Analyzing the stability of a failing rock slope for suggesting suitable mitigation measure: a case study from the Theng rockslide, Sikkim Himalayas, India

At km 87.20 near Theng on the North Sikkim Highway, India, severe rock falls and slides in recent years rendered 180 m stretch of this important road corridor in the Sikkim Himalayas risky and vulnerable to the commuters. The rockslide at Theng has, thus, been studied in detail at 24 discrete locations using the ground-based topographic, lithologic and structural data with an objective to evaluate the mechanisms of rock sliding for suggesting the most effective mitigation measure. In this study, large scale geological mapping and site-specific kinematic analyses in different spatial domains corroborated that within this 180 m stretch, despite having a competent lithology (quartzo feldspathic gneiss and quartzites), the possibility of both plane and wedge failures are high because of gentler and unfavourably oriented planar discontinuities with respect to the available steep topography. The rock slope failure analysis both at discrete locations and in a distributed manner in a Geographic Information System further revealed that unfavourable geometric disposition of discontinuities at Theng rendered multiple modes of failure at critical locations, which is also corroborated by a very low Slope Mass Rating value (4.0) determined in this area, thus, making it difficult to suggest any cost-effective surface protection measure. However, analysis suggests slope dressing or re-excavation to contain future rock sliding in such cases, but the same appears to be difficult to implement on the ground because of unfavourable kinematic conditions of the vulnerable rock discontinuities. Accordingly, after evaluating the site conditions in detail, a short road tunnel with its probable tunnel rockmass conditions is proposed to bypass this active rock sliding stretch.