Yesterday a wise professor made a presentation that caputured an enduring fact and a recent change in tailings analysis and modeling. He did this at a meeting I happened to be in, and I repeat below from his PowerPoint presentation.

  • In 1943, Terzaghi wrote: "Soil Mechanics is the application of the laws of Mechanics and Hydraulics to engineering problems dealing with sediments and other unconsolidated accumulations of solid particles."
  • It was difficult to do this for even the simplest problems until the development of the computer and specific codes in the past 20 years.
  • We have used the Computer Code FLAC, Fast Lagrangian Analysis of Continua, for design of the cover of the tailings pond as the code incorporates these laws of mechanics and hydraulics in a rigerous manner.

The problem is, do you use total or effective stresses in these computer codes?  Here is one way to easily conceptualize the issues:

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.

The point is to be very careful when dealing with oil sands tailings in a computer code.