You mentioned durability thats all. I thought you implied that the earthworker would as a result gain more hp. I simply thought you meant that be increasing its HP, its durability would go up. Sorry for the misunderstanding... didn't mean nothing by it.
I'm just thinking about the earthworker from a personal usage standpoint. I like to remove tubes for cosmetic reasons and to allow walls to be closer to a guard post (more efficient with space) and don't like having to repair them constantly because removing tubes burns health like crazy. And by reducing the damage percentage dealt to earthworkers it allows them to remove more tubes between repairing.
Ex. Earthworker has 125 hp, 40% of 125 is (50 hp). So alternatively you could say removing a tube deals 50 damage to the earthworker. By changing the % to 20% of 125hp (25hp), the earthworker takes 25 damage per tube removal. This also helps when you move it to be repaired, as it takes a much lower speed reduction from 20% reduction in hp.