Deep foundations can be used to transfer the loading to a deeper, more competent strata at depth if unsuitable soils are present near the surface. They include piles, piers and caissons or compensated foundations using deep basements and also deep pad or strip foundations. They are often needed on soft or loose soils with low bearing capacity as they can spread the loads over a larger area.ĭeep foundations are those founding too deeply below the finished ground surface for their base bearing capacity to be affected by surface conditions, this is usually at depths >3 m below finished ground level. Raft foundations have the advantage of reducing differential settlements as the concrete slab resists differential movements between loading positions. It may be stiffened by ribs or beams incorporated into the foundation. They are used when column loads or other structural loads are close together and individual pad foundations would interact.Ī raft foundation normally consists of a concrete slab which extends over the entire loaded area. Raft foundations are used to spread the load from a structure over a large area, normally the entire area of the structure. Strip foundations are used to support a line of loads, either due to a load-bearing wall, or if a line of columns need supporting where column positions are so close that individual pad foundations would be inappropriate. Pad foundations are usually shallow, but deep pad foundations can also be used. They usually consist of a block or slab of uniform thickness, but they may be stepped or haunched if they are required to spread the load from a heavy column. They may be circular, square or reactangular. Pad foundations are used to support an individual point load such as that due to a structural column. Shallows 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. Shallow foundations (sometimes called 'spread footings') include pads ('isolated footings'), strip footings and rafts. 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 are those founded near to the finished ground surface generally where the founding depth (D f) is less than the width of the footing and less than 3m.
Shallow foundations (sometimes called 'spread footings') include pads ('isolated footings'), strip footings and rafts.ĭeep foundations include piles, pile walls, diaphragm walls and caissons.