Foundation is the part of the structure that supports the load of the super-structure and transferred the load to the underlying soil or rock beneath the ground.(wiki) Foundation can be classified on different basis like on the basis of material, the foundation may be wooden, steel, concrete and composite. In general, all types of foundation are classified into two categories: shallow and deep.
Types of foundation:
Depending upon the ratio depth to the least lateral dimension of the foundation, foundation is of two types:
- Shallow foundation
Df/B less or equal to 1
2. Deep foundation
Df/B greater than 1
Shallow foundation located just below the lowest part of the superstructure they support; deep foundations extends considerably deeper in to the earth.
Let’s further classified the types of foundation;
|S N||Types of shallow foundation:||Types of deep foundations|
|1||Spread footing ( isolated column footing)||Pile foundation|
|2||Strip footing ( wall or continuous footing)||Pier foundation|
|3||Combined foundations||Well or caissons|
|4||Strap or cantilever footing|
|5||Mat or Raft foundation|
Types of Shallow foundation:
Shallow foundation is defined as that foundation whose width is greater or equal to the depth and transmit structural loads to the soil strata at relatively small depth.
1. Spread footing:
Spread footing is basically a pad used to spread out loads from columns over a sufficiently large area of the foundation soil. It is constructed as close to the ground surface as possible consistent with the design requirements. Isolated footing are used to support an individual point load such as that due to structural column. They may be circular or rectangular/ they usually consist of a block or the slab of uniform thickness but may be stepped or hunched if they have to resist heavy load from the column.
2. Strip footing:
Strip or Wall foundation support a line loads which may be load-bearing wall or a line of columns. Column are in line position footing if their positions are so close that individual isolated foundations are inappropriate.
3. Combined footing:
A combined footing supported two or more column in a row when the areas required for the individual footings are such that they come near each other. They are mostly preferred in situation of limited space on one side owing to the existence of the boundary line of the privacy property.
4. Strap or cantilever Footing:
Strap footing comprises two or more footings connected by the beam called strap. Strap footing is required for that exterior column which cannot extend into an adjoining property due to limited size of plot area.
5. Mat Footing:
A raft or mat foundation id a large footing usually supported walls as well as several columns in two or more rows. This is adopted when individual column footings would tend to be close or to tend to overlap. Further, this is considered suitable when differential settlements arising out of footing on weak soils are to be minimized. It is required when the loads are heavy and the soil is very weak or highly compressible.
A mat foundation is a thick reinforced concrete slab which supports the entire load (from bearing wall and column loads) of a structure or a large portion of the structure. When the base area required for the individual footing exceeds about one half of the area covered by the structure, raft/mat foundation comes to be more economical than the individual footing .
Types of deep foundation:
Deep foundation is a types of foundation which transfers all the load from the superstructure to the deep down underneath soil from the surface than the shallow foundation. The types of deep foundation on the basis of construction and appearance.
A pile is a long vertical or inclined slender structural member which is made up of steel, concrete or wood and used to transfer load to a deeper strata below ground surface. It is basically a substructure that is supported by a group of piles which may be either driven in the ground or bored and then cast in situ. Pile foundation are used to transfer loads from soil of low bearing capacity to stiff layers of soil or the rock with higher bearing capacity. Piles may experiences vertical or lateral loads or a combination both types of loads. Pile foundation is further sub-classified into two types on the basis of its load resisting mechanism:
- End bearing pile
End bearing piles are those pile foundation which transfer the load of super-structure to deep down hard strata. The bottom end of pile rest on the layer of hard strata takes all the loads. This types of case arises when the load bypasses the weak layer and transferred to the strong layer.
- Frictional pile
Frictional piles are those piles which transfer the load of substructure to the soil across the full height of the pile by friction. The entire surface of pile works to transfer the load to underneath soil.
In reality, the pile transfer load through both the mechanism and resist load by the comb ination of friction and end bearing.
2. Pier foundation:
Pier foundation are somewhat similar to pile foundation but typically larger in area than piles. To construct pier foundation, an opening is drilled to the required depth and fresh concrete is poured. Usually, pier foundations are used for bridges.
3. Well foundation:
A caisson is the structural box or chamber that is sunk into place or build in place by the systematic excavation below the bottom Caissons are classified as open caissons, pneumatic caissons and box or floating caissons. Open caissons may be box types of pile foundation.
- Open caissons: The top and bottom are open during installation for open caissons. The bottom may be finally sealed with concrete or may be anchorage on the rock.
- Pneumatic caisson: it is one in which compressed air is used to keep water from entering in the working chamber, the top of the caisson is closed. Excavation and concreting is facilitated to be carried out in the dry. As the excavation proceed, the caisson is sunk deeper. And the working chamber is filled with concrete when the excavation is reached to the final position.
- Box or floating caisson: it is on in which bottom is closed. It is cast on land and towed to the site and launched in water, after the concrete has gone cured. To keep the caisson into position, sunk is filled with sand, gravel, concrete or water. False bottoms or the temporary bases of timber are sometimes used for floating the caisson to the site.