Dear Poormousavian,

The MC model dose not consider the strain-dependent stiffness behavior or the small train characteristics that invole high stiffness modulus at small strain levels of soil. The MC model only uses a single Young's modulus and dose not also distinguish between loading and unloading stiffnesses. In the HS model, soil stiffness is calculated much more accurately by using three different stiffnesses (triaxial loading secant stiffness, triaxial unloading/reloading stiffness and oedometer loading tangent stiffness). The MC model represents Young's modulus of soil in the in situ stress state. On the other hand,  the HS model represents its three moduli at the reference pressure, and these moduli at the in situ stress state are automatically calculated as a function of the current stress state.

I suggest this paper:

"Evaluation of Constitutive Soil Models for Predicting Movements Caused by a Deep Excavation in Sand"

Mehdi,

In majority of cases it would be more logical to obtain smaller displacements with HS than MC model, when dealing with excavations. Which deformation parameters have you been using to model MC behavior (compression or unloading modulus, etc.)? The reason is that MC does not account for unloading in its formulation. The soil behaves much stiffer during excavation. Of particular importance in these cases is small strain stiffness (very high value) at some distance from the excavation. HSsmall model can account for small strain stiffness along with nonlinear stress-strain dependency. 

Dear Poormousavian,

The MC model dose not consider the strain-dependent stiffness behavior or the small train characteristics that invole high stiffness modulus at small strain levels of soil. The MC model only uses a single Young's modulus and dose not also distinguish between loading and unloading stiffnesses. In the HS model, soil stiffness is calculated much more accurately by using three different stiffnesses (triaxial loading secant stiffness, triaxial unloading/reloading stiffness and oedometer loading tangent stiffness). The MC model represents Young's modulus of soil in the in situ stress state. On the other hand,  the HS model represents its three moduli at the reference pressure, and these moduli at the in situ stress state are automatically calculated as a function of the current stress state.

I suggest this paper:

"Evaluation of Constitutive Soil Models for Predicting Movements Caused by a Deep Excavation in Sand"

Thank you for your attention Mr.Zoran and Mr.Sadegh

I used E50 for MC model as a soil stiffness and the displacements gained from MC model are much higher than HS model but what surprise me is about safety factor. Isn't logical when you calculated higher displacement, you anticipate lower safety factor?

In calculation, HS and MC safety factors are approximately identical.

Do different soil models ( HS or MC) have effect on C/phi reduction method?

You should distinguish between ultimate (failure) and serviceability (deformations) limit states. During phi/c reduction (failure analysis) both models behave similar, i.e. they obey linear Mohr-Coulomb failure criterion (tau=c+signtgphi).

(Do not get confused with MC linear failure envelope and MC model - they are not the same, MC model is elastic-perfectly plastic stress strain model and MC criterion is simple linear failure envelope).

In Plaxis deformation parameters such as E50, Eun, Eoed, v and etc. should not have any influence on FoS value. (In reality higher displacements could lead to reduction in shear strength form peak to residual for brittle clay materials with plateay grains, but this phenomenon-progressive failure is a subject of displacement analysis, and failure is characterized by displacement vector plots not the value of FoS).

Displacements you are obtaining when you plot sumMsf vs displacements plot are unrealistic and show only that solution converged.

As far as displacements of the model are concerned you should treat them from a serviceability point of view. If for e.g. your excavation has lateral deflections of 10 cm and it did not fail you should make a decision whether this is admissible or not for your structural support system.  

Just to make it clear regarding no.4 paragraf in my previous statement. FoS is obtained when convergence could not be met in phi/c reduction process. The sumMsf vs displacement plot (sumMsf stays constant with increasing displacements) shows that you have reached steady FoS value.