Uttarakhand tunnel collapse: ‘In building Himalayan tunnels, support and monitoring key’, say experts

Accounting for shear zones

R K Goel, former chief scientist at the Central Institute of Mining and Fuel Research, said that with tunnel construction in the Himalayas, the collapse of weak rock mass is common.

“A shear zone is created when there is movement between two rocks and the rock gets crushed. This crushed rock can change its behaviour, it may have clay, or it may get weathered over time. Additionally, there are ground stresses — how this shear zone behaves under these stresses is important,” he said.

Various methods are used to protect weak sections. “This depends on the length and strata of the shear zone. If it’s a small shear zone, forepoles and rock bolts are used. Some of these decisions are taken at the designing stage — if the shear zone is already known, it is incorporated in the design. If these sections are not known in advance, protection measures are kept ready,” Goel said.

Data is gathered during construction and this could now be crucial in determining what may have triggered the collapse, he said.

The need for monitoring

The Uttarakhand government has formed an expert committee with the director, Uttarakhand Landslide Mitigation and Management Center, and experts from the Wadia Institute of Himalayan Geology, Dehradun, the Indian Institute of Technology, Roorkee, the Central Building Research Institute (CBRI), Roorkee, and the Geological Survey of India to investigate the causes of the collapse.

Shantanu Sarkar, who retired as chief scientist from CBRI Roorkee, and heads the expert committee, told The Indian Express: “We need data to see why it happened. We will have to see whether any measures were taken — there was a shear zone, but they seem to have already crossed it. They had already placed some primary lining and moved ahead. It is not that they have only excavated to the point where the tunnel collapsed. So, we don’t know why this has happened.

“Identification of the weak zone happens during geological mapping before excavation. These zones are visible at the top, and accordingly they design. The question is whether they took the right protection measures. Periodic monitoring is also very important, since there is always a possibility of geological surprise,” he said.

Shear zones, Sarkar said, are commonly encountered.

“Geologically, there are faults or weak/ shear zones, where there is stress accumulation. Material along the shear zone is loose, pulverised. When a tunnel is excavated or a road is being built in the mountains, you’re destabilising the slope — so, that needs to be studied and protection measures need to be designed and taken. It’s not about avoiding the zone, but taking protection measures and strengthening it using steel ribs, rock bolts, or shotcrete. It needs to be seen if the contractor was following this, and whether it was being monitored. Monitoring should be done periodically with instrumentation and sensors, so that if there is any displacement, it can be recorded,” he said.

Having an escape tunnel helps

On whether an escape tunnel should have been provided, Goel, who has been part of the design consultant team for tunnelling projects including the 9-km Chenani-Nashri tunnel in Jammu and Kashmir, said: “With the Chenani-Nashri tunnel, the escape tunnel was excavated simultaneously with the main tunnel. Excavation of the escape tunnel was, in fact, 600 to 700 m ahead of the main tunnel since it’s usually a single-lane tunnel that is narrower than the main one, and can tell us the geology before the main tunnel is excavated. Exits from the main tunnel are provided into the escape tunnel, in case of emergencies like a fire.”

Guidelines on safety practices in tunnel construction prepared by the International Tunnelling and Underground Space Association say that particular attention should be given “to the means of escape in an emergency situation (specific escape routes should be marked by signs),” in contingency planning.

Presence of Barkot thrust

S P Sati, Professor of Environmental Sciences at the Uttarakhand University of Horticulture and Forestry, said, “In this area, a prominent thrust (or fault) called the Barkot thrust passes, and because of the thrust, there may have been shear zones. Tunnels are risky ventures, and they may not have had necessary emergency preparedness and safety measures.”

The geology report on the Silkyara bend-Barkot road tunnel, which was submitted by the National Highways and Infrastructure Development Corporation Limited to obtain forest clearance, said that near the Barkot bend, rocks are “weak to moderately strong, slightly weathered to moderately weathered.”