Types Of Foundation In Construction
The Detailed Guide Of Types Of Foundation
Foundation is broadly classified under two heads
|Shallow Foundation||Deep Foundation|
According to Terzeghi’s theory, a foundation is said to be shallow if its depth is equal to or less than its width. And for a deep foundation, the depth is equal to or greater than its width.
From the point of view of design, shallow foundations may be of the following types;
- Spread Footing
- Combined Footing
- Strap Footing
- Mat Foundation
A brief description is given below.
Spread footing are those which spread the superimposed load of wall or column over a larger area. Spread footings support either a column or wall. Spread footing are divided into subcategories;
- Single Footing
- Stepped Footing
- Sloped Footing
- Wall Footing without step
- Stepped Footing for wall
- Grillage Foundation
The picture below shows a single footing for a single column, in which the loaded area ( b x b) of the column has been spread to the size B x B through a single spread. The base of this footing is generally made of concrete.
Stepped footing for the heavily loaded column, which requires a greater spread. The base of the column is made of concrete.
In a slopped footing the concrete base does not have a uniform thickness but is made slopped, with greater thickness at its junction with the column and smaller thickness at the end.
Wall Footing Without Slope
Spread footing for a wall consists of the concrete base without any steps. Usually, masonry walls have speed footing with a concrete base as shown in the picture.
A steel grillage foundation for a steel stanchion carrying a heavy load. It is a special type of isolated footing that is generally provided for heavily loaded steel stanchions and used in those locations where the bearing capacity of the soil is comparatively low. The depth of such a foundation limited to 1 to 1.5 m.
A load of the stanchion is distributed or spread to a large area by means of two or more tires of rolled steel joints, each layer being laid at a right angle to the layer below it. Both the tires of the joists are embedded in cement concrete to keep the joists in position and to prevent their corrosion. Grillage foundation is also constructed using timber beams and planks.
A spread footing which supports two or more columns is termed as combined footing. The combined footings are invariably constructed of reinforced cement concrete and they are maybe of the following kinds;
- Rectangular combined Footing
- Trapezoidal Combined Footing
- Combined Column wall Footing
Rectangular Combined Footing
The combined footing is constructed in a rectangular shape to carry equal loads. The design of rigid rectangular combined footing should be done in such a way the center of gravity of column loads coincides with the centroid of the footing area.
Tropizodial Combined Footing
When columns of two different shapes or sizes, carrying unequal loads in such cases construction of trapezoidal combined footing is preferred. the column which carries the greater load is located at the wider side of the footing and the less load carrying column is located at the front which is a comparatively narrow area.
Combined Column And Wall Footing
Sometimes it is essential to provide a wall in a combined footing to keep the properly balanced loading to the footing. In such footing, a wall is constructed above the footing along with the combined column in the same base of the footing. The shape of those footing could be rectangular or could be trapezoidal.
In the independent footing, two columns are connected by a beam, it is called a strap footing. A strap footing may be used where the distance between the columns is so great that a combined trapezoidal footing becomes quite narrow, with high bending moments.
In that case, each column is provided with its independent footings and the beam is used to connect two footings. The strap beam does not remain in contact with the soil and thus does not transfer any pressure to the soil.
The strap, assumed to be infinitely stiff, serves to transfer the column loads to the soil with equal and uniform soil pressure under both footings.
The above pictures show that the footing for two columns A and B. Column A is near to an existing wall that the footing of the wall does not permit the independent footing of column A to spread out towards the wall, though it has freedom in other direction.
A mat foundation is a combined footing that covers the entire area beneath a structure and supports all structures and columns. When the allowable soil pressures low, or the building loads are heavy, the use of spread footing will cover more than one half the area it may prove more economical to use mat or raft foundation.
They are also used where soil mass contains compressible lenses or the soil is sufficiently eccentric so that differential settlement would be difficult to control. The mat or raft foundation tends to bridge over the eccentric deposits and eliminates the differential settlement.
Raft foundation is also used to reduce settlement above highly compressible soils, by making the weight of structure and raft approximately equal to the soil excavated.
