Rock slopes aren't formed only by one huge block of intact rock. They're usually found in blocks separated by the sets of joint that are part of it. These joints changes the behavior of how the rock responds to stresses. That's why Hoek came up with the knowledge of brittle failure and Brown with the knowledge of the jointed rock mass behavior and, together, they identified a empirical relation of rock mass field descriptions (Rock Mass Rating) with a failure criteria for rock masses. Their research, along with the research of other authors (Palmstrom, Bieniawski, Barton, etc), is brilliant! By applying the equations derived by their results geoscientists can estimate under which stresses will a rock mass enter the failure state with only scrupulous field observations, accurate measurements and an attentive reading of their articles. In this tutorial, I will go through the process of calculating the Hoek-Brown Failure Criteria using Python.