Feb. 7 update: Two factors contributed to tunnel stoppage

Posted on Feb 7 2014 12:06 PM

For two months, the contractor on the SR 99 Tunnel Project, Seattle Tunnel Partners (STP), has been working to determine what caused the stoppage of the SR 99 tunneling machine on Dec. 6, 2013. WSDOT and STP have identified two contributing factors: a clogged cutterhead and high-temperature readings indicating there were other factors to explore.

A clogged cutterhead

Tunnel workers performed 158 hours of hyperbaric inspections between Jan. 17 and Jan. 28. They found that many of the cutterhead openings were clogged with dirt and other material. A clogged cutterhead can affect the tunneling machine’s performance in the same way that a major obstruction would affect its performance. Once the hyperbaric work was completed, it was determined that a major obstruction was not the cause of the mining difficulty. The more likely cause was the clogged cutterhead.

During the 12 days of hyperbaric work:

Tunnel workers removed the clogs (view video on YouTube) from the cutterhead openings.

No major obstructions were found inside or in front of the machine.

Several cutting tools were replaced (view video on YouTube).

STP and WSDOT will continue to review the data and information gathered from the hyperbaric inspections.

High-temperature readings

After the cutterhead was unclogged, the contractor moved the machine forward an additional 2 feet and installed one of the concrete rings that line the tunnel. On Jan. 28 and 29, higher-than-normal heat sensor readings appeared like they did on Dec. 6, 2013. In the course of investigating the temperature readings, STP discovered damage to the seal system that protects the tunneling machine’s main bearing.

The main bearing is what allows the cutterhead to spin. It is similar to the bearing on the axle of a car, which is protected by a seal that keeps lubrication in and road grime out. The tunneling machine’s main bearing is protected by seals that function the same way - they keep the bearing lubrication in and the tunnel muck out. Investigations have shown that portions of the seal system have been damaged and need to be repaired or replaced. STP and its tunneling experts are working with the machine’s manufacturer to determine the best fix for this issue. They are currently assessing the extent of the damage and the best path forward.

Cost and Schedule

STP is the design-build contractor of this project, which requires cutting-edge engineering and design practices. The tunneling machine is owned by STP. It is STP’s responsibility to determine the scope of the issue and the best options to repair it and get the machine moving again. WSDOT expects to receive specific information on the impact to the project schedule and costs once STP determines a solution to the current situation. WSDOT oversees this mega-project and has both independent experts and onsite tunneling inspectors providing continuous analysis and review. We all share the same goal - to build the tunnel and remove the Alaskan Way Viaduct in a timely manner.