<P> 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 . The contact stress on the spheres decreases as the beaker is filled to the top of the spheres, but then nothing changes if more water is added . Although the water pressure between the spheres (pore water pressure) is increasing, the effective stress remains the same, because the concept of "total stress" includes the weight of all the water above . This is where the equation can become confusing, and the effective stress can be calculated using the buoyant density of the spheres (soil), and the height of the soil above . </P> <P> 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). Using our spheres as a model, this is the same as injecting (or withdrawing) water between the spheres . If water is being injected, the seepage force acts to separate the spheres and reduces the effective stress . Thus, the soil mass becomes weaker . If water is being withdrawn, the spheres are forced together and the effective stress increases . </P> <P> Two extremes of this effect are quicksand, where the groundwater gradient and seepage force act against gravity; and the "sandcastle effect", where the water drainage and capillary action act to strengthen the sand . As well, effective stress plays an important role in slope stability, and other geotechnical engineering and engineering geology problems, such as groundwater - related subsidence . </P>

Increase in effective stress on a soil mass