Q. Fixed vs pinned connection between piles and pile caps?

The type of connection amid piles and pile caps affects the load carrying capacity of pile groups. Fixity of pile head into pile cap, in place of pinning into pile cap, improves higher lateral stiffness of the pile groups. For example, for same deflections, a cap with fixed connected piles can withstand far more loads than that of pinned connected piles. To satisfy criteria of fixed connection, minimum embedded length of piles into pile caps must be at least two times the diameter of piles. 

Furthermore the fixed connection of piles at pile caps allows significant bending moment to be transmitted through the connections when compared with pinned connections.