The boring of under-construction Bangabandhu Tunnel's second tube at the bottom of the River Karnaphuli ended on Thursday - three months ahead of deadline.
The excavation was expected to be completed by December this year.
Mentioning that 73% of the construction work for the tunnel is done, Project Director Harunur Rashid Chowdhury said the entire project will be completed by December 2022.
The tunnel will then be opened to traffic, he added.
The tunnel, at a depth of 18-43 metres below the riverbed, will connect Anwara and Karnaphuli upazilas on the south bank of the Karnaphuli River with the port city.
The project was conceived to develop the Chattogram city on the "One City, Two Towns" model of Shanghai in China.
The tunnel will reduce the distance from Chattogram to Cox's Bazar by 40 kilometres, with vehicles plying it reaching speeds of 80km per hour.
Approved in 2015 at an estimated cost of Tk8,447 crore, the project was supposed to be completed by 2020.
Amid various complications, however, in 2018 the completion date was extended to December 2022.
The total cost of the revised project also rose to Tk10,374 crore.
To cover the cost, the Exim Bank of China is giving a hard loan – one secured by real property – of Tk5,913 crore at 2% interest rate, while the Bangladesh government is providing Tk4,461.23 crore.
The tunnel was promised by Prime Minister Sheikh Hasina at a rally at Laldighi in Chattogram before the 2008 elections. When the Hasina-led Awami League came to power soon after, work on the tunnel began.