A diaphragm wall is a deep, reinforced-concrete wall cast panel by panel in a slurry-supported trench in the ground, serving as a permanent earth-retaining and/or water-retaining wall for deep excavations, basements, underground car parks or tunnels. It is one of the most widely used techniques for deep underground construction in built-up areas.
How a diaphragm wall is built
Construction proceeds panel by panel, typically 2.5 to 7 metres wide:
- Guide walls — two parallel concrete guide walls are cast at the surface; they keep the wall on line and define its top edge
- Trench excavation under bentonite slurry — a clamshell grab or hydromill excavates the trench to depths of 20–60 m. At the same time the trench is filled with bentonite slurry, whose density supports the soil walls
- Reinforcement cage — a prefabricated rebar cage is lowered into the slurry-filled trench
- Concrete placement — concrete is poured through a long tremie pipe from the bottom upward, displacing the lighter bentonite, which is recovered and recycled
- Joint waterproofing — joint profiles between panels ensure water-tightness
Each panel is built in the same order, from guide walls to watertight joints. li>
Application
Diaphragm walls are used for:
- Underground car parks — often 2–4 storeys deep in city centres
- Basements of high-rise buildings — permanent retaining wall during and after construction
- Tunnels — as walls of bored or cut-and-cover tunnels
- Quay walls and locks — where great depth is required
- Excavations in soft ground — where sheet piles cannot reach deep enough
Advantages
- Combined temporary and permanent — the wall acts as earth support during excavation and remains as basement wall afterwards
- Low vibration and noise — suitable for city-centre work
- Watertight — well-built walls are virtually impermeable, suitable below groundwater
- Great depth possible — up to 80 m achievable
- Heavy loads — can also carry vertical load as part of the foundation
Disadvantages
- High cost — more expensive than sheet piling or berlin wall
- Specialist equipment — only large, experienced contractors can build them
- Working space at surface — large rig required
- Bentonite waste stream — must be reprocessed or disposed of
Related terms
- Sheet pile wall
- Excavation
- Berlin wall
- Combi-wall
- Bentonite
Want to know more about construction terms? Visit our knowledge base on fredsdiyplans.com.