LA tunnel work remains halted after partial collapse

Dive Brief:

• A Los Angeles area wastewater tunnel partially collapsed July 9 in a section “where the ground exerted high pressure on the area,” according to a letter from Robert Ferrante, chief engineer and general manager of the Los Angeles County Sanitation Districts.
• No one has yet been able to enter the tunnel or assess the breach location, Los Angeles County Sanitation Districts spokesperson Michael Chee said in an email on July 28, and the tunnel boring machine remains in place where it stopped mining. “Work on the tunnel remains at full stop,” Chee said. “Red tag placed by CalOSHA remains in force.”
• The pressure issue first surfaced in February, per the letter dated July 17, “resulting in the ‘squeezing’ of the segment rings creating ovalization of those rings and some cracking.” At the time, a tunnel liner engineer reviewed the problem and Broomfield, Colorado-based GC Flatiron/Dragados adopted the suggested measures to mitigate it.

Dive Insight:

The investigation into the breach of the Los Angeles Effluent Outfall Tunnel is ongoing. The tunnel boring machine had advanced 4,100 feet away from this initial squeezing area when the partial breach occurred 370 feet below ground, according to the letter. Flatiron/Dragados is currently working on an access plan for the tunnel, and more answers will become available once crews can reenter.

Twenty-seven workers escaped without major injury from the tunnel in Carson, California, by climbing over a large mound of loose soil and emerging at the only entrance 5 miles away, The New York Times reported. Four others who went in to help with rescue efforts also emerged unharmed. The tunnel is designed to carry treated effluent, not sewage, and the partial collapse does not pose a risk to the environment, the agency said.

The $630 million tunnel, which broke ground in 2019, is part of the Sanitation Districts’ Clearwater Project to protect local waterways, according to the project website. The new 7-mile tunnel will transport treated wastewater to the ocean and will replace two aging tubes. It will be built to current earthquake standards and with enough capacity to protect against overflows from major storms.

The tunnel alignment is primarily within the public right-of-way and not under any homes, and “there is no information to suggest that any local streets or surface areas above tunnel construction have been compromised,” per the letter. Nonetheless, survey crews will monitor the area daily.

“We are committed to prioritizing worker safety and will pursue any and all corrective measures necessary to complete construction of this vital infrastructure project once all safety concerns have been thoroughly addressed,” the letter reads.

A 2022 Mining Engineering magazine article noted that the tunnel route passes through two active fault zones — the Palos Verdes Fault zone and the Cabrillo Fault zone — and numerous geotechnical challenges would be encountered in its path.

The project is being constructed by Flatiron West and Dragados USA, operating as a joint venture. The two parent firms merged last July to form FlatironDragados.

Work was expected to be completed in 2027 but it is not yet clear how much the breach will delay the project, according to the letter. Construction prep work at the tunnel boring machine arrival location at Royal Palms Beach in San Pedro continues as scheduled, according to Chee.

A town hall meeting to give updates on the investigation and timeline is scheduled for Aug. 9, according to the project website.