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Why Seattle’s Downtown Sinking Has Little To Do With Actual Tunnel Digging

Settling ground is affecting the Alaskan Way viaduct, Pioneer Square buildings and underground water pipes, Seattle utilities officials said Monday. Engineers think the sinking is connected to the Highway 99 tunnel project, but it probably has little to do with actual digging.

The likely culprit is groundwater pumping.Crews are lowering the groundwater as they dig an access pit from the surface down to where the tunneling machine known as Bertha is stranded. The dewatering is necessary to prevent water from filling in the pit or blasting mud up through the bottom.

Soil inundated with groundwater weighs less than dry dirt, because underwater soils are buoyant. It’s the same principle that makes a person suspended in water weigh less than he or she normally would.

Draining The Pool

If you walk into a swimming pool with water up to your neck, and stand on a scale on the pool floor, you would weigh much less than you normally do. If you then begin to drain the pool, as more of your body emerges from the water you will begin to weigh more, and the needle on your scale will go up.

In similar fashion, as crews lower the water table the underground soils themselves weigh more, and compress under their own weight.

The soils that are in play are not the less consolidated fill material near the surface, but rather the tightly packed glacial soil sitting 3,000 feet deep on bedrock. This soils has already been compacted by thousands of feet of ice during the last glacial period, but even this dense layer gets denser when the water is removed. Compressing that soil even by a fraction of a percent of its previous volume adds up to significant settling at the surface.

No New Settling

State transportation officials say that downtown buildings and infrastructure have sunkbetween one and 1.4 inches. They say the settlement occurred in November, around the time crews began pumping water near the access pit.

Contractor Seattle Tunnel Partners is reportedly making contingency plans in case they are forced to stop pumping groundwater.

At Monday’s city council hearing, Washington transportation secretary Lynn Peterson said the sinking appears to have stopped around late November and no new settlement has been detected. Crews are continuing to investigate and monitor 29 Pioneer Square buildings, along with the viaduct and water mains.

Gabriel Spitzer is a former KNKX reporter, producer and host who covered science and health and worked on the show Sound Effect.