Types of Foundations

Foundations

Definition

A foundation is that part of the structure which is indirect contact with ground. It transfer the load of the structure to the soil below so as to avoid over loading of the soil beneath.

Function of foundation

The following are the main functions of foundation.

   • To transmit and distribute the total loads of the structure to a larger area of underlying support.
   • To prevent the differential settlement of a structural.
   • To provide stability tot he structure.
   • To prevent cracks due to movement at moisture in case of weak or poor soils.
   • To prepare a level and hard surface for concreting and masonry work.
   • To support the structure.

Types of Foundations

Foundation may be broadly divided into two groups.

    1. Shallow foundation.
    2. Deep foundation.

1. Shallow foundation : 

The foundation whose depth is not more than its width is called a shallow foundation. The various types of shallow foundation are 

Wall footings : These footings can be simple or stepped . The base course of these footing can be concrete. The width of concrete base should beat least equal to twice the width of wall.

                                                                                                              

Isolated or Column footings : They are used to support individual columns. They can be either of stepped type or have projection in the concrete base. Generally 15 cm offset is provided on all side of concrete bed. For heavy loaded columns, steel reinforcement is provided in both the direction concrete bed.

Combined footing : 

A combined footing support two or more columns in a row. The combined footing can be or in trapezoidal in shape.

Inverted Arch Footings : 

This type of construction is used on soft soils to reduce the depth of foundation loads above an opening are transmitted from supporting walls through inverted arches to the soil.

Continuous footings : 

In this type of footing a single continuous R.C slab is provided as foundation of two or three or more columns in a row. This type of footings is suitable at earth quake zone and also prevents differential settlement in the structure.

Cantilever footings : 

Cantilever footings consist of two or more individual footings connected by a beams called strap. This type of footing may be used where the distance between the columns is so great that a combined trapezoidal footings becomes quite narrow with high bending moments.

Grillage footing : 

This type of footing is used to transmit heavy loads from steel columns to the soils having low bearing capacity.This footing is made up of rolled steel joists as Grillage beams provided in single double tiers. In double tier arrangement the top tier is laid perpendicular to the bottom one. The total footing is embedded in cement concrete.

Raft foundation :

A raft or mat is a combined footing that covers the entire area beneath a structure and supports all the columns. A raft foundation is also used to reduce settlement above highly compressible soils by making the weight of structure and raft equal to the weight of soil excavated. A raft may
under go large settlement without causing harmful differential settlement.

Deep Foundation

These foundations are suitable when buildings are constructed in low bearing capacity of soil (or) hard rock is only available in high depth.

It is classified in two types.

I)                   Pile Foundation

II)                Well foundation

I)  Pile Foundation:

A pile may be defined as a long vertical load transfer element composed of timber , steel, concrete or a combination of them. The main function of pile is to carry load of the structure which cannot be sufficiently supported at a certain level due to its poor bearing capacity to a depth at which good bearing capacity is available.

Types of Piles: In general, there are two types of piles.

      1. Bearing Piles: 

When a pile passes through poor material nut its tip (bottom end) penetrates a small length in to a stratum of good bearing capacity, and then it is called a bearing pile. The hard stratum or bed rock must be at a reasonable depth and there should be no too soft stratum below the hard stratum. Bearing piles transfer their load to the hard stratum.

      2. Friction piles: 

When a piles passes through deep strata of limited bearing capacity and develop their carrying capacity by friction on the sides of the pile (circumference of the pile), then they are called fiction piles. The friction pile derives its support mainly from the surrounding soil through the development of friction between the soil and the periphery of pile. A very small percentage of the load is carried by the soil near the lower tip of the pile. Friction piles are used when hard stratum of bed rock is at a large depth. The length of friction pile depends upon the type of soil, amount of load and the size of pile.

       3. Other Piles: 

Besides these two main types of piles, there is a number tip of the pile. Friction piles are used for specific purposes. They are compaction piles, Anchor piles, Dolphin and Batter piles.

Compaction piles are used to compact loose granular soils there by increasing the bearing capacity. When friction piles are driven in coarse grained soil, the process of driving such piles close to each other in group greatly reduces the porosity and compressibility of the soil within and around. Thus they compact the soil.

Compaction piles do not carry any load themselves.

Tension piles anchor down the structures which are subjected to or due to overturning movement.

II. Well Foundation (or) Caisson Foundation

The foundation structure constructed under water and sandy areas is called Caisson.

These are three types. 1) Box Caisson 2) Open Caisson 3) Pneumatic caisson.

      1) Box Caisson

Box caissons are used where hard rock is available at lower depth. Caisson is a strong water tight vessel open at top and close at bottom.

      2) Open Caisson (or) Wells

The open caisson is a well which is open at top and bottom. These foundations are constructed in the ground where high erosive sand and soft bearing stratum occurs. These are constructed in the shape of square, Rectangle and Circle.

      3) Pneumatic Caisson

In this type of foundation bottom portion is open and top portion is closed. We can use this when digging well is not possible. These are constructed when availability of water level is at 12 m from ground.

Foundations Constructed in Black Cotton Soil

Black cotton soil has got useless property. This type of soil expands when moisture rises. when moisture reduces it shrinks. According to the variations of temperature shrinkage and expansions occurs alternatively. Due to the weight of the building structure it immerse in the ground because of shrinkage of soil.

POINTS TO BE NOTED WHEN FOUNDATIONS ARE CONSTRUCTED IN BLACK

COTTON SOIL

I)                   It depth of the black cotton soil is less than 1m it can be rugged and removed and other hard soil is filled in that place.

II)                It should be noted that the depth of foundation should be below the depth of black soil.

III)             In order to prevent the force acting on the soil the load of building should not exceed 5000 N/m².

IV)             To resist the longitudinal force acting on the foundation RCC can be used. While providing this concrete a layer of sand should be filled between the layers of concrete and black soil.

V)                It is an alternative for Raft foundation, Pile foundation and grillage foundation.

VI)             Under reamed pile with concave base is the best option for this type of soil.

Under Reamed Piles with Concave Base

These foundations are the best for the black soil regions. As shown in the figure one or two more shell like concave base are in these types of piles. These type of shells increases the bearing capacity of soil. Diameter of pile is 20 cm and diameter of shell is approximately 50 cm are made for these type of piles. These types of piles are useful for the sandy regions and high water level regions.

Stepped Foundations

When the ground is sloping, it becomes uneconomical to provide foundation at the same level. In such cases, the correct levels of the sloping ground in which the building is to constructed are taken and a longitudinal section is prepared accordingly. The stepped foundation are then provide

Following points should be noted:

    1.      The overlap between two layers of foundation concrete should be equal to the depth of foundation concrete or twice the height of the step, whichever is greater.

    2.      A minimum depth of 800 mm should be provided at all points so as to protect the foundation from weathering effects.

   3.      The depth of foundation concrete should be in even number of the masonry course.

   4.      The distance of the sloping surface from the lower edge of the footing should not be less than 600 mm for rock and 1m for soils.

   5.      When footings are heavily loaded, it becomes necessary to carry out a slope stability analysis.