The breakthrough of the Zojila Tunnel, a 13-km passage being carved through the Himalayas at an altitude of over 11,500 feet, is a major milestone for one of India’s most challenging infrastructure projects. But unlike tunnels bored through relatively stable rock formations elsewhere, excavation in the Himalayas comes with a unique set of geological and environmental risks.

We explain why tunnelling through the Himalayas is particularly difficult, and how engineers navigated these challenges while excavating the Zojila Tunnel.

Multiple reasons make tunnelling through the Himalayas a challenging feat. The altitude — the Zojila tunnel is at 11,578 feet — lends to difficult working conditions. So do the temperatures, which fall to as low as -30℃.

Arnold Dix, geologist, engineer, and former president of the International Tunnelling and Underground Space Association, said that working in these conditions is difficult for both workers and combustion engine machines. The harsh winters often bring along avalanches, which can be fatal. Two workers died in an avalanche at Sarbal, near Sonamarg, in January 2023, a few days after which 172 workers were trapped and rescued in a subsequent avalanche.

Additionally, the rock strata in the Himalayas are highly variable, even differing metre to metre, said Dix. And unlike tunneling through the hard basalt rock underground in Mumbai, this means a long list of surprises lie in store for the engineers as they drill through. Loose rock, boulders, water ingress: the rock formations can be delicate and susceptible to disasters. Each variation requires a different treatment for structural integrity.

What makes the Himalayas particularly risky to tunnel through?

The Himalaya range is a relatively new mountain range, making it far more tectonically active. “To a geologist,” said Dix, “the Himalayas are ocean floors that have been pushed upwards.”

This rock strata the engineers can encounter can be soft rock, characteristic of a sea bed, to boulders and everything in between. The way the mountain has formed could have left cavities and cracks.

A lot of water — even up to an ocean’s worth — is stored in the mountains. Due to its age, there are sheer zones where there is higher strain on the rock and tunnelling can lead to instability. The inside of the mountains can be highly sensitive.

Dix described the mountain range as “alive,” moving and responding to pressures it feels.

What methods were used to safely excavate the Zojila tunnel?

The Zojila tunnel was excavated using the New Austrian Tunneling Method, tailor-made for such conditions. The method involves selective blasting of the rock, followed by securing the tunnel wall with shotcrete (sprayed concrete) and rock bolts. The excavation is done in two steps, wherein the top half of the tunnel is excavated first, followed by the bottom half.

How can structuring systems be altered to meet the conditions of rock strata?

The mountains, especially as they were not so long ago a part of the sea bed, store a lot of water in them as well as have melting snow coming through. Depending upon the level of water ingress, engineers will fit pipes and allow the water to drain out in a gradual manner so as to not allow water pressure to build up in the mountain, which could potentially lead to a collapse or flooding.

Cracks in the rock formation will alter the number, size, and spacing of the rock bolts, while the shotcrete glues the rocks together. Even the tunnel’s alignment can be tweaked to bypass a weak segment of the mountain and skirt around it. The tunnel’s shape and configuration is also moulded as per site conditions.

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“The tunnel cannot hold the mountain up, simply because it is too big. The only way to tunnel through a mountain then is to do it in a way which upsets the tunnel the least, and ensure the weight falls on to the wayside. It is like conducting an operation without anesthesia. This is why tunneling in the Himalayas has to be done slowly and carefully,” said Dix.

What other safety systems were in place?

Three shafts were built across the length of the tunnel to ensure ventilation and create space for any rescue attempts.

The first, also the longest in India, went 474.3m deep. The second is 367.5m, and the third is 213.5m deep.