GUWAHATI, June 29 - Places located in the hill districts of the State and the hilly areas of both the Kamrup districts are vulnerable to landslides, found in an ongoing countrywide survey conducted by the Geological Survey of India (GSI), a Union government agency.
The survey, launched by the GSI in the 2014-'15 financial year, aims to cover the hilly areas of the entire country.
As of now, the National Highway (NH)-37 (erstwhile NH-53) is the most landslide-prone area beyond Maibong, between Mahur to Jatinga and between Retzawl/Miyungkro to Bara Narainpur (near Bandarkhal). These two zones contain seven critical sinking zones, which have troubled the construction of the Express Way for a decade and caused havoc to Northeastern Frontier Railways. Both these areas contain abundant black shale (fine-grained, clastic sedimentary rock composed of mud) which swells in presence of water and shrinks and becomes brittle when dry.
However, it should be kept in mind that because of heavy and prolonged monsoon rains and high seismicity (Zone-V) activities, the entire NE region of India is vulnerable to landslide incidences, said official sources here.
Speaking on the factors that trigger landslides, sources said 90 per cent of the landslide incidents are generated by anthropogenic activities while undertaking road construction works, etc., which result in unscientific and extensive slope cutting and selection of wrong route (often geologically weak and fragile) without any slope protection. Developing human habitations with unmanaged waste-water disposal is also a leading cause.
The sources added that the amount and duration of rainfall, angle of slope, direction of slope (valleyward or hillward), thickness of overburden material, composition of overburden material, dip angle, direction, weathering condition, number of fractures/joints of the underlying rock, its spacings, opening, its fillings/gouge, type and height of vegetation cover, accessibility to the spring water or influence of groundwater table, drainage density, nature of dissection, etc., are the factors found so far at the root of such incidents.
It is also pertinent to mention here that low and persistent drizzle not only soaks and saturates the overburdened materials and make them heavy, but under the impact of the gravitational pull, such materials tend to creep down rock surface. The materials which form the slope are also crucial in this respect, sources added.