Environment

Char Dham: How geological instability and blasting are causing landslides in Tota Ghati

Any hard rock valleys yet to be widened for the project need to be critically evaluated

 
By YP Sundriyal, S P Sati, Shubhra Sharma
Published: Thursday 17 December 2020
Char Dham: How geological instability and blasting are causing landslides in Tota Ghati

The Char Dham project, inaugurated by Prime Minister Narendra Modi in 2016, aims to widen roads in India’s Himalayan state of Uttarakhand. However, the study of rocks in a key area where blasting is underway shows that the project is responsible for many of the landslides have been occurring recently.

The area in question is Tota Ghati (‘Parakeet Valley’), a less-than-10 kilometre stretch that lies between Kodiyala and Sakhni Dhar on National Highway 58 in Tehri district, Uttarakhand.

In this stretch, the highway passes through a complex geological terrain, with multiple rock types and slopes susceptible to ‘failures.’

A slope failure is a phenomenon in which a slope collapses abruptly due to weakened self-retainability of the earth under the influence of a rainfall or an earthquake.

Tota Ghati, where there are seemingly stable rock slopes, has been in the news of late for having had a number of slope failures recently.

The narrow stretch of Tota Ghati is dominated by limestone and interbedded shale rocks, shattered light grey dolomite, with occasional pockets (as fracture filling) of gypsum and purple grey shale and limestone. This is called a ‘Karol formation’ in geological terms.

The calcareous (calcium containing) rocks are highly jointed (two to three sets of intersecting joints), fractured and sheared due to three ‘thrusts’ passing proximally to Bayasi, Shaknidhar, and Teen Dhara.

Field investigations reveal that Tota Ghati’s rocks are widely sheared, faulted and fractured. Often, the competent (strong) rock types such as quartzite also occur as shear bands and completely crushed rocks in the stretch according to Geodata India Pvt Ltd, 2014.

At places, cavities filled with secondary carbonate precipitate have developed in the rocks, as the rocks dissolved locally. The beds around Tota Ghati are varied due to folding. However, a dominant dip is towards the road / valley (south-east).

Geologically, rocks with fractures and joints are highly susceptible to failures, which may be triggered even due to the vibration induced by traffic flow.

A recent study by SP Pradhan and Tarique Siddique (2020) published in the Journal of Rock Mechanics and Geotechnical Engineering assessed the slope stability of the rocky slopes in the region in terms of the ratio of the actual shear strength of the slope forming material to the minimum shear strength required to resist failure.

They employed the Critical Factor of Safety (FoS) analyses to assess slope stability via numerical methods to predict the risk due to slope failure. Their analyses showed that for the slopes around Tota Ghati, the FoS lay between 1 (unstable) to 1.3 (marginally stable).

One reason could be the widening of the joints due to blasting, as extensive mechanical excavation aided by improper blasting generates secondary fractures within the rock masses.

The above photograph shows a location that is being frequently obstructed by rock falls during the last couple of months. Under the Char Dham project, people were given an impression that the use of blasting would be prohibited as it disturbed a large area and createed cracks and fractures which could widen in the future and create a rockfall / slope failure zone.

However, in Tota Ghati, the blasting holes were drilled in the rocks (Karol limestone), and subsequent blasting reduced the slopes to a crumpled mass.

Although it requires detailed scientific validation, these inferences suggest that inherently weak rocks of variable competency and blasting, coupled with heavy mechanical excavators are responsible for the current slope instability in Tota Ghati.

Blasting not only would have weakened the slopes along the road, but the impulse of the explosion and vibration generated by heavy excavators possibly would have transmitted upslope. Therefore, this road segment would have been transformed into a chronic landslide zone. A detailed geological investigation supported by potential remedial measures needs to be done to stabilise the slopes.

The Tota Ghati example warns us that stable, hard rock dominated valley slopes, if not investigated critically for their geological fragility, lead to unprecedented slope instability having a cascading impact on the terrain.

It is a classic example that we must learn from so that such tragedies are not replicated in other valleys. Similar geological tragedies are reported from Tanakpur to the Pithoragarh sector (NH-125), which has near-identical geological and structural settings.

Therefore, any hard rock valleys that are yet to be widened for the Char Dham road project need to be critically evaluated for geological and structural stability before subjecting them to large-scale mechanical excavation and blasting.  

YP Sundriyal is Prof and Head, Dept. of Geology, HNB Garhwal University, Srinagar

S P Sati, Head, Department of Basic and social science, College of Forestry, Ranichauri, Tehri Garhwal (VCSGUUHF, Bharsar)

Shubhra Sharma, Assistant Professor, Department of Geography, BHU

Views expressed are the authors’ own and don’t necessarily reflect those of Down To Earth

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