All 31 workers escaped without injuries from a collapsed industrial tunnel in Los Angeles' Wilmington area after scrambling over a tall pile of loose underground soil, city officials have said.

The trapped workers were shuttled back to the tunnel's entry point, more than eight kilometres away from the affected area, after they escaped the collapsed section and met several coworkers in the unaffected part of the tunnel, the Los Angeles Fire Department (LAFD) said.

The tunnel, which had a diameter of 5.5 metres, trapped 27 individuals, while four workers entered the damaged section to assist with rescue, LA Fire chief Ronnie Villanueva said.

"The workers had to climb through debris. They had to make themselves out through," before they were assisted out, Villanueva said.

Robert Ferrante, chief engineer and general manager of Los Angeles County Sanitation Districts, said a section of the already built part of the tunnel experienced squeezing ground conditions and partially collapsed.

"LAFD has just reported that all workers who were trapped in the tunnel in Wilmington are now out and accounted for. I just spoke with many of the workers who were trapped. Thank you to all of our brave first responders who acted immediately," Los Angeles mayor Karen Bass said.

The collapsed section was a part of the Los Angeles County's Clearwater Project, where the new 11km tunnel is being built to upgrade the region's sewer system, officials added. (Reuters)