Update on Washington Park reservoirs: 'Concrete for the next two years'
The Portland Water Bureau is three-eighths of the way through its Washington Park Reservoir Improvement project.
Now that some of the parking restrictions around the Park Place entrance have been lifted, tourists and locals alike have been calling the water bureau more frequently to know what's going on. Both reservoirs are still without water, although now the activity at the bottom of them is heating up. On a recent tour of the site, two machines were drilling four-foot-wide shafts 90 feet through the dirt and the bedrock at the upper (and most visible) Reservoir #3. So far 39 of the eventual 140 holes have been drilled.
On the dry bed of the lower Reservoir 4 ironworkers were building steel rebar cages. These are trucked up to the upper reservoir and dropped into place in the holes which are then filled with concrete.
The result will be 140 pilings that will resist two big forces, and are the reason all the work is being done. A slow, gradual landslide toward downtown is taking place at an eighth of an inch per year, and a big earthquake would cause a lot of shaking and more downhill movement.
The new underground reservoirs will be large boxes held up by these pilings, steady in the bedrock. Where the boxes meet the new shotcrete wall, there will be a two foot-thick layer of absorbent material, similar Styrofoam, which will save the concrete from cracking under pressure in an earthquake.
The reservoirs will be divided into two cells, north and south, so one can be drained for maintenance without shutting the whole system down.
On the west side, where the landslide builds up pressure on the reservoirs, there will be a five-foot thick layer of the same material. The landslide will push against this material over the material's 75-year lifespan, squishing it. Then it will be dug out and replaced.
"We're not trying to stop the slide, we're trying to manage how we can control it pushing against the reservoir," said the water bureau's Stan VandeBergh, Lead Project Engineer, formerly of the Portland Water Bureau, now with West Yost Associates. Gesturing to a sensor on a pole overlooking the site, he says, "We monitor is 24/7 so we know exactly what is happening with that slide."
The reservoirs were built in 1894 with thin concrete (and not much steel rebar) poured over the dirt of the hillside. Over time, they have begun to crumble, and cannot stand an earthquake. After the Sept. 11 attacks of 2001, fear of terrorists poisoning the open reservoirs gripped the Portland City Council, and that remains one reason for burying the reservoirs. On the surface, an 18-inch reflecting pool and decorative cascades will keep the area an attractive feature of Washington Park. The main reason, however, is structural: They will crumble sooner or later.
The project, slated to span eight years with a maximum approved cost of $152 million, will be completed by Hoffman Construction Company. The project is part of the Portland Water Bureau's Capital Improvement Program, funded by revenue bond proceeds paid back with the utility ratepayers' fund.
Reservoir 3 will maintain the historic drinking water function. The lower reservoir, Reservoir 4, will be disconnected from the public drinking water system and transformed into a lowland habitat bioswale.
The 140 pilings are carefully placed.
"They modelled the hell out of this project," says VandeBergh. "And whenever (engineering company AECOM) changes the parameters it takes 16 hours for the model to run."
The project will take a two-year break in 2020 and 2021 while the land settles and then be completed in 2023.
Other aesthetic concerns include the pump houses, which look like stone but are poured concrete that was sculpted to look like masonry. They are historically protected and will be restored. There will be more walking paths, places for sitting and watching the water and the view east.
The reservoirs have been empty for almost three years. Normal demand for water is satisfied by the flow through pipes — the reservoir levels stay constant. The water bureau needs to draw down from the reservoir in emergencies, such as when the fire department is putting out a big fire, or if there is a disaster and the 12 million gallons are needed during several days. The reservoirs serve downtown and much of the west side, including several hospitals and will hold 12.5 million gallons.
VandeBergh explains: "This isn't serving everyday demand. The pipes themselves are serving that need. What we're talking about is emergency supply, a major fire or earthquake. We're trying to store as much as we can, and coupling this with a hardened pipeline, a river crossing, which is going to give us 60 million gallons a day in that crossing. Right now, we have seven river crossings, but they're vulnerable to a subduction zone breaking, bridges coming down on them. The new pipeline is being planned now, it will be about four years out."
Normal summer peak demand is all handled by the redundancy in the interconnection.
Portland's water flows by gravity from the source in Bull Run, just west of Mt. Hood. The force of gravity is enough to push the water down through the pipes under the Willamette River and up again into the West Hills. Pumps are used to raise some of the water up to higher neighborhoods where it is stored in green metal tanks on stilts.
Pouring concrete for the reservoir itself will begin at the end of November.
"Now it's just concrete for the next two years," said Cary Bubenik, operations manager for Hoffman Construction.
The site was chosen in 1894 because of its topography. Isaac Smith, who chose the Bull Run source, figured out it could be delivered by gravity, but he needed a site with the two ravines in it at the right elevation. (This was around the same time when the zoo was just a few caged animals on the hillside, and there was a cable car for tourists.)
"All the backbone of the system is pointed to this direction; all the large transmission mains are still pointed to this site," says VandeBergh. "That drove the decision to stay here." They would have to build many major pipelines if they were to give up on the Washington Park site. Having said that, the water bureau is building new pipes under the city to boost delivery.
VandeBergh sums up the challenges:
"This project is challenging because it has a lot of constraints, it's the landslide and the earthquake, and the topography with really high hillsides and big loads, and then this is moving all the time, all those lead up to a more challenging reservoir. If we were to have a green field, we'd never make this reservoir this tall, we wouldn't make it this shape, but because we're working with all these constraints, we've been solving the problems all along."
Reporter, The Business Tribune
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