Why in driven piles, the allowable set is limited between 25mm and 50mm per 10 blows ?

 The max. limit of50mm/10 blows is established in the code principally to avoid very heavy hammer impacting at pile head so that the pile head may be damaged due to extra compressive stress

Agreed with Mr.Yaqoob's answer as well as Mr.Fouad's full answer, thanks.

Thank you my dear brother for your invitation ,,,,.  It is commonly to the limit pile set between25mm/10 blows and50mm/10 blows. In final set table . Because of the provision for lower limit of set value of25mm/10 blows is to avoid possible damage at pile toe owing to compressive stress reflected at the bottom of pile. The upper limit of 50mm/10 blows is established principally to avoid very heavy hammer impacting at pile head so that the pile head may be damaged owing to excessive compressive stress . These limitations are believed to be applied  originally to precast concrete piles.

i agree with engineer Yaqop and Foaad

I agree with the experts, to avoid any possible damage caused by force or hammering. 

Thank you for invitation:

there is no answer after our teachers En.Jafar, En.Fouad and En. Yaqob..

I support all answers of you.