Effective stress

Usually this applies to sand, soil, or gravel, as well as every kind of rock and several other porous materials such as concrete, metal powders, biological tissues etc.

Alec Skempton in his work in 1960,[5] has carried out an extensive review of available formulations and experimental data in literature about effective stress valid in soil, concrete and rock, in order to reject some of these expressions, as well as clarify what expression was appropriate according to several work hypotheses, such as stress–strain or strength behaviour, saturated or nonsaturated media, rock/concrete or soil behaviour, etc.

In 1962, work by Jeremiah Jennings and John Burland examined the applicability of Terzaghi’s effective stress principle to partly saturated soils.

[citation needed] Consider a grouping of round quartz sand grains, piled loosely, in a classic "cannonball" arrangement.

[citation needed] If we then have these spheres in a beaker and add some water, they will begin to float a little depending on their density (buoyancy).

With natural soil materials, the effect can be significant, as anyone who has lifted a large rock out of a lake can attest.

[citation needed] The concept of effective stress truly becomes interesting when dealing with non-hydrostatic pore water pressure.

Under the conditions of a pore pressure gradient, the ground water flows, according to the permeability equation (Darcy's law).

If water is being injected, the seepage force acts to separate the spheres and reduces the effective stress.