Seattle tunnel progress a major contrast to past problems
Bertha, the Seattle tunnel boring machine, is now almost halfway done with the 385-foot distance under the Alaskan Way Viaduct. And for the first time in more than two years, it looks like part of the project could be done on time.
When tunneling started in 2013, Bertha was expected to go 35 feet per day. But progress was much slower than that because the first 1,000 or so feet were expected to be “Training wheels” while Seattle Tunnel Partners made sure everything was ship shape.
The problem: Bertha ran into trouble about 1,000 feet in after chewing through a pipe and stopping for repairs — for two years.
The State Department of Transportation would not answer any questions about why tunneling progress is currently going so smoothly and a spokesperson said no one from Seattle Tunnel Partners was available for interviews. However, interviews with KIRO Radio over the past week break down the stretch of success into four factors:
• Better soil:
When tunneling first started, the machine ran into loose, wet, sloppy soil that caused significant delays.
“Coming out of the launch pit, the contractor was in loose, hydraulically-placed fill. The Denny Regrade, if you will: the old estuary deposits, old man-made fill. So the ground was much softer, wetter, looser. When they got to about King Street the face of the machine was fully into this over-consolidated glacial, hard soil. And that makes conditions much easier to manage,” Sowers said.
More than 1,000 feet later, Bertha is in the middle of the kind of soil the machine was designed for: hard glacial till and packed clay.
“These soil conditions are actually, in some ways, ideal,” Sowers said. “The sands don’t run, it’s not real loose and soft. The hard material is allowed to sort of stay vertical longer, and they’re more forgiving in terms of grout subsidence.”
Seattle Tunnel Partners had two years to repair Bertha as the tunnel boring machine sat immobile under Seattle.
“The machine has gone through an extensive amount of rehabilitation, if you will, to get it ready after damage was found. So, the contractor’s very confident that they’ve done a prudent amount of effort to get the machine repaired, strengthen it,” Sowers said.
Back in December of 2013 Bertha ran into several problems that led to a tunneling shutdown. After overheating several times, engineers discovered problems with the giant tunnel boring machine’s seals. Bertha had run into a 119-foot steel pipe – one of the only materials Bertha could not cut through. Although there was speculation that the pipe caused Bertha’s breakdown, that has not identified as the cause of mechanical failings.
Hitachi-Zosen, which designed and built Bertha, manufactured a new bearing-block, outer seal ring, and a new seal system that is more accessible for crews to access in case repairs are needed. STP also replaced the center pipe on the machine, among additional repairs discovered during inspections.
Tunneling started again at the end of December 2015. Before diving under the Viaduct, Bertha underwent even more repairs, maintenance checks, and testing.
“They got out in front of the machine, they did inspections of all the cutting tools that they could access – there’s over 700 cutting tools on the cutter head and they replaced about 11 of them,” Sowers said.
After a sinkhole surfaced in January, Governor Jay Inslee and the State Department of Transportation ordered tunneling to stop. After that, Seattle Tunnel Partners made plenty of changes so they could get approval to get back to work.
“They’ve made operational changes, they’ve made some equipment modifications, and they’ve even changed out some personnel,” Sowers explained. “The team that they have on right now and the operators they have, have proven over the last couple hundred feet that they can be successful mining in these types of difficult soil conditions.”
New personnel included four tunneling experts, among them a globally recognized tunneling expert with more than 30 years of experience.
• Looking ahead:
Both Seattle Tunnel Partners and the State Department of Transportation have the benefit of hindsight after a steel pipe helped derail tunneling two years ago.
This time, they didn’t take any chances.
“After encountering the test well, we did go and do an exhaustive LiDAR-ture [ground penetrating radar] search to make sure there were no other test wells in the tunnel horizon,” Sowers said.
Since mining resumed last Friday, Bertha has averaged 23.7 feet per day. The only time she’s moved faster was in 2013, before the machine was damaged when STP representatives say there was a month where tunneling clocked a faster pace.
Though tunneling seems to be going well now, it’s important to remember this is just a two-week chunk of a project that isn’t slated for completion until sometime in 2018. So, project administrators are crossing their fingers that this kind of progress continues for the rest of the next 8,000-feet that Bertha still has to go.