I went out drinking with a colleague last night. He works in the oil sands industry. We got to arguing about total and effective stresses in soil mechanics and tailings engineering. I illustrated the differences between total stress, effective stress, and pore pressure via our drinks, thus:
Add ice to your brandy, and you have a fluid in which the pore pressure is equal to the total stress; the ice cubes are not carrying load but are just floating or bobbing in the water, thus the effective stress is zero.
Conversely take your brandy on the rocks and the effective stress is pretty much equal to the total stress, and unless you have overdone the brandy, the pore pressure is pretty nearly zero. The ice carries the load and the brandy is nice and cold for drinking.
The point is that for both forms of brandy and ice, the effective stress law is valid. Altough if the ice is floating in the brandy, the effective stress equation may simply be irrelevant.
Of course you can think of this in terms of phase diagrams as mechanical engineers do when they plot regions for steam, water, and ice: the same stuff, just that different equations are more or less important depending on temperature and pressure.
We concurred on these concept. And we drank and chatted about total stress, effective stress, pore pressure, or common old garden stress in a fluid like brandy with a few clumps of ice where the effective stress concept and law are simply irrelevant—until that terrible time came when we had drunk too much brandy and the individual ice cubes were in particle to particle contact and the brandy bottle was empty. And the dominance of the effective stress induced sobreity panic.
