Closed End Pile in Alluvial gravel

[designrider]

Driving 24" pipe pile to 850kip ultimate capacity using gates equation. Piles driven open-ended through alluvial gravel with numerous cobbles up to 12 inches in diameter as well as occasional boulders. Pile embeds are going much deeper than expected, 80ft embed estimated, actual 140ft+. SPT N-value=50 at these depths. Is driving closed-end pile (flat plate at tip, not cone) a legitimate option? Realistically, shouldn't a reliable soil plug have already formed at these embeds? Will the cobbles and boulders compromise the performance too much causing piles to go rouge, misalignment issues, etc? If we try closed-end, is it acceptable to try a pile with a 10ft open section at the tip (i.e. cap plate installed up the pile length). The open section used to assist in a more reliable pile alignment at the start of driving. Will the trapped air in that pile length compromise performance (i.e. compressed air dissipates over time leaving void inducing settlement)? Pile are temporary, service life of <3yrs.

Thanks in advance for any input!