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Tunnelling under the city

Journey made possible with virtual underground models

Underground visualization shows the SR 99 tunnel’s route beneath downtown Seattle and its proximity to other underground structures. (Images courtesy of WSDOT.)
Underground visualization shows the SR 99 tunnel’s route beneath downtown Seattle and its proximity to other underground structures. (Images courtesy of WSDOT.)

For 60 years, the Alaskan Way Viaduct has been a fixture of the Seattle waterfront and a vital link in the State Route (SR) 99 corridor through the city. As the 20th century drew to a close, the double-deck highway structure was already showing its age. But in 2001, the 6.8 magnitude Nisqually earthquake sealed its fate, sinking sections of the viaduct several inches. After the earthquake, crews immediately stabilized the structure and everyone starting asking, “Now what?” Fast forward 12 years and we know the answer: a two-mile bored tunnel beneath Seattle 200 feet underground at its lowest point - connected to a mile of new highway in the southern end of the city. But getting to that answer took more than 90 design alternatives that included combinations of new elevated roads, surface streets, bridges and tunnels.

Project challenges

Since its construction, the viaduct has separated Seattle from its waterfront. Therefore opening waterfront access was critical for the success of any replacement strategy. Tunnelling under the city keeps the traffic moving on the viaduct during construction and avoids creating a new elevated barrier to the waterfront. However, the lack of space in downtown Seattle would force the tunnel alignment directly below the most congested area of the city. This underground area is filled with building foundations, utilities, several existing tunnels for railway and public transit. The long-term road closures required for a cut and cover tunnel would wreak havoc on traffic as well as on these underground utilities. A bored tunnel deep beneath the city avoids some of these obstacles, but not at the tunnel portals. Moreover, to keep costs contained, the diameter of the bored tunnel must be optimized, keeping enough room for four lanes of traffic, breakdown shoulders, egress facilities, fire and lifesafety equipment and ventilation ducts. 

A model solution

For 10 years, the Washington State Department of Transportation (WSDOT) worked with King County, the City of Seattle, the Port of Seattle, the U.S. Federal Highway Administration, and a team of private consultants led by Parsons Brinckerhoff (PB) to develop solutions for the replacement of the viaduct. As a part of its effort, the team utilized software from Autodesk - including Autodesk AutoCAD Civil 3D, Autodesk Revit, Autodesk Navisworks, and Autodesk 3ds Max Design - to integrate disparate data sources and create intelligent, digital 3D models that were used for the design, visualization and contract packaging of all of the major parts of the project. This model-based planning approach helped the design teams, the project stakeholders, and the general public better understand and evaluate more than 90 design alternatives -  leading to faster, more informed decision-making.

Building consensus

“During the early stages of the planning effort, all options were on the table - from closing the viaduct and not providing any alternative, to building a whole new elevated structure or tunnelling under the city,” recalled Alec Williamson, engineering manager, Alaskan Way Viaduct project at WSDOT. “All the design proposals had to be judged in terms of fulfilling the major project goals: replacing vulnerable highway infrastructure, improving access to Seattle’s waterfront, and keeping traffic moving within the city.” The team developed 3D models of the proposed design alternatives in the context of the surrounding cityscape and aggregated existing GIS and utility data to help create an underground model of the city’s infrastructure. PB then used these models to generate visualizations of the design options in the form of still renderings, animations, and simulations of the new roads, tunnels, and interchanges from a driver’s perspective. This visual communication of the design proposals improved everyone’s understanding and helped build consensus on the emerging design solutions. “Even when the bored tunnel option came to the forefront, project visualizations were still critical for getting buy-in from project stakeholders and the public,” said Williamson. “3D models helped them more clearly picture the interchanges required at both ends of the tunnel - the southern end in particular since it borders a very large, active port.”

