Role of RMR in underground design

The RMR system is one of the most widely used tools for design of underground openings (tunnels, caverns, mines, shafts) because it turns field observations into numbers that can be used directly in design.

Role / importance

1. Quality assessment: divides the rock into five classes (very good to very poor), so the tunnel can be divided into sections of similar behaviour (geotechnical zoning).
2. Stand-up time: gives the unsupported span and time the rock will stand, which decides how soon support must be installed.
3. Support design: the class gives guidelines for rock bolts (length, spacing), shotcrete thickness, steel ribs and wire mesh.
4. Excavation method: suggests full-face, top heading and bench, or multiple drifts and the length of advance per round.
5. Strength parameters: cohesion and friction angle for each class, and the modulus of deformation $E = 2\,RMR - 100$ (GPa, for $RMR > 50$) for numerical models.
6. Orientation adjustment: the tunnel axis direction relative to joints (favourable to very unfavourable) can reduce RMR by up to 12, which helps choose the best alignment.
7. Cost estimate and planning: quantities of support and construction time can be forecast; it is also used at the face for quick checks during construction (updating the design).
8. Communication: a simple common language between geologists, designers and contractors.

It is used together with the Q-system for cross-checking, and the empirical relation $RMR = 9\ln Q + 44$ links the two.