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 Viaduct.

Concrete grout must be injected to fill any gaps — not only in the ring, but several yards to the south.

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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.

Mike Lindblom: 206-515-5631 or