Bengaluru’s 18.5 km Underground Tunnel Project Under Scrutiny: Panel Warns of Design, Cost, and Drainage Issue

Karnataka's ambitious plan to build an 18.5 km underground tunnel connecting Silk Board Junction and Hebbal Junction has encountered significant challenges. A government-appointed expert panel, led by Siddanagouda Hegaraddi of BMRCL, has highlighted numerous deficiencies in the detailed project report (DPR). These issues span technical, financial, and environmental aspects, leading the panel to suggest a complete overhaul of the plan.

The committee criticised the DPR's geotechnical investigations for relying on data from only four soil-test boreholes. It recommended comprehensive soil testing along the entire alignment to understand varied subsoil and rock conditions better. Drawing from Bengaluru Metro's tunnelling experience, experts noted the absence of electrical resistivity tomography (ERT) in mapping groundwater and aquifers. They warned that ignoring this could result in structural flaws and waterlogging during construction.

Environmental Concerns at Lalbagh

The proposal to locate a tunnel shaft inside Lalbagh Botanical Garden faced strong opposition from the committee. They deemed it "environmentally unacceptable." Additionally, major flaws were found in the stormwater drainage plan, which lacked necessary hydraulic calculations. The proposed diversion of a drainage channel near Hebbal Lake was labelled "impractical" and unsupported by data.

The panel also raised concerns about the tunnel's alignment running almost parallel to the Namma Metro corridor, potentially rendering the project redundant. Traffic forecasts in the DPR were criticised for not considering commuter shifts to Metro, suburban rail, or BMTC buses, leading to inflated traffic estimates.

Design and Cost Estimation Issues

A mismatch was identified between the DPR's 3+3-lane tunnel design and its horizon year of 2041, which is just over a decade from expected completion. The committee suggested that a 2+2-lane configuration would suffice. It recommended revising either the horizon year or design parameters for engineering and financial viability.

Concerns were also raised about cost estimation processes in the DPR. The cost of tunnel boring machines (TBM) was based on a single vendor quote, violating PWD norms. Experts advised obtaining quotes from at least three suppliers and providing a detailed cost breakdown instead of rounded-off estimates for land acquisition, utilities, and toll systems.

The entry and exit ramps near Mekhri Circle and other signal-controlled junctions were flagged as potential traffic bottlenecks and safety hazards by the committee. These concerns add another layer of complexity to an already challenging project.