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Los Angeles County Sanitation Districts provide wastewater collection and treatment for 5.6 million people in the region. Its A.K. Warren Water Resource Facility in Wilmington, California, (formerly known as the Joint Water Pollution Control Plant [JWPCP]) processes the bulk of that.
The two existing tunnels that transport treated water from the plant to the ocean date back 60- 80 years. This aging infrastructure, which crosses two faults, is
not built to current earthquake standards and cannot be taken out of service. There is also growing concern about potential overflow events due to climate change. The capacity of the tunnels has nearly been exceeded during major rainstorms on more than one occasion. Should this ever happen, untreated wastewater would end up discharged to local waterways.
Completed tunnel through the soft ground section
A new JWPCP effluent outfall tunnel is under construction to provide redundancy to the existing tunnels, which can then be taken offline for repairs, as well as providing additional capacity for the network. This new tunnel is approximately 7 miles (11km) long with a finished internal diameter of 18ft (5.5m) and no intermediate access. Contractor Dragados USA is using a slurry TBM to excavate the tunnel through both soft ground and rock conditions as part of its contract worth $630.5 million.
Varying geologies
The owner aimed to place as much of the alignment in the public right of way as possible, in response to public input and to limit potentially time- consuming negotiations with private landowners. This is part of the reason for the tunnel’s length, geometry, the 31 curves the TBM will navigate, and the fact that there are no intermediate shafts along the 7-mile alignment.
For approximately half of the tunnel drive, the TBM is expected to pass through heterogeneous soils of alluvium, the Lakewood formation and San Pedro sand. Around 5% of the alignment goes through very weak transitional rocks of Lomita Marl, Malaga Mudstone and Valmonte Diatomite. The remaining, roughly 42%, of the alignment is through weak and intensely fractured Altamira Shales and Point Fermin Sandstone.
Dragados has been working with geotechnical consultant Pini Group, initially to review geotechnical documents as part of the tender and to create a plan for TBM operations. Once the TBM launched, the Pini team stayed on board providing review and interpretation of TBM operating parameters, analysis of TBM interaction with the ground, and technical advice.
TBM launch and project work site
Martino Scialpi, technical director with Pini Group USA, explains the cover is relatively shallow in the soft ground section—just enough depth to safely advance with active face pressure and mitigate settlement. “You wouldn’t want to go any deeper because the second half of the drive is already under high cover. We’re talking about almost 500ft [150m] of overburden in extremely poor ground conditions that could produce a variety of challenging ground behaviors, including severe squeezing.”
The soft ground portion comprises flat terrain, shallow cover and a more urban environment, and the TBM completed this portion of the excavation in September 2023.
It couldn’t be more of a contrast to the hard rock section yet to come under the Palos Verdes hills, a low mountain range along the coast. “It’s still very urbanized, but settlement is not as much of an issue because of the increasing depth,” Scialpi explains in December 2023 as the TBM began to reach the rock portion of its drive mining through the transitional geologic units.
“Right now, the rock is extremely weak. It’s basically behaving like soft ground.”
The zone ahead is characterized by very heterogenous sedimentary deposits with a wide range of strength and major fault zones that can produce heaving, blocky and squeezing behaviors. “This is all considered part of the Altamira Shale,” he explains, “a mix of sandstone, siltstone, claystone, dolomite, and basalt intrusions that may present themselves at the face all at the same time, making the advance really difficult.”
The tunnel is lined with post tensioned segments
The increasing cover and the weak rock mass, combined with the progressively higher water table (up to 280ft [85m] above the tunnel crown) could require the TBM to advance with face pressure close to 10 bar.
In addition, the tunnel is considered gassy according to Occupational Safety and Health Administration’s (OSHA) classification, and encountering volatile organic compounds (VOCs) is a concern as well. Dragados mitigated much of that risk by selecting a slurry TBM manufactured by Herrenknecht. This classification has been changed for the second part of the tunnel (the rock section).
“It has the benefit of keeping all the material in the slurry lines, which lowers the exposure to the men,” explains Matt Kendall, project manager for Dragados.
