A caisson cellar (also known as a sinking cellar) is a type of basement that is constructed at ground level and then gradually sunk into the earth by excavating soil from beneath it. This technique allows builders to create underground spaces without the need for deep open excavations, making it particularly useful in dense urban areas or locations with high groundwater.
How it works
The cellar is first built as a complete concrete box on the ground surface. The walls are reinforced and the bottom edges are shaped into a cutting shoe — a tapered edge that helps the structure cut through the soil. Once the concrete has cured, soil is systematically removed from inside the cellar. The weight of the concrete box causes it to sink steadily into the ground. When the desired depth is reached, a concrete floor slab is poured inside, sealing the bottom and creating a watertight basement.
Application
Caisson cellars are used in several situations where traditional excavation methods are difficult or impractical:
- Residential basements — Homeowners in areas with limited space or difficult soil conditions use this method to add underground rooms, storage, or living space.
- Urban construction — In cities where neighbouring buildings are close by, a caisson cellar minimises vibration and soil disturbance, protecting adjacent structures.
- High groundwater areas — The sealed concrete box can be designed to resist hydrostatic pressure, making it suitable for sites below the water table.
- Renovation projects — Adding a cellar beneath an existing building is sometimes achieved by sinking a caisson under the structure.
Advantages and considerations
The main advantages are minimal disruption to the surroundings, no need for sheet piling, and a watertight result. However, careful engineering is required to control the sinking process evenly and to ensure the structure does not tilt. Ground conditions must be thoroughly investigated before construction begins.
Related terms
- Caisson sinking
- Waterproof basement
- Sheet piling
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