Longitudinal crack in PHC piles

21 blows for 20mm penetration followed by 59 blows for 42mm penetration. IMHO, the piles were needlessly overdriven... causing the damage.
What are the specs for the hammer?

'seemed to be within permissible limits' - thats your issue right there

You need to do a GRL WEAP pile drivability analysis and check what stresses you are generating. This should have been done to select the hammer prior to driving.

You should also have some small strain pile testing to verify driving stress