About 30 buildings in Seattle’s historic Pioneer Square neighborhood will be inspected both inside and out for damage after the soil deep below slumped an inch from Highway 99 tunnel work.
Todd Trepanier, program administrator for the Washington State Department of Transportation (WSDOT), described this step Monday while briefing the City Council.
Groundwater pumping beneath stranded tunnel-boring machine Bertha, from wells as deep as 185 feet, is the suspected cause of the troubling soil settlement.
The affected area runs from Yesler Way to King Street, and from Alaskan Way to First Avenue South, said Trepanier.
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It’s not clear when the inspections will be done.
The Alaskan Way Viaduct also sagged 1 to 1¼ inches in mid-November, the state reported earlier.
But nearly all that settlement is evenly distributed, so it’s not causing new cracks or strains on the 61-year-old elevated highway, said Tim Moore, senior WSDOT bridge engineer.
“The viaduct is safe to drive on,” Trepanier told reporters.
Excavation stopped several days ago in the deep shaft that’s being built in front of the buried tunnel machine, where the front end eventually will be lifted to the surface for repairs.
Groundwater pumping is continuing, contrary to a state news release Sunday night that said contractor Seattle Tunnel Partners (STP) “is taking the prudent step to stop dewatering.”
The state, STP and city transportation staff are meeting daily to discuss the data and strategies.
So far, STP has excavated 84 feet of the planned 120-foot-deep, ring-shaped pit near Bertha’s front end. When that’s done, the giant tunnel drill will advance through a concrete wall and about 40 feet into open air.
In some positive news Monday, the soils appear to be in equilibrium around Bertha, even as water pumping continues, said Robert “Red” Robinson, a soils expert and vice president of Seattle-based Shannon & Wilson. About 600 gallons per minute are currently being removed from a deep water table, a sustainable pace that would enable workers to dig the entire 120 feet down, he said.
“We’re pretty sure that it’s not continuing to settle,” Robinson, who is working as a WSDOT consultant, said after the council briefing.
Excavation could resume if things remain stable. Robinson wouldn’t speculate on how long the team needs to wait and study, while Trepanier said WSDOT needs further data before making any plans to resume pit work.
Survey crews on Sunday confirmed the viaduct has sustained only a negligible amount of uneven settlement, on the order of one- or two-tenths of an inch in one section, Dave Sowers, geotechnical engineer for WSDOT, said Monday.
Uneven settlement poses the risk of cracking or weakening the old viaduct, while sinking evenly is considered more benign.
Nonetheless, the data are disturbing because officials do not entirely understand the causes and solutions. The area has long been understood to present high risk of soil settlement, when state lawmakers chose a tunnel in 2009 to replace the viaduct.
Seattle City Councilmember Jean Godden criticized WSDOT for not notifying the council sooner about the viaduct issues.
Trepanier apologized and said the state had difficulty with its email system Friday.
And Councilmember Mike O’Brien asked: “At what point has it gotten so unsafe, we consider something else?”
Moore, the bridge engineer, gave several technical responses that boiled down to this: The viaduct is safe unless cracks and strains appear — which they haven’t.
“I’m 95 percent confident, in the sense it’s going to provide that service,” Moore said. The other 5 percent represents the odds of an earthquake, he said.
He explained that the columns are sturdy, and expansion joints reduce pressure. The vulnerable parts are some long girders that don’t meet modern strength standards, he said.
Meanwhile, Trepanier told the council that uneven soil settlement occurred beneath Pioneer Square buildings.
Over at the J&M Cafe, a brick building 124 years old, a 3/4-inch-wide crack has appeared in a basement wall.
“The crack was half this size three months ago,” said general manager Mike Petrone. It isn’t known whether vibrations from construction, or recent soil settling, is the culprit, he said. Petrone said rats showed up in the neighborhood around First Avenue South earlier in the tunnel work, a fright that he didn’t encounter before.
About $20 million was spent by the tunnel team to install lasers, targets, meters, buried gauges, satellite monitoring and other devices to monitor buildings and streets above the tunnel route. But two-thirds of the Pioneer Square buildings are outside the monitoring zone, which was designed for straight-ahead tunnel boring, not this unplanned repair-pit operation, Trepanier said.
The WSDOT and STP likely will expand the monitoring program, he said. Tunnel contractors are not commenting.
Tunnel builders can’t simply call an abrupt halt to the groundwater pumping because there could be unintended consequences, such as an upward heave in the soil, Robinson said.
The purpose of deep groundwater pumping is to reduce pressure below the access pit; otherwise the wet soil could exert hydrostatic pressure four times that of the atmosphere, he said. Halt the pumping, and soil would burst upward through the bottom of the pit.
Last December, a temporary, shallower groundwater removal caused a few buildings to sag one-quarter inch, but they recovered immediately afterward.
There are 15 shallow and deep wells in and around Bertha’s front end, Trepanier said.
Bertha has been stuck for a year, since a bearing filled with grit and overheated Dec. 6, 2013, and the cutter failed to scoop dirt.
Art Skolnik, a Seattle architect who studied retrofitting the old viaduct, said Monday the tunnel reminds him of the Washington Public Power Supply System, which finished only one of its five nuclear reactors in the 1980s. He reiterated his call to reinforce the foundations, as the city did at its Spokane Street Viaduct on soft soil.
“It’s possible the viaduct would come down before the tunnel is finished, and in that case, you have a worse traffic problem than when you started,” Skolnik said.
Moore told the council the viaduct is currently safe but will not reach its full 75-year design life.
A short segment near Yesler Way has sunk more than 6 inches since the 2001 Nisqually earthquake, but the foundations and columns were reinforced.
Tunnel supporters point to the project’s long-term benefits — to remove the threat of a quake ruining the viaduct, to reduce noise and visual blight, and to allow redevelopment of parks and buildings.
About $1 billion has been paid to STP for the $1.44 billion construction contract. Workers continue to build the north and south portals despite the delays in tunnel boring. The state’s current goal is that the four-lane, tolled highway open for traffic by late 2016.