Explain the types of building foundations

(11.09.2015, modified 12.09.2015)
A.
[Introduction
The building foundation interacts with the earth. Therefore, this interaction may result in the choice of the foundation and its advantages. After this, there is a brief explanation of different types of foundation structures. This is the work of the structural engineer who specializes in foundations.

General Information on Foundations]
Foundations are one of the most important aspects of construction. This is the part of the structure which interacts with the earth. At first blush, this "interaction" may not appear to be significant. However, one must realize that the surface of the earth is covered by many different materials (soils). These materials (soils) have vastly different properties and react quite differently when loads are placed upon them. The problem is further complicated when we disturb these soils and move them from one location to another. This practice is generally referred to as "cutting and filling." Read this article for more information on cutting and filling.

Footing and foundation systems, when properly designed and constructed, allow us to construct buildings which will withstand the powerful forces of nature. Some of these forces are quite impressive. Gravity, soil swelling, frost heaving, hydrostatic pressure, etc. are just a few of the subtle hidden forces that can affect your house or building. These forces are powerful and should not be underestimated. For example, I have witnessed firsthand the ability of a 2' x 2' area of soil to raise 1,200 lbs of steel and concrete 1/2". This occurred after the soil underneath a column footing had been saturated with water after a moderate rainfall. I was quite impressed. You must respect these forces.

Let's start with the basics. Footings are the structural members which transmit the concentrated loads of the structure to the soil. These members come in various shapes and sizes and are generally constructed of steel-reinforced concrete. The footings are generally a minimum of two to three times wider than the width of the foundation wall. The thickness of the footer is a function of the weight of the structure above and the strength of the soil below the footer. A thicker footer (10 -12") will be stronger than a thinner (6 - 8") footer. The footer is usually installed immediately after excavation. The foundation is then constructed on top of the footer. Generally speaking, the footer is constructed independently of the foundation.

Foundations are also structural members. They basically are nothing more than giant beams. They carry loads, similar to structural steel I-beams. Foundations can be constructed from a variety of materials. They can be made from concrete, stone, concrete block, wood, steel, etc. Since they are difficult to replace, it is a wise idea to construct foundations using strong, durable, water resistant materials. The design criteria concerning foundations is very technical. You should consider consulting a structural engineer prior to building a foundation. This person will properly size the foundation and specify the materials from which it should be constructed.

Additional Reading

The following books offer a wealth of information concerning footings and foundations. In the event you cannot locate these books, your local library will probably have many similar books on the same topic. Good Luck!

Residential Foundations
Jim Carr
Home Builder Press, 2000

Simplified Design of Building Foundations, 2nd ed.
James Ambrose
John Wiley & Sons, Inc. 1988
Foundations and Concrete Work
Fine Homebuilding Magazine
Taunton Press, 2002

Column B10

Ref:
[X] Own account
House Foundations and Footing. Ask the Builder. Available at,

House Foundations & Footings

[Below is a good write up by AboutCivil.org:]

Shallow Foundations :
Foundations provided immediately beneath the lowest part of the structure, near to the ground level are known as shallow foundations. Such foundations are mostly placed on the first hard and firm strata available below the ground level. Shallow foundations are further classified into the following types:

1. Spread footing or open trench foundations
2. Grillage foundations
3. Raft foundations
4. Stepped foundations
5. Inverted arch foundations

1) SPREAD FOOTING FOUNDATIONS:

Foundations constructed by increasing the are at the base of the structure by means of offsets, are called spread footing foundations. In such foundations, spread is given under the base of a wall or a column by providing offsets. This spread is known as footing and the foundation itself is called spread footing.

Broadly speaking, all types of shallow foundations can be referred to as spread footing foundations. However ,from design and construction point of view, they have been designated separately. The various types of spread footing foundations are ;

a) Wall footings:

This is a common and the simplest type of spread footing foundation; It consists of a number of courses of bricks, the lowest being usually twice the thickness of wall above. In this type of foundation, the base width of the wall is increased by providing 5cm (one-fourth of brick length) offsets on either side of the wall. The depth of each course is usually 10 cm.

In some cases, however, the bottom course is made 20cm deep. In the case of footing for store walls, the size of offset is slightly more than that of the brick wall footing.

Method of Construction of Wall Footing:

In no case, the depth of the concrete bed should be less than its projection beyond the wall base. For foundations over firm soil or compacted ground, the concrete bed below the brick wall footing may be dispensed with. After laying the concrete for constructing the concrete bed, it should be properly compacted and cured before laying the base course of the wall.

