“That means long trips for commuters,waits sitting on Victoria Bridge and on the road network.”

The council expects the tunnel to improve on-time running by 50 per cent.

Murphy said the tunnel needed to be excavated slowly because it was shallow,the rock known as Brisbane Tuff was highly weathered and relatively soft,and the route must avoid building foundations.

“In Brisbane the vast majority of tunnels are dug through hard rock - projects like Clem 7 and Legacy Way - so this is an extremely delicate project and at any one time we are only metres away from car parks,and the foundations of other buildings,” he said.

Project manager Jose Antonia Sanchez said it was “quite a challenging tunnel,” having to be built beneath temporary 16-metre steel canopy support tube beams.

“It is very shallow,only a few metres under the street,and it is probably the first tunnel in Australia done in soft ground,” Sanchez said.

Sanchez said the cautious approach meant slow progress - the team was tunnelling “one metre at a time” to ensure there was no slippage.

“We have used this technology under the Opera House in Sydney. It was in different conditions,but it was the same challenges.”

The work will take about 18 months,with seismic movement monitored daily before tunnelling commences.

Subsidence in August 2022 required the closure of part of Adelaide Street at peak hour,causing two-hour traffic delays.

Excavations have unearthed old bricks and bottles from Brisbane’s convict-era days and a convict-era wall from one of Brisbane’s original hospitals.

The project has also increased in cost,from $980 million to $1.4 billion and now $1.7 billion,which includes a new station at the Gabba.