Plastic analysis & safety analysis - different tolerance

Dear All

I am running a Plaxis 3D safety analysis for a shallow foundation with skirts for a subsea structure. I have the follow query and really appreciate if anybody can help

In Plaxis, the safety analysis can only start after a successful plastic analysis run.

My plastic analysis fails to reach the full load; by inspection it is believed due to the local soil failure around the foundation skirt - see attached screen shot (Figure 1); so I have slightly changed the tolerance from the default (0.01) to a slightly large number (0.02, up to 0.05) – this have made it a pass (See Figure 2).

then I run the safety analysis, at the beginning it gives below 1.0 and then it go back to 1.3 ( I stop the run, let’s say it become stable). see attached Figure 3. It is noted when i run the safety analysis, i used the default tolerance (0.01). the relative stress ratio after the safety run is shown in Figure 4.

Can I have you thought about this. can I take the stable value (say 1.3 ) as my safety factor? 

Also, having read some early discussions, I will add a nil- step between the plastic and safety analysis - hopefully it will help sort some out of balance force out?

Noted all the below pictures attached are from a worst case. for other cases, the plastic run failed marginally (reach 98% full load) and I have just needed to increase the tolerance to 0.02. can you advise if there are any limit set for this.

Thanks all for your help - look forward for feedback

Figure 1 plastic analysis 

Figure 2 plaxis analysis with updated tolerance 0.05

Figure 3 – Safety analysis with tolerance 0.01

 Figure 4 – Safety analsysi with tolerane 0.01 (stress mobilisation ratio)