Shallow 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.
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.
Pad foundations Pad foundations are used to support an individual point load such as that due to a structural column. They may be circular, square or reactangular. 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.
Pad foundations are usually shallow, but deep pad foundations can also be used. Strip foundations 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. The main area of strip foundation are buildings with heavy walls (brick, concrete, stone), and heavy floor slabs. Strip foundation is located on the perimeter exterior walls, and placed under the supporting inner walls of buildings or other areas with increased load.
Also strip foundation is well-suited for homes, which is planned to create basement, garage or basement. Strip foundation is also suitable in the case of a possible slight deformation of the base. Typically, strip footing located below the level of soil freezing at 20 cm of sandy soil You can lay the strip foundation and higher levels of freezing, but not more than 60 cm from ground level. For deep-freezing and highly swelling soils usually strip foundation shall not apply. Process itself strip foundation construction is simple, on the other hand it requires a fairly high cost of materials and work on the construction of the foundation.
There are criteria for the minimum thickness of the foundation, which can be used building a house. Thus, the minimum thickness of reinforced concrete strip foundation – 10 cm of concrete – 25 cm, rubble concrete foundation – 35 cm, well and laying of natural stone – 50 cm in the construction of strip foundation to be also take into account the permissible load on the soil under the foundation. For clay soils at a depth of 80 cm load should not exceed 2 kg/cm2. In addition to pouring the foundation concrete mixtures, often strip foundations erected using ready-made foundation blocks or bricks.
This speeds up the process of building a foundation. It should also be noted that during the construction of strip foundation is an opportunity to save money raising the foundation to the full depth only during the construction of heavy structures. There is still the most reliable option strip foundation remains solid foundation, more time-consuming and requires prior creating formwork and reinforcement training design to increase strength strip foundation. And in this case, the valves must be connected to each other by welding. Raft foundations
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. Deep foundations * Piles Deep 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. They include piles, piers and caissons or compensated foundations using deep basements and also deep pad or strip foundations. 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.
Piles are relatively long, slender members that transmit foundation loads through soil strata of low bearing capacity to deeper soil or rock strata having a high bearing capacity. They are used when for economic, constructional or soil condition considerations it is desirable to transmit loads to strata beyond the practical reach of shallow foundations. In addition to supporting structures, piles are also used to anchor structures against uplift forces and to assist structures in resisting lateral and overturning forces.
Piers are foundations for carrying a heavy structural load which is constructed insitu in a deep excavation. Caissons are a form of deep foundation which are constructed above ground level, then sunk to the required level by excavating or dredging material from within the caisson. Compensated foundations are deep foundations in which the relief of stress due to excavation is approximately balanced by the applied stress due to the foundation. The net stress applied is therefore very small. A compensated foundation normally comprises a deep basement. Types of pile
Piles are often used because adequate bearing capacity can not be found at shallow enough depths to support the structural loads. It is important to understand that piles get support from both end bearing and skin friction. The proportion of carrying capacity generated by either end bearing or skin friction depends on the soil conditions. Piles can be used to support various different types of structural loads. End bearing piles End bearing piles are those which terminate in hard, relatively impenetrable material such as rock or very dense sand and gravel.
They derive most of their carrying capacity from the resistance of the stratum at the toe of the pile. Friction piles Friction piles obtain a greater part of their carrying capacity by skin friction or adhesion. This tends to occur when piles do not reach an impenetrable stratum but are driven for some distance into a penetrable soil. Their carrying capacity is derived partly from end bearing and partly from skin friction between the embedded surface of the soil and the surrounding soil.