This document discusses rock mass classification based on RQD (Rock Quality Designation). It provides the equation for calculating RQD based on the length of intact core pieces over 10cm long as a percentage of total core length. It also includes a table classifying rock quality based on RQD percentages from very poor to excellent. Additionally, it notes that in underground openings it is possible to view the rock mass in 3D, so a 3D RQD may be used which can be calculated using the provided formula involving the number of joints.