Cellular raft foundation

Foundations provide support for structures, transferring their load to layers of soil or rock that have sufficient bearing capacity and suitable settlement characteristics to support them.

Very broadly, foundations can be categorised as shallow foundations or deep foundations:

• Shallow foundations are typically used where the loads imposed by a structure are low relative to the bearing capacity of the surface soils.
• Deep foundations are necessary where the bearing capacity of the surface soils is not adequate to support the loads imposed by a structure and so those loads need to be transferred to deeper layers with higher bearing capacity.

Raft foundations are a type of shallow foundation. They are typically formed by reinforced concrete slabs that cover a wide area, often the entire footprint of a building. They spread the load imposed by a number of columns, walls, and so on, over a large area, and can be considered to ‘float’ on the ground in a similar way to a raft floating on water. Also known as mat foundations, they are often used for lightly-loaded buildings on weak or expansive soils such as clays or peat.

A cellular raft foundation consists of two slabs with two-way interlocking ground beams. The upper (suspended) slab and the lower (ground-bearing) slab are usually incorporated within the beams to form I-sections, with voids between them.

Cellular raft foundations are very rigid, making them suitable for heavy loads or loose soil that can be subject to uneven settlement. Where ground heave is likely to be a problem, cellular raft foundations may be installed to reduce the upward force of heave from transmitting to the structure above.

They are commonly found on sites that have been used for heavy mining activity or in poor ground where large bending moments need to be resisted. While often being an economic solution for such situations, they are one of the most expensive foundation types.

Bearing capacity can often be increased by the removal of overburden (waste or spoil), which is the material that lies above an area to be developed.

The required depth of the raft, in the case of overburden removal, may be influenced more by the amount of excavation required to achieve an adequate load reduction than by the resistance of the cellular raft to the bending moment. However, if the raft is required to be resistance to seismic loads of mining subsidence, then the depth of the raft can be influenced more by the likely bending moments that will be induced.

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