What is negative skin friction on piles and how is it addressed?

Negative skin friction (downdrag) occurs when soil settles relative to pile, adding downward force instead of supporting load. It happens in: compressible soil layers, fill placement, groundwater lowering, or adjacent loading. Downdrag force can equal or exceed structural load. Mitigation methods: preloading to complete settlement before piling, coating pile shaft with bitumen or other slip coatings, using larger pile sections, ignoring shaft capacity in settling zone, or using settlement-reducing foundations (rafts).