Stress distribution in soil

A load placed on the ground surface spreads out as it goes down. Directly beneath a footing the added stress is high near the base and falls with depth and with horizontal distance. Knowing that distribution lets engineers estimate settlement of each layer and judge how deep investigations must go.

Boussinesq's solution treats the ground as a uniform elastic half-space and gives the vertical stress below a point load as a function of depth and radial offset. Integrating it gives stresses under uniformly loaded circles, rectangles and strips. Newmark's influence chart and Fadum's charts made these integrations practical by hand. Westergaard's version suits layered soils with stiff horizontal seams that restrict lateral strain. Contours of equal stress form the pressure bulb; the zone within about 0.1 to 0.2 of the surface pressure, roughly 1.5 to 2 times the footing width deep for a square footing, is where most settlement occurs.

A quick approximation is the 2:1 method, which spreads the load at a slope of 2 vertical to 1 horizontal, so a B × L footing's load acts on (B + z)(L + z) at depth z. It is good enough for preliminary work. Closely spaced footings have overlapping bulbs, so their stresses add, and a raft creates a much deeper bulb than an individual footing, important where weak layers lie at depth.