimensions that the excavator will dig. The excavation pit is stabilized with bentonite slurry and the concrete is poured using the tremie technique, as with cast-in-situ piles.
Guide Wall Construction
Before starting the diaphragm wall excavation, two temporary parallel concrete beams are constructed to guide the excavator and stabilize the top. Guide walls are cast-in-situ or pre-constructed reinforced concrete elements. The diaphragm provides horizontal alignment of the wall and wall continuity while these walls are used to prevent superficial floor collapse, position the panels, and support the steel lattices. This temporary support is important as slurry levels change during construction and the wall tends to be unstable.
The dimensions and shape of the guide walls may vary depending on the nature of the floor surface. As with bored piles, the bentonite slurry in the excavation pit should always be kept a few meters above the water table height. In some special cases where the water table is very close to the surface and the ground has poor mechanical properties, guide walls can be built at a higher height compared to the ground plane to maintain the above-mentioned height difference.
A single panel is dug down using the grab until the desired level is reached. With the use of bentonite slurry support, the collapse of the trench is prevented during excavation, reinforcement and pouring. The slurry forms a thick layer on the walls of the trench, which balances the inward hydraulic forces and prevents water flow into the trench.
“Grab” is a diaphragm wall digging tool consisting of two jaws that can be opened and closed to collect excavations and empty the excavated trench.
The outside of the jaw is equipped with teeth for cutting the ground. The excavation work cycle begins with the grab sitting on the ground, the teeth penetrating the ground due to the weight and speed of the tool. By activating the closing system, the grab digs the ground and retains some of it in the jaws. Usually, one jaw has an odd number of teeth and the other an even number of teeth to ensure perfect closure.
The asymmetrical configuration of the jaws tends to deviate from flatness and therefore the grab is not always used in the same direction but is used alternately in both directions, turning 180° each time the grab is lowered into the excavation. The grab is lifted with the jaws closed and the ground is emptied into a designated area. This work cycle is repeated several times until the excavation is complete. Excavation of the primary panels is started first and the secondary
panels are built between the primary diaphragm wall panels. After panel excavation is complete, before concrete is poured, the slurry liquid must be cleaned and regenerated so that its density, sand content, viscosity and pH are at specified levels.
The junction points of the panels, one of the most sensitive issues in diaphragm wall construction, are represented by waterproof connections. There are several types of couplings available, and their choice is dependent on excavation equipment as well as contractor preference. One of the main functions of the diaphragm wall is to support the ground up to the final excavation, as well as to prevent settlement of buildings and superstructures near the excavation and also to waterproof the wall. Once the bottom of the panel has been reached and cleaned, the reinforcement cage can be lowered into place. Reinforcement cages have significant size and weight.
As a general rule, the cage is built by laying its components on the ground and then connecting them together. The concrete is placed in a vertical steel tube with an open, funnel-shaped end called the tremi. The concreting of the diaphragm wall with the tremiler starts from the bottom and the tremises are gradually removed as the concrete level increases. Simultaneously as the concrete is placed, slurry is pumped through the panel for replenishment and reused in the next panel excavation.