Deciding the Depth of a building foundation
We know that every building basically rests on the foundation of the building. And the stability of building is mainly dependent on the strength of the foundation. And the entire load of structure is generally taken by these footings i.e., foundations ultimately. And if the soil is not having sufficient strength over which footings were laid upon the the real problem comes i.e., the structure constructed over it is not at all safe. Because it may undergo settlement due to which cracks may occur over the walls, beams etc. And some times due to the excessive settlements the structure also fails. And hence we must take utmost care while designing and placing the foundations.
Now the question that arises immediately in our mind is what should we do so that we can prevent the settlement of footing and subsequent failure of the structure....???? And what are the various factors we must consider while placing our foundation...?
Answer is very simple we must select a correct type of foundation at a sufficient depth. And what is the satisfactory depth we must provide>..?
Basically there are 2 types of foundations depending on depth. If the depth of footing is more than the width of the footing it is called deep foundation. And if the depth is less than the width of footing it is called shallow foundation. Depending on the structure type, soil nature and loading characteristics etc we will adopt a particular type of foundation
FACTORS AFFECTING THE DEPTH:
the various factors that mainly affecting the depth of a footing are explained below.
1. Scour Depth : At river beds and canals the soil at top layers is generally formed by erosion and deposited soils. When the river or the stream is in full swing this soil cannot be in a stable state and quickly erodes or moves away. Due to this there will not be any lateral support to the foundations laid with in this loose soil which is vulnerable to scour(erosion by streams). And hence the footing of a foundation must be laid below the scour depth.
2. Water table depth : The depth of ground water from the surface of ground is called water table depth. If the water table is very nearer to the footing it causes increase of pressure and loading on the footing and hence the footing must be laid above the water table.
3. Topsoil: The top soil generally consists of organic materials and various types of other loose materials and deposits. This soil is very weak and cannot serve as a foundation soil. And hence the depth of footing must extend beyond the depth of top soil so as to get adequate bearing capacity and as well as lateral support.
4. Zone of volume change : some times the top layers will contain clayey soil layers and organic swelling property soils. When the conditions are dry all will go on fair...but when water comes in contact swelling takes place and building will be lifted up and in the subsequent stage after escape of water it will shrink...thus volume changes in soil takes place making the building lifting and sinking. this will damage the life of the building . And hence while laying footings care must be taken so as to surpass this zone of volume change.
5. Fault zones : some times the building foundations are subjected to damage due to the active faults present at the area. Hence care must be taken to safeguard this faults in rocks beneath the footing.
And also there are so many factors which one must consider while laying a foundation. However all situations will not be similar and hence an engineer must decide the suitable and appropriate measures and precautions while placing a footing keeping in all the factors mentioned above.