Get off to a good start.
Foundations are the starting point in construction and a foundation transmits the building spread-load onto the earth. There are many different types of foundations designed to deal with differing earth conditions and types of buildings occupying the foundation. Each of the following classification are also sub-divided to allow a certain use. At the moment we only need concern ourselves with “Shallow Foundations” and one particular one at that – The Raft.
- Strip Foundations
- Shallow Foundations
- Raft Foundation
- Pad Foundation
- Piled Foundations
Shallow foundations are those founded near to the finished ground surface; generally where the founding depth (Df) is less than the width of the footing and less than 3m. These are not strict rules, but merely guidelines: basically, if surface loading or other surface conditions will affect the bearing capacity of a foundation it is ‘shallow’. Shallow foundations (sometimes called ‘spread footings’) include pads (‘isolated footings’), strip footings and rafts.
Shallow foundations are used when surface soils are sufficiently strong and stiff to support the imposed loads; they are generally unsuitable in weak or highly compressible soils, such as poorly-compacted fill, peat, recent lacustrine and alluvial deposits, etc.
Raft foundations are used to spread the load from a structure over a large area, normally the entire area of the structure. They are used when column loads or other structural loads are close together and individual pad foundations would interact. A raft foundation normally consists of a concrete slab, which extends over the entire loaded area. It may be stiffened by ribs or beams incorporated into the foundation. Raft foundations have the advantage of reducing differential settlements as the concrete slab resists differential movements between loading positions. They are often needed on soft or loose soils with low bearing capacity as they can spread the loads over a larger area.
The raft excavation should be a minimum of 60mm wider and longer than the proposed buildings footprint in all directions.
The raft should be excavated to a minimum of 600mm and filled with compacted MOT (Hardcore).
This should be blinded over with sand or ash and a DPC membrane overlaid.
The fill should then be overlaid again with re-enforcing mesh raised on concrete cups.
A shuttering of 150m should be constructed and a C10 or C15 Concrete mix poured to level, and vibrated to remove any air pockets.