Bengaluru's tunnel plan must consider geological, hydrological risks

The proposal for a 14.55-km vehicular tunnel from the Hebbal Esteem Mall junction to the Silk Board KSRP junction, to be constructed at a depth of 120 feet below the surface, lacks crucial information about the most important geological and geohydrological issues and their potential adverse impacts.

Bengaluru, a rapidly expanding megacity, is grappling with severe traffic congestion despite the expansion of the Metro and Bengaluru Metropolitan Transport Corporation services. With over 15 million vehicles now plying the city's roads, the Bruhat Bengaluru Mahanagara Palike (BBMP) has identified high-congestion zones and is considering constructing a vehicular tunnel along certain key corridors. In pursuit of this plan, the BBMP received a final feasibility report in December 2024 from a New Delhi-based consultancy, Comprehensive Bengaluru City Traffic Management Infrastructure Plan Proposals for Vehicular Tunnel/Grade Separator/Road Widening for the Selected Corridors.

The report includes a proposal for a 14.55-km vehicular tunnel from the Hebbal Esteem Mall junction to the Silk Board KSRP junction, to be constructed at a depth of 120 feet below the surface. However, the feasibility report lacks crucial information about the most important geological and geohydrological issues and their potential adverse impacts.

Having conducted intensive geomorphological, geological, and hydrogeological studies in the BBMP area for over a decade, the authors strongly emphasise the need for an in-depth investigation into the geological viability of the project and its potential impact on Bengaluru's aquifers that are already stressed by ongoing mining of groundwater resources.

Bengaluru is spread over 800 sq km at an altitude ranging between 780 and 954 metres above mean sea level (AMSL), forming a basin divide created by nearly 45-km-long and 4-km-wide, centrally run, north-south-disposed granite high ground. The divide separates the Arkavathi catchment with its principal tributary (365 sq km), Vrishabhavathi, in the west, and the Ponnaiyar catchment (435 sq km) in the east, demonstrating a stark contrast in geological, geomorphological, and geohydrological features between the Arkavathi and Ponnaiyar river catchment parts of the city.

The Vrishabhavati stream, originating about a kilometre northwest of Laggere, flows southward over a length of 30 km with a 100-metre gradient drop. The rapid stormwater flow, in rock-cut valleys, is mainly amid hills, mounds, and undulating terrain. Structurally trained flow systems are masked by urbanisation.

East of the granite highland, the Ponnaiyar catchment with gneissic rocks, chemically altered to saprolites for a thickness of 20 to 30 metres, between 885 m and 850 m AMSL, forms a plateau-like topography with a gentle slope towards the east and southeast, predominated by Hebbal and Bellandur valleys, respectively. The flow paths of the drainage system in the Ponnaiyar part are characteristically broad, shallow, and unstable. The flow path of Hebbal and Agara-Bellandur-Vartur main drains is trained by the east-west fractures. The erosion of laterites has left behind clayey, porous, but impermeable saprolites—reddish brown to greyish white in colour—covered by 2-4 metre-thick in situ, or transported, red laterite soil.

Bengaluru's urbanisation with a mosaic of concrete buildings, asphalted roads, and an adverse hydrogeological setup of massive rock zones and saprolites with clayey laterite soil cover has resulted in poor groundwater recharge. Parched water conditions are common in the Ponnaiyar catchment area.

The city, developed on a basin divide without any river water resource of its own except the rainwater, has been allowed to expand horizontally and vertically without crucial infrastructural facilities. The green-belt area of over 400 sq km has all but disappeared. The stormwater drains of first and second orders as well as the lake beds are encroached. SWDs are now the perennial flow path for partially treated and/or untreated urban sewage, and the centuries-old lakes constructed across such drains are the repositories of such discharges. This has reduced the carrying capacity of the SWDs, causing flooding in low-lying and encroached areas. The international airport near Devanahalli has brought an unprecedented growth of apartments. Many of the multi-story apartments are constructed in the unstable saprolite zone of the Ponnaiyar catchment part. Instances of a few apartments collapsing were reported between September and October 2021, when the area received heavy rains.

The proposed tunnel runs along fractured or near the eastern contact of N-S granite belt with gneissic rocks altered to saprolites. This raises several concerns. The feasibility report must disclose the carrying capacity of Bengaluru to sustain the erratically induced urban pressure in the name of development and viability to solve the critical engineering geological aspects. The saprolites, upon contact with water, may cave in and collapse. Tunnel construction may disturb the natural flow system of the surface drainage network conditions, ponds and lakes, and nearby groundwater aquifers. Lineaments play a key role in the vertical infiltration of rainwater and surface bodies. Tunnel construction may affect hundreds of water-yielding borewells. Already under stress due to continued over-exploitation of groundwater, many of these borewells may dry up. Tunnels affect the water infiltration into the aquifers.

Before proceeding, the authorities must commission detailed geological, geotechnical, and hydrogeological investigations by experts, involving spot tests carried along and across the proposed tunnel area to ascertain ground conditions and its suitability.

(The writers are hydrogeologists)