PRESSURE DISTRIBUTION UNDER DIFFERENT SOILS AND THE DEPTH OF FOUNDATION REQUIRED.

BEARING CAPACITY OF THE SOIL AND ANALYSIS:-- The shape and size of the foundation depends on the permissible bearing capacity of the soil .Total load per unit area under the footing must be less than the permissible bearing capacity of the soil to prevent for the excessive settlement.In general foundations have to resist vertical load , horizontal load and the moments.For the purpose of the design of the foundations,values of density , bearing capacity,, coefficient of the internal friction etc of the soil are required.These data must be known reasonably accurately prior to the design of the footing.Two terms are virtually used for the bearing capacity ie the Gross bearing capacity and the net bearing capacity.Gross bearing capacity is the total safe bearing pressure at the bottom of the footing including the load of the superstructure,weight of the footing and that of the earth lying over the footing.Net safe bearing capacity is the safe bearing pressure at the bottom of the footing additional to the weight of the earth which existed at that level before the trench for footing was dug .Thus the net safe bearing capacity is the gross bearing capacity minus the weight per unit area dug out of trench .Typical values of the net bearing capacity for different types of the soil may be taken as for different types of Rocks from 3300 to 450 kN /mt square and for the non -cohesive soils it may be assessed 450 to 100 kN / mt square and for the cohesive soils it may be taken from 450 to 50 kN / mt square.The depth of the foundation is governed from the following factors (1) to secure the safe bearing capacity,and (2) to penetrate below the zone where seasonal weather changes are likely to cause significant movement due to swelling and the shrinkage of soils and to (3) to penetrate below the zone which may be affected by frost .An estimate of depth of footing below the ground level may be obtained by using the Rankine formula from where minimum depth of the foundation may be calculated by using density of soil , angle of repose of the soil,gross bearing capacity of the soil and net bearing capacity.