Lithology
One of the formulas
Hard
H l i = 100 ∑ M 0.15 ∑ M + 3.12 ± 11.18
H l i = 30 M + 10
Medium hard
H l i = 100 ∑ M 0.23 ∑ M + 6.10 ± 10.42
H l i = 20 M + 10
Weak
H l i = 100 ∑ M 0.31 ∑ M + 8.81 ± 8.21
H l i = 10 M + 10