By selecting a slurry machine, the contractor was able to apply for a variance from OSHA dismissing the requirement to probe out in front of the TBM. “That’s kind of counterproductive when the whole objective of a pressurized machine is to keep gases and water and potential risks of inundations away from the personnel.”
Geology along the tunnel alignment
Existing tunnels
The two tunnels currently in operation conveying treated effluent from the plant to the outfall are an 8ft (2.4m) diameter horseshoe tunnel from 1937 and, adjacent to that, a 12ft (3.7m) diameter tunnel from 1958. These are located west of the new tunnel under construction and are shorter at 6 miles (9.7km) due to their more direct alignment— there being fewer property owners at the time of their construction compared to today.
The project team has access to field reports and drawings from these projects that were incorporated into the geotechnical baseline report, as well as photographs and other documentation of how the previous contractors dealt with squeezing conditions and floor heave.
“There is clear evidence in these reports that they had squeezing ground,” Kendall says. “I think a big factor of that was the amount of freestanding time that these tunnels were in. With the excavation methods back in the mid ‘30s, the advance was relatively slow. The tunnel had a lot of time before the final lining was actually installed, which
allowed the ground to relax and I think they had more issues with converging ground because they couldn’t control the ground quickly enough. We’re optimistic that, at our mining rates, it won’t be a factor, but it’s still a risk.”
Scialpi agrees. “You always wonder, ‘is that true or are they just being super conservative?’ I’ve been on both sides, so I’m the first to criticize our tendency as engineers to be overly conservative at times.
“The fact is that there are two previous tunnels that actually go weren’t required, the segmental liner comprises steel fiber reinforcement. As the excavation transitions to rock, the segments now have a rebar cage reinforcement designed to handle the thrust capacity.
TBM launch event in April 2021
“So far we haven’t had any issues with squeezing ground,” Kendall says, “and all indications are, to date, that we’re operating fine and the void measuring system on the TBM is working well. As we get into the rock zone and start crossing into some of these faults that’s where the squeezing potential is extremely high.”
The project team hypothesizes that this section of rock may possibly be the most challenging squeezing ground a TBM has encountered in a very long time. They know the industry is curious to find out what happens in these extremely difficult ground conditions, but that will have to wait until the project is completed.
Advancements in Lining
The JWPCP is using post tensioned (PT) segmental rings for the lining in the first half of the tunnel alignment, which has been an additional challenge for the team. The project specifications define a target of 75% installation concurrent with tunneling.
Initially, the segmental rings were supposed to be tensioned on both ends. The team studied another option, later producing a paper and performing laboratory testing to show it can achieve the required tension by only tensioning one end of the tendon instead of both, speeding up the process.
Those studies aimed to engineer out the force strand zones. “There were certain zones in the PT segment where you had two, three or up to four strands per duct and there are two ducts per segment. Obviously the four strand ducts are slower and harder to do than the two strands and we were able to get the quantity of three strand and four strands reduced, and increase the quantity of two strands.”
Another issue that hadn’t been anticipated, Kendall adds, is the impact of steering the TBM and ring selection especially in the tunnel’s many curves. “Unfortunately, a lot of these
duct access points in the curves weren’t always in the horizon where we can reach them. That also contributes to quantity of post tensioning that we’re going to have to do at the end.”
The project team was able to get very close to the 75% target while mining the soft ground portion of the tunnel and is developing a special gantry to better enable post tensioning the remaining part of the segments, achieving that specific target while completing the rock section.
Conclusion
When the tunnel is completed, Dragados will construct all of the infrastructure works to tie-in to the existing plant and the outfalls at Royal Palm Beach adjacent to the Pacific Ocean. This includes a bypass structure that will allow crews to close the values to the two existing tunnels, which will be drained. Then, the team will demolish the existing concrete manifold, enter, inspect and repair the existing tunnels, followed by replacing the demolished concrete manifold with a new steel manifold. Once all of that is done, the bypass can be disconnected and flow re-established in the two existing tunnels, utilizing the new manifold.
“Yes, this is a tunnel project. It’s a large, long tunnel. But the reality is, the tunnel’s just an interim milestone,” Kendall says, “because we have so much more work to do at the end of the job.”
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