Built by a geologist
Q-System Rock Mass Calculator
The Q-system is an empirical rock-mass classification developed by the Norwegian Geotechnical Institute for stability assessment of underground openings. This calculator follows the NGI handbook revised in June 2022 and exposes the full equation: Q equals RQD divided by joint set number Jn, multiplied by joint roughness number Jr divided by joint alteration number Ja, multiplied by joint water reduction factor Jw divided by stress reduction factor SRF. The first quotient is a relative block-size expression, the second represents inter-block joint friction, and the third represents active stress from water and the selected stress regime. These are classification factors, not direct measurements of block volume, shear strength, pressure, factor of safety, or support demand. RQD is entered from zero through 100 percent. NGI requires an effective RQD of 10 in the Q equation whenever the reported or measured RQD is between zero and 10, including zero. GeoMiner preserves the entered value and displays the effective value so this floor is never hidden. RQD is directional and affected by drilling, logging, natural-versus-mechanical break decisions, core diameter, run selection, orientation, and sampling scale. Joint set number Jn is selected from mapped joint geometry, not merely the number of orientations seen across an entire project. The NGI base table spans 0.5 to 20, with a multiplier of two for portals and three for tunnel intersections. The relevant sets must occur at the same location and form blocks at the scale of the opening; combining sets from unrelated sections can inflate Jn. Joint roughness Jr describes the least favourable relevant set with attention to small-scale roughness, large-scale planarity or undulation, wall contact, block size, and likely sliding direction. The base table spans 0.5 to 4 and includes a conditional note that can add one when spacing exceeds three metres. Thick soft filling that prevents wall contact changes the interpretation: roughness may no longer control. Joint alteration Ja describes coating or filling thickness and character. Published values extend from 0.75 for tightly healed hard filling to 20 for the upper end of thick swelling-clay zones. Visual labels alone are insufficient; mineral identification, wall contact, thickness, continuity, weathering, and softening behaviour can matter. Jw ranges from one for dry exca