A diaphragm wall is a deep reinforced concrete wall built underground before the main excavation begins. It is used for basements, subway stations, underground parking, tunnels, and waterfront structures where soil and water must be held back safely. The wall is made in narrow vertical panels, so workers can build a strong underground barrier without first digging a huge open hole.
This method matters because it lets engineers build deep structures in crowded cities while limiting ground movement near roads and buildings.
The process usually starts with shallow guide walls that keep the digging tool aligned at the surface. A grab or hydraulic cutter excavates a vertical trench, while bentonite or polymer slurry fills the trench to support the soil and prevent collapse. A steel reinforcement cage is lowered into the slurry-filled trench, and concrete is placed from the bottom upward using a tremie pipe.
As concrete rises, it displaces the slurry, leaving a deep reinforced concrete wall that can resist earth pressure and groundwater pressure.
Key Facts
- A diaphragm wall is a cast-in-place reinforced concrete wall built underground in separate panels.
- Guide walls control trench position and help keep the excavation tool vertical.
- Slurry pressure supports the trench when p_slurry is greater than or close to the surrounding earth and water pressure.
- Hydrostatic pressure increases with depth: p = rho g h.
- Concrete is placed by tremie pipe from the bottom upward so it displaces slurry instead of mixing with it.
- Lateral earth pressure often increases with depth and can be estimated by sigma_h = K sigma_v.
Vocabulary
- Diaphragm wall
- A deep reinforced concrete wall built underground to retain soil and water before or during excavation.
- Slurry
- A thick fluid, often bentonite or polymer based, used to support the sides of a trench during excavation.
- Hydraulic cutter
- A machine with rotating cutting wheels that excavates hard soil or rock for deep diaphragm wall panels.
- Tremie pipe
- A vertical pipe used to place concrete underwater or under slurry from the bottom of a trench upward.
- Reinforcement cage
- A prefabricated steel bar assembly lowered into the trench to give the concrete wall tensile strength.
Common Mistakes to Avoid
- Thinking the trench is left open with air inside. This is wrong because deep narrow trenches can collapse unless slurry or casing supports the soil pressure.
- Pouring concrete from the top like normal surface concrete. This is wrong because top pouring through slurry can cause mixing, segregation, and weak zones, so tremie placement from the bottom is used.
- Ignoring groundwater pressure in wall design. This is wrong because water pressure increases with depth and can push strongly on the wall or cause instability during excavation.
- Assuming all panels are built at once. This is wrong because diaphragm walls are usually constructed in separate primary and secondary panels to maintain stability and control quality.
Practice Questions
- 1 A slurry has density 1100 kg/m^3. What is the slurry pressure at a depth of 25 m? Use p = rho g h with g = 9.8 m/s^2.
- 2 A diaphragm wall panel is 6 m long, 0.8 m thick, and 30 m deep. What volume of concrete is needed for one panel, ignoring overbreak?
- 3 A construction site is next to an old building and has a high groundwater table. Explain why engineers might choose a diaphragm wall instead of sloped open excavation.