PART 2: ON THE CUTTING EDGE
Looking at the technology that will build the tunnel
By Ruth Dusseault • Bay City News
October 23, 2024
IF THE DELTA Conveyance Tunnel is granted all necessary permits; if the California Department of Water Resources can create a plan to raise $20 billion; if the Water Resources Control Board extends water rights to the State Water Project; and if a dozen or more lawsuits are won; then construction on one of this century’s most ambitious civil engineering projects will commence.
The year would be 2035. It would be preceded by five years of infrastructure upgrades in the Delta region. Stronger bridges and streets will lay the way for machines of every scale to safely traverse the tunnel’s 45-mile path from Sacramento to the Bethany pump station at Stockton.
The climax event for the tunnel’s chief engineer Steve Minassian will be the arrival of the first of four tunnel boring machines, the primary piece of equipment for the Delta Conveyance Tunnel. Minassian has used the machines to build water and transportation tunnels in several places around the world, including Miami, Abu Dhabi and New York.
The operation done by a tunnel boring machine is more like arthroscopic surgery than open heart surgery. The land will not be cut and splayed open to lay down the tunnel. It would all be done using a series of 10 deep holes, 60 to 100 feet wide, where operators can reach down to manage the tunnel boring machine on its underground path between shafts.
The tunnel boring machine looks like a giant sand worm that travels 200 feet below ground. Its flat circular drill head, called a cutter head, is the shape of a bank vault door but with the diameter of two school buses. Its long body begins with a single concrete ring that snaps onto another ring, and another, for miles.
“A total of 45 miles, but that’s in four different reaches,” said Minassian. “So, the shortest reach is eight miles. And the longest reach is 14 miles. The critical path runs from Lower Roberts to Bethany pump station. That’s the 14-mile one, so that’s the one we’re going to do first.“
At the same time, they will launch a second long machine that will run 12.7 miles from Twin Cities to Terminus.
“Then the third machine will start at Lower Roberts and go north to Terminus. The shortest stretch will be from the Twin Cities site to the Sacramento River, to the intakes. That’s going to be launched last. At one given point, all four of them will be tunneling simultaneously,” he said.
Like a mechanical earthworm
Each drill head has hydraulic legs that retract and brace each new concrete ring, which it uses to push itself forward. Its flat cutter face lacerates the alluvium soils — the clay, sand, gravel — underneath the pressure of 200 feet of earth.
The cutter head ingests the soils, which are softened with water. It swallows the earth down an auger throat to a conveyor belt esophagus that carries the dirt to its tail, where it is elevated up the shaft to an open-air pile.
“Some of the soil will be reused throughout the project. Some of it will be used for restoration, if they’re clean,” Minassian said. “But the majority of it is going to be stockpiled at Twin Cities and Lower Roberts. And I feel that as an engineer, it would be a great resource for the Delta community for later, if they want to use it to strengthen the levees 10 or 20, 30 years from now, it’s going to be there for them to use it.”
Up the shaft comes raw earth and down the shaft goes precast cement ring sections that are carried by rail down the belly of the beast to the head, where they’re lifted by a robot that snaps them into a new ring. Eight pieces makes one ring. The machine advances, one ring after another. It moves 40 feet a day, two miles a year, beneath the levees and the farms, building the concrete tunnel as it goes.
Will there be any vibrations on the surface?
The cutter head ingests the soils, which are softened with water. It swallows the earth down an auger throat to a conveyor belt esophagus that carries the dirt to its tail, where it is elevated up the shaft to an open-air pile.
“Not at these depths. You won’t feel it, not even the fish,” said Minassian. “First of all, we’re on soft ground. And second of all, we’re more than 100 feet deep.”
The tunnel will have no pumps. The flow will be gravity fed from the intake point near the town of Hood on the Sacramento River to the pumps at Bethany.
Minassian was hired by the Delta Conveyance Design and Construction Authority (DCA), a joint powers authority governed by a seven-member board. They represent 16 of the 27 water agencies that rely on the state delivery system. None of the state water contractors have yet committed to funding the tunnel’s construction, but 18 have funded the tunnel’s research and design.
An ever-changing landscape
The California Department of Water Resources approved the tunnel’s environmental impact report just days before Christmas 2023, allowing the authority to seek permits. But the move was followed by a barrage of lawsuits from environmental groups and local water agencies, saying it violated the state’s Environmental Quality Act.
Recently, the tunnel engineers started taking deep core samples to find out what soils are down there, but that was stopped by the groups in a June injunction by a Superior Court in Sacramento.
Local tribes and farmers are against the tunnel over concerns of its impact on their historical water rights and the potential disruption to cultural landmarks. In 2019, the Sacramento-San Joaquin Delta was designated a National Heritage Area. Small villages dating from the early 20th century still stand as a living archive of California’s agrarian architecture.
Much of what threatens the Delta can be traced back 150 years to its agricultural development. The Delta is a landscape of farms that sit below sea level surrounded by high levee walls, which are aging. According to Minassian, the landscape of the Delta has always been changing.
“The Delta has evolved even before people showed up.” said Minassian. “Anytime you’re in a Delta situation, whether it’s the Mississippi Delta, the Nile Delta or the Sacramento, deltas usually sink.”
“It’s sinking, and people don’t realize it. If you leave it alone, you don’t build levees or anything. Naturally, as it sinks, the river brings more silt and rebuilds it. But once you build the levees, like we have in the Mississippi and in New Orleans or in Sacramento, once you do that, then you’re not allowing the river to bring the material back.
“So, what we’re building becomes part of nature,” he said. “The ground isn’t going to know whether we have a tunnel or not.”
Audio credits
Additional audio licensed under Creative Commons public domain: SwayingKeysRev1 by Bigvegie, CC0; lowgrowl_drift.wav by wgwgsa, CC0; Beeps & Brakes ~Construction Site Trucks by Bon_Vivant_Pictures, CC0; guitar by Nick Wenner.
This series is a production of Bay City News, presented in collaboration with Climate One and Northern California Public Media. For more on this story and other news in the Greater Bay Area, visit localnewsmatters.org.