TOPIC 1: BUILDING FOUNDATIONS

Definition:

Foundation is the lowermost structure in any building, it will transfer the load from superstructure to the soil (substructure). There would have been no need of foundation if the soil is good in shear.

Reasons why foundations are placed below the ground level

a)      The surrounding ground provides stability

b)      Protection from impact

c)      Protection from extremes weather conditions

Although depths of foundation vary according to the site conditions, the best load bearing ground is normally 900mm below the surface.

Foundation is usually divided into 2 types:

1. Natural foundation. This is the ground underneath the base of the building after the excavation is completed.

2. The artificial foundation: This is the structure that lies between the building and the natural foundation.

Foundation Loads.

1.      Dead loads-these are the weight of all the fixed parts of the building eg, walls, floors, ceiling, services like sanitary fittings and plumbing.

2.      Superimposed or live loads: these are the weight of the people, furniture and machines.

3.      Wind loads; these are the pressures on the walls and roof from the wind.

Conditions that affect foundations.

The following conditions should be considered when choosing the correct type of foundation. These include:-

1.      The load-bearing capability of the ground;

2.      The depth where you will find  suitable load-bearing soil;

3.      The distance from the trees which can affect the stability of the soils;

4.      The level of the water table ;

5.      The Normal variation in the water table;

6.      The total weight of the building.If the building is heavier than the soil that was removed, then there will be some settlements of the soil adjusts to the new loads.

Choosing the best foundation type.

The choice depends on:-

·         the strength of the natural FD

·         the weight of the building and its loads.

Types of Foundation

1.      Shallows Foundations

2.      Deep Foundations

Shallow foundation

Shallow foundations are also called spread footings or open footings. The 'open' refers to the fact that the foundations are made by first excavating all the earth till the bottom of the footing, and then constructing the footing. During the early stages of work, the entire footing is visible to the eye, and is therefore called an open foundation. The idea is that each footing takes the concentrated load of the column and spreads it out over a large area, so that the actual weight on the soil does not exceed the safe bearing capacity of the soil.

Shallows foundations are used when surface soils are sufficiently strong and stiff to support the imposed loads; they are generally unsuitable in weak or highly compressible soils, such as poorly-compacted fill, peat, recent lacustrine and alluvial deposits, etc.
There are several kinds of shallow footings: individual footings, strip footings and raft foundations.

In cold climates, shallow foundations must be protected from freezing. This is because water in the soil around the foundation can freeze and expand, thereby damaging the foundation. These foundations should be built below the frost line, which is the level in the ground above which freezing occurs. If they cannot be built below the frost line, they should be protected by insulation: normally a little heat from the building will permeate into the soil and prevent freezing.