(SPU photo inside West Seattle Reservoir in May 2010, not long before it was filled)
By Tracy Record
West Seattle Blog editor
Only four years after it went into service, the underground West Seattle Reservoir in Highland Park needs a $7.6 million earthquake-resistance retrofit.
Work will start this summer, while the city and its consultants determine the scope of retrofitting that is also expected for our area’s other underground water facility, Myrtle Reservoir in Gatewood, as well as for two others in the city, Beacon and Maple Leaf.
We talked today with Seattle Public Utilities and its consultants about the problem, the solution, and the work ahead.
This was foreshadowed a year and a half ago; we made note of it here in November 2012, following up after The Seattle Times (WSB partner) broke the news that the reservoirs’ designer, MWH, had told the city about what SPU calls “possible seismic deficiencies in their work.” SPU subsequently announced that testing would be done to find out about those potential deficiencies and what would be needed to remedy them.
They didn’t expect the testing would take as long as it did. West Seattle Reservoir is the first one for which a seismic review has been finished and a retrofit plan laid out.
We found out about it when a public notice seeking bids on the West Seattle Reservoir work appeared online last week. Our subsequent inquiries with SPU led up to today, when we and a Times journalist met at the Municipal Tower downtown with SPU’s supervising civil engineer Aziz Alfi and media liaison Andy Ryan as well as two managers from the consulting firm CH2M Hill, which has led the seismic review, Wally Bennett and John Spencer.
First thing you should know is: SPU says the reservoirs “are in no danger of catastrophic failure,” even without the retrofits. But the testing showed that a massive quake – one that would make the 2001 Nisqually quake look like a gentle shake – would likely render them unusable, and that is not acceptable to the utility.
It was stressed that none of the reservoirs will have to be dug back open for these retrofits – the work will be done inside, with crews and equipment accessing through existing maintenance hatches. The work will take the reservoirs out of service for months – not concurrently (the timeline is explained later) – but that won’t lead to any water restrictions or shortages; SPU says it will simply route water service other ways.
Spencer explained the backstory: Once the designer (MHW) disclosed the reservoirs were not likely able to meet seismic expectations, the discussion, as well as a review by outside experts, led to a plan for 3-D modeling.
This led to what was in essence the invention of a type of computer analysis that so far has been completed only on West Seattle Reservoir, with the other three to follow. The central square below correspond with the square outlined in red in the aerial photo above.
The computer modeling looked at the reservoir structure piece by piece (see the squares in the renderings above and below) and analyzed how each of those pieces would react in an earthquake of the type that might happen only every 2,500 years.
We’re told there is no magnitude that can be assigned to that hypothetical quake – just, much stronger than Nisqually, and of a different type. Each reservoir was analyzed in the prism of the type of quake most likely to be the strongest potential quake in its geographic area; for the West Seattle Reservoir, that’s a Seattle Fault quake.
Other firms were brought in for the computer analysis, including SC Solutions of Sunnyvale, California, which has experience in “high-end finite-element computer models, and specific experience with buried structures.” The analysis was so complicated, using “supercomputer”-type functionality, that just “one run” resulted in a terabyte of data.
What they had to test, explained Spencer, was whether a catastrophic earthquake would still leave the 30-million-gallon West Seattle Reservoir functional – would it hold water so that drinking and fire-fighting water would still come from it? The initial test runs revealed: No, it wouldn’t. The consultants say it would not burst “like a dam” – but it wouldn’t hold its water, so to speak. It would drain out, mostly into the earth, within hours/days.
Once they got the problem sorted out, they started testing possible solutions. A conclusion: This catastrophic quake would lead to the West Seattle Reservoir’s floor slipping – almost an inch, seconds into the quake, Bennett said – and its north wall cracking. So a “joint” in the reservoir’s floor has to be covered in an additional 8-inch-thick slab of concrete to make sure it doesn’t slip much if this kind of quake happens, since the testing determined that retrofit would help the structure “gain a lot more resistance to movement,” resulting in fewer stresses to other parts of the reservoir. Two other areas of the floor will get an added slab, too. And the north wall will be covered in a polyurea liner that will keep water from getting out even in the event of major cracking of the wall.