Going underground

“Given the protracted debate regarding the viaduct’s replacement, we had time to undertake an extensive amount of subsurface exploration, mapping and modelling,” explained Williamson. “We developed a comprehensive model of existing underground structures - from utilities and tunnels, to building foundations and the viaduct’s pile foundations. These models helped us define the alignment and profile of the tunnel, propose mitigation and inform our decision-making.” During the design of the bored tunnel and its access points, this underground model was also instrumental in helping to avoid future construction conflicts. The team used Autodesk project coordination software to aggregate the underground models and the design models, and perform clash detections - helping WSDOT to identify and resolve clashes during design and set the stage for a smoother design-build process. The team also used Autodesk model-based civil design software to design subsurface utilities, earthworks and other infrastructure. During the design of the tunnel itself, the team modelled the interior of the structure to fit four lanes of traffic and all the requisite facilities and utilities within the smallest possible diameter. “Trying to find space for all the fire, safety, and ventilation equipment and systems was a challenge,” Williamson said. “Virtual modelling and clash detection made this process much more efficient.”

Tunnel realignment

In 2007, WSDOT crews began a separate project to replace the southern end of the viaduct. “When the bored tunnel option emerged, we selected a tunnel alignment running underneath First Avenue, a very busy road through the heart of the city’s historic district,” said Williamson. This part of Seattle used to be tidal mud flats and many older structures were built on wooden piles resting in these mud flats. Gradually, the mud flats were filled and when buildings (now at grade) were replaced, the old wooden piles remained buried in the sediment and turned into fibrous knots. Unfortunately, the tunnel boring machine, which can eat through boulders up to three feet in diameter, does not digest fibre very well. Given those obstacles, WSDOT called a time-out and worked with its partners to study alternative alignments at the southern end - using the underground models developed by PB to quickly formulate and evaluate a series of design alternatives. After a month of analyses, the team settled on a new alignment that heads underground in the area of the demolished portions of the viaduct, runs under the still-standing section of the viaduct, and then veers off to First Avenue, avoiding  the wood piles.

The results

This $3.1 billion program, which includes the SR 99 tunnel and nearly 20 other projects, is now in full swing. “Bertha”, the world’s largest tunnelling machine began drilling the two-mile-long tunnel in July 2013, with an expected opening in late 2015. To date, 3D digital modelling technology has helped develop early design concepts, validate the viability of the new and final alignment location, and create hundreds of photorealistic images and dozens of animations that supported public outreach and fostered project consensus. Virtual project models continue to play an important role in helping to communicate the program and its status to the public.

“Bertha,” the world’s largest tunnelling machine, is digging a tunnel beneath downtown Seattle to replace the SR 99 Alaskan Way Viaduct. Built for the dig by Hitachi Zosen, Bertha measures 57 feet in diameter, 325 feet in length and weighs 7,000 tons.

For 60 years, the Alaskan Way Viaduct has been a fixture of the Seattle waterfront and a vital link in the State Route (SR) 99 corridor through the city. As the 20th century drew to a close, the double-deck highway structure was already showing its age. But in 2001, the 6.8 magnitude Nisqually earthquake sealed its fate, sinking sections of the viaduct several inches. After the earthquake, crews immediately stabilized the structure and everyone starting asking, “Now what?” Fast forward 12 years and we know the answer: a two-mile bored tunnel beneath Seattle 200 feet underground at its lowest point - connected to a mile of new highway in the southern end of the city. But getting to that answer took more than 90 design alternatives that included combinations of new elevated roads, surface streets, bridges and tunnels.

Project challenges

Since its construction, the viaduct has separated Seattle from its waterfront. Therefore opening waterfront access was critical for the success of any replacement strategy. Tunnelling under the city keeps the traffic moving on the viaduct during construction and avoids creating a new elevated barrier to the waterfront. However, the lack of space in downtown Seattle would force the tunnel alignment directly below the most congested area of the city. This underground area is filled with building foundations, utilities, several existing tunnels for railway and public transit. The long-term road closures required for a cut and cover tunnel would wreak havoc on traffic as well as on these underground utilities. A bored tunnel deep beneath the city avoids some of these obstacles, but not at the tunnel portals. Moreover, to keep costs contained, the diameter of the bored tunnel must be optimized, keeping enough room for four lanes of traffic, breakdown shoulders, egress facilities, fire and lifesafety equipment and ventilation ducts. 

