Poisson's Ratio For Gravity Loading

Question

Hi all, 
 
 I have to define the soil with ko = 0.8. Since the soil layer is not horizontal I need to use gravity loading to generate stress at the start. horizontal stress can be generated using poisson's ratio (v) in gravity loading (k = v / (1-v). Therefore for ko = 0.8 , v = 0.445, but when I assigned this one Plaxis said v is too high.

Please explain why is this and how to generate the stress for ko = 0.8 using gravity loading (as the soil layer is not horizontal. 
 
 Thanks.

Dennis Waterman · Accepted Answer

Hi, 
 The message occurs because a Poisson's ratio of close to 0.5 indicates that the soil is almost incompressible, which usually means that it's undrained. Since you're supposed to give an effective Poisson's ratio the software warns you that what you're entering more feels like an undrained Poisson's ratio. If you have a K0 of 0.8 there are two possibilities: 1) either the soil is normally consolidated and has a friction angle of 11 degrees 2) the soil is overconsolidated In case 1 you could decide to ignore the message and it will give you the correct K0. But since you're clearly the Mohr-Coulomb model, which is an elastic model, the Poisson's ratio determines the K0 but also the compressibility and you kind of say now that your soil is almost incompressible. If that is not what you want you should move into using Hardening Soil where those are not coupled and you can just specifiy K0=K0nc=0.8 and a Poisson's ratio of 0.2. 
 In case 2 you shouldn't use Mohr-Coulomb to start with....since Mohr-Coulomb can't handle properly overconsolidated soil. For overconsolidated soil K0 and K0nc are not the same, while for Mohr-Coulomb they are and also linked to the Poisson's ratio. Hence, use the HS model, or HSsmall, or Soft Soil .... 
 With kind regards, 
 Dennis Waterman

Dennis Waterman · Answer

Dear Nitharshan, 
 You can't ensure it .... but you as a engineer / PLAXIS user should recognize the situations where the K0-procedure is almost correct, or when K0 would really generate incorrect stresses. This depends on soil layering, water levels, existing (initial) structures etc. 
 With kind regards, 
 Dennis Waterman

Dennis Waterman · Answer

Hi, 
 1) Please check the Material models manual, chapter 2, section "On the initial stresses". 
 2) The value of K0nc is linked to the stiffness E50ref through the shape of the compression cap. So not just any combination of Eoedref, E50ref and K0nc can be used. On the other hand, I see you want to enter a K0nc value of 0.7, which is really high, even for a very soft soil. And you also specify a friction angle of 30 degrees .... this is not a realistic combination. With kind regards, 
 Dennis Waterman

Poisson's Ratio For Gravity Loading

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