A deadly accident inside the tail-race tunnel of the 520-megawatt Vishnugad-Pipalkoti Hydroelectric Project in Uttarakhand’s Chamoli district has once again raised questions about the risks of large-scale subterranean excavation in the fragile Himalayas.

Torrential monsoon rain had saturated the unstable mountain slope above the tunnel alignment, creating conditions for water to accumulate underground. Without warning, a section of rock collapsed, sending a surge of high-pressure water, mud and muck through the tunnel.

The deluge trapped 22 workers, leaving ten dead and sweeping several others through the narrow concrete chamber. Surviving workers described the sudden ingress as a subterranean flood, with water, air and debris rushing through the tunnel like a dam burst.

This was not merely an isolated engineering accident. It points to a wider and worrying pattern of water ingress, aquifer punctures and tunnel instability across hydroelectric and transport projects in the Himalayan fold-and-thrust belt.

The hidden water inside the mountains

The Indian Himalayas, often called the planet’s Third Pole, store enormous volumes of freshwater. Much of this water is visible in glaciers, snowpack and surface ice. But an equally important hydrological network lies hidden within the mountains themselves.

The Himalayas are young fold mountains, still being shaped by the collision of the Indian and Eurasian tectonic plates. Their rock strata are heavily sheared, jointed and fractured. Major tectonic zones, including the main central thrust, main boundary thrust and north almora thrust, cut across the region.

Over millenia, seasonal snowmelt and rainfall have percolated into these fractures, fault lines and rock troughs, forming pressurised subterranean aquifers at high elevations.

Under normal conditions, this underground hydrology helps maintain a delicate balance. Water trapped within fractured rock exerts pore-water pressure, which contributes to the internal support of mountain slopes. These underground reservoirs also feed surface ecosystems, releasing water through natural channels that sustain traditional drinking water sources such as dharas, or natural stone spouts, and naulas, or aquifer-fed wells. They also provide critical non-monsoon baseflow to Himalayan rivers.

When multi-kilometre tunnels are bored through these hydrogeologically complex zones, using tunnel boring machines or drill-and-blast methods, that balance can be disturbed. Piercing a water-bearing fault zone can turn the tunnel into an artificial drain, drawing groundwater out of the surrounding rock.

This can lead to large-scale dewatering, lowering local water tables and weakening rock structures that had been supported by internal water pressure. As pore pressure declines, fractured and fragile rock masses can settle or shift, increasing the risk of surface subsidence and structural damage.

This process has been discussed in connection with the land subsidence and structural cracking seen in Joshimath, where changes to subsurface water flow have been identified as one of several contributing factors.

Warnings from earlier projects

The risk is not new. In December 2009, during construction of the nearby Tapovan-Vishnugad Hydroelectric Project, a tunnel boring machine reportedly breached a pressurised aquifer more than 900 metres below the surface. The puncture released a continuous burst of groundwater, estimated at 600 to 700 litres per second, for months. Large volumes of groundwater were drained, and local hill springs were reported to have dried up.

The Pipalkoti accident suggests that the lessons from earlier projects have not been fully absorbed. Once again, high-pressure water and mud entered a tunnel, overwhelming lining operations and submerging sections of the underground cavity.

Such incidents underline a basic problem: Himalayan tunnels are not being cut through inert rock. They pass through fractured, water-bearing, seismically active mountain systems. Treating them as conventional engineering projects, without giving equal weight to hydrogeology, can leave workers, communities and ecosystems exposed to serious risk.

Risks of pushing deeper into the High Himalayas

The latest disaster also raises concerns about future infrastructure plans in Uttarakhand, including proposals to extend railway networks beyond Karnaprayag towards the pilgrimage centres of Badrinath and Kedarnath, both located at elevations above 3,000 metres.

High-altitude rail infrastructure in this terrain brings significant hydrogeological challenges. Comparisons are sometimes drawn with systems such as the Qinghai-Tibet railway, but that comparison has limits. Tibet is a relatively stable, high continental plateau, where much of the railway runs over open surface terrain. The High Himalayas, by contrast, are an active collision zone, marked by steep slopes, narrow valleys, crushed rock, active faults and unstable drainage systems.

The 125-km Rishikesh-Karnaprayag rail line already places more than 100 km of track inside tunnels. Extending rail infrastructure further towards Badrinath and Kedarnath would require deeper excavation through high mountain catchments and recharge zones for alpine aquifers.

Such tunnelling can alter subsurface drainage paths, create new fractures and affect springs far from the excavation site. At elevations above 3,000 metres, tunnels also face extreme overburden pressure. In weak, water-saturated Himalayan rock, this can create ‘squeezing ground’ conditions, where tunnel walls deform under stress and destabilise the surrounding rock mass.

The repeated disasters, groundwater depletion, and tunnel collapses across Uttarakhand indicate that conventional engineering models — which treat mountains as static blocks of concrete and stone — are fatally flawed. Surface surveys and sparse test boreholes consistently fail to detect pressurized aquifers hidden within fault zones.

Continuing to drill hundreds of kilometers of deep tunnels through the High Himalayas without comprehensive real-time hydrogeological mapping and advanced probe-drilling risks permanently destroying the subterranean water architecture that sustains life across the region.

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