The repair pit for tunnel-boring machine Bertha requires watertight walls, to keep groundwater surges from thwarting the work, or causing the ground to settle.
Sylvia Plummer: Here's something we should be concerned about, since the SR-710 tunnel will go thru two aquifers.
By Mike Lindblom, March 25, 2014
As engineers design a
pit from which to repair tunnel-boring machine
Bertha, they must conquer the threat of groundwater losses that might
sink Pioneer Square buildings.
The emerging plan calls for a watertight ring of buried shafts,
allowing the dirt within to be scooped away, to form a circular work
zone. Bertha would then grind its way into this concrete-walled pit from
the south, before repairs begin midyear.
Most of the shafts will be 7 feet in diameter, to resist tremendous
soil and water pressures. Part of the ring will include not only a row
of new columns, but have a dual row, by incorporating some of the buried
columns that were previously installed to protect the Alaskan Way
Concrete grout must be injected to fill any gaps — not only in the ring, but several yards to the south.
The worries are twofold.
Groundwater might pour into the repair pit, hampering workers and
equipment as they remove the giant cutterhead, and fix or replace the
damaged main bearing.
And if groundwater suddenly rushes in, the water loss elsewhere could
destabilize old buildings in Pioneer Square, or the viaduct itself.
Previously, Seattle Tunnel Partners (STP) installed a protective
north-south line of buried pillars, to shield the viaduct from Bertha’s
vibrations. Gaps of 5 inches were left between them, to let groundwater
migrate more-or-less normally. Now in a design change, STP’s pit
designer, Brierley Associates, is proposing to fill those gaps with grout. That way, water wouldn’t follow Bertha into the pit.
“Part of the plan includes sealing the pilings placed along the
tunnel route, to prevent water from getting inside the access pit when
the machine breaks through the wall,” said Matt Preedy, deputy Highway
99 administrator for the Washington State Department of Transportation.
Groundwater cannot be taken lightly, the project’s history shows:
• Tests performed in 2002 and
2010 indicated the water content near 35 percent in soil where Bertha
is stranded, at South Main Street. This generates a pressure nearly
three times that of the atmosphere.
• Inspectors inside the machine on Dec. 7, the day Bertha overheated and stalled, report that the cutting head was too flooded for workers to open a hatch in the machine to take a look.
• On Dec. 23-24, when crews pumped away groundwater (in test runs for
January inspections) some buildings in Pioneer Square temporarily sank
by up to a quarter-inch, says a report
by Bender Consulting, of Camano Island, released to The Seattle Times
under a public-records request. Afterward, they rebounded to a net loss
of only one-tenth inch.
• A similar Canadian repair pit in 1994 in Sarnia, Ontario, caused ground sinkage several yards away, while crews added extra pilings to fend off oozing mud.
The repair ring in Seattle will be 83 feet in diameter and 120 feet deep, according to diagrams Preedy showed the City Council on Monday.
“The designers are currently predicting not only what sort of effects
the viaduct might see from that shaft, but also any buildings in
Pioneer Square,” he said.
Also, contractors are designing a temporary noise wall, in hopes of
working through the night. STP has said drilling might restart by Sept. 1
at the earliest.