Southern California transportation officials have begun route analysis and public outreach for a plan to build a tunnel that would carry trains along a portion of the nation’s second-busiest passenger rail corridor. The single-bore or twin-bore tunnel. which would be as deep as 305 ft below sea level in the southern section of the San Diego-Los Angeles-San Luis Obispo corridor, could cost from $3 billion to $4 billion.

The 351-mile ail corridor carries an estimated $1 billion in goods each year and some 8 million annual passengers on Amtrak’s Surfliner. The Del Mar section, which rises 50 to 70 ft above the Pacific Ocean, has been periodically closed over the past two decades due to erosion of bluffs that loom over the tracks. 

The San Diego Association of Governments, working with the North County Transit District, the California Dept. of Transportation and the U.S. Dept. of Transportation, launched a $3.2-million study in summer 2020 to evaluate five tunnel alignments originally identified in 2017. Since then, the state has awarded $300 million to fund the project from preliminary engineering through final design.

The association has ramped up public meetings and workshops, mostly in Del Mar where the tunnel alignment would be. At an Oct. 4 meeting, Sharon Humphreys, its director of engineering and construction, told residents that the bluffs are eroding on average by about 6 in. per year. “The tracks can’t stay where they are,” she said. “Mother Nature will win.”

Stabilization and emergency repairs have been done over the last two decades, years, including a $19-million job in 2020 and a current $78-million effort. “We’ve designed these columns for a 20-year life,” she added. “They’re not going to last forever.”

In addition to higher costs of temporary stabilization with columns, “we have other vulnerabilities” like century-old wooden trestles, said Danny Veeh, association rail planning project manager, at an Oct. 18 virtual meeting.

Joe O’Carroll, a San Diego-based senior vice president for Mott MacDonald, explained at the meetings that a tunnel boring machine might dig either a single-bore, 47-ft-dia. tunnel or twin-bore 31-ft-dia. tunnels. The approximately 4-mile tunnel would be 305 ft below sea level, cutting through layers of sandstone, sand cobbles and silty clays.

A tunnel is preferable to an at-grade alignment due to the region’s hilly topography and the need for freight trains to not exceed a 2% grade, said Veeh. The depth would also mitigate noise and vibration concerns, officials said.

Officials expect to conduct geotechnical work, a cost-benefit analysis and begin preliminary engineering/environmental phases next year.

Construction is anticipated to begin in 2028 and last seven years.