Wednesday, June 29, 2016

Under pressure with Bertha

By Mike Allende

Not all routine maintenance is considered equal. While Bertha, the SR 99 tunneling machine, will be at a planned maintenance stop for the next several weeks, there are some unique challenges to that work.

Bertha has tunneled more than 1,500 feet since its last pit stop, and the front end of the cutterhead is now about 120 feet beneath Spring Street near Post Avenue in downtown Seattle. While performing regular maintenance is a crucial part of keeping the machine in good working order, working on a machine that deep underground is no easy task.

The steps needed for crews to do pressurized work on Bertha deep under downtown Seattle.

First, crews need to stabilize the ground in front of the machine by injecting a type of clay known as bentonite into the front end of the machine. This creates a seal that prevents water and soil from entering – and air from escaping – their work space.

Next, they'll over-pressurize the space by introducing compressed air, which pushes against the bentonite to counteract the ground and water pressure at the front end of the machine. This creates a "hyperbaric" work space with pressure levels that are higher than regular atmospheric pressure. This is basically like conditions during an underwater dive.

Each person on the seven, five-member teams doing the work has to spend several minutes in a special chamber to prepare for the greater pressures they'll feel working in the hyperbaric conditions. When this type of work was done earlier on Bertha, they were able to spend up to an hour in these conditions before decompressing and returning to the surface, but the amount of time can vary depending on the pressure in the work space. Workers expect to begin the hyperbaric work next week.

The length of the maintenance stop depends on how much work is needed. Seattle Tunnel Partners' previous routine stop near Yesler Way lasted about six weeks. You can stay updated on the progress on the Alaskan Way Viaduct Replacement website.


Donald Merry said...

Why is the space between the cutter head and the tunnel pressurized?

WSDOT said...

Donald, crews introduce compressed air into the work space to prevent ground and water from entering through openings in the cutterhead.

whamp said...

1) Is this pressurizing technique done while the drill is normally operating?

2) Was this same pressurizing technique used when the bus tunnel was dug under the railroad tracks north of the Union Station?

WSDOT said...


1. No, it's only necessary to keep out soil and water so crews can work within the space behind the cutterhead during matinenance stops.

2. We can't speak to the specific construction methods used during construction of the bus tunnel since it wasn't our project. Hyperbaric work is a common practice on tunnel boring machines, though.

Alex Short said...


2) Hyberbaric work was used on Sound Transits light rail extension to the University of Washington and I believe when they bored the tunnel under beacon hill. It was also used on a tunnel in Washington DC for DC Water. As WSDOT said it is a very common practice for this style of tunnel boring machine known as Earth Pressure Balance or EPB.

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