Geo scientific causes behind Nigulsari landslide in Himachal : Steeply inclined slope & incessant rainfall

In a 12 page report prepared by the Geological Survey of India on the Nigulsari landslide incident in the Kinnaur district last month revealed steeply inclined slope & incessant rainfall as geo-scientific causes behind the incidents.
A team comprising of Geological Survey of India Northern region, Director S R Mahapatra, Geologist Atul Kohl, Assistant Geologist
Vijit Thakur and Abhinav Poonia conducted the reconnaissance study on the rock slide incident of  August 11 at National Highway -5near Nigulsari village of Nichar Tehsil of Kinnuar district.
Report submitted to the state government recently, a copy of which is with our team, found the rainfall as a triggering factor and also steeply inclined slopes geo scientific reason behind this catastrophic incident.
The team of GSI visited the spot and found that incessant rainfall from the last three days before the main incidence (intimated by Gram Panchayat and local residents presents at the time of field study) was the triggering factor to cause the landslides which continue for few days.   
The field observation also recorded that based upon the Macro scale Landslide Susceptibility Mapping( NLSM) 
studies carried out during 2016-17, the incidence spot fall in a high susceptibility zone (Vadakkedath and Biswal 2017).
 ” From the field traverses also , the site appears to have been perceived as a location vulnerable to landslide hazard as the same is characterized  by the presence of moderately to steeply inclined and dissected hill slopes, bottomed by steeper cut slopes and further bottomed by  steeper natural slopes(para-6). The rocks exposed are highly foliated and jointed with open joints at several places. ” study mentioned. 
The report further revealed that the actual slope failure incidence took place from natural slopes above the cut slopes. Voluminous debris comprising rock blocks of varying dimensions as well as soil, mostly with coarse fraction, failed in the incidence. 
” Immediately  after the incident of Aug 11, 2021 another incident took place on Aug 13, 2021 in which another HRTC bus had a minor escape. ”
” At  the time of field observation, although water was neither observed flowing nor trickling, both from crown area and from rock slopes, wet and damp conditions were observed immediately above the crown . In all, a total of 400 meter stretch along NH-5(including the main failure zone), is characterized by the presence of highly jointed and foliated rock mass as observed along the cut slopes. ” the study elaborated. 
” Although the rock failure had resulted in the road blockade and the failed material had already been removed before traverse , due to  continuous falling of loose rock blocks , officials were deployed by the administration above the crown and along the road to clean the road for smooth flow of traffic and to avoid any mishap.” the report said.
” Based upon the photograph of electronic media and field observations of rock blocks lying close to the site of failure , it can be inferred that the size of the failed blocks could have been as large as a few cubic meters.” 
”The crown of the slide is about 120 meter above the road (visual estimate) and the bottom end of the zone of depletion
is above the road level. The width of the crown area is about 25 meter . The maximum depth of the depletion is about four meter in the crown part. ”  the study pointed out.
Report said that based upon the material and type of movement involved the slope failure incidence can be classified as Rock slide( in upper parts of the zone of depletion) and associated rock fail (in most part of the zone of depletion). The discontinuity ( joint) along which the movement occurred in the upper reaches of the zone of depletion , dips valley ward at moderate to steep angle.
 ” The crown portion of the slide shows traces of contour parallel tension cracks , piping holes in in-situ soil , traces of rills, tilted trees, partly shaved off  vegetation , exposed roots lying over the plane of sliding which might have played a role in widening of the joint planes. ”
” The density of vegetation over the rocky slopes can be at best termed as sparse, whereas in the upper reaches of the hill section, it is moderate. The spacing, continuity and mutually interfering spatial disposition of these joints have made the rock mass as assemblage of individual blocks varying in the size from few cubic mcm to few cubic meters, cemented by relatively weaker links along and across the joints. ” 
” A rock block with some in-situ soil developed over it was observed protruding from the valley ward. slightly towards the left side of the crown  which appears to be highly vulnerable based on field observation. In addition , several insecurely positioned blocks on cut slopes were observed .”the study report indicated.
Preliminary inferences drawn from the end of the reconnaissance 
1 Prima facie the failure appears to be a natural rock slopes failure with no signatures of anthropogenic activity seen in or adjoining areas of the crown , from where the failure was initiated in per the field observation. 
2. There appears to be no preventive measures in place that are capable of reducing uncontrolled  flow of water over the moderately to steeply inclined slopes or valley facing joint planes above the cut slope section. 
The report said that on the basis of reconnaissance , suggestions could be proved useful in minimizing the slope failure incidence and risk
to commuters. These suggestions need to be converted into actionable points only after adequate investigations wherever necessary.

Suggestion based on present field investigation 

1  Vigil along the slope failure affected stretch as well as nearby vulnerable stretch and slope failure or landslide location along NH-5 irrespective of whether or not the preventive measures are in place . This math help in early detection of likely failure and measures such as closure of landeslikely to be affected for specific time etc.  can be implemented.
2. Identification and removal of rock blocks that are insecurely positioned and are likely to fall on the road alignment as far as possible,blasting should be avoided for carrying out the removal of such blocks. This includes a vulnerable block ready to fall above the crown.
3. Contour parallel drain : at least two to three contour parallel drains all along the crown at appropriately identified levels to divert surface water toward nalas on either side.
4. Immediately filling of tension cracks with suitable impervious material 
5. Protective measures for cut slopes : possibility of  double layer shotcrete or wired mesh can be explored for the cut slopes section as several loose rock blocks are appearing  that may fall at  any point of time especially during the course of incessant or perennial rainfall spell.Detailed studies , however, may  be carried out for the same . 

6. The slope paredness is a long term process and such susceptible  slopes  can be identified with a fair degree of confidence  through the process of  landslide \susceptibility or zonation on different scales. However it must be understood that supsecpitblity may classify the  area on  as is basis ( like as napshot) andlow  susceptibility  does not rule out landslide occurrence .
The study said that geomorphology of the spot is highly dissected hill and denudational hill slopes. The landslide and land cover is moderately vegetated and agriculture. Hydrological condition is wet and damp at places in and around the crown area.
The dimension of the landslide is about 250 meter in length and 70 meter in width along the road. The height is 135 meter with a run out distance of 450 meter .
The rock type exposed in the area is Jeori-Wangtu gneiss. The Litho units are traversed by three sets of joints that have been noticedin the area. The prima facie geo scientific cause was steeply inclined slope and incessant rainfall from the last three days before the main incident .  
The remedial measures being explored are provisions of mesh trapping and afforestation and crate wall and trock fall shelters. The landslide awareness programme should be launched in the form of a mission for all villages located in sucha fragile geo environmental condition. Report sound alert of scale one in the region. 
The rock fall incident took place on NH-5 road on August 11, 2021 in district Kinnaur.of Himachal Pradesh  as per the media reported 28 death occurred and 17 injured and damage to HRTC bus and private vehicles.