Workers in 1958 in Los Angeles County, California, place rebar during construction of a tunnel that will carry treated effluent to an ocean outfall. Courtesy LACSD
A new 18 ft diameter tunnel stretching 6.9 mi would carry treated effluent to existing ocean outfalls, relieving two existing tunnels dating to 1937 and 1958.
August 27, 2013—Design and geotechnical work is under way by the Sanitation Districts of Los Angeles County (LACSD) on a 6.9 mi long, 18 ft diameter tunnel that will carry treated effluent from the Joint Water Pollution Control Plant (JWPCP), in Carson, California, to existing ocean outfalls at Royal Palms Beach. When the project is complete in 2022, the ocean discharge system will have a capacity of 1,080 mgd.
The system currently services 4.8 million residents living in 73 cities and unincorporated areas in Los Angeles County. Effluent is currently discharged into the ocean via two aging tunnels that nearly reached their full capacities during a major storm event in 1995.
“The population in our service area is projected to increase from about 4.8 million people today to 6.3 million by the year 2050, based on regional growth forecasts,” says Steve Highter, P.E., BCEE, M.ASCE, a supervising engineer for the LACSD. “The two existing tunnels were completed in 1937 and 1958, and they flow full every day. We haven’t been able to get in and inspect them for over 50 years. At this point we don’t know their true condition.”
In addition to concerns about capacity and what effect age has had on the infrastructure, the project is also driven by seismic matters. The tunnels cross two fault lines: the Palos Verdes Fault is considered active, while the Cabrillo Fault is considered inactive but would likely move sympathetically with the Pales Verdes Fault during an earthquake.
The new tunnel will present several engineering and design challenges for the team. The new tunnel will be constructed using gasketed concrete segmented liners. The external diameter of the tunnel will be from 20 to 22 ft depending on results of the geotechnical program.
The tunnel alignment will begin at a working shaft constructed at the JWPCP. A tunnel boring machine (TBM) will be employed to carve an opening through soils of various types. The north end of the alignment goes through alluvium materials of the Upper Pleistocene Lakewood Formation. The south end consists of rocklike soils.
“Because we don’t have an intermediate shaft, the cutter configuration for the TBM is going to be unique,” says David Haug, P.E., BCEE, M.ASCE, a senior engineer with the LACSD. The soft alluvium materials would require cutter teeth while the rocklike soil, depending on how hard they prove to be during geotechnical investigation, might require robust cutter disks.
The new tunnel, with an interior diameter of 18 ft, will extend 6.9
mi from the Joint Water Pollution Control Plant (JWPCP) in
Carson, California, to existing ocean outfalls at Royal Palms
Beach. Courtesy LACSD
“This tunnel is almost 7 miles long and is being constructed from one working shaft site with no intermediate access or ventilation shaft sites,” Haug says. “That is definitely one of the largest challenges.” He explains that the ventilation fans must be carefully calibrated to work in concert together without creating a great enough negative pressure on the ductwork to collapse it.
At the JWPCP working shaft, the invert of the tunnel will be approximately 100 ft below mean sea level—a depth of 130 ft below ground surface. At the end of the alignment—at Royal Palms Beach—the invert of the tunnel will be approximately 28 ft below mean sea level, a depth of 50 ft below ground surface. When the tunnel extends under the hills of the Palos Verdes peninsula, it will be about 450 ft below the ground surface.
The team will encounter varied water table conditions along the alignment, including a passage through the Dominguez Gap Barrier Project, which injects freshwater into the groundwater table to combat infiltration by sea water.
“We could see a very high pressure on the face of the machine as we go through this zone,” Haug says. The pressure will also be high in the rock formations of the Palos Verdes peninsula, where fractured rock—and possibly possible deep open gaps extending to the surface—could create 11 to 12 bars of pressure on the face of the TBM.
“The profile was chosen so we could drain the tunnel in the future for inspections,” Haug says. “But it will also allow for any groundwater that may enter into the tunnel [during construction] to flow by gravity back to the shaft, so it can be pumped out at the shaft as opposed to being pumped out at the face of the machine.”
The team worked through 50 alternatives to reach this alignment, which is the only one that didn’t require the construction of a new ocean outfall structure and the expense and environmental impacts that would have accompanied it.
Highter says an inspection and core samples of the existing ocean outfalls determined them to be in good condition, with an expected lifespan beyond 2050. Further, a hydraulic analysis of the system by component found that a bottleneck existed in the 8 ft diameter tunnel, not the outfalls, as originally believed.
“By building a new onshore tunnel, we can alleviate that hydraulic bottleneck, and we can tie into the existing outfalls,” Highter says. “That’s how we’re going to achieve this more than 1,000 mgd capacity.”
Once the new 6.9 mi tunnel is completed, the LACSD will divert flow from the older tunnels and inspect their condition for the first time in half a century. The initial plan is to repair and seismically retrofit the tunnels and keep them available as standbys.
“The first question that needs to be answered is, just how active is the Palos Verdes fault?” Haug says. “We are currently underway doing our geotechnical and geophysical program for the final design. We are going to be doing a lot of work on the fault to better address that. How active the fault could potentially be will dictate what kind of seismic retrofits we would do with the existing tunnels.”
One solution under discussion is the installation of a steel liner that could deflect during a seismic event while providing support to the older tunnels.
The LACSD has hired Parsons, of Pasadena, in association with Jacobs Associates, of San Francisco, to provide final design services for the tunnel and connection facilities. Parsons’ work will include tunneling, civil, structural, mechanical, electrical, instrumentation, and control engineering services.
Geotechnical work, conducted by Fugro Consultants, Inc., of Houston, is slated to be complete in two years, with construction beginning by the spring of 2016. The tunnel is slated to be complete in 2022.