Raft foundation may be divided into three types based on their design and construction
- Solid Slab System
- Beam Slab system
- Cellular System
All three types are basically the same, consisting of a large, generally unbroken area of slab covering the whole or the large part of the structure. The thickness of the slab and the size of the beams will be governed by the spacing and loading of the column and the degree of rigidity required in the raft.
A deep foundation is divided into some subcategories, those are;
- Deep strip, rectangular or square footing.
- Pile foundation
- Pier foundation or drilled caisson foundation
- Well or caissons foundation
Deep Strip, Rectangular Or Square Footing
As we discussed earlier, the usual strip, rectangular or square footings considered under the shallow foundation in general but when the depth is greater than its width of the footing it is considered to be a deep foundation.
Pile foundations are the type of deep foundation in which the loads are taken to a low level by means of vertical members which may be of timber, concrete, or steel.
Pile foundations could be adopted;
- Instead of a raft foundation where no bearing strata exists at responsible depth and the loading is uneven.
- When a firm bearing stratum does not exist but a deep strip or spread footing construction is uneconomical
- When pumping of subsoil water would be too costly or timber shoring to deep excavations is too difficult and expensive to permit the construction of normal foundations.
Piles constructed in a building could be of four types ;
- End Bearing Pile
- Friction Pile
- Combine End Bearing And Friction Pile
- Compaction Pile
End Bearing Pile
End Bearing piles are used to transfer load through water or soft soil to suitable bearing stratum. Such piles are used to transfer load safely to a hard start. Multistoried buildings are invariably founded on end bearing piles so that the settlements are minimized.
Friction piles are used to transfer loads to a depth of a friction-load-carrying material by men’s skin friction along the length of the pile. Such piles are generally used in granular soil where the depth of the hard stratum is very great.
Combine End Bearing And Friction Pile
A pile that transfers the superimposed loads both through side friction as well as end bearing. Such piles are more common, especially when the end bearing piles pass through the granular soils.
Compaction piles are used to compact loose granular soils, thus increasing their bearing capacity. The compaction piles themselves do not carry a load. Hence they may be of weaker material ( such as timber, bamboo sticks, etc.) sometimes of sand only. The pile tube, driven to compact the soil, is gradually taken out and sand is filled in that empty space thus forming the ‘sandpile’.
One interesting fact is –The Taj Mahal which was built during 1631-1648 was constructed with the Sand Pile Foundation.
A pier foundation consists of a cylindrical column of large diameter to support and transfer the superimposed loads to firm strata below.
The difference between the pile foundation and pier foundation lies in the method of construction. Through pile, foundations transfer the load through friction and/or bearing, pier foundations transfer the load only through the bearing. Generally, the pier foundation is shallower in-depth than the pile foundation.
Pier foundation is preferred in allocation where the top strata consist of decomposed rock overlying strata of sound rock. In such a condition, it becomes difficult to drive the bearing piles through decomposed rock. In the case of stiff clays, which offer large resistance to the driving of a bearing pile, pier foundation can be conveniently constructed.
Pier foundation is can be categorized further;
- Masonry or Concrete Pier
- Drilled Caissons
Masonry Or Concrete Pier
When a good bearing stratum exists up to 5 m below ground level, brick masonry or concrete foundation piers in excavated pits may be used. The size and spacing of the piers depend upon the depth of the hard bed, the nature of overlying soil, and the superimposed load.
The term drilled caissons, foundation pier, Sub-piers are interchangeably used by engineers to donate a cylindrical foundation. A drilled foundation is largely a compressed member subjected to an axial load at the top and reaction at the bottom.
Drilled caissons could be of three types;
- Concrete caisson with an enlarged bottom.
- Caisson of steel pipe with concrete-filled in a pipe.
- Caisson with concrete and steel core in steel pipe.
Well foundations or caissons are box-like structure-circular or rectangular-which are sunk from the surface of either land or water to the desired depth. They are much large in diameter than the pier foundations or drilled caissons. Caisson foundation is used for a major foundation works, such as for.
- Bridge piers and abutments in rivers, lakes, etc.
- Wharves, quay walls, docks.
- Breakwaters and other structures for shore protection.
- Large waterfront structures such as pump houses, subjected to heavy vertical and horizontal loads.
Well, foundations or caissons are hollow from inside, which may be filled with sand, and are plugged at the bottom. The load is transmitted through the perimeter wall, called steining.