Mumbai: On the surface, the noise may be deafening at times, but what goes unheard is the daily grind of the huge man-force and machines quietly digging a tunnel about 100 feet underground for the ambitious Metro-3 corridor.

So far, 220 metres of the 33.5 km-long underground corridor, connecting Colaba in South Mumbai to SEEPZ in suburban Andheri, has been dug up to construct the tunnel, the first such in the city, with the help of boring machines.

The corridor, expected to be a "game-changer" for the city's transportation system and also reduce pollution, is likely to pass over around 2,000 buildings.

The Mumbai Metro Rail Corporation (MMRC) has devised a comprehensive plan for digging underneath while allying fears that the vibrations caused by the work may affect the heritage and dilapidated structures.

"Nearly 1,500 project-affected families have been shifted and a manpower of 7,600, including skilled and unskilled, is on the job," the state-owned agency implementing the project, said recently.

The Metro-3 corridor will zip through Colaba up to SEEPZ (Santacruz Electronics Export Processing Zone, a special economic zone), making halts at 27 stations, out of which 26 will be underground.

Some activists and citizens' representatives have raised concerns over the safety and sustainability of the old and heritage buildings ever since the construction work began.

Dispelling the fears, MMRC Managing Director Ashwini Bhide said the "best internationally tried and tested" technologies are being used and thus, no such question arises.

"Over 20 types of monitoring tools, which are internationally tried and tested, have been pressed to ensure the longevity and safety of the buildings coming in way. The vibrations caused during the piling work are being regularly measured and recorded by advanced instruments," she said.

Bhide said the records show the vibration levels to be even less than one-fourth of the permissible safe limits allowed for heritage or weak buildings.

Underlining the safety measures taken up by the MMRC, she said, "We have identified the zones falling in the stretch of 50 metres on either sides of the route alignment and categorised them under various levels of strengths."

The corporation has hired the best available design consultants who are extending their professional expertise to the contractors, she assured.

Appealing activists and other stakeholders to cooperate in executing the project, she said the Metro services help reduce carbon footprint as well as pollution, and constitute the greenest mass rapid transit system.

"This Metro will subside the burgeoning load over the local rail, which is unfortunately leading to deaths in thousands every year due to over-crowding.

Eventually, this is going to be a game-changer in terms of the transportation scenario in the country's financial capital," Bhide said.

Japan's International Co-operation Agency (JICA) is providing a loan assistance of Rs 13,235 crore, which accounts for 57.2 per cent of the total cost. The remaining funds would be made available by the state and the Union governments.

The MMRC has been constituted as a joint venture between the Centre and the Maharashtra government. It has set 2021 as the deadline to get the entire stretch operational.

Chief Minister Devendra Fadnavis earlier said that by the end of 2022, over 90 lakh people would travel in the Metro trains.

The Ghatkopar-Andheri-Versova Metro corridor is already running since the last three years, attracting a huge ridership.