Mumbai: How engineers carved tunnel through fort precinct

Nestled in the heart of South Mumbai’s heritage precinct, the construction of Hutatma Chowk station on Metro Line 3 was anything but straightforward. Beneath the colonial-era facades and bustling D N Road, engineers faced one of the most intricate tunneling challenges in the city’s history — balancing the precision of underground construction with the fragility of 100-year-old heritage buildings above.The fully underground station lies just 23 metres below the surface and is hemmed in by narrow lanes, vintage stone arches, and footpaths teeming with pedestrians. Its 233-metre-long station box had to be carved with surgical accuracy — often just metres beneath Grade II A structures.“The biggest challenge wasn’t just engineering,” said S K Gupta, director (projects), Mumbai Metro Rail Corporation. “It was respecting the historical character of Fort while safely inserting 21st-century infrastructure beneath it.”

A hybrid construction technique was adopted to suit the cramped topography— one platform built using cut-and-cover, and the other using NATM (New Austrian Tunneling Method). NATM tunneling, typically used in mountainous terrain, had to be adapted for the city core. Its tunnel crown was a mere 16 metres below ground, offering little buffer between excavation and heritage foundations.To reduce impact on these structures, engineers reinforced arches above ground with steel supports.

Controlled micro-blasting — more than 10,000 precisely measured detonations — was deployed to loosen rock without jolting buildings. A “free face” groove was carved before each blast to dissipate shockwaves. Post-blast excavation used a drum cutter instead of traditional machinery to further minimize vibration.“In a city like Mumbai, vibration is asdangerous as a collapse,” Gupta explained. “So we kept it to less than 2.54 mm/sec — half of what’s normally permitted.”To stitch together different underground elements, eight narrow adits — side-entry tunnels — were built. Each had to be aligned with millimetric accuracy. Monitoring devices, including tiltmeters, vibration sensors and seismographs, dotted the area. Sensitive heritage structures were under round-theclock surveillance, with real-time data guiding every blast, every dig.Even the tunnel boring machines (TBMs) were fine-tuned for the Fort zone.

Their RPM was reduced, anti-roll systems were added, and partial excavation was done manually to counter possible misalignments.A comprehensive building condition survey documented existing cracks. Based on this, buildings were categorized by fragility, and construction methods were tweaked accordingly. Alert systems were set up — if any structure approached risk limits, work would stop until mitigation plans were implemented.“It was an endeavour of precision, planning and patience,” Gupta said. “But we couldn’t afford even the slightest error here.” The 3.8km stretch between CSMT and Kalbadevi posed the biggest engineering challenge during the tunnelling of Metro 3. The tunnel boring machine (TBM) Vaitarna-1 had to navigate a congested corridor lined with heritage, dilapidated and old buildings.