Application of Rock Mass Classification Systems to Assess Slope Stability Along the <scp>NH</scp> ‐109, Kumaun Himalaya, India

ABSTRACT The Himalayan Mountain system was formed by the collision of the Eurasian and Indian plates, creating a relatively young folded mountain belt. Over the past few decades, this neotectonically active Himalayan belt has frequently experienced geohazards, including flash floods, landslides and earthquakes. Anthropogenic activities, coupled with natural triggering factors such as intense rainfall, seismic forces, freeze–thaw cycles and tectonic discontinuities, substantially exacerbate landslide hazards. The present study comprehensively undertakes a comparative slope stability assessment to address this multifaceted issue using multiple well‐established rock mass and slope mass classification systems. These include the basic rock mass rating (RMR basic ), geological strength index (GSI), continuous slope mass rating (CSMR), slope mass rating (SMR), rock slope instability score (RSIS) and Q ‐slope approaches. Twelve slope sections of National Highway 109, from Niglat to Manrasa in the Lesser Kumaun Himalaya, were evaluated for assessment of slope vulnerability. Kinematic analyses provided insights into potential failure modes, such as planar, toppling and wedge failures. The RMR basic values ranged from 57 to 82, and the CSMR, which uses a continuous function, demonstrated better performance than the SMR. The Q ‐slope method classified slopes as unstable, uncertain, or stable, whereas the RSIS assessed stability on a scale from completely unstable to partially stable. This study contributes to an enhanced geo‐mechanical understanding, supporting improved excavation techniques, stabilisation measures and slope design strategies in this seismically active and precipitation‐intense Himalayan terrain.