How to estimate the reduction of "Negative Skin Friction" for the soil (sand fill overlaying the clayey material) that have been consolidated for several years.
The existence of downdrag assumes that the settling soil moves downward more than the pile top does under the structural dead and live loads. Have you checked to see if this is the case? There is a detailed discussion of how to compute downdrag in the recently-issued FHWA publication FHWA-IF-99-025, by O'Neill and Reese, August, 1999, pages 297-307. This book can be purchased from the ADSC at 214-343-2091, or