Local failure in safety analysis

Question

Hi 
 I am finding a local failure in the interface between a layer (top about 0.5m) considered elastic (pavement) and the next below which is under Mohr-Coulomb model (soil). Even after I created a cluster and unchecked "apply strength reduction" I still note the local failure in the interface between the layers (Incremental shear strain output). Because of this, I am needing a lot of steps to get a global FoS and it is taking too much time. 
 I would appreciate any help 
 
 Regards,

Stefanos Papavasileiou · Answer

Dear Eduardo, 
 From the screenshot, it is difficult to fully understand what the failure mechanism is over there and especially since we have no information on the material parameters used. 
 Naturally, the pavement cannot fail as it is modelled as elastic (Linear Elastic?). A Mohr-Coulomb material model can fail, of course, and it can be that a failure mechanism would continue all the way up to the pavement material but it cannot as the pavement is elastic. 
 I can see that there are quite some big elements in the area of the high shear strains, so possibly refining can help identify better the failure mechanism. 
 The fact that the failure mechanism occurs when the "Apply strength reduction" option is unchecked, is not odd, as this option simply instructs the cluster not to "participate" in the strength reduction. However, if the failure mechanism already develops before, or the strength reduction on the clusters below has an effect on the new cluster, this would still develop shear strains in this area, too. 
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