Local surface failure in safety calculation

I'm a relatively new PLAXIS 2D user and I can't fix this issue on my own.

I'm modeling gravel excavation slope.

I'm trying to uncheck "Apply strength reduction" option to remove local surface failure from a safety calculation.

After unchecking "Apply strength reduction" option, a failure surface forms around unselected soil cluster. 

After widening the cluster the same happens.    

Is there any other way to remove these kinds of surface erosion failures from a safety calculation?

The safety calculation starts with a decent looking fail surface but after 20 or so steps, safety factor drops and fail surface is replaced with the surfaces described above.  

Any input or help is appreciated.

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