Pre-boring for HP piles vs. soil-pile properties

[Inzynier]

1) How to account for reduced capacity of steel HP pile, which is inserted into prebored hole to minimize driving resistance?
Preboring or pre-drilling is used frequently on bridge projects to facilitate installation of steel piles in fills or hard clays. Piles are inserted in holes prebored to natural ground elevation shown in geotechnical report. After pile is inserted into the hole the space around the pile is filled with concrete (bottom 3 ft). Prior to conrete setting, the pile is driven to refusal.The the rest of the hole is filled up with sand all the way up. The diameter of the hole is slightly larger than the cross dimension of the pile.
2) If preboring is specified, is there any reason to use pile shoes?

Thank for your help.

 

[72shortbox]

For the prebored section I typicaly neglect any frictional resistance, however if I needed more capacity I would have the entire hole filed with concrete and design the top as a caisson
The the material being driven to will control if pile tips are needed.