Talk:Effective stress

Untitled

I'm starting a rewrite of this series, to be more consistent with a general 'discovery channel' level, and pretty pictures. Zeizmic 14:49, 11 September 2006 (UTC)

Recommend adding a link to "Stress" http://en.wikipedia.org/wiki/Stress_%28physics%29

Maybe some more explicit definitions would help

I have the feeling this article is a bit unclear. First Terzaghi's effective stress is introduced. Then a very vague definition is introduced: "Stress causing displacement", "average stress carried by the soil skeleton". Later the effect of buoyancy is introduced in the discussion. Then Darcy's law is introduced, and forces related to movement is discussed. This is really a mixup of different things.

Terzaghi's effective stress is simply the normal stress corrected for buoyancy (archimedes law). (You can work that out simply by balancing the forces acting on the solid from the weight of the total column minus the weight of the displaced pore fluid.) However, it is assumed that there is zero frictional forces or any other forces involved in this force balance. Furthermore, the effective stress concept is always specific to both the property we are after and the material itself; 1) The effective stress that causes volume strain is not necessarily the same effective stress that causes failure. 2) The effective stress is intimately linked to the material: it is a constitutive concept. As an example of the second point: the effective stress regarding volume strain for an elastic material is not the same as a viscous material. However, the form may be the same, i.e., a Biot like formulation:

effective stress = total stress - (fluid pressure)*(Beta)

where Beta depends on tons of stuff. It will though approach zero as the porosity approach zero, because than there is no more pore fluid to buoy up the solid. As friction etc increases Beta will decrease as the pore fluid becomes more and more involved in other force (grain-water-grain)balances: Effective stress is a macroscopic static concept from upscaling the microscopic force balance.

However, Darcy's law and the rest of the discussion does not belong to the effective stress article. Darcy's law is about the conservation of momentum of the pore fluid. (a similar law will apply for the matrix). The interaction forces due to a relative velocity between the solid and fluid and phenomena such as fluidization and liquifaction are completely separate things.

PETRSCIENT (talk) 13:52, 2 August 2009 (UTC)

Maybe an introduction would help

I added one, although I wasn't sure of the details. a once-over by an expert would be good.

I thought the pic was a good illustration of it. OsamaBinLogin (talk) 20:15, 10 December 2011 (UTC)

Define variables! (and fix formula)

Please define the variables in the equations! H? gamma?

129.170.241.32 (talk) 01:32, 22 January 2012 (UTC)

looking at the equations i would say H is thickness of overlying solid/fluid and gamma is density of the overlying solid/fluid — Preceding unsigned comment added by Cookstein (talk • contribs) 19:11, 16 August 2012 (UTC)

In addition to the lack of definitions the fact that σ and u can both be written as products of H:s and w:s strongly suggest that it should be σ'=σ - u (or possibly σ'=σ + u) and that σ'=σ u should not be understtood as σ'=σ * u. 150.227.15.253 (talk) 10:21, 20 January 2021 (UTC)

There is some ambiguity as to whether the stated equation is in a tensor format or not. I'd recommend writing it in Einstein notation. — Preceding unsigned comment added by 128.220.159.7 (talk) 16:12, 16 August 2022 (UTC)

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