Our field trip to Bellevue, WA was worth the long roundtrip drive. The group departed Corvallis on Friday, June 1st at approximately 5:40am. We were greeted by Archie Kollmorgen and his colleagues from Atkinson Construction and were kindly provided with pizza for lunch. During lunch, we listened to a brief overview of the Sound Transit tunnel project. The Sound Transit East Link Extension project shall connect Seattle with Redmond via light rail. The connection in downtown Bellevue will be made possible with a 2,000 feet long tunnel. The excavated dimensions are about 27 ft high and 34 ft wide.
How do they dig the Bellevue tunnel?
The method used is called SEM – sequential excavation method. Imagine you are sitting in a large excavator. You’ve exchanged the front shovel for a digging tool with some teeth on it. You push it into the glacial till and sand to take it down: not all at once, just bit by bit. You drive backwards to give some space to the front loader, so the soil can be removed, and your fellow laborers place some steel sets and spray concrete on the walls. The complete tunnel cross section can be excavated in several steps – first the top part (top heading), then the middle part (bench), and lastly the bottom part (invert). With this piece-by-piece excavation it is possible to excavate a tunnel in urban areas with high-rises and very little soil between the road surface and the tunnel structure.
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| Figure 2. From left to right: Atkinson Construction Employee, Sean Iiams, Maggie Exton, Nathan Jones, Stephie Lange, Nick Carter, Tucker Jones, Clinton Lindgren, Yingqing Qiu, Aleyna Donaldson, Amalesh Jana, and Christopher Roach |
How does a tunnel function in an emergency, such as during a fire?
There is a shaft that goes from the ground surface to the tunnel at the half-way point along the tunnels length. The shaft has emergency fans to remove deadly fumes from the construction site and also serves as an access path for rescue parties. This shaft construction is done with partial downward excavation, which is similar to the tunnel excavation method. A part of the shaft hole is excavated, steel is placed around the hole, a shotcrete layer is applied over the steel, and the cycle is repeated. See figure 8 below.
How does it look, sound, and smell?
The air quality is surprisingly great. The exhaust air and dust is vented through large ventilation ducts and fresh air is automatically sucked in from the entrance of the tunnel excavation from figure 3 by the pressure difference. It is loud with all the machinery operating. It smells like dirt and concrete mixed with some truck exhaust, but the air on the surface above smells worse due to all the cars and vehicles driving their daily commute in the sun. Luckily, the tunnel excavation hasn’t encountered ground water so far. Ground water makes excavation much more difficult.
I’d like to thank GIGSO (Geo-Institute Graduate Student Organization) of Oregon State University (OSU) for their financial support. The group would like to thank Atkinson Construction with Archie and his colleagues for their time, knowledge, and warm welcome.
I’d like to thank GIGSO (Geo-Institute Graduate Student Organization) of Oregon State University (OSU) for their financial support. The group would like to thank Atkinson Construction with Archie and his colleagues for their time, knowledge, and warm welcome.
Stephie
Impressions from the tunnel construction visit:
Impressions from the tunnel construction visit:
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