So this work is what has gone out to bid, with bids due to be opened on July 2nd, and work to begin by fall, lasting about five months. It’s not affecting the park-building project on the West Seattle Reservoir lid, SPU says.
Meantime, the Maple Leaf reservoir is being analyzed now, and whatever retrofit work it needs is expected to be done next year. Myrtle Reservoir in Gatewood – by far the smallest undergrounded reservoir, at 5 million gallons – is also expected to have a 2015 start for whatever retrofitting is needed, possibly by the end of next year. Beacon Hill Reservoir would follow in 2016.
The full price tag could be up to $30 million for all four reservoirs, according to SPU, which says it has been “working collaboratively with the reservoirs’ designer to recover the costs of the repairs.”
You might recall another reservoir problem first reported here back in 2009 – waterproofing leaks at Myrtle (where the undergrounding was completed in 2008) and Beacon Reservoirs. That wasn’t related to this, but Ryan reiterated today that the costs were covered by the contractor, and that cost recovery is being pursued just as intensely in this case. Alfi says MHW already has made “some payments.” SPU says that “to date, the overall cost of building and repairing the four reservoirs is about $133.5 million – $15.5 million under the original budget.”
The retrofitting would not prevent all reservoir damage in case of a catastrophic quake, but it would leave the reservoir in “repairable” condition, not losing a lot of water, because, as Bennett said, “our expectation is that the structure and the soil (would) move together.”
We asked how much of the technology and methodology would be reusable for the studies of Myrtle, Beacon, and Maple Leaf reservoirs. Each is unique, the consultants explained; each will behave differently, and each faces a different kind of potential quake threat. But, Spencer said, “We have learned a lot in the course of doing this work, so we’ll be able to (move) more quickly on each one of (the remaining three).”
The order chosen for studies and repairs is in part because of “water supply considerations,” Alfi said. While the retrofitting work is under way, the reservoir will have to be drained and kept out of the system. That requires planning and rerouting comparable to “traffic controllers,” he added. “Instead of detouring cars, we’re detouring water.” They’re also coordinating the water “detours” with other SPU projects.
But all the work will be done inside the reservoir, said Bennett. “No new holes in the surface – no excavation. It will go inside through three existing hatches. The contractor will mobilize the site so you’ll see trailers and typical construction (crews/equipment).”
The mobilization for West Seattle Reservoir is expected to start in August; it would be completely empty from September through January. The draining, said Alfi, will mostly happen through regular usage, with the water level allowed to continue falling as people use the water, and whatever finally remains at the end would be routed to some other system. (If the reservoir ever had to have its contents drained to the storm-drain system, it feeds into the Duwamish River, we were told.)
At that same time, the contractor for Seattle Parks is expected to be doing landscaping atop the reservoir.
So why did the safety analysis take so much longer than first anticipated? The piece-by-piece review was something that had never been done before, for one, but a breakthrough finally happened when a new staff member at SC Solutions applied a model used in analyzing car crashes. And it might not have been so complex if not for the fact the city had four reservoirs with potential seismic deficiencies; if it had only been one, the project team said, they might have taken a different approach.
Will the lessons learned here be applicable to any future SPU projects? Ryan noted that Volunteer and Roosevelt reservoirs remain offline as SPU continues to evaluate whether the system is OK without them ever going back into service, and that evaluation has another year or so remaining, so currently there is no anticipation of additional undergrounding.
Ryan says the City Council has been updated on all this, particularly the Public Utilities and Neighborhoods Committee. A Council decision in 2004 led to the undergrounding of six reservoirs including the four involved in this review, with health and security concerns in mind.
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