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Landslides are all about water in the ground.

This fact is front of mind for leaders of the Washington Park Reservoir Improvements Project, the eight-year, $190 million effort to bury a major source of drinking water in the hills west of Portland. The whole job site lies in the path of an ancient, slow-moving landslide, and the six months since the project began have been six of the wettest on record.

Project managers and consulting engineers are eyeing multiple monitoring systems, on high alert for irregular movements in the slide.

Cement is being added to the fill dirt for stability. Loaders are working in up to two feet of water.

鈥淚’ve worked in Alaska 鈥� I’ve worked everywhere, and I tell you, it’s tremendously wet,鈥� said Cary Bubenik, project manager for Hoffman Construction. 鈥淚t’s the toughest job, weather-wise, that I’ve ever done.鈥�

Watered down

Leaders of the Washington Park Reservoir Improvements Project say it will involve, for the most part, mostly uninteresting, below-the-radar earthwork. But there will be milestones when the thrust of work shifts. With a project as complex and challenging as this, those moments also represent challenges overcome, Bubenik said.

Last week featured one of those moments, as a crew with Gonzales Boring & Tunneling concluded jacking and boring three tunnels beneath the park’s historic dam 鈥� a critical piece of prep work. Eventually, almost all water poured from faucets in Portland west of the Willamette River will first flow through these inlets.

The main goal of the project is to construct a 12.5-million-gallon drinking water reservoir below the footprint of the city’s existing Reservoir 3 and add a reflecting pool on top. The reservoir will be engineered to withstand the ongoing landslide occurring at the site as well as effects of a major earthquake.

Then Washington Park’s other reservoir, Reservoir 4, will be disconnected from the city’s grid, and a stormwater management area will be built.

Heavy work on the reservoirs is scheduled to continue until 2020, when a two-year pause will begin to allow soils to settle. Work will resume in 2022 with construction of the 鈥渋nterpretive features,鈥� like the reflecting pool.

鈥淲hat soil does when it’s unloaded is rebound and swell a little bit,鈥� said Stan VandeBergh, head of the Portland Water Bureau’s capital projects division. 鈥淎nd when you put stuff on it, it goes back down. So if we don’t let it settle before we do the reflecting pool, it would all crack.鈥�

Adding to the complexity are the site’s relatively small size (only between 75 and 100 workers have been working), access issues with the surrounding residential streets, and mandates to preserve historic features and relocate more than 170 trees.

But the biggest issue, by far, for construction manager-general contractor Hoffman Construction has been the weather, which has been unpredictable, though predictably wet.

Robotic survey monitoring equipment is spread around the site, sending out prisms to different points and up to satellites in space.

鈥淲e can look at the movement of the slide real time; I mean, it’s going on right now,鈥� Bubenik said. 鈥淗ow many projects are you constantly monitoring a landslide with both piezometers and vibrating wires?鈥�

The city even has retained a landslide stabilization consultant (Cornforth Consultants) to do work that would normally be done in-house.

Bubenik, VandeBergh and much of the team came directly from the Kelly Butte Reservoir Replacement Project. That means the team is familiar with the challenges of covering a drinking water reservoir VandeBergh said. But nothing can really prepare a team for a project like the one at Washington Park.

鈥淚’ve been with the Water Bureau for 34 years, and this is by far the most challenging project I’ve seen,鈥� he said.

Material advantage

The schedule is highly segmented, with the next big segment involving cutting the earth down to where the reservoir will go and forming a toe-block wall tied into the landscape with cables and rods.

The first step was drilling 172 pilings to a depth of 100 feet below where the reservoir will go. At 3.5 feet to 5 feet in diameter, they are the 鈥渂ig boys,鈥� according to Bubenik.

Necessitating this heavy work is the fact that far below the now-damp park, soils under Washington Park are flaky, and not competent enough to support the 50-foot-tall reservoir.

The walls, floor and ceiling of the basin will be made of reinforced concrete 鈥� around 30,000 yards 鈥� between 2 and 5 feet thick. The No. 18 rebar needed to reinforce it will require use of a tower crane because human workers can’t work with steel so thick. Two cranes are called for (foundations for them are being dug).

Engineers opted to design the reservoir in a dogleg shape, which led to a design problem in anticipating how it will perform in an earthquake.

鈥淭he engineer of record told me he had to run 85,000 iterations of this project in the modeling every time we’d make a change,鈥� Bubenik said. 鈥淭hat wouldn’t occur if it was a square or a rectangle or a circle.鈥�

With the landslide moving about one-eighth of an inch annually, engineers opted to encase the reservoir in a compressible foam, which will be crushed slowly until it needs replacement 鈥� in about 75 years. It’s a highly-advanced, lightweight plastic. Just don’t call it Styrofoam around VandeBergh.

Portlanders don’t mess around with their drinking water, and might not feel comfortable with the thought that it’s being protected from an earthquake by a relatively thin layer of Styrofoam, he said.

But this foam is strong 鈥� it’s nearly impossible to dent samples kept at the Portland Water Bureau job trailer. It only looks like Styrofoam.

鈥淚t’s an engineered plastic,鈥� VandeBergh said, 鈥渘ot Styrofoam.鈥�

Eliminating the slide altogether would have been 鈥渢remendously expensive,鈥� he added.

But the project’s steep price tag reflects the seriousness of its goal. With reservoirs in Arlington Heights being temporarily bypassed, the system is operating at a risk. The project is also just one chunk of a larger effort to shore up the city’s drinking water system. Another major one 鈥� providing a new pipe crossing of the Willamette River 鈥� is in early design stages, with a request for proposals set to go out soon.

鈥淲e’re fairly vulnerable at the moment,鈥� VandeBergh said. 鈥淚f a subduction zone event occurred today, I’m not going to say we wouldn’t have water (on the west side of the Willamette River), but if I was a betting person, I’d say it would be pretty limited.鈥�

Washington Park Reservoir Improvements

Cost: $190 million

Contractor: Hoffman Construction

Architect: city of Portland

Consulting engineer: Cornforth Consultants

Structural engineer: AECOM

Expected completion: 2022