adjoining bored tunnel project required revising and resubmitting 75 percent of the design documents, which included changing and reprint- ing 1,400 documents over three months. “One of the most challenging aspects was dealing with and still meet- ing project milestones,” Lalonde said. “Even though final alignment decisions were delayed, the project had to remain on schedule.” Digging Bertha After years of planning and careful exploration of more than 75 pos- sible options to replace the Alaskan Way Viaduct, the decision was made to bore a double-decked tunnel through downtown Seattle to replace the viaduct. The tunnel design met stringent seismic standards capable of with- standing a 2,500-year earthquake. It also would feature smart transit technology, including more than 300 cameras that monitor traffic, safety conditions and security. Digging began in July 2013 as “Bertha” began to make its way un- derneath Seattle. The tunneling boring machine (TBM) was named for Bertha Knight Landes, Seattle’s first female mayor. Standing five- stories tall and weighing some 7,000 tons, Bertha was the world’s larg- est TBM at the time digging began. Bertha’s enormous size provided the capacity needed to use one ma- chine to drill a tunnel large enough for four traffic lanes – two lanes stacked upon two others – thus eliminating the need for a second bore. The upper deck carries southbound traffic, while the lower deck carries the northbound traffic. Tunneling moved forward until December 2013, when the TBM failed. At that time, only 10 percent of the tunnel had been built. The contrac- tor, Seattle Tunnel Partners, and the machine manufacturer, Hitachi Zosen developed a bold plan to raise the machine from below ground for extensive repairs above. Bertha was fully repaired and resumed digging in February 2016, and on April 4, 2017 broke through into the disassembly pit—building the tunnel’s final ring shortly after. Throughout the dig, more than 4,000 monitoring devices kept watch for ground movement along the two-mile tunnel alignment. Using a variety of above-ground devices, 158 buildings – including historic buildings – were monitored in real-time. Ground and vibration moni- tors operated 24 hours a day for the length of the project. The extensive monitoring along the tunnel alignment would allow an immediate response if movement exceeded contract specifications. In the end, all the monitoring systems confirmed an amazing feat – there was no surface ground movement under Seattle buildings or streets caused by tunneling. When the TBM’s tunneling and ring-building work ended, the massive machine had to be taken apart to allow road building to continue. Bertha’s signature 57-and-a-half-foot steel cutterhead was cut into pieces and hauled to a local steel recycler. Hitachi Zosen salvaged
Photo: Washington State Department of Transportation (WSDOT)
WSP created dynamic, three-dimensional animations to illustrate con- ditions at the site and help WSDOT officials visualize the impacts of proposed solutions. “Design visualizations were an integral part of the program, used ex- tensively to educate and inform the public, and as a primary design tool to effectively determine the viability of proposed concepts and eliminate conflicts between design elements for the replacement,” said Ginette Lalonde, current project manager for WSP. WSP created a video simulation to show, from a "worm's eye view," the proposed route for the tunnel and its proximity to other underground structures, including utilities and buildings. Another lifelike video pre- pared by WSP and released by WSDOT in 2009 showed a simulation of the viaduct’s collapse from a hypothetical seismic event similar to the Nisqually Earthquake. “The viaduct posed an immense public safety risk, and we all knew we were racing against Mother Nature,” said Mike Rigsby, who served as WSP’s project manager during the early stages of the program. “The video simulation attracted huge media attention and heightened public awareness about the life-safety risk of the seismically vulnerable structure.” WSP drew upon computer-aided design (CAD) and geospatial data to develop digital 3D and BIM for all major parts, pieces and processes of the project. These models enabled the project team to understand the project, design alternatives, and view traffic impacts and construction phasing from any point of view, allowing for more informed decision making by multiple stakeholders, while supporting public outreach efforts. An example of the team’s extraordinary effort was with an early de- sign-bid-build enabling project—the Holgate-to-King Street Viaduct Replacement. While the project was out for bid, a late change in the
20
csengineermag.com
may 2020
Made with FlippingBook Annual report