compressive strength gives a visual comparison of the strength and modulus values of different rocks. ! M R= E t50 / σ a (unconfined compressive strength) - Modulus Ratio – Deere and Miller Classification 50%! Strain! s! Slope of the line tangent at 50% of the ! unconﬁned compressive strength = E t50! 34! Engineering Classiﬁcation!