The quantitative classification system of rock mass - Guidelines, Part 1: RMR for predicting engineering properties
1998 Edition

The Rock Mass Rating (RMR) as per IS 13365 Part 1 is derived by considering the following key factors: (1) Uniaxial Compressive Strength (UCS) of the intact rock, (2) Rock Quality Designation (RQD), (3) Average spacing between discontinuities, (4) Condition of discontinuity surfaces, and (5) Groundwater conditions. These parameters are summed to yield the basic RMR value. Subsequently, an adjustment factor based on the orientation of discontinuities relative to excavation geometry is applied to obtain the final RMR. The Rock Condition Rating (RCR) excludes UCS and orientation adjustments, focusing on other parameters. This comprehensive approach enables estimation of support needs, stand-up time, and excavation strategies.

Groundwater presence significantly impacts the Rock Mass Rating (RMR) and rock mass properties. According to IS 13365 Part 1, pore water pressure should not be counted twice; if considered in structural stability analyses, it should be excluded from RMR adjustments. Saturation usually lowers rock mass strength and deformation modulus, especially in weaker rock types with water-sensitive minerals. For saturated soft rock masses, the modulus of deformation decreases with time and depth, following empirical relationships. Shear strength parameters like cohesion and friction angle also diminish under saturated conditions. It is recommended to perform in-situ tests such as uniaxial jacking shortly after excavation in saturated zones to capture diminished stiffness accurately.

IS 13365 Part 1 outlines a systematic method to evaluate discontinuities: First, measure the linear spacing between adjacent discontinuities such as joints, faults, and foliations. Next, assess the condition of these discontinuities in terms of roughness, weathering, separation, and infilling material. Orientation is then recorded using strike and dip angles relative to magnetic north and excavation axis. The favorability of joint sets is determined through tables provided in the standard to evaluate their impact on stability. Finally, the basic Rock Mass Rating is adjusted based on orientation factors. This stepwise approach ensures comprehensive characterization essential for rock mass classification and design.

The Rock Mass Rating (RMR) facilitates the estimation of the in-situ modulus of deformation (Ed) through empirical correlations. For hard jointed rocks (uniaxial compressive strength ≥ 50 MPa), Ed can be approximated by formulas such as Ed = 2 × RMR - 100 GPa or Ed = 10 × (RMR - 10)/40 GPa. For dry soft rocks at depths over 50 m, Ed depends on depth and RMR according to Ed = 0.3 × z^a × 10^((RMR-20)/38) GPa, where a ranges between 0.16 and 0.30. These correlations assist preliminary design stages, while in-situ testing is needed for final validation. The standard also notes that saturation and time can reduce modulus values, especially in poor-quality rock masses.

IS 13365 Part 1 presents empirical relationships to estimate short-term support pressures in tunnels, particularly for steel rib supports in conventional blasting operations. A key formula is: Proof pressure = (7.5 × B^0.1 × H^0.5 - RMR) / (2 × RMR) in kg/cm², where B is the tunnel span in meters, H is the overburden depth typically exceeding 50 m, and RMR is the Rock Mass Rating. Additionally, tunneling ground conditions are predicted through correlations involving overburden and tunnel width, categorizing conditions as self-supporting, non-squeezing, or varying degrees of squeezing. The Q-system (IS 13365 Part 2) with appropriate Stress Reduction Factor (SRF) provides refined support pressure predictions.