Types of Foundations

 TYPES OF FOUNDATIONS

Based on the depth, foundation is broadly classified into two types. If the depth of the foundation is less than or equal to its breadth, then it is called as shallow foundation. If the depth of the foundation is greater than its breadth, then it is known as deep foundation. The following are the different types of foundation:

SHALLOW FOUNDATIONS

The depth of the shallow foundation is less and is economical to build lightweight structures. It depends on the ratio of the depth and width of the foundation of the structure. They are constructed where soil layer at shallow depth (upto 2m) is able to support the structural loads. The depth of shallow foundations is generally less than its width. Various types of shallow foundation are described below:

❖ Isolated Footing: Isolated footing is also known as spread footing or individual footing. It is the type of footing which supports either one wall or one column spread the superimposed load of wall or column over a brs suas large area. In this footing the bearing capacity of soil is high. If the shape of footing is in rectangular or square, then it is called as pad footing. Sometimes, it is stepped or haunched to spread the load over a large area and it is termed as stepped footing. If the footing is sloped, then it is called as sloped footing.

❖ Combined footings: If two to three columns connected collectively with one single footing is called combined footing. It is used when the two nouson role columns are so close to each other that their individual footings would overlap. Also this footing is necessary when a column is very close to the boundary of the property and hence there is no scope to project footing much beyond the column face. A combined footing may be rectangular or trapezoidal in plan.

❖ Strip footing: A strip footing is provided for a load-bearing wall. The width of the footing is generally two three times the thickness of the wall. A strip footing is also provided for a row of columns which are so closely spaced that their spread footings overlap or nearly touch each other. The broader base of the footing area provides more stability to the structure. It can also be used for retaining wall foundation. It is more economical to provide a strip footing than to provide a number of spread footings in one line. A strip footing is also known as continuous footing.

❖ Cantilever Footing: This type of footing consists of two isolated footings connected with a structural strap or a lever. The strap connects the two footings such that they behave as one unit. The strap is designed as a rigid beam. The individual footings are so designed that their combined line of action passes through the resultant of the total load. A strap footing is more economical than a combined footing when the allowable soil pressure is relatively high and the distance between the columns is large. This type of footing is also known as strap footing.

❖ Mat Foundation: A mat or raft foundation is a large slab supporting a number of columns and walls under the entire structure or a large part of the structure. A mat is required when the allowable soil pressure is low or where the columns and walls are so close that individual footings would overlap or nearly touch each other. Mat foundations are useful in reducing the differential settlements on non-homogeneous soils or where there is a large variation in the loads on individual columns. It is also known as raft or Grid foundation.

DEEP FOUNDATIONS

Deep foundation is used for where hard strata are not available within the mit of shallow foundation. This type of foundations is used when top soil is not capable of taking the load of the structure even at 3 to 4m depth. Deep foundation helps to prevent the structure from the uplift. Various types of deep foundation are described below:

❖ Pile Foundation: In this type of foundation, the load is transmitted by a vertical member. This vertical member is known as a pile. These piles are generally made of steel, concrete and wooden. Nowadays precast members are also used. Pile foundation is further classified as follows:

According to Function

Bearing pile: These piles are used if the hard strata are available at reasonable depth. The load is transmitted by columns to the hard layer of soil.

Friction pile: These piles are used where the soil is soft at a considerable depth. Friction piles transfer the load to the soil by the friction between soil and the pile. The friction developed is to be properly assessed before deciding the length of the pile. The surface of such piles is made rough to increase the skin friction so that required length of pile is reduced.

According to Material

Concrete pile: The piles which are made with the help of concrete are known as concrete piles. The diameter of these pile varies from 30 to 50 cm. Minimum length of these pile is not taken less than 20 meters and maximum it can be taken till 30 meters. These piles may be either precast or cast-in-situ. Precast piles are manufactured in the factory which is further transported to the construction site where ever it is required. These piles can bare load up to 800 KN. Cast-in-situ piles are made or manufactured on site where it is to be installed. It saves money as the transportation cost is reduced. These files bare load up to 750 KN.

Timber piles: As the name suggests these piles are made up of wooden so they are known as wooden or Timber piles. For these piles, seasonal timber wood is used. The diameter of the timber pile varies in between 20 to 50 cm. The maintenance cost of these piles is more because as it is wood if it comes in contact with water then it can be damaged by fungus or white ants. So care has to be taken.

Steel piles: These Piles are generally in shape of 'I' or hollow section. It can be easily driven in the soil because it has a very small cross-sectional area. These piles can be used as a bearing pile but cannot be used as friction piles because if we use them as a friction pile it can sunk in the soil due to structural load.

Composite pile: When the piles are made from more than one material they are known as composite pile. These piles are made from concrete and wood. These piles are used in those areas where the water table is up. These piles are used in such conditions just because concrete and wood both are good water absorbers.

❖ Pier Foundation: A pier foundation is a vertical column of relatively larger cross-section than a pile. If the diameter is greater than 0.6 m or equal to 0.6 meters then it is termed as a pier. The shape of the pier foundation is cylindrical, and it is supported with the help of piles. The load coming from the superstructure is carried to the hard strata through these vertical piles. They are generally cast on site. It is used for a heavy multi-storey structure where the load of the structure is hefty, and it is generally used in bridges and fly-overs to resist the heavy traffic loads. This type of foundation is chosen when the depth of the hard strata is at 5 meters or less than 5 meter. Also, this type of footing is done when not much heavy load is coming from the superstructure. The size of excavation depends upon the level at which hard strata exists. The size and shape of this concrete pier depend upon the level of hard strata is present.

❖ Well foundations: It is also called as drilled shaft or caisson foundation. It is generally used for bearing the very heavy loads of the structure. It is a cast-in-situ foundation and also available as pre-cast material. Construction of this foundation is done with the help of auger and used where the pile foundation is not enough to bear the heavy load of the structure. The depth of the drilled shaft foundation is more than the pile foundation. And it is not preferable to use in water-bearing granular soils, loose and soft clay soils. This foundation generally used in underwater project like river, lake and sea. It is used as pier of bridge, building and construction of the dam. It is constructed with the help of water shoring. This foundation works as a compression member. Drilled Caissons are classified into three categories as:

* Concrete caissons with enlarged bottom: In this at top which is at ground level of cap is provided. Above that cap brickwork is carried out. Below this cap a pier is constructed which is further followed by the enlarged bottom which is also known as bell. The angle of this bell at bottom is 60 degree.

* Caissons of steel pipe with concrete filled: In this also at the top which is at ground level, a cap is provided. Below this cap, at both extreme ends, a steel shell is created. This steel shell is the outer portion. Inside this steel shell concrete is filled.

* Caissons of steel pipe with concrete and steel core: The assembly of this type is also the same as the caissons of steel pipe with concrete-filled. But the only change is that in the central portion of the steel core or a rod is fixed which gives more stability to the structure as the weight taking capacity increased due to the steel core.

FACTORS FOR SELECTION OF SUITABLE FOUNDATION TYPE FOR A BUILDING

 The type of foundation is selected based on the following factors:

❖ Building height

❖ Load condition

❖ Soil type

❖ Type of building (residential, administrative, warehouse)