Tunnel vision

With over 15 million vehicles plying the roads of Bengaluru, the rapidly expanding megacity no doubt grapples with severe traffic congestion despite the expansion of public transport facilities like the metro-rail services. The 2024 congestion level index by TomTom, a Dutch location technology developer, says Bengaluru is the third-slowest city for traffic in the world, behind only Barranquilla in Colombia and Kolkata. So, to decongest the Silicon Valley of India and ensure a “seamless travel solution”, the city’s civic body, Bruhat Bengaluru Mahanagara Palike (BBMP), has unveiled the blueprint of an ambitious public infrastructure plan that involves constructing elevated corridors, under-passes and even tunnel roads between key corridors, such as Hebbal Esteem Mall junction and Silk Road KSRP junction. A New Delhi-based consultancy firm has submitted its feasibility report for the project to BBMP in December 2024.

However, a quick analysis of the feasibility report shows that not much emphasis has been placed on the ground-truth data related to the region’s lithology (which explains the characteristics of rocks) and hydrogeology (movement of groundwater and how it interacts with soil and rocks). Such data is particularly necessary to assess the viability of tunnelling projects that are influenced by the geological condition of the region and may lead to hazards such as subsidence and landslides. It is also important because of the city’s complex geology.

Bengaluru is spread over 800 sq km at an altitude ranging between 780 m and 954 m above the mean sea level. A granite high land, nearly 45 km long and 4 km wide, extends through the city in the north-south direction, separating the Arkavathi river catchment on the west from the Ponnaiyar river catchment on the east. There is a stark contrast between the geological features of these two parts of the city. For instance, on the west of the granite belt, the Vrisha-bhavathi stream, a tributary of the Arkavathi, originates at 920 m above the mean sea level and flows southward for over 30 km with 120-m gradient drop. Such rapid flow has carved rock-cut valleys and undulating terrain along its path that remain masked by urbani-sation. Besides, the western contact of the granite belt with gneissic rocks (a primordial rock formation on which Bengaluru and much of the Peninsular India rests) is sheared and faulted. Building structures near such faulted rockmass terrain face the risk of damage during earthquakes and other ground movements.

On the east of the granite belt, the Ponnaiyar catchment forms a plateau-like topography with a gentle easterly slope. The Ponnaiyar drainage system is dominated by Hebbal and Bellandur valleys, in the north and south respectively, with broad, shallow and unstable catchment where the gneissic rocks have weathered to form saprolites, which is clayey, highly porous but impermeable. At places, saprolites are covered by 3-4 m thick red laterite soil, which is also clayey and impermeable. When wet, saprolites can cave in and collapse.

Now, consider the cumulative impact of such vast saprolite terrain, massive rockmass terrain which is faulted, and the mosaic of asphalt roads and concrete structures that have expanded both horizontally and vertically. Together, they have restricted the recharge of aquifers in an area that has no river water resource of its own. The carrying capacity of the city land is also increasingly under pressure.

So, before going ahead with such mega tunnel projects, particularly in the heavily crowded parts of Bengaluru, the authorities must investigate a few issues to ensure that it does not add to the list of hazards that already ail the city.

First, alignment of one proposed tunnel between Hebbal Esteem Mall and Silk Road KSRP junctions appears to be along or nearby the eastern contact of the granite belt with gneissic rocks, which have been weathered to saprolites up to varying depths. The part of the saprolite that comes in contact with water during the tunnel works, may cave in and collapse. Such a cave-in and collapse was witnessed in 2022, while laying a tunnel for Yettinahole drinking water supply project in Hassan district of Karnataka. Second, the tunnel construction is likely to disturb the natural flow of the surface drainage systems and perched aquifers (a shallow zone of groundwater that sits a layer of lower permeability material like saprolites). At certain points the geological lineaments, aligned in northeast-southwest direction, have abruptly ended in gneissic rocks in contact with granites. These lineaments, though masked by the urban concrete mosaic cover, have a role in the vertical infiltration and movement of groundwater. Any disturbance to them will affect water infiltration into aquifers. Hundreds of borewells on either side of the proposed tunnel area may get affected.

Hence, the authorities must obtain transparent opinion of experts and conduct tests along and across the proposed tunnel areas to know ground-truth conditions and ascertain ground suitability.

(G V Hegde and K C Subhash Chandra are hydrogeologists and have conducted several studies in Bengaluru region)

This was first published in the 16-31 March, 2025 print edition of Down To Earth