Shallow Foundation

Bearing Capacity of Soil

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1. Gross Bearing capacity or Gross loading Intensity(q_g):

For a footing constructed with its base at a depth D_f below the ground surface, the total pressure, q_g at the base of the footing will be due to the weight of the superstructure, self-weight of the footing, and the weight of soil fill over the footing. This total pressure is known as Gross Pressure of Gross loading intensity(q_g)

${q_g} = {P \over {{B^2}}} + \gamma {D_f}$

It is the total pressure intensity at the base of the footing.

2. Net Pressure intensity or Net loading intensity(q_n):

It is generally the loading intensity at the base of footing in excess of the load intensity that the soil was originally subjected to that causes deformation in soil.

${q_u} = {P \over {{B^2}}}$

${q_n} = {q_g} - \gamma {D_f}$

For safe design, qn≤ Nett allowable bearing pressure i.e. ${P \over {{B^2}}} \le qnet - allowable$
From the above formula calculation of B i.e proportioning of the foundation is done.
If after excavation of soil, the foundation is not backfilled then ${q_n} = \left( {{P \over {B2}} - \gamma {D_f}} \right)$

$\left( {{P \over {{B^2}}} - \gamma {D_f}} \right) \le {q_{net - allowable}}$

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