IS 13365 (Part 1) : 2000The quantitative classification system of rock mass - Guidelines, Part 1: RMR for predicting engineering properties

IS 13365 Part 1:2000 gives the quantitative classification system of rock mass — guidelines for Rock Mass Rating (RMR) for predicting engineering properties. It is the rock-engineering classification used to characterise a rock mass for tunnels, underground caverns, rock slopes, dam foundations and deep rock excavations — turning qualitative geology into a number that drives support and design decisions.

It is read with the rock and geotechnical stack:

• IS 8764 — point load strength index of rocks (a feeder input)
• IS 13365 Part 2 — rock mass quality (Q-system) — the complementary classification
• IS 4464 / IS 5878 — core logging / tunnelling codes · IRC tunnel/slope references
• IS 2720 — soil tests (the soil counterpart of rock characterisation)

RMR is the sum of ratings for measurable rock-mass parameters, adjusted for joint orientation:

• Uniaxial compressive strength of the intact rock (or point load index)
• RQD — Rock Quality Designation (sound core ≥ 100 mm as a % of run)
• Spacing of discontinuities (joints/bedding)
• Condition of discontinuities (roughness, aperture, infilling, weathering)
• Groundwater conditions
• Adjustment for discontinuity orientation relative to the works (favourable → unfavourable)

The total (0–100) places the rock in a class (I Very good → V Very poor), which is correlated with stand-up time, recommended excavation method, and the type/quantity of support (rock bolts, shotcrete, ribs) for tunnels, plus shear-strength estimates for slopes/foundations. It converts site logging into a design-usable engineering rating.

Data from a mapped tunnel reach: intact UCS ≈ 100 MPa; RQD ≈ 75%; joint spacing 0.3–1 m; joints slightly rough, slightly weathered, < 1 mm aperture; damp; joint orientation fair relative to drive.

Step 1 — sum the parameter ratings (UCS + RQD + spacing + joint condition + groundwater) → a base rating in the ~65–70 band.

Step 2 — orientation adjustment: apply the (negative) adjustment for the *fair* joint orientation → adjusted RMR ≈ 60.

Step 3 — class & implications: RMR ≈ 60 → Class II–III (good to fair rock) → a meaningful but limited stand-up time, with systematic rock bolting + shotcrete support typical (light ribs only in poorer zones).

Step 4 — design use: feed the class into the support standard for the span, estimate rock-mass shear strength for any portal slope, and re-map and re-rate every reach — RMR is reach-by-reach, not one number for the tunnel.

1. One RMR for the whole tunnel/slope. Rock mass varies along the alignment — RMR must be re-evaluated reach-by-reach; a single average hides the dangerous poor zones.

2. Bad RQD. RQD is sensitive to core handling and the 100 mm rule — sloppy logging or drilling-induced breaks corrupt the rating.

3. Ignoring the orientation adjustment. The same rock with joints daylighting into the excavation is far worse than with favourable orientation — skipping the adjustment over-rates the mass.

4. Using RMR alone for support. Read it with the Q-system (Part 2) and engineering judgement; the two classifications cross-check each other.

5. Treating groundwater as static. Inflow during excavation can drop the rock class sharply — design for the construction-stage water condition, not just the dry log.

• IS 13365 Part 2 — rock mass quality (Q-system) classification
• IS 8764 — point load strength index of rocks
• IS 4464 — code of practice for presentation of drilling/core information (logging)
• IS 5878 (parts) — construction of tunnels · IS 11315 — rock-mass description methods
• IS 13063 / IS 14448 — rock-slope and rock-anchor references
• IS 1893 — seismic input for underground/rock structures

IS 13365 Part 1 (RMR) and Part 2 (Q-system) are the Indian-codified forms of the internationally standard Bieniawski RMR and Barton Q rock-mass classifications — reaffirmed and still the routine basis for tunnel and rock-slope support design in hydropower, metro and highway-tunnel projects. They remain *empirical correlation* tools, not analyses: their value is converting consistent field logging into a defensible support class and a first estimate of rock-mass strength.

The practitioner essentials: classify reach by reach, log RQD and joint conditions honestly, always apply the orientation adjustment, and cross-check RMR against Q before fixing support. For major works these empirical classes are confirmed by numerical modelling and, decisively, by instrumented observation during excavation (the observational method) — the rock that is exposed always overrules the rock that was logged from a borehole.