Uttarakhand tunnel collapse caused by a lapse in judgement

In the early hours of November 12 came the news that a portion of the under-construction Silkyara Bend-Barkot tunnel in Uttarakhand collapsed, trapping 41 workers. Seven days later, after efforts to drill through the tunnel hit multiple roadblocks, government agencies put rescue efforts on hold to reassess their approach. As of November 22, the workers are still inside the tunnel and being supplied food, water and medication through small pipelines.

The 4.5-km tunnel, which is part of the Union government’s Char Dham Pariyojana to improve connectivity to four pilgrimage sites in the state, is located in the Uttarkashi district and is being constructed by the National Highways and Infrastructure Development Corporation Ltd (NHIDCL), set up by the Union government in 2014. The tunnel was at a shallow depth of 140 metres, hence one does not expect much stress. However, Uttarkashi is in the upper Himalayan region and close to the Main Central Thrust (It is a geological fault where the Indian Plate is pushed under the Eurasian Plate. This fault spans more than 2,000 km along the Himalayas in the northwest to southeast direction). Shear stress accumulation around this thrust fault is very high and it sees continuous tectonic plate movement.

This writer has been part of several projects where excavation and tunnelling is required. Generally, tunnelling is much safer than excavation for roads. Open excavation for road construction can lead to landslides, and lacks adequate safety measures. However, tunnels are better protected due to their support system, which comprises rock bolts and steel ribs reinforced in areas at risk of a collapse.

The process of designing a support system begins with understanding the class and behaviour of rock mass. When the construction area has both hard, strong rocks and soft, weaker rocks, one cannot design a uniform support system. The weaker sections require additional support. Hence for such projects we use the New Austrian Tunneling Method (NATM) that has a flexible support system of girders, shotcrete (concrete or mortar applied with high velocity to a surface) and rock bolts.

The Silkyara Bend-Barkot tunnel, too, is in an area with hard and soft rocks, and hence needed additional support. Post the tunnel collapse this writer attended a video conference meeting with other experts organised by the chairperson and managing director of Rail Vikas Nigam Limited, where it was suggested that this project has used a conventional support system.

The second factor that helps protect a tunnel from collapse is instrumentation for real-time monitoring during construction. This is a crucial step that must be followed every day to check for deformation inside the tunnel and alert project proponents to augment the support system. However, it appears that with this tunnel, the contractors have not provided instrumentation, which means deformation would be impossible to detect.

Monitoring also helps check whether the allowable limit of convergence in the tunnel is incorrect, that is, whether the rock mass is becoming heavier than the support system. Designers sometimes provide more allowable limit in their calculations. If a tunnel collapses, it is clear the calculations are wrong.

Elevation adds risks

Tunnelling in elevated Himalayan areas like Uttarkashi adds some aspects that necessitate extra precaution during construction— the shearing effect of tectonic movements, the rock mass and type of the area, and the hydrological factor or the presence of groundwater.

There are protocols in place to account for these aspects. These are standard practices which must be followed while making detailed project reports that government agencies assess before approving the project. During investigation of tunnel alignment for the detailed project reports, an exploratory drift sized 2 m-by-1.8 m is created and the rock masses are mapped and assessed on a 1:100 scale. This way, variations in the rock mass are assessed by identifying the zones with good strength or those that are weak. This helps specify the proper support system needed to ensure stability during construction of the proposed tunnel. For longer tunnels, we have to depend on data obtained by drilling, along with geophysical and seismic surveys that give us information about the rock material and rock mass behaviour. A similar process is undertaken to check for groundwater by drilling probe holes.

But there are always surprises during construction that we do not anticipate but need to be mentally prepared for. This is why constructions should not be done too speedily.

Newer methods like tunnel seismic prediction (TSP), which involves seismic survey before initiating a new phase of the tunnel, also help maintain safety during construction. TSP is used to analyse what lies 100 m ahead of the completed zone.

India has all the necessary systems in place to ensure tunnels are properly made. Yet we have incidents like this tunnel collapse due to the carelessness of project proponents and of contractors in ensuring that all the precautions are taken and standard protocols followed. 

P C Nawani is an engineering geologist and chairperson of the wrd 05-Geological investigation and subsurface exploration sectional committee of the Bureau of Indian Standards. He is also former director of the Geological Survey of India and the National Institute of Rock Mechanics

This was first published in the 1-15 December, 2023 print edition of Down To Earth

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