The quantitative classification system of rock mass-Guidelines, Part 3: Determination of slope mass rating
1997 Edition

According to IS 13365 Part 3, the Slope Mass Rating (SMR) is derived using the formula:

[ \text{SMR} = \text{RMR}_{basic} + (F_1 \times F_2 \times F_3) + F_4 ]

where:

• (\text{RMR}_{basic}) is the basic Rock Mass Rating from IS 13365 Part 1.
• (F_1, F_2, F_3) are multiplicative adjustment factors based on discontinuity orientation relative to the slope face and failure mode.
• (F_4) is an additive correction for excavation methods and site-specific conditions.

The SMR thus refines the RMR value by incorporating geological and engineering factors relevant to slope stability, enabling classification of slope conditions from fully stable to very unstable.

IS 13365 Part 3 identifies three principal failure mechanisms for rock slopes:

1. Wedge Failure: Occurs along the intersection of two discontinuity sets where the line of intersection dips towards the slope but with a plunge less than the slope angle, being the most frequent failure type.

2. Toppling Failure: Involves rotation about steeply dipping joint sets that face outwards relative to the slope, often developing slowly over time.

3. Planar Failure: A special case involving sliding along a single discontinuity plane that is approximately parallel or subparallel to the slope face.

These failure modes are determined through field data collection and stereonet analysis of discontinuity orientations.

Joint orientation adjustments (factors F1, F2, F3) significantly affect slope stability ratings by quantifying the spatial relationship between rock discontinuities and the slope surface. Favorable orientations reduce the risk of failure, reflected by lower factor values, while unfavorable alignments increase instability.

Excavation methods contribute through the additive factor F4, where techniques such as natural slopes or presplitting improve stability (positive adjustments), whereas poor blasting practices can degrade slope integrity (negative adjustments).

Together, these factors modify the basic RMR, resulting in a comprehensive SMR that accounts for both geological structure and engineering interventions.

The standard recommends collecting between 100 to 500 measurements of geological discontinuities including dip, dip direction, and strike across the rock slope. These data are plotted as poles on an equal area stereonet, and contouring the pole concentrations helps identify maxima which indicate potential failure modes such as planar, wedge, or toppling failures.

This graphical approach aids in understanding the spatial distribution of discontinuities controlling slope stability and supports informed engineering decisions.

For slopes rated as very unstable (SMR < 20), major interventions such as re-excavation to adjust slope geometry and extensive stabilization measures are necessary.

Unstable slopes (SMR between 21 and 40) require significant corrective actions including installation of rock bolts, anchors, drainage systems to mitigate seepage, and possible slope regrading.

Partially stable slopes (SMR 41-60) may be stabilized with systematic support systems like rock bolts and anchors, while stable or fully stable slopes (SMR above 60) generally require minimal or no support.