Whilst many building projects are relatively straightforward in terms of construction techniques, others require a more specialised approach, in order to create a structure that is safe and strong enough to withstand the pressures that it will be subjected to once complete. One example is where extremely deep foundations are required for a construction project, or underground retaining walls are needed. In this scenario, diaphragm walls are typically used, creating incredibly strong underground structural elements that can retain enormous loads, provide permanent foundation walls and even act as groundwater barriers. In this article, we take an in-depth look at exactly what a concrete diaphragm wall is, what kinds of projects require this type of construction, and what is involved in diaphragm wall construction.
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In simple terms, a concrete diaphragm wall (sometimes also known as a slurry wall) is an underground structure composed of multiple panels, and it typically acts as a foundation wall or a retaining wall. Diaphragm walls are generally very deep structural elements – they can range from 10 metres to 50 metres in depth, but significantly deeper diaphragm walls are not uncommon. Diaphragm walls require less soil to be excavated than other foundation types, making them a good option for many deep construction projects.
Each diaphragm wall is made up of a series of panels of the same width and depth. Apart from the end panels, each panel will have a profiled joint on one end, to allow the next panel to be fixed permanently to it. Compared with a secant wall, a concrete diaphragm wall will have fewer joints, reducing the chance of water ingress. Panels can vary in length from around 3 metres to 7 metres, and the width of the panels can vary from 600mm to 1800mm, depending on the requirements of the build.
Excavation of the trench for a diaphragm wall requires specialist hydraulic or mechanical grabs, or piling rigs. Because the trenches are so deep, pressure must be applied to the trench walls to prevent them caving in or collapsing. This pressure is applied using a support fluid, which is typically a bentonite slurry mix. This specific mixture is used because of the pressure it exerts on the walls of the trench, which exceeds the hydrostatic and earth pressures on the trench walls.
After the excavation for one panel is complete, a stopend is installed, which contains a profiled joint form to connect to the next panel, as well as a water bar. Once the support fluid is cleaned up, the reinforcement cage is positioned in the trench and the concrete is poured. With such deep trenches, pouring the concrete correctly is vital and special pipes are used to reach down to the trench floor. As the concrete is poured, the length of the pipe is slowly reduced, always ensuring that the pipe outlet remains below the surface of the poured concrete.
When the concrete is fully poured and the panel is complete and dry, the stopend is removed, exposing the concrete joint that will connect to the next panel. The process is repeated along the length of the wall, completing one panel at a time.
At the end of the run, closing panels are poured. These do not require stopends with profiled joints, since they are contained within short end panels that have already been constructed.
As with all concrete construction and reinforcement of concrete structures, there are specific standards which must be complied with. In the UK, these include BS EN 1538:2010+A1:2015 (Execution of special geotechnical works. Diaphragm walls). This comprehensive standards document covers everything from geotechnical investigation to the materials and products used in the diaphragm wall construction process, including bentonite, aggregates, water, concrete and more. It also covers design considerations, trench stability, reinforcement cages, and joint formations. The standards should be a key part of the design process and should be referred to throughout what is undoubtedly one of the more complex construction processes in use in modern building.
There’s a very useful video published by Malcolm Drilling on Youtube which explains the process well.
In commercial and infrastructure projects, a diaphragm retaining wall is often used in the construction of underground train or subway stations, underground car parks, underpasses, railway tunnels and many other types of underground structure. They can also be used in water treatment stations and as ventilation and access shafts in deep construction projects. In smaller projects, diaphragm walls are typically used to construct basement walls.
Since diaphragm walls are expensive to construct, they are only used where they are really necessary, or where they can be used to form part of the actual construction, such as basement walls, and for underground settings such as subway stations.
For domestic or commercial basements, diaphragm walls are the perfect solution. Advantages include the fact that you can achieve significant depths with diaphragm walls, and construction can be achieved with minimal disruption to neighbouring buildings and in low headroom situations. Installation produces low vibration levels and can be completed relatively quickly, keeping disturbance of neighbours to an absolute minimum. Since concrete diaphragm walls can act as water barriers too, they are perfect for basement applications, where damp penetration is often an issue.
In some commercial projects, construction of diaphragm walls for the basement level of the building can take place simultaneously with the construction of the above-ground elements of the building. This can lead to significant cost and time savings.
Diaphragm walls are a central part of many commercial and infrastructure building projects, offering a reliable means of construction at a very significant depth and with extremely high load-bearing capacities. Without diaphragm walls, we would not see many of the large-scale skyscraper buildings that populate our urban landscapes, nor would we enjoy the benefits of modern, connected transportation systems such as roads, subways and rail networks.
If you would like further information about reinforcement products that are suitable for use in diaphragm wall construction, please get in touch. Our expert team has extensive knowledge of a wide variety of construction techniques and applications, and can provide in-depth guidance on all reinforcement matters.