Poisson's Ratio For Gravity Loading

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

Dear Mr. Dennis, 

Thanks for the reply and please clarify point no 1, how did you say the friction angle should be 11 degrees?

Thanks. 

Just follow Jaky's rule :  K0 = 1 - sin(phi)  

Ah okay. Thanks 

Dear Mr. Dennis, 

Please clarify this doubt also. 

01. In advanced model like HS model, konc represents the normally consolidated at rest coefficient and ko is correlated as shown below (based on the plaxis reference manual).

How this equation is obtained?

02. If I enter konc value, after certain range, error message came, what is the reason and how konc is interconnected with the modulus?

03. How can ensure in ko procedure, whether the stresses are required only small equilibrium corrections or stresses are out of equilibrium?

Thanks. 

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

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

Thanks for the reply, there is one more question in the previous message, I typed here again 

03. How can ensure in ko procedure, whether the stresses are required only small equilibrium corrections or stresses are out of equilibrium?

Thanks.

Dear Mr. Dennis, 

Please reply to this question (One person put another (unwanted) message after my message).

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

Noted with thanks