Rockslide causes: Rains, rock type and human interference

KOHIMA— Incessant rains coupled with human and other natural interferences could have caused the July 4 rockslide tragedy near Chathe Bridge in Chümoukedima on the Kohima-Dimapur four-lane road, which killed two people and injured three others, a geologist said.

Dr. Meniele Nuh, assistant professor of Geology, Kohima Science College, was opined that another cause of rockslides in Nagaland can be attributed to the types of rock available in the state.

She informed that rocks in the state are “very young and unstable” compared to the ones found in other places.

She elucidated that the rock formation along the Dimapur-Kohima highway are mostly sedimentary, which are not hard but “cracked.” Moreover, the formation of rocks along the highway is tilted towards the road, thereby increasing the risk of rockslides, Nuh said.

Due to faults and joints in the rocks, she explained how gaps are created in between the rock formation and during the rainy season, water starts trickling inside the cracks, making it more fragile.

The geologist also said that one of the major factors that is posing threat to life and property is the manner in which the slopes have been cut along the highway in Pagala Pahar areas.

“The problem along the highway is the slope cutting made by the road construction company, which is not at all right,” Nuh said while wondering whether they are just waiting for the rain to lubricate those rocks at the slopes alongside the road to fall.

The geologist informed that experts were startled at the sight of how the slopes have been reshaped in such a threatening way, while the use of safety measures such as geo jute and netting in some of these areas were “not at all safe.”

While the rain is also a contributing factor in triggering rockslides or rock-falls, the slope cutting is equally giving chances to such similar incidents, she said.

She informed how fellow geologists have also raised reservations against the slope cutting along the highway and “ineffective preventive measures” set up in some of the affected areas.

“It’s (accident) going to happen again and again,” she said and added “the incident is just the starting; more will occur if proper actions are not taken immediately.”

Suggesting that the slopes have to be lowered and at places where there are risk factors, she said there are many mitigating methods to prevent rockslides.

“Safety measures in our terrain are possible; there is no excuse or chance for blaming our topography and geology,” Nuh added.

Meanwhile, Pro Vice Chancellor of Nagaland University, Kohima Campus, GT Thong, said that the whole area near Chathe river bridge “is a very risky zone.”

He explained how the friction holding together the rocks is lost due to the presence of water, which often leads to rockslides.

The senior geologist also said that the rocks in these areas are unstable; while due to the constant movement of the crust coupled with too much rain, the plains are fractured, causing the rocks to get jointed.

“The water pressure inside the rocks has loosened the material and as a result, big chunks of these rocks collapse,” Thong explained, adding that due to the presence of water, such rocks also get weathered to clay. He went on to explain how these clays start expanding so much so that fractures become bigger as years goes by.

While the rocks get withered to clay because of the presence of water, the clay becomes lubricant where boulders get detached and collapse from time to time with too much rainfall, he added.

Nuh also said that rock formation is naturally horizontal and not vertical. However, due to disturbances over a hundred or a million years, the rocks are tilted instead of the natural way. Likewise, she said the rock formation along the Kohima-Dimapur road has moved in such a way that the horizontal has been tilted, and instead of the young rocks, the old ones are at the top.

“Some of our areas along the highway are like a pregnant mother, ready to fall,” she said in a lighter note.

Also read: Nagaland rockslide tragedy: Traffic hits alternate route; condition of 3 injured stable