Q. Define Hammer efficiency and coefficient of restitution?

Hammer efficiency refers to ratio of kinetic energy of hammer to the rated energy (or potential energy). Basically it is undoubtedly that certain energy losses are induced by the hammer itself prior to the actual impact on the driven piles. For example these losses may comprise the misalignment of the hammer, energy losses because of inaccurate dropping height,guiding friction etc.

Coefficient of restitution refers to a value representing the strain energy at the time of collision regained after the bodies reverting back to their original shapes. If coefficient of restitution is equal to unity, it means that collision is elastic and all energy has been returned after the impact action. Therefore this is an index showing the degree the impact action in terms of elasticity.

In mathematical forms, 

Coefficient of restitution = - (v₁-v₂)/ (u₁-u₂)

Where u=initial velocity and v=final velocity after impact