Suitability:
This is the cheapest type of spread footing foundations and is largely used for walls of ordinary buildings.

c) Concrete Column Footing:
These are either stepped type, slate type or slope type, having projections in the vae concrete. To support heavy loads, reinforcement is also provided at the base. The reinforcement provided is in the form of steel bars and is placed in both directions. Concrete column footings may be either isolated footings or combined footings. Isolated footings are used to support the individual columns, whereas combined footings are used where projections of different columns are not possible on all the sides, due to limited space.

The combined footings are generally rectangular in shape when both columns are of same sections and trapezoidal in shape when the columns are of different sections.

2) GRILLAGE FOUNDATION:

OBJECT :
The object of this type of foundation is to spread the load over a large horizontal area at the base of a structure.

TYPES :Depending upon the material used in construction, grillage foundations are further classified into two types.

1. Steel Grillage Foundation
2. Timber Grillage Foundation

Steel Grillage Foundation

Method of construction of steel grillage foundation:

1. For constructing steel grillage foundation, a trench is excavated to the calculated width and about 0.90 m - 1.50 m deep. Bottom of the trench is rammed and leveled.
2. Then a layer of lean cement concrete ( 1: 8 : 16 ), about 30cm thick, is laid and compacted.
3. After this, a layer of rich cement concrete (1: 2 :4 ), about 15cm thick is spread and compacted to form a concrete bed.
4. Over the concrete bed thus prepared, the bottom tier consisting of a number of steel I - beams of designed dimensions are placed at specified distance apart, using spacer bars. The space in between and around the steel beams is then filled with cement concrete.
5. On this bottom tier, a second layer of steel I - beam is placed, if required.
6. The entire space is then filled with cement concrete 1: 2 : 4. On the grillage bed thus prepared, the structure in the form of a steel stanchion, column, pier is built.

 

This type of foundation is useful for public buildings, office buildings, school buildings, residential quarters etc, where the ground conditions are very poor and bearing power of the soil is so low that individual spread footing cannot be provided.

 

4) STEPPED FOUNDATION:

For constructing this type of foundation, excavation is done into steps having short length and uniform thickness and the masonry work is done on the horizontal bed of concrete thus prepared.

If there is any possibility of slipping of the structure bodily, R.C.C piles can be driven along its base concrete on the sloping side.

5) INVERTED ARCH FOUNDATION:

The foundation consisting of inverted arches between the piers is known as inverted arch foundation. In this type of foundation, the load from the piers is transferred to the soil by constructing arches in inverted position at their at their base. The rise of the inverted arches is about one - fifth to one - tenth of the span and they are usually build in half-brick rings. The position of arches may be either along the row of piers or across the row of piers in both the directions depending upon the nature of soil and the type of land to be taken by them.

B) METHOD OF CONSTRUCTION:

For constructing this type this type of foundation;

1. Excavation is done to the required depth.
2. The bottom of the excavation is leveled and compacted
3. Then the foundation concrete is laid to the required thickness and finished according to the layout of the arches to be constructed under the piers.
4. The arch rings, usually half brick thick, are then built in cement mortar and piers are constructed.
5. The end piers should be designed and constructed to take the outward thrust caused by arch action.

Suitability:

This of foundation is not commonly used for buildings, but it is quite suitable for other structures like bridges, reservoirs, tanks, supports for drainage lines etc.

Deep Foundation:

The foundation constructed sufficiently below ground level with some artificial arrangements such as piles, wells etc, at their base are called deep foundations. Deep foundation are further classified into the following types ;

1. Pile foundation
2. Well foundation
3. Caisson foundation (sometimes 'Cassion' is used)

1) PILE FOUNDATION:

A foundation ( spread footing or grillage ) supported on piles is called a pile foundation. A pile foundation usually consists of a base of spread footing or grillage supported by piles at their bottom. Piles distribute the load of structure to the soil in contact either by friction alone or by friction combined with bearing at their ends.

Suitability:

This type of foundation is suitable under the following situations ;

1. When the soil is very soft and solid base is not available at a reasonable depth to keep the bearing power within safe limits.
2. When the grillage and raft foundation are very expansive.
3. When the building is very high carrying heavy concentrated loads.
4. When it is necessary to construct a building along the sea shore or river bed

Ref:

Types of Foundation and Methods of Construction of Foundation, AboutCivil.org, available at

http://www.aboutcivil.org/foundation-types-construction-methods.html