Reason for this type of failure when doing safety analysis

Hi all, 

I'm doing MH analysis using axisymmetric model. There are 2 materials, top part is 20mm thk steel plate and bottom 250mm thk RC as shown below. 

While doing un-drained analysis, during safety analysis case, the failure surface looks like near the meeting point of steel and RC part (not happened in drained analysis) and it reduces the value so much (i guess this failure can't be occurred). Please tell what is the reason for this?

Thanks. 

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