A model solution

For 10 years, the Washington State Department of Transportation (WSDOT) worked with King County, the City of Seattle, the Port of Seattle, the U.S. Federal Highway Administration, and a team of private consultants led by Parsons Brinckerhoff (PB) to develop solutions for the replacement of the viaduct. As a part of its effort, the team utilized software from Autodesk - including Autodesk AutoCAD Civil 3D, Autodesk Revit, Autodesk Navisworks, and Autodesk 3ds Max Design - to integrate disparate data sources and create intelligent, digital 3D models that were used for the design, visualization and contract packaging of all of the major parts of the project. This model-based planning approach helped the design teams, the project stakeholders, and the general public better understand and evaluate more than 90 design alternatives -  leading to faster, more informed decision-making.

Building consensus

“During the early stages of the planning effort, all options were on the table - from closing the viaduct and not providing any alternative, to building a whole new elevated structure or tunnelling under the city,” recalled Alec Williamson, engineering manager, Alaskan Way Viaduct project at WSDOT. “All the design proposals had to be judged in terms of fulfilling the major project goals: replacing vulnerable highway infrastructure, improving access to Seattle’s waterfront, and keeping traffic moving within the city.” The team developed 3D models of the proposed design alternatives in the context of the surrounding cityscape and aggregated existing GIS and utility data to help create an underground model of the city’s infrastructure. PB then used these models to generate visualizations of the design options in the form of still renderings, animations, and simulations of the new roads, tunnels, and interchanges from a driver’s perspective. This visual communication of the design proposals improved everyone’s understanding and helped build consensus on the emerging design solutions. “Even when the bored tunnel option came to the forefront, project visualizations were still critical for getting buy-in from project stakeholders and the public,” said Williamson. “3D models helped them more clearly picture the interchanges required at both ends of the tunnel - the southern end in particular since it borders a very large, active port.”

Going underground

“Given the protracted debate regarding the viaduct’s replacement, we had time to undertake an extensive amount of subsurface exploration, mapping and modelling,” explained Williamson. “We developed a comprehensive model of existing underground structures - from utilities and tunnels, to building foundations and the viaduct’s pile foundations. These models helped us define the alignment and profile of the tunnel, propose mitigation and inform our decision-making.” During the design of the bored tunnel and its access points, this underground model was also instrumental in helping to avoid future construction conflicts. The team used Autodesk project coordination software to aggregate the underground models and the design models, and perform clash detections - helping WSDOT to identify and resolve clashes during design and set the stage for a smoother design-build process. The team also used Autodesk model-based civil design software to design subsurface utilities, earthworks and other infrastructure. During the design of the tunnel itself, the team modelled the interior of the structure to fit four lanes of traffic and all the requisite facilities and utilities within the smallest possible diameter. “Trying to find space for all the fire, safety, and ventilation equipment and systems was a challenge,” Williamson said. “Virtual modelling and clash detection made this process much more efficient.”

Tunnel realignment

In 2007, WSDOT crews began a separate project to replace the southern end of the viaduct. “When the bored tunnel option emerged, we selected a tunnel alignment running underneath First Avenue, a very busy road through the heart of the city’s historic district,” said Williamson. This part of Seattle used to be tidal mud flats and many older structures were built on wooden piles resting in these mud flats. Gradually, the mud flats were filled and when buildings (now at grade) were replaced, the old wooden piles remained buried in the sediment and turned into fibrous knots. Unfortunately, the tunnel boring machine, which can eat through boulders up to three feet in diameter, does not digest fibre very well. Given those obstacles, WSDOT called a time-out and worked with its partners to study alternative alignments at the southern end - using the underground models developed by PB to quickly formulate and evaluate a series of design alternatives. After a month of analyses, the team settled on a new alignment that heads underground in the area of the demolished portions of the viaduct, runs under the still-standing section of the viaduct, and then veers off to First Avenue, avoiding  the wood piles.

The results

This $3.1 billion program, which includes the SR 99 tunnel and nearly 20 other projects, is now in full swing. “Bertha”, the world’s largest tunnelling machine began drilling the two-mile-long tunnel in July 2013, with an expected opening in late 2015. To date, 3D digital modelling technology has helped develop early design concepts, validate the viability of the new and final alignment location, and create hundreds of photorealistic images and dozens of animations that supported public outreach and fostered project consensus. Virtual project models continue to play an important role in helping to communicate the program and its status to the public.

Company info

111 McInnis Parkway
San Rafael, CA
US, 94903

Website:
en.autodesk.ca

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