The South Coast Water District board of directors unanimously approved an addendum to the environmental impact report for its tunnel stabilization and sewer pipeline replacement project at its March 28 board meeting in Dana Point.

District officials approved a recent proposal to realign a 725-foot portion of a 2-mile tunnel under the bluffs at Three Arch Bay. The tunnel, whose interior is composed of earth, rock and timber supports, houses a sewage pipeline from Three Arch Bay north to Aliso Creek Beach. The pipeline handles 1-million gallons of sewage per day.

The water district must secure a development permit from the California Coastal Commission for the work in the private gated community, according to a project summary.

"The district and its team of geologists and engineers determined that 725 feet of the bluffs along Three Arch Bay were prone to erosion and might impact the tunnel over time," said Joe McDivitt, the district's director of operations. "We analyzed numerous alternatives to ensure the stability of the tunnel and determined it was prudent to realign it further inland under South La Senda Drive."

The project proposal calls for enlarging and stabilizing about 3,200 feet of the existing tunnel, realigning 725 feet of the tunnel under South La Senda Drive and installing a new pipeline. This work is scheduled for the fourth and fifth years of the five-year project.

District officials estimate starting construction in 2014. The first phase includes building the project's main staging area on district land at Fourth Avenue and South Coast Highway. Project plans call for constructing a 100-foot-deep access shaft for workers, equipment and materials to enter and exit the tunnel. This work, and 6,100 feet of tunnel stabilization and pipe installation from Aliso Beach to the beginning of Three Arch Bay, requires a coastal development permit from Laguna Beach. District officials submitted their request to the city on March 13, according to the project summary.

The district also needs additional underground easements from private property owners on bluffs from Aliso Beach to the south end of Three Arch Bay so it can add 5 additional feet to those existing underground easements.

As of Monday it secured 162 of 183 easements from private property owners, according to the project summary. The district also needs five easements from the county to enlarge and stabilize the tunnel within ocean bluffs at public beach access points.

CalTrans approved its one easement for construction below South Coast Highway between Fourth and Fifth avenues. Plans call for crews to build a 320-foot connector tunnel under South Coast Highway that links the access shaft at the main staging area to the tunnel inside the bluffs. The district still needs city approval and hopes to begin this phase in 2014.

The district board authorized the interim general manager to negotiate with Kiewit Infrastructure West Co. on a contract proposal for the work on Jan. 15. District staff will return to the board for approval on a final construction contract award later this year.

The tunnel's interior will be strengthened with a reinforcing material known as "shotcrete" — a type of concrete sprayed onto wall panels —throughout the 2-mile tunnel.

Bryce.alderton@latimes.com

Twitter: @AldertonBryce