Verifying Piping by exit gradient in Plaxis 2D

Dear all,

I am checking piping for an open excavation (20m deep) with side slope by applying Fully Couple Flow analysis in Plaxis 2D V20. The excavation work will be carried out through 20m thick of clay layer then contact sand layer. When I show the output contour of gradient and gradient for whole domain and at the top of sand layer where the final excavation reaches,and the piping could occur, I found some problems. Firstly, In terms of theory, the highest hydraulic gradient at the top of sand layer should have been at around the location of toe slope, however, it only true for the later excavation steps that nearly touch the sand layer, in some intermediate excavated levels, the greatest gradient situates in other places. I am curious about the accuracy of this. Secondly, for intermediate excavation steps that do not contact to sandy layer, the critical gradient areas place at clay layer(undrained behavior) right overlying in sand (drained behavior). Is this right? In one layer the hydraulic gradient at the top is usually higher than those of bottom and even those of top of underlying layer. I wonder if this right? Finally, should I check the exit hydraulic gradient at top of sand layer ( right at the transition of clay and sand layer) or a bit lower in sand layer ( around 1mm below the top of sand layer)?

there are some file in the link below you could find for more information.

https://drive.google.com/drive/folders/1yzoNoz2dMstAbrUR0Obf0ErOy4f__jK4?usp=sharing

thank you.