This phenomena doesn’t apply to just dangerous work. One should expect any provision in a contract to be the norm. If a contract specifies that work on Saturday

This phenomena doesn’t apply to just dangerous work. One should expect any provision in a contract to be the norm. If a contract specifies that work on Saturdays is ok, assume every Saturday that you will work. Adjust your pay requirements to reflect this. Don’t let the other party to convince you that “it’ll only be sometimes” or “just in case”. For the 50% stuff, the real answer is empower the worker and train people to sign 2 different contracts for seemingly unrelated jobs. If that screws them out of benefits, then ask for more money. At the very least, it prevents the low pay for hard labor.