Walnut Creek commuters heading west through the Caldecott Tunnel will lose one bore on Wednesday, Sept. 23.

Caltrans District 4 announced that Bore #3 on westbound SR-24 will close overnight from 10 p.m. to 4:30 a.m. for electrical equipment replacement and routine maintenance. The agency described the work as a UPS and breaker replacement, referring to the tunnel's backup power and electrical switching systems.

All other bores will stay open.

511 SF Bay confirmed the same closure window, describing it as the left bore of the westbound tunnel.

The Wednesday night shutdown follows a pattern of overnight single-bore closures Caltrans has run through the tunnel in recent months. Eastbound Bore #1 closed Sept. 15 and Sept. 16 for lighting upgrades and routine maintenance. In late July, Caltrans shut down one bore per night over three consecutive evenings: Bore #1 eastbound on July 27, Bore #2 eastbound on July 28 and Bore #3 westbound on July 29. Each of those closures ran the same 10 p.m. to 4:30 a.m. window.

The overnight work comes as Caltrans prepares for a much larger rehabilitation of Bores 1, 2 and 3. That project, funded by Senate Bill 1 and the federal Infrastructure Investment and Jobs Act, aims to repair cracked concrete, address rust and water damage, upgrade electrical and lighting systems, and install new jet fans for ventilation and a fire-suppression sprinkler system. Construction is tentatively set to begin in fall 2026 and wrap up by fall 2029, according to the Caltrans project page. A construction contract award is tentatively scheduled for September 2026.

Project partners include Caltrans, the Federal Highway Administration, the State Fire Marshal, Alameda Fire Department and Contra Costa Fire Department. Hector Chinchilla, public information officer for Caltrans District 4, is the project contact for the rehabilitation.

Drivers who want advance notice of future Caldecott closures can sign up for traffic advisories through Caltrans.