A decades-long effort to stabilize a coastal bluff in Southern California beneath a 1.6 mi section of a critical rail line is moving into its next stage. Phase 4 of the Del Mar Bluffs Stabilization Project is currently under construction and expected to be completed by year’s end, notes John W. Haggerty, P.E., M.ASCE, the director of engineering and construction at the San Diego Association of Governments (SANDAG), the primary public planning, transportation, and research agency in the San Diego region. The busy rail section, in Del Mar, carries multiple passenger and freight trains daily.
SANDAG is working on the bluff stabilization project in collaboration with the North County Transit District, which controls train operations on a roughly 60 mi long segment of a 351 mi long corridor from San Diego to San Luis Obispo, California. The corridor is considered the second-busiest passenger corridor in the United States, carrying Amtrak service, COASTER and Metrolink commuter lines, and freight trains operated by BNSF Railway. Some of the existing infrastructure dates to 1912, when a predecessor of BNSF constructed the rail line along the bluffs.
In addition to the bluff stabilization efforts, required because of ongoing slope erosion, phase 4 of the project also involves the repair or replacement of aging drainage systems along the bluffs and a retrofit of the existing soldier piles and other protection systems at the base of the roughly 60 to 70 ft high bluffs, which overlook the Pacific Ocean.
Coordinating with Trains
Among the many challenges that must be accommodated is the fact that the beach at the base of the bluffs is often inaccessible, especially in winter, because of high tides and wave action. Neither the city of Del Mar nor the California Coastal Commission—a state agency that regulates land use and public access in the California coastal zone—wants any new permanent structures constructed along the beach. And, of course, the work site involves an operating railroad. “Essentially, we have rush-hour commuter trains, Amtrak trains, [and] BNSF service during the day and night,” Haggerty says. Although the COVID-19 pandemic has reduced rail traffic on the corridor, it can still be a challenge to accommodate the needs of both the railroads and the construction crews. There are only limited times in the year when the tracks can be shut down, and when they are operating, the trains are often forced to slow down through the work zones and crews must move equipment out of the way, Haggerty says.
The key private firms involved with the phase 4 work include the international engineering firm HNTB Corp., responsible for civil and structural engineering; Leighton and Associates, based in Irvine, California, the geotechnical engineer; San Diego-based Blue Pacific Engineering and Construction, the prime contractor; and Condon-Johnson & Associates Inc., headquartered in Oakland, California, in charge of drilling operations.
SANDAG has been studying and working on the Del Mar bluffs for the past two decades. Phases 1, 2, and 3—completed between 2000 and 2012—included the installation of more than 200 soldier piles as support columns to stabilize the full height of the bluffs and the installation of new drainage infrastructure.
In November 2019, heavy rains washed out portions of the embankment in two locations along the 1.6 mi segment of the bluffs, forcing a “significant interruption” of operations and requiring emergency repairs, Haggerty says. The damage might have been worse, but fortunately the areas affected had previously been stabilized by the soldier piles. Thus, the phase 4 work is designed specifically to reinforce the soil to prevent it from washing onto the track bed, Haggerty notes.
This work will involve the installation of a series of cast-in-drilled-hole steel piles made from W12 steel beams, ranging from 17.5 to 19 ft in length and featuring 2 ft diameter concrete foundations. Of these piles, 13 will be installed along the upper bluff, and 93 will be installed along the beach at the base of the bluff. Because the phase 4 piles are intended to stabilize localized areas against erosion, they will be smaller than the soldier piles used in the earlier phases for track bed support, which were up to 65 ft long with 3 ft diameter foundations, Haggerty says.
Dealing with Drainage
Drainage in the area presents significant challenges because stormwater runoff is not contained or controlled as it comes off the city streets. The existing public storm drains are minimal and undersized, Haggerty explains, so the runoff from the streets “flows at high velocity and overtops the channels.” This runoff often flows over the existing, unprotected slopes into the railroad right-of-way, eroding the slopes and filling trackside ditches.
The planned drainage improvements will include the demolition of an existing concrete drainage channel, roughly 12 ft wide, that is under capacity and has needed to be patched over the years. The new channel—measuring up to 13 ft wide and featuring splash walls and gravity walls—will transition into a grouted riprap channel to connect with additional existing drainage structures. The ground around the new channel will be regraded to better direct water into the channel and prevent water from running down the upper bluff into the track bed area, Haggerty notes.
New drainage pipes will also be installed, and a damaged storm drain chute that extends to the base of the bluffs will be repaired. The foundations of the chute will be retrofitted to support the deteriorated bowl at the bottom of the structure, which is generally exposed during the winter months when the surrounding sand on the beach washes away. Eventually the chute will be replaced entirely with an underground pipe, Haggerty says.
Sections of a concrete headwall that have broken off will be replaced by a new soil nail headwall. Shotcrete and timber lagging will be replaced in existing retaining walls along the toe of the bluffs, and tiebacks might need to be installed.
Two more phases of work are planned. These concentrate on additional slope stabilization and drainage improvements as well as measures to protect against sealevel rise at the toe of the bluff. All phases of the project are being designed for a service life of just 30 to 50 years, Haggerty adds. That is because the long-term goal is to eventually relocate the rail line off the bluff entirely and send it through a tunnel that will likely pass beneath Del Mar.
This news article first appeared in the July/August issue of Civil Engineering.