Introduction to terms
Hardness is related to compressive strength, but there are also great differences. The rock hardness is the resistance of the local part of the rock surface to another object when it is pressed in, and the rock strength is the resistance of the whole rock when it is broken, so the uniaxial compressive strength of the rock cannot be taken as an indicator of the rock hardness. According to theoretical analysis, the rock indentation hardness is (1+2m) times of the uniaxial compressive strength. The experiment shows that the ratio of rock indentation hardness to uniaxial compressive strength is about 5~20 times. For drilling broken rock, the rock indentation hardness is more practical than the uniaxial strength. Because the tool's crushing method for rock at the hole bottom is local crushing in most cases, the rock hardness index is closer to the difficulty of drilling broken rock.
