Firefighters said the tunnel collapse occurred 10km from the only access point of the tunnel.
However, just about an hour after the tunnel collapsed, all construction workers inside the tunnel were rescued, according to Los Angeles County Supervisor Janice Hahn.
Firefighters said that construction workers climbed through a 3.6 to 4.5m high embankment to escape the collapsed tunnel.
No one of the 31 construction workers trapped in the tunnel was injured.
Los Angeles mobilized large amounts of resources, including all Urban Search and Rescue teams, to the 1700 block of South Figueroa as the tunnel collapse occurred.
The tunnel collapse occurred at the $630.5 million Los Angeles Wastewater Discharge Hospital under the Clearwater Project of the Los Angeles Ward Toilet facilities.
Flatiron Dragados, the project's main contractor, informed that the tunnel is about 11.27km long, about 5.49m wide and located about 137m below ground.
The project is expected to be completed in 2027 and the new tunnel will be activated in January 2028.