Modelling a pile in slide which improves your safety factor is a fallacy. The first big problem is 2D v 3D. Are you modelling the pile as a soil element and having high (few thousand?) cohesion value?

All this will do is force the failure plan to extend below your pile tip.

In short, limit equilibrium stability programs are not a good way to model retaining walls. Soil nail anchors, etc yes, embedded retaining walls no IMO.

Agree with EireChch 2D slope stability programs are not a good way to look at piles. They are good at modelling uniform slopes, where the represntative section is at least 4 times the height of the slope. At lower ratios 3d effects offer significant resistance.

To respond to why the FOS increased you want to look at the graph which shows you the shear resistance for each slice along the failure plane with and without the pile for the exact same slip surface.

I agree with what has been said above.

In addition if you need/want to model a pile in a 2D software program, my suggestion is to model the pile as a reinforcing load acting perpendicular to the slide plane. Determining the appropriate load is difficult, but can be done using other software programs such as LPile.

Mike Lambert

Thank you for your professional answers
I will try other analysis software for slope and pile modeling