Beneath downtown Seattle, Bertha faces biggest test
The Seattle Mariners and the tunnel boring machine Bertha are following similar arcs lately. After years of wallowing in failure, both have shown a bit of a heartbeat in 2016.
Earlier this month, Bertha burrowed beneath the Alaskan Way viaduct without causing catastrophic damage. The elevated highway opened a week early and Bertha just kept on going. After a brief pause, the machine is moving again, drilling under downtown toward South Lake :union:.
Several tunnel engineers say the progress is proof that we should put more trust in the machine’s operator, Seattle Tunnel Partners. “These machines are highly specialized,” says Dr. Reza Hedayat, an assistant professor at the Colorado School of Mines.
But a pattern of mechanical problems and operator errors have drained optimism in the machine, Seattle Tunnel Partners and the Washington Department of Transportation, which hired the company. As a result, we have dinner theater City Council meetings in which elected officials demand explanations about how they will safely burrow beneath downtown, only to have engineers respond, “It’s complicated.”
Many Seattleites simply wonder what will go wrong next.
A tunnel boring machine has two very important functions: to remove dirt and to prevent the dirt above from collapsing into the hole it’s just created. Few of us cared much about how Bertha did this until January, when the machine created enormous sinkholes near one of the most precarious major highways in the world. It turns out that the details are pretty important.
There are two types of tunnel boring machines, each of which deals with the whole collapsing earth thing a little differently. For the Seattle tunnel, WSDOT went with an Earth Pressure Balance Machine (EPBM). There is some disagreement surrounding whether that was the right choice, but we’ll get back to that.
To understand how an EPBM works, imagine pushing a couch up a flight of stairs: What’s keeping the pleather davenport from crashing down is the counteracting force of your body. Digging a tunnel project is like pushing a really heavy couch. The pressure in some cases can be many times normal atmospheric pressure.
MONDAY 23, MAY 2016