Geological report on tunnel shows weak rock, slip circle failure near Silkyara | Latest News India - Hindustan Times

Geological report on tunnel shows weak rock, slip circle failure near Silkyara

Nov 27, 2023 05:28 PM IST

The vertical drilling of the hill in Silkyara commenced on Sunday afternoon, with around 110 meters of the hill to be dug out to rescue the trapped workers. In a fast-paced operation, the machine has already drilled 20 meters of the hill, with around 86 meters still left

A geological report submitted to the ministry of road transport and highways (MoRTH) before the commencement of the Silkyara-Barkot tunnel project shows that the proposed tunnel could encounter weak rocks and adequate support structure needed to prop up the weak rocks.

Uttarkashi tunnel site (File Photo)
Uttarkashi tunnel site (File Photo)

“From the surface geology, it may be anticipated that the rock type to be encountered along the diversion tunnels would be 20% good (Class 2), 50% fair (Class 3), 15% poor (Class 4) and 15% very poor (Class 4),” the report said.

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According to the, the major rocks in the area were ‘weak sedimentary rocks’, such as slate and siltstone and the formation of wedges in the crown and side wall cannot be ruled out. “This has to be taken care of during construction planning,” the report said.

The formation of wedges in the context of geological or geotechnical considerations typically refers to the creation of wedge-shaped masses within rock formations or soils.

These wedges can have implications for slope stability, rock mechanics, and geotechnical engineering, officials said, adding that the exact cause of the tunnel collapse would be known only after 41 workers are evacuated.

The under-construction tunnel from the Silkyara side had collapsed on November 13 morning trapping 41 workers in the 2 km stretch towards the Barkot side.

Multiple attempts to rescue the workers using the augur machine have failed due to several hurdles and additional collapse of the tunnel taking the collapsed part from 55 meters on the first day to over 80 meters. The tunnel is part of the 880-km Char Dham project, which was not environmentally appraised.

On Monday, the Silkyara tunnel collapse site entered day 16. With the failure of drilling using the auger machine, the rescuers have begun vertical drilling. The vertical drilling of the hill in Silkyara commenced on Sunday afternoon, with around 110 meters of the hill to be dug out to rescue the trapped workers. In a fast-paced operation, the machine has already drilled 20 meters of the hill, with around 86 meters still left.

The project areas fall both in lesser as well as higher fragile Himalayan zones, which are prone to landslides, and coincide with a major tectonic boundary known as the Main Central Thrust (MCT), which has witnessed several mild earthquakes in recent years.

Although the inquiry committee is yet to submit its report on the reasons for the tunnel collapse, the geological report highlighted that the mountain rocks on the Silkyara side were weaker than on the Barkot side and pointed towards the possibility of “slip circle failure” on the Silkyara side.

A slip circle failure, also known as a rotational or circular failure, is a type of landslide that occurs when a mass of soil or rock rotates along a curved surface and is considered bad for tunnel construction. “Slip circle failures, often associated with landslides, can pose significant risks and challenges for tunnel projects,” said experts.

For the preparation of the geology report, exploratory drilling was done at the Silkyara, Barkot, and Radi top. Exploratory drilling is a technique used to obtain sub-surface information about the geology, composition, and structure of the earth’s crust. It involves drilling into the ground to collect samples of soil, rock, or other materials for analysis.

Dr Naveen Juyal, a geologist with expertise in the Himalayan region, said, “First of all if the 4.5 km road tunnel project in the sensitive Himalayas region was taken up based on this geological report, it is insufficient. One can’t know the type of rocks by just three exploratory drilling. Further, the report clearly says there was no very good quality rock in the area where the tunnel was being built. Only 20% of rock is of good quality, the rest is fair and poor and very poor. The report admits that the area wasn’t geologically stable.”

He, however, clarified that he was not saying the tunnel could not have been built in that area and questioned the method used in building the tunnel.

Another geologist YP Sundiryal said, “According to the report, the type of rock in the area where the tunnel is built is weak like slate, siltstone. It needs proper planning and a support system during the construction. It seems the implementing agency of the Silkyara ignored it.”

Anshu Manish Khalkho, director of project proponent, NHIDCL, did not respond to several phone calls. Another NHAI official, however, said that the inquiry report would tell whether the tunnel construction was done as per the guidelines or not. “We should not jump to any conclusion now,” he said.

Another report related to the project on debris dumping highlighted that an escape passage was proposed in the project report but was not built. As per the government guidelines, there has to be an escape passage for tunnels longer than 1.5 km. The Silkyara tunnel is 4.5 km long.

“Keeping in view the economics of the construction, considering local conditions and construction practice, the standard profile proposed for the tunnel driven by the conventional tunnelling method is a Modified Horse Shoe Shaped, rather than circular shaped produced by a Tunnel Boring Machine (TBM). The tunnel dimensions are also based on the requirements to provide a 7.0-metre carriageway width and a 3.5-metre escape carriageway. 0.75-metre side walk-walks, 5.5 m (minimum) clear headroom and space for ventilation channels for a fully transversal system of ventilation,” the report said.

On the question of no escape passage in the tunnel, Mahmood Ahmad, additional secretary, MoRTH while addressing the press conference on Sunday said, “A committee has been constituted to look into such aspects related to tunnel construction. But, as of now, our immediate priority is to save the trapped men.”

In support of tunnel construction, the authorities said the construction of the tunnel will provide all-weather connectivity between Silkyara and Barkot will reduce travel time by an hour and also reduce environmental impact on the river flowing 200 metres below the tunnel. The report mentioned that the tunnel construction can breach a water aquifer and it needs to be taken care of during construction.

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