Opportunity
In 1954, workers dug by hand for two miles (3.2 kilometers) through the breccia, sandstone, and siltstone that make up the beach cliffs of the San Onofre Formation. The tunnel they created, running parallel to the oceanfront, protected a vitrified-clay sewer pipe that ran to the Coastal Treatment Plant in Alison Canyon.
As of 2012, reported the Orange County Register, “the tunnel is crumbling in places, leaving the 24-inch sewer pipeline inside vulnerable. More than a million gallons of wastewater run through the pipeline each day, and in its current state, workers would have a tough time accessing the source of any leak.”
“It would be an environmental disaster,” said Mike Dunbar, then General Manager of the South Coast Water District. “Let alone that, my concern is that we have anybody that’s trapped in that tunnel.”
Dunbar noted that the tunnel was built when the area was rural and undeveloped. “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.”
In later years, luxury hillside estates were built over the route of the tunnel, complicating the challenge of inspecting and repairing it. Residents were represented by well-organized homeowner associations and community groups.
Solution
In 2006, Mott MacDonald inspected the overall condition of the tunnel. “This was an end-to-end assessment, with the inspection team walking through the tunnel,” said Joe McDivitt, the South Coast Water District’s Director of Operations and project leader.
“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.”
In January 2013, Municipal Sewer & Water magazine reported that the project was of immediate concern that required much repair.
Contractor access to the beach was limited to a very steep set of public stairs, so all construction equipment and material were transported to the beach site using workboat barges that supplied a small staging yard….
“With the workboat, all of the equipment and the sudden arrival of crews, it looked like the landing at Iwo Jima,” says Dunbar.
The emergency repairs included improvements to Adit 15, concrete encasement of the existing pipeline, removal of the timber sets and lagging, detailed excavation to a modified horseshoe shape, and installation of a fiber-reinforced shotcrete lining.
Planned rehabilitation of the entire tunnel will include improvements to selected adits and portals, construction of a new access shaft, and carefully coordinated and sequenced staging and construction activities. Each section of the tunnel was assigned a rating regarding its condition and the time frame in which repairs should be carried out.
Mott MacDonald's key project tasks include the following:
- Geotechnical site investigations and tunnel condition survey
- Development and evaluation of alternative repair concepts
- Preliminary and final design of the preferred repair methodology
- Preparation of cost estimate and construction schedule
- Bid phase support and inspection of emergency repairs
- Preparation and maintenance of a risk management plan including a risk register
- Design of the full tunnel rehabilitation and pipeline line replacement:
- Portal and adit reconstruction
- New access shaft and tunnel
- Tunnel rehabilitation
- New 24-inch (61-cm) pipeline and lateral connections
- Cured in place pipe (CIPP) of existing pipeline
- Traffic analysis and construction maintenance of traffic (MOT) plan
- Preparation of a Geotechnical Baseline Report
- Coordination of Early Contractor Involvement
- Support of other project related activities including these:
- California Environmental Quality Act (CEQA), Coastal Commission, and CalTrans permitting
- Acquisition of permanent and temporary construction easements
- Public outreach: creation of a flyover and animation visualization video, presentations at hearings and community meetings and conducting tours of the tunnel
- Future: Construction management services for full tunnel rehabilitation and pipeline replacement
Outcome
The five-year rehabilitation project is expected to provide 100 years of service to the community, at less expense than constructing a new sewer line.
According to a paper presented at the Rapid Excavation and Tunneling Conference in 2011, Mott MacDonald's involvement was "extremely effective in the identification of issues that might otherwise be left to a later time.”
Materials were reused wherever possible. The existing 24-inch pipeline will remain for future use as a backup pipe, and spoil material is reserved for use by local contractors for District future projects.
The South Coast Water District is proud to have experienced zero spills onto the beach or ocean since the sewer line opened in 1954. “The tunnel rehabilitation project is clearly one of the most important projects any water agency in the area has been charged with,” said McDivitt.
Award
The project received a 2014 Merit Award for Engineering Excellence from the American Council of Engineer Companies, California. In its statement, ACEC-CA said, "The resultant project exemplifies what every owner strives to achieve: an operationally robust, environmentally and socially responsible, cost effective design with certainty of delivery."