Low Msf due to local failure in PLAXIS safety calculation

I have a question on safety calculation in PLAXIS 2D. I used safety calculation in PLAXIS to check the overall stability of retaining wall and got a very low Msf value. By checking the results, a local failure occurred on the ground surface, for example the edge of surcharge. Besides a higher FoS is achieved at the intermediate calculation steps, but then declined to a lower values. The mesh quality is very good at the area surcharge applied. So I don't think it is due to a mesh quality issue. Is there any way to avoid such local failure in the safety calculation?

  • Dear Zhilong Liu,

    There are some possible reasons, why PLAXIS gives local failure mechanism.

    Firstly, we should check soil parameters and phase sequence.

    Secondly, we should eliminate the impact of mesh.
    On screenshot I can see relatively fine mesh at the bottom part of pit. But on the left on the ground surface the mesh is quite coarse: it can be seen via amount of load arrows. Maybe it would be better to refine the mesh in this part and check the result again.

    Thirdly, we can change some iterative procedure settings. 
    For example, in this article PLAXIS suggests to use Max unloading steps or Arc length control settings.
    The user should evaluate the impact of such changes and carefully recheck other results in Output.

    Finally, PLAXIS has a good instrument to exclude some parts of soils, that have mathematically predetermined local failure mechanism.
    In Selection explorer you can deactivate option called Apply strength reduction:

    By default it is activated for all soil clusters.
    If you deactivate it, the PLAXIS will not reduce strength parameters in this cluster.

    So in common case you can draw a small cluster, which limits local failure area:

    and then deactivate Apply strength reduction option on Safety phase.

    By the way, before this option has appeared in PLAXIS 2D, users had artifically increased strength parameters in this part =)

    More information about Apply strength reduction option you can find in Reference manual.

    I hope this information will help you.

    Answer Verified By: Zhilong Liu 

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