Are you refering to the Ensoft program L-Pile? It uses p-y curves internally generated or from user inputs. With p-y curves you're trying to model soil response more accurately as non-linear. Some methods go with a simplier linear soil response which may be accurate enough for some applications. I've used an older version of L-Pile for drilled shaft (augered pile) retaining walls and I believe the p-y curves allow you to get a better handle on the deflection response of the wall.
I wrote down the wrong thing. I actually meant Evans and Duncan charts. They were made using a series of p-y method computer analyses. It looks like they have a wide variability in deflection results, but pretty accurate results with moment tests. The charts are really easy to use though and I don't have L-Pile.
We are wanting to get deflection results because the piers are supporting some retaining walls. If the retaining walls move enough, then people look at it and think it's the end of the world.
Cantilever retaining walls are going to move and there is a good deal of uncertainty in the expected deflection because of variability and uncertainty with respect to soil properties. I believe you'd be correct to recognize that soil properties do not have a huge effect on maximum moment in the piers.
If I understand your problem correctly you are concerned about lateral movement of a cantilever retaining wall on piers. Have you considered battering the front piers to resist lateral movement at the base of the wall?
As to building the p-y curves, I just go with the correlations provided with program documentation. The engineers at the Ensoft company are leading experts in the field of laterally loaded piling. I'm not about to second guess them. I put most of my effort into analysing soil boring data to decide what layers are most representative of the resisting zones on the shafts.
I'm not qualified to advise on how to build p-y curves but there is plenty of instruction and advice available in print. There is an excellent text by Reese and Van Impe "Single Piles and Pile Groups Under Lateral Loads".
