Written Response to City and County of Los Angeles

This is a copy of our response sent to Supervisor Janice Hahn and LA City Councilmember Tim McOsker's office in reply to their request received by the Sanitation Districts on July 10th.

Response to Request from Supervisor Hahn and City Councilmember McOsker
July 17, 2025

Dear Direct Hahn and McOsker:

In response to your letter dated July 10, below is an update based on the most current information. Our investigation is active and ongoing, and we will share updates with your offices as available. We appreciate that you understand the magnitude of this situation and our commitment to thoroughly investigate all potential causes related to the partial collapse (breach). 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 Sanitation Districts and our contractor, Flatiron/Dragados (FD), understand the concerns expressed in your letter dated July 10, 2025. This response is limited to the facts currently known until a more comprehensive and detailed investigation can take place. This will commence after FD is able to re-enter the tunnel. We anticipate approval to re-enter will be granted once CalOSHA reviews and approves FD’s access plan. We will provide updates as the re-entry plan develops and next steps are determined or approved.

Question No. 1: What caused the partial collapse of the tunnel?

At this point, FD knows only that the partial breach occurred in a section of the tunnel where the ground exerted high pressure on the area. This issue first appeared in February 2025, resulting in the “squeezing” of the segment rings creating ovalization of those rings and some cracking. FD’s tunnel liner engineer reviewed and provided measures to mitigate further ovalization and these measures were implemented by FD. The Tunnel Boring Machine had advanced 4,100 feet from the initial squeezing area when the partial breach occurred on July 9.

Question No. 2: How will you ensure the safety and security of the existing portions of the tunnel that have already been constructed?

Currently, FD is working with CalOSHA on an access plan for the tunnel. Once approved, this plan will govern FD’s re-entry into the tunnel leading up to repair of the damaged area and the recommencement of tunneling. FD will conduct an examination of the completed tunnel segments up to the breached area as part of the re-entry. Additional measures will be put in place as needed based upon the initial survey of the completed portion of the tunnel.

Question No. 3: How long do you anticipate the project will be delayed?

Re-entry activities will begin as soon as allowed by CalOSHA. Re-entry will be a phased process as the required ventilation and electrical systems are brought back online so that the Sanitation Districts and FD can safely reach and investigate the breached area. Thereafter, a repair plan will be developed. At this time, the full extent of the delay cannot be known; however, FD is committed to working with the Sanitation Districts to minimize the impacts of this incident.

Question No. 4: Does the incident compromise any of the local streets (i.e., potential sinkholes)?

Based on the preliminary information we have, it appears the tunnel breach occurred directly under Western Avenue, approximately 600 feet south of Weymouth Avenue and 370 feet below ground. The tunnel alignment is primarily within the public right-of-way and not under any homes. FD and Sanitation Districts survey crews at the site have verified there is no surface damage or discernible movement at this location and the area is safe.

Sanitation Districts’ survey crews monitor the road above the tunnel alignment before and after the tunnel boring machine passes through the area. Surveying involves checking the elevations of different locations along the route to ensure there is no movement as tunneling is occurring below the surface. This area has been monitored continuously since September 2024 and as recently as April of this year with no discernible movement. Our survey crews will monitor the area daily, 300 feet before and after this point. Readings will be taken across the street in 50-foot increments.

Based on physical inspection and ground monitoring data, there is no information to suggest that any local streets or surface areas above tunnel construction have been compromised. As previously mentioned, there are no homes within the surface area above the tunnel construction and therefore no impact is expected.

We assure you that the Sanitation Districts and our construction partners will continue to provide critical updates as they become available. Sanitation Districts will contract with additional engineering consulting firm(s) with expertise on tunnel construction as necessary to provide independent review and added expertise. We intend to develop a more comprehensive report once a complete investigation of this matter is finished. We are available to meet with you and the public and provide updates as available. Please contact me if you have questions.

Sincerely,

Robert Ferrante
Chief Engineer and General Manager