The tunnel has been closed for years. Here, we take a glimpse inside the once well-used walkway.
The closure, between New Bonny Street and York Street, will be put in place on Monday and will remain until March 26 while the tunnel is filled in.
The road will be managed by two-way traffic signals either side of Chapel Street junction. The west end of Chapel Street will be closed and a diversion will be in place.
The subway, which has been disused for years, is closed to the public as the council says it is too dangerous to be reopened.
Light-weight foam concrete will be used to fill in the subway, as repair and ongoing maintenance of it would be expensive. This will increase the loading capacity of Chapel Street and the Promenade to 40/44 tonnes.