Structures that are built close to existing retaining or sea-walls generally have surcharge load limitations imposed on their construction. This will require the load which exceeds the allowable KPa surcharge load to be transferred to a zone of soil which is at least 0.5m below the imposed line of influence. This line will extend from the nearest face of the foundation of the existing retaining wall. Load transfer needs to occur below that depth.
There is a stark difference between conventional bored concrete piles versus screwpiles in this senario in that a concrete pile will, by way of simple design, transfer load all the way along its shaft located in the upper zone as well as in the lower zone. The only way to avoid this is to install a typical PVC or polyproporlyne outer sleeve in the upper zone to minimise the large skin friction due to the concrete pile shafts diameter.
No so with screwpiles designed without helix's in the upper zone. A screwpiles shaft is slended in comparison to concrete bored piles and skin friction is already minimal negating the need for expensive liners. Typically a screwpile solution in this situation will be significantly shorter as it is performing as an end bearing pile.
Compounding the issue is the likleyhood of geo-grid (Tensars) layers tying the walls back into the embankment. Piling to transfer surcharge loads will need to pass through the layers without affecting their intended performance. Screwpile leading and cutting edges are cut sharp to ensure penetration through the polypropylene layers is clean.