Retaining wall built along bluff-top train tracks
Train service resumed through Del Mar on Monday, Dec. 16, after a weekend shutdown to build an 80-foot-long retaining wall in an area washed out by a powerful November storm.
Storm run-off exposed the tops of eight concrete piles that were installed more than a decade ago to protect the tracks on the coastal bluffs near the end of 15th Street, said Stephen Fordham, North County Transit District’s director of capital projects.
“The original idea was to build a wall once the erosion reached the piles,” Fordham said.
The piles are 30 to 36 inches in diameter and 10 feet apart, sunk up to 45 feet into the ground. Workers anchored reinforcement bars into the exposed tops of the piles, welded more steel reinforcement to the bars, installed water drains, and then covered the entire assembly with sprayed-on concrete called “shotcrete.”
Three truckloads of concrete, each carrying about 10 cubic yards, were used to build the wall, Fordham said. Another truckload of concrete slurry was used to fill space between the wall and the cliff. All of the concrete was delivered to the site in trucks loaded onto a flatbed railroad car.
The structure is sculpted and textured to blend in with the surrounding cliffs, Fordham said.
Studies show the coastal bluffs erode an average of 6 inches annually. However, the rate of erosion varies depending on many factors and can occur more rapidly during wet winters.
Del Mar received a total of 2.5 inches of rain in late November, most of it on Thanksgiving Day. The area has an average annual rainfall of less than 12 inches. The National Weather Service says another significant storm could arrive next week.
Steel plates and concrete slurry were used in an earlier round of repairs completed Nov. 30 and Dec. 1 on a nearby section of the cliff.
NCTD worked with the San Diego Association of Governments, consultants HNTB Corporation and Leighton Engineering, and the Mid-Coast Transit Constructors to design and complete the most recent emergency repairs. Mid-Coast Transit is the company building a $2 billion expansion of the San Diego trolley system.
During both weekend construction projects bus service was provided for Coaster and Amtrak passengers between the Solana Beach train station and the Santa Fe Station in downtown San Diego.
NCTD and SANDAG have been working on a series of projects since 2003 to stabilize the bluffs at Del Mar. The fourth phase of that work is scheduled to begin in January and will be focused primarily on improving drainage structures.
— Phil Diehl is a reporter for The San Diego Union-Tribune
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