I noted that people would use different expressions for set criteria of driven piles. For example, 4 mm/blow or 40 mm/10 blows. Can I say they are the same? Thanks.
The best way to answer the question "Can I say they are the same?" is "Not really."
Let me explain it in US units, as I'm more familiar with the problem there.
Let's assume that we have a refusal criterion of 120 blows/ft. In theory we could also say 10 blows/inch; however, if we're driving to a dense/hard layer, the decrease in set may take place within the span of the foot. So the set could decrease substantially from the point where the pile toe entered the foot in question to where it left (or driving stopped.) Different sets imply different pile ultimate capacities, so whether you're using an inch, foot, 25 mm or 300 mm could make a difference.
How much of a difference it makes will depend on how much the soil profile varies. That's a job specific decision, but it's one that needs to be adhered to consistently once it's determined.