Survey report of Tupul landslide trigerred on 29th-30th June

The team comprising of Prof. Arun Kumar MU, Dr Manichandra Sanoujam MU,  Dr Thingujam Dolendro Geoconsult, M Jagajit Singh Computer Gallery Imphal, Dr N Sanamacha Singh ECC, SK Gainaimei ECC,RK Bikramjit ECC visited the spot with Dr RK Ranjan
Contd from previous issue
Causes of the Massive Landslide : We have assumed the probable cause of the landslide: Geologically, non-resistant lithologies (shale inter-bedded with mudstone, siltstone and sandstone) of Barail Group exposed in the study area after the modification of the slope geometry that impedes the mountain's ability to resist the existing gravitational force may become one of the causes of mass wasting/landsliding. The process might have been initiated long back, and the entire mass wasting slump was settled along the slope of the hill. It seems that the recent precipitation during May-June 2022 became the threshold for ongoing massive landslide hazards in the study area. And due to the Ijai fault which is 11 km long NE-SW trending fault which gets abruptly terminated against the Tupul fault north of Tupul village at the confluence of Ijai and Tupul rivers (after GSI Misc. Publication 1992, Acc. No. 2028).
Secondly, the fluctuation in the ongoing precipitation pattern is the source of availability of more water along these slopes to percolate below the slope surface. The fine grained lithologies (clay and silt) being less permeable becomes more plastic and reduces the stability of the slope and initiate the sliding, where the resistant lithologies (sandstone and mudstone) still hold water in their pore spaces.
These two contrasting combinations of litho units in the western Manipur as well as in the study area seems to be one of the causes for slope instability.
Thirdly, Manipur state is included in the High Seismic Hazard (Zone V, Seismic Zonation Map of India) and hence, the micro seismicity is active besides a large earthquake of 2016 (M6.7) which is near to the recent landslide area. Based on the fault plane solutions (Global CMT Project) of earthquake data, the principal P-axis is towards north, indicating the compression direction, resulting in the extension along east-west.
The resulting creeping of micro deformation towards the western slopes of the terrain including the recent landslide area is aligned with the principal T-axis (Kumar et al 2011). It is also seems to be one of the triggering factors for the hazard as few minor cracks are observed in the cut slopes of the railway yard.
Fourthly, changing in the land use patterns on the top of hill like deforestation, Jhum cultivation, banana cultivation, which also accelerated the soil erosion process and might also be the secondary factors for the recent landslides.
We observed that the lithological trend of the Marangching is along NW-SE which is one of the factors for ongoing deformation processes and initiating slope instability as a continuous phenomenon. We have observed during the site visit and compared with existing terrain maps as well as DEM data that the slope of the Makhuam hill along with the first order stream drainage lines.
The seasonal first order streams along the slope of the hill are the main source of water percolating on these slopes, which become more prone to mass wasting because gravity has an easier time pulling materials down a steep slope as compared to a gentle slope. Heavy incessant rainfall and too much water also weakened its ability to resist gravity.
However, a little bit of water actually made it easier for soil particles to bond together and keep the mountain intact. Generally, soil particles lose this bond and break apart after excessive rain.
The primary effect of water within the slope was to reduce shear stress (reduced cohesion) acting along the potential slip surface, thereby reducing the shear strength along the surface.
The driving force (or moment) is due primarily to the component of the slump block weight acting parallel to the potential slip surface. Movement could be triggered if the ratio of driving to resisting forces (or moments) is altered by adding water by rainfall (60% above the normal rainfall in the last two months) to the slope. This seems to be the main causative factor for the recent massive landslides.
Envisaged Future Strategy for NF Railways:
In this preliminary study using standard soil parameters (at present no soil parameters are available) the results of 2D finite element modelling of the slide area are presented. The analysis results validate and confirm the already initiated failure zones and expected progressive failure zones as observed in the Drone image, Drone generated DEM and field. It may be inferred that the geometry of the slope, material profile and characteristics along the slope are the most crucial data for accurate modelling of a potential landslide. We can argue that, the predicted slope failure and actual landslide incidence have matched.
It is observed that finite element-based modelling is a quite powerful tool to simulate the near-real model of a landslide, if the modelling parameters are actual based on the field and laboratory investigations.
Modelling has manifold utility for landslide disaster mitigation in terms of identifying stress accumulation zones and also the extent of displacement expected to occur for locating and planning strengthening measures at such zones within a specific landslide.
It can also help in accurately locating the sites for placement of sensors for instrumentation and monitoring of actual ground movements in a landslide.
On the basis of finite element analysis with shear strength reduction method, it can be concluded that the existing yard cutting area is not stable as upper slope of the area may undergo sliding in a rainfall event. And The landslide area was earlier investigated and identified as high to very high hazard under the unpublished doctoral thesis submitted and awarded by Manipur University in the Department of Earth Sciences in 2007 (Thingujam Dolendro, 2007).
On the basis of the above site survey, DEM data analysis and field observations we could able to infer the massive landslide incidence was triggered after excessive rainfall in typical geological and geomorphological characteristic features of the terrain which is vulnerable to recurrent landslide. The entire western Manipur has typical geological formations vulnerable for such landslides, land subsidence hazards and more obviously, the National Highways and Railway network routes require regular monitoring of the slopes in order to mitigate the future loss of properties and casualties.
The Jiribam -Tupul-Imphal Railway Project is progressing well and the recent landslide has affected the railway formations in the proposed Tupul Station yard. Since, it will be one of the major life-line for the state in order to transport goods and passengers from rest of the country to the state, therefore, considering the success and longevity of the ongoing project for several decades, it is necessary to adopt the regulatory monitoring of the vulnerable slopes along the railway line where the cut portion and yards are exposed.