An appraisal of Pre- and Post-failure regimes of a hillslope using stability, runout, and structural implications; A case study from the NW Himalaya

The Himalayan Mountain range is infested by natural disasters annually. Mass movement and hillslope instability are among the most common phenomenon witnessed in those regions. The propensity to such activities is perplexing since it is not attributed to a definitive factor. The reason being the complex geological condition of this mountain range, topography, aided by anthropogenic intervention. A number of landslides reported in these regions are often rainfall induced landslides. However, there is always an interplay of geomorphology and tectonics which contribute to such mass movement.

On September, 2017, a colossal landslide occurred along the char Dham (Hindu pilgrimage) corridor, located near Wazri (locally called Ojari) village in Uttarakhand (India). Resulting in the blockade of road for four consecutive days and stranding ~1313 pilgrims of the Char Dham Yatra, and as many as 14 villages were cut-off from the main district headquarters. In order to evaluate the landslide, slope stability, runout, structural, and precipitation analysis were carried out for both the pre- and post- failure event. In the pre-failure analysis, results showed that the maximum shear strain of about 0.16-0.26 was developed parallel to the slope and a total displacement of about 1.8-8 m was likely to take place, parallel to the slope. In the post-failure analysis, results indicated that the maximum shear strain had decreased to 0.14, with a decrease in total displacement to 4.4m. Indicating that much strain was still accumulated in the existing slope. This still implied that the slope was unstable and had accumulated much load on the slope, with high chance of further failure. This model was testified by the incident in the consecutive year of July 11, 2018, where the landslide was triggered due to incessant rainfall. Resulting in the disruption of the daily traffic and stranding ~90 pilgrims of the Char Dham Yatra. The debris flow simulation of the loose unconsolidated material, in case of excessive rainfall effect, revealed a flow accommodating 7-24 m height, 3-4 m/sec velocity, and 100-800 kPa pressure along the Yamuna River. To analyze the role of deformation in the landslide, the study area was subdivided into three distinct zones; (i) fault core, (ii) fault damage zone, and (iii) the intact zone (undeformed host rock), following the evidence from fields. From the visual observations, it was evident that the landslide was confined to the Main Central Thrust (MCT) damage zone.