Rock Mass Classification: RMR and Q-System

Rock in mines and tunnels is cut by joints, faults and bedding planes, and it may be weathered or wet. Laboratory tests on small samples cannot capture this. Rock mass classification gives engineers a practical, repeatable way to describe rock quality from core logging and mapping, compare sites, estimate rock mass properties and choose support, especially at early design stages.

Rock Quality Designation (RQD)

Developed by Deere in the 1960s, RQD is the percentage of a drill core run made up of intact pieces 100 mm or longer:

$$RQD = \frac{\sum \text{lengths of core pieces} \geq 100\ \text{mm}}{\text{total core run length}} \times 100\%$$

RQD is simple but ignores joint condition, orientation and water, so it is used as an input to fuller systems.

Download diagram (WebP)

Bieniawski’s Rock Mass Rating (RMR)

The RMR system (1973, updated 1989) sums ratings for five parameters, then applies an adjustment for joint orientation relative to the excavation:

RMR classes:

RMR also gives estimates of rock mass cohesion and friction angle, and through correlations, the rock mass deformation modulus.

Barton’s Q-system

Developed at the Norwegian Geotechnical Institute in 1974 from many tunnel case histories, the Q-system rates rock quality as:

$$Q = \frac{RQD}{J_n} \times \frac{J_r}{J_a} \times \frac{J_w}{SRF}$$

• RQD/Jn: block size, where Jn is the joint set number.
• Jr/Ja: inter-block shear strength, from joint roughness Jr and joint alteration Ja.
• Jw/SRF: active stress, from joint water reduction Jw and stress reduction factor SRF, which covers weakness zones, high stress and squeezing or swelling rock.

Q ranges from about 0.001 (exceptionally poor) to 1000 (exceptionally good). The Q support chart plots Q against the equivalent dimension, span divided by an excavation support ratio (ESR) that reflects how critical the opening is, and recommends bolt spacing and length and shotcrete thickness.

A widely used correlation between the two systems is RMR ≈ 9 ln Q + 44.

Geological Strength Index (GSI)

Introduced by Hoek, GSI rates rock mass from 0 to 100 using a chart of structure (blockiness) against surface condition of joints. It is the main input to the Hoek-Brown failure criterion for estimating rock mass strength and modulus in numerical models.

Using rock mass classification in design

• Preliminary support design for tunnels, drives, shafts and caverns.
• Stope design with modified systems such as Laubscher’s MRMR for caving and the stability graph method for open stopes.
• Slope design with Romana’s slope mass rating (SMR), relevant to open pits.
• Estimating rock mass properties for numerical modelling.
• Choosing mining methods, since strong rock allows large open stopes while weak rock needs fill or caving, as discussed in surface vs underground mining.

Collecting the data

Classification inputs come from several sources. Core logging records RQD, fracture frequency, joint roughness and infill along every drill hole. Face mapping during excavation records joint sets, spacing, persistence and water. Laboratory tests give intact strength, and point load tests offer a quick field estimate. Consistent logging standards and photos of every core box make ratings repeatable between geologists and over time.

Limitations and good practice

Classification is a starting point for design, not a substitute for engineering analysis. Ratings should always be recorded with the data behind them so that others can review and update them as excavation reveals more of the ground. For critical or unusual excavations, numerical modelling and site specific testing are essential.

Classification systems simplify complex ground. They may not capture stress driven failures in deep mines, time dependent squeezing, or individual wedges defined by specific joints. Good practice is to use more than one system, map rock carefully during excavation, monitor displacements with extensometers and convergence measurements, and update designs. Rock quality also influences drilling and blasting design and the stability of foundations, much as soil properties control bearing capacity in civil works.