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Tucked away in Portland’s West Hills beneath the International Rose Test Garden, two reservoirs for more than a century sat quietly at Washington Park. That is now changing.

The two reservoirs, operated by the Portland Water Bureau, have provided water to residents since 1894. But one is being replaced and the other is being taken offline. The challenge for the project team is that a gravity-fed system that allowed efficient operations also placed the reservoirs in the path of an ancient landslide. And that means construction of a new reservoir is complicated.

鈥淚t’s nice to build reservoirs in gullies and ravines because then you only have to dam one side, so it’s a nice place for water storage,鈥� said Thomas Gilman, an engineer with the Portland Water Bureau. 鈥淲hat geologically existed here was a stream, and the landslide would move and block the stream and the water would come and wash the soil out and it would move again. When they started construction in 1894 the landslide hadn’t moved; it had filled in and was in the process of being washed out.鈥�

Excavation for the original project, however, reactivated the landslide and forced engineers to come up with a solution on the spot. A series of dewatering tunnels bored into the hillside did the trick. The slide was slowed to a geological crawl and the reservoirs went on to serve without incident for many years.

When crews began construction in 2016 on a replacement reservoir, however, that slide once again had to be taken into consideration. Building at a new location was considered and rejected for a variety of reasons, said Teresa Elliott, the chief engineer for the water bureau.

鈥淲e would still have had to do something about the slide because it has to have weight on it,鈥� she said. 鈥淎nd all of our infrastructure goes through here, so if we built somewhere else we would have had to build new infrastructure, new piping and everything to get there.鈥�

In the end the estimated $190 million project to replace the two old open-air reservoirs with a new 12.4-million-gallon underground reservoir got started in 2016 with excavation and site work as well as construction of a massive temporary shoring wall on the east side of the site.

Reservoir 4 on the south side was also disconnected from the water supply infrastructure, excavated and used to hold earth excavated from Reservoir 3. It will also support a future decorative reflecting pool.

There is an eight-year timeline for construction of the new reservoir. Work started in 2016 and now has progressed to the point where crews from general contractor Hoffman Construction, Western Rebar and other subcontractors are now roughly 40 percent of the way finished pouring slab-on-grade sections for the four-foot-thick floor of the reservoir. Other crews are now busy constructing massive steel wall forms for the equally thick south walls.

A total of 176 shafts were drilled into bedrock to anchor the reservoir structure into the hillside. On the site’s west side, a new concrete, mechanically stabilized earth (MSE) wall has been constructed to anchor the toe of the landslide.

鈥淭his project has caused it to move faster than it had been,鈥� Gilman said. 鈥淏ut it’s still well within our criteria for what is allowable.鈥�

A second tower crane with a luffing jib has been erected at the north end of the construction site and will supplement the first crane operating at the south end of Reservoir 3.

鈥淚t took about two years to get to the point where they could start building the concrete structure itself after all the shoring, walls and the shafts,鈥� Gilman said.

鈥淲e’re now working on the end walls, and that’s what you’ll see,鈥� Elliott added.

Two historic gatehouse structures fronting the old reservoirs are being refurbished in order to house control valves for the new reservoir.

鈥淭he gatehouse is one of those things they wanted us to preserve,鈥� Gilman said. 鈥淪o we gutted it out and brought it up to seismic standards.鈥�

Construction of the new reservoir is scheduled to be completed by early 2020; then two years will be allowed for the soils to settle and the massive concrete vault to do the same. Crews will then return to the site to build the decorative reflecting pools atop the old reservoir footprints, along with a new grand staircase and other features. Completion is expected in 2024.