Gross Pressure Intensity
It is the total pressure at the base of the footing due to the weight of the superstructure and earth fill, if any.Net pressure intensity. It is the total pressure at the base of the footing due to weight of the super structure. It is the increase in pressure at foundation level, being the total weight less than the weight of the soil permanently removed.
- Before removal of site, stress at foundation level (So)
s = ? x D
- After excavation, some soil is removed which was imposing a stress equal to ? x D.
qnet = qgross - ? x D
If qgross = ? x D
Qnet = 0
Since net pressure increases qnet = 0, this implies that settlement should theoretically be zero because settlement is caused due to increase in pressure (qnet). However, when this much soil is removed, the soil mass at the bottom of excavation will expand (heave) from loss of overburden pressure.Any mass placed on this expanded soil will result in recovery of some of this expansion.