Evaluating the Failure Mechanism of an Active Retrogressive Landslide in Himalayas

Saibal Ghosh Indranil Chakraborty Geological Survey of , Kolkata

Abstract

Land sliding is a perennial hazard in the Darjeeling-Sikkim Himalayas where monsoon precipitation is unprecedentedly higher (2500 – 4500 mm/ within June- October in every year), which acts as a prominent trigger in Eastern Himalayas. Owing to the complicated geological setting within an active fold-thrust belt, steep topography and rapid changes in land use pattern inherently make this fragile terrain vulnerable to different types of landslides. Amongst them, the larger landslides in this terrain are mostly retrogressive in nature and shows rapid rate of retrogression in shortest possible time. Similar to this, Gayabari (14th Mile) Landslide, situated at km 35.0 on National Highway 55 (Hill ) cum Darjeeling Himalayan Toy Train Tract is a perfect example of such retrogressive landslide, which is evident from its fast retrogressive movement within a span of only nine years (2003-2012). Prior to 2003, it was a small rock wedge much below (100-125 m) the NH-55 road bench, later in 2006; it retrograded up to the road level. Since 2010, due to some rapid retrogressive movements, the crown of 14th Mile landslide retrograded about 125 m above the NH-55 road bench and snapped this vital communication corridor for a length of about 450 m (Fig. 1).

Fig. 1: Retrogression of Gayabari (14th Mile) Landslide, , .

To evaluate in detail the failure mechanisms of Gayabari (14th Mile) Landslide, detailed scale (1:1000) topographic and geological mapping of the affected slope, followed by a deterministic slope stability analysis using the geotechnical parameters of insitu soil and rock samples were carried out. Detailed scale geological mapping revealed that the 14th Mile landslide is primarily a retrogressive rockslide and has exposed the underlying bedrock due to sliding at two distinct locations (Crown and near toe areas). The intervening slope is covered with thin to thick (max. 10 m) unconsolidated slide debris material fallen from the crown region.

Topographically, this landslide is situated within slopes having varied slope gradients (30o-55o), predominantly moderately steep to steep in nature. Tectono- stratigraphically, the area of 14th Mile landslide is traversed by a highly sheared and fractured rock mass belonging to the Main Central Thrust (MCT) rendering its bedrock to be highly fragile, sheared and fractured. The rock mass near the crown is a high-grade, sheared gneiss with inter bands of highly schistose and crenulated metapelitic layers and quartzites. The rock mass exposed near the right toe part is highly fissile phyllite and quartzites with schist interbands. Due to intense shearing, fracturing and weathering, the rock mass at 14th Mile is inherently weak in nature and is perennially prone to land sliding. Moreover, unfavourable disposition of topography and planar discontinuities (foliation parallel and other prominent joints) facilitate at many locations possibility of both planar as well as wedge failures. As such, joints are closely spaced and rough to smooth planar in nature, rendering moderate to lower Rock Quality Designation (RQD) and Rock Mass Rating (RMR) values that ultimately give rise to lower Slope Mass Ratings (SMR) (34-37) at 3 distinct locations explaining the prevalent failure forms and its corresponding failure mechanisms. In those areas, if not treated, fresh rock sliding cannot be ruled out.

In debris-covered areas, deterministic slope stability analysis following the Infinite Slope method exhibited that predominant areas under debris cover is potentially landslide prone having factor of safety values lesser than 1.0 owing to lower C & φ values of slope forming material and steep gradient. The debris and scree-covered areas of the present 14th Mile landslide are also potentially landslide-prone as evident from the results of the Infinite Slope method of slope stability analysis applied in this landslide, wherein both on dry and water-saturated scenarios, substantial areas within this loose and unconsolidated debris are found unstable. The most conspicuous result of this analysis is observed for the scree-covered areas adjacent to left crown, where due to saturation, a substantial part becomes critically vulnerable to future land sliding. This numerical aspect was also corroborated during active monsoon of 2012 when large areas adjacent to left crown failed due to fresh land sliding.

All the above analytical techniques facilitated to understand the prevalent failure mechanisms of 14th Mile Landslide, which ultimately guided the authors to suggest both short-term and long-term remedial measures to road-maintaining authorities.