Bearing Capacity of Soil
1. Gross Bearing capacity or Gross loading Intensity(qg):
- For a footing constructed with its base at a depth Df below the ground surface, the total pressure, qg 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(qg)
$${q_g} = {P \over {{B^2}}} + \gamma {D_f}$$
- It is the total pressure intensity at the base of the footing.
2. Net Pressure intensity or Net loading intensity(qn):
- 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$${q_n} = \left( {{P \over {B2}} - \gamma {D_f}} \right)$$