Light at the End

The South Coast Water District is working to stabilize a half-century-old tunnel that forms the backbone of its sewer system.
Light at the End
Sancon Engineering contractors install steel supports in the South Coast Water District’s two-mile-long sewer tunnel.

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California's South Coast Water District (SCWD) is blessed with a spectacular view that cuts a picturesque swath along the Pacific Ocean 60 miles south of Los Angeles. The district also benefits from a man-made feature — a two-mile tunnel carved through the ocean bluffs of South Laguna — that protects the sewer line serving a third of its customers.

The SCWD's mandate is to ensure reliable, adequate supplies of water and the safe collection and treatment of wastewater for residents and businesses in Dana Point, South Laguna and areas of San Clemente and San Juan Capistrano. The gravity sewer line inside the tunnel primarily serves northern Dana Point and South Laguna, carrying 1.1 million gallons of wastewater per day for treatment.

"The tunnel was built in 1954 when the area of Laguna Beach was rural and undeveloped," says Mike Dunbar, recently retired SCWD general manager. "The South Coast Highway (SCH) wasn't yet complete and dirt roads served a few dozen or so resort homes that belonged to celebrities like TV star Ozzie Nelson. The district blasted the tunnel about 6 feet in diameter and dug out the rock by hand, then installed a 21-inch vitrified clay pipe (VCP) operated entirely by gravity. The foresight of that project has saved the district more than a million dollars by eliminating the need for five to seven lift stations along the coast and the electricity required to operate them."

While the sewer tunnel was built to last, it featured typical creosote-covered wood beams commonly found in mine tunnels. It also offered a few access points in case the tunnel or sewer line ever required repair. The clay sewer line was crushed in place and replaced by a 24-inch Techite reinforced plastic mortar pipe in 1974.

Sewer tunnel assessed

The tunnel system was assessed in 1997 and inspected several times in the early part of the following decade while ongoing repairs were carried out. The sewer line, however, remained intact with no spills. In 2006, engineers from Hatch Mott MacDonald (HMM) carried out an overall condition assessment of the tunnel.

"This was an end-to-end assessment, with the inspection team walking through the tunnel," says Joe McDivitt, director of operations/Tunnel Stabilization Project leader with SCWD. "There were a few areas where the ceiling had fallen in around the sewer pipe, but we had everything inside the tunnel mapped out as a result of that inspection."

Of immediate concern was a 400-foot section of tunnel that required emergency repair. HMM designed repairs in which the rotting timber supports and loose rock were removed and the damaged sections were repaired by contractors in 2007. Contractor access to the beach was limited to a very steep set of public stairs, so all construction equipment and materials were transported to the beach site using workboat barges that supplied a small staging yard.

A tunnel entryway, Adit 15 at Thousand Steps Beach was enlarged to allow better access to workers and equipment. Inside, a small skid-steer loader with a bucket attachment mucked out one face of the tunnel, while a Takeuchi TB016 excavator outfitted with a rock breaker attachment excavated the tunnel. As fresh supplies arrived, the workboat removed construction debris.

"With the workboat, all of the equipment and the sudden arrival of crews, it looked like the landing at Iwo Jima," says Dunbar.

Once the tunnel was excavated, the contractors lined the interior with shotcrete and installed a new concrete floor that encased and protected the 24-inch sewer pipe. "That left us with a flat bottom and a tunnel that we refer to as a horseshoe shape," says McDivitt.

Tunnel rehab recommended

The tunnel condition assessment conducted by HMM recommended that the entire tunnel be rehabilitated. Each section of the tunnel was assigned a rating regarding its condition and the time frame in which repairs should be carried out.

"Within the environmental im-pact reporting regulations required by the California Environmental Quality Act, we analyzed project alternatives and consulted with stakeholders, engineers, consultants and contractors," says McDivitt.

Alternative construction ap-proaches that were considered included: no action; construction of a new tunnel built above, below or inland of the existing tunnel; construction of a new tunnel and pipeline following the SCH; construction of a new sewer line in the SCH with new lift stations on the coast; construction of one lift station and a smaller sewer line next to the existing line; and filling the entire tunnel with concrete and sliplining the old sewer line.

"We eventually finalized the design work, building upon the basic construction design methods employed in 2007, and received approval to move forward from the district's board of directors in November 2010," says McDivitt. "We struggled with the idea of how to keep the existing pipe in service while construction work was ongoing and we ultimately decided to install a new 24-inch sewer line off to one side, on top of the new concrete floor. We're pretty well built out in this area, so a line of that diameter will continue to serve roughly 15,000 people with capacity to spare. With the new line in place we'll be able to use the lower line until construction is completed and then switch back to it as a redundant system if we ever need to take the new line out of service due to maintenance."

The interior of the new tunnel will measure about 8 feet high and 8.5 feet across, retaining the horseshoe shape achieved in 2007. However, many of the project challenges of the 2007 emergency repair remain in place.

No blasting options

"There will be no blasting like they employed in 1954. The area is settled and there are million-dollar mansions and other properties overhead," says Dunbar. "We plan to use a small track hoe to chip at the rock, or a road-header tunneling machine with a grinding head to bore out the tunnel."

"However, to stabilize and enlarge the tunnel we must acquire underground easements from private property owners on the bluff. It's a challenge to acquire 200 easements, with an offer and a package for each owner."

The SCWD has also purchased a small vacant lot on SCH inland from the coast for its main construction staging area. Construction of a shaft on that lot will descend to a depth of 100 feet, and a connector tunnel under SCH will give crews necessary access to the tunnel.

"We'll still be in permitting with the California Coastal Commission for another year," says Dunbar. "They are very specific in their permitting and usually require a lot of mitigation efforts to impacts in and around the facility. Inland, traffic concerns are more of an issue, but there's a heightened consciousness of ocean water quality with the agencies along the coast."

The cost of the five-year reconstruction project, which should provide 100 years of service to the community, is estimated at $50 million.

Sewer line maintenance a priority

While the tunnel rehabilitation is the top capital priority of the district, the routine cleaning and maintenance of the 140-mile sewer system remains an important operational priority, as does the relining and repair of sewer infrastructure. The sewer infrastructure dates back as far as the 1930s and is made primarily of VCP, ductile iron and PVC.

The district uses outside contractors for cured-in-place pipe lining, but performs short line repairs up to 25 feet in length using in-house crews.

SCWD owns and operates two Vac-Con combination trucks and a jetter. It also performs its own CCTV inspections using a CUES sewer mainline and lateral inspection system with Granite XP software.

"We inspect and clean the entire system over a five-year period and we focus on certain hotspots, where we expect to find grease buildup, for example, as often as quarterly," says McDivitt. "Overall, we have a good track record with staying on top of system maintenance, with perhaps one or two sewer spills a year, some caused by contractors. However, zero spills remains our standard and goal."

The district has never experienced a sewer spill onto the beach or ocean from the tunnel sewer line since it opened in 1954.

"Our tunnel is located 30 or 40 feet from the ocean," says Dunbar. "If we were to ever get a call that there was a sewage spill it would literally be 'all hands on deck' to stop it. The access issues alone are challenging and it would take from 18 to 36 hours to control a pipeline break. By that time, 800,000 gallons of sewage or more would have spilled into federally protected marine life habitats and fisheries. The tunnel rehabilitation project is clearly one of the most important projects any water agency in the area has been charged with. With rigorous planning, we'll get it right."



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