A pressure bulb is a stress isobar that is a line containing all the points of equal stress. For a given load, an infinite number of isobars can be drawn.
Seat of settlement
Terzaghi recommended that vertical stress is considered of negligible magnitude when smaller than 20 % of the applied contact pressure and that about 80% of the total settlement takes place due to compression of soil within this pressure bulb and the region within the isobar is known as seat of settlement or active zone.
For an elastic isotropic, semi-infinite solid media, the depth of the 20% of the applied contact pressure is about 1.5B where B is the width of the loaded area of footing. Thus the wider the loaded area, deeper is its effect.
The depth within which soil compression contributes significantly to surface settlement is called as critical depth. For fine-grained soils the Critical depth extends to the point where the applied stress decrease to 10 % effective over burden pressure. For coarse grained soils, the critical depth extends to a point where the applied stress decreases to 20 % of the effective overburden pressure.