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.
[edit] Find out more
[edit] Related articles on Designing Buildings Wiki
IHBC NewsBlog
18th-century hospital in York to become sustainable homes
A former mental health establishment founded by a Quaker in 1792 is to be converted into 120 energy-efficient homes in York.
Context 180 Released - Where Heritage and Nature Meet
The issue includes life, death, Forests, bats, landscapes and much more.
Church architecture awards 2024: now open
The National Churches Trust has announced three awards, all of which are run in partnership with the Ecclesiastical Architects & Surveyors Association (EASA).
The essential sector guide includes officers' updates and a foreword by EH Chair Gerard Lemos.
Historic England opens nominations for the National Blue Plaque Scheme
The scheme is open to nominations to celebrate people from all walks of life.
Striking photos show nature reclaiming brutalist concrete
‘Brutalist Plants’ explores nature’s links to the architectural style characterised by imposing form and exposed concrete.
Purcell’s guidance on RAAC for Listed Buildings in England & Wales
The guidance specifically focuses on Reinforced Autoclaved Aerated Concrete (RAAC) in listed buildings.
IHBC Membership Journal Context - Latest Issue on 'Hadrian's Wall' Published
The issue includes takes on the wall 'end-to-end' including 'the man who saved it'.
Heritage Building Retrofit Toolkit developed by City of London and Purcell
The toolkit is designed to provide clear and actionable guidance for owners, occupiers and caretakers of historic and listed buildings.
70 countries sign Declaration de Chaillot at Buildings & Climate Global Forum
The declaration is a foundational document enabling progress towards a ‘rapid, fair, and effective transition of the buildings sector'.