To build itself up, San Francisco digs down
April 4, 2016
As the world urbanizes, empty lots are vanishing under relentless development pressure in many cities. Governments looking for space for critical infrastructure often find themselves tightly constrained.
As a result, their attention is turning downward. “In developed areas, underground infrastructure may offer one of the few acceptable ways to encourage or support the redirection of urban development into more sustainable patterns because new support infrastructure can be added relatively unobtrusively,” claimed a recent National Research Council report, Underground Engineering for Sustainable Urban Development. Land-starved Singapore developed an ambitious planning program incorporating belowground areas. In Helsinki, designs for a two-tiered underground system placed utility tunnels deep below the surface with pedestrian tunnels above. Lima, Peru, is building a freeway beneath a riverbed.
In San Francisco, a massive new transit hub sunk 65ft into the earth will soon bring millions of passengers to and from the heart of the city each year.
Billed as the “Grand Central of the West,” the Transbay Transit Center (TTC) will accommodate train traffic below the ground and buses above it, capped by a 5.4-acre rooftop park.
San Francisco’s most complex excavation
“Improvements in underground technologies have enabled great strides in urban development in recent decades,” the National Research Council report notes, “but the complexity and unpredictability still inherent in underground construction are indications that many challenges remain.”
The transit center dig bears this out. Given the site’s massive footprint and crowded urban setting — three blocks long and half a block wide, it’s traversed by three of San Francisco’s busiest streets and flanked by some of its tallest towers — no aspect of the design could be taken for granted.
Before shovels hit the dirt, Arup’s team spent more than three years studying the site. This phase included extensive consultation with three of the world’s most respected geotechnical experts: Thomas Denis O’Rourke, Edward Cording, and Youssef Hashash — all veterans of the most comparable excavation in the United States, Boston’s Big Dig.
“This project sets an example of what our industry is going to be looking at in the future,” said Stephen McLandrich, Arup’s project manager for the design and construction phases. “The stakes just keep getting higher and higher.”
Watch and learn
Estimating the behavior of soil and water, the chief variables at play in geotechnical design, requires a mix of art and science.
Engineers have historically taken one of two approaches to excavations, noted pioneering soil mechanics specialist Karl Terzaghi in a 1945 essay: either “adopt an excessive factor of safety, or else… make assumptions in accordance with general, average experience.” Both are problematic; the former wastes resources while the latter can introduce excessive risk.
As a result, Terzaghi advocated for a technique that has since become standard practice on digs around the world. Known as the observational method, it allows engineers to move past the initial phases of a design despite the presence of some unknowns.
“The procedure is as follows,” he wrote. “Base the design on whatever information can be secured. Make a detailed inventory of all the possible differences between reality and the assumptions. Then compute, on the basis of the original assumptions, various quantities that can be measured in the field.” Continual monitoring of these variables during construction gradually makes these unknowns known, permitting the engineers to modify the design as needed.
The growing complexity of urban excavations has made the observational method more important than ever. For the TTC, it allowed the team to improve the design as the project progressed.
To gather the necessary information, Arup decided to use monitoring tools known as theodolites to record movements of the construction site and surrounding buildings as small as 1/100th of an inch. This system, collectively referred to as automated motorized total stations (or, more commonly, AMTS), is becoming the industry standard for excavations in dense city environments.
The team then used an online web portal to access near-real-time data about what was happening on the site.
From the outset of the project, protecting surrounding buildings from excavation-induced movement was a top priority. As the heaviest building around the site’s perimeter, the 58-story Millennium Tower received particular scrutiny.
Following an extensive analysis of the local ground conditions and the required excavation, the geotechnical engineers designed a massive reinforcement system — the largest secant-pile buttress ever installed in the United States — which effectively replaced the soil beneath the eastern half of the TTC site.
The contractor installed the buttress by drilling 182 overlapping shafts, then filling them with high-strength concrete. The construction took more than a year.
The buttress then supported the adjoining property during the excavation.
After the excavation was complete, special care was taken to prevent the force of the surrounding earth from collapsing the walls of the massive underground space.
In standard subterranean structures, a horizontal layer of steel pipes, referred to as a brace, is incorporated into each below-ground-floor level in order to prevent the walls from caving in. Given the unusually high ceilings required for the California High-Speed Rail platform — 35ft, as opposed to the standard 9ft required in most subterranean parking lots — a typical brace would not have provided sufficient control over excavation-induced movements. The designers therefore specified two levels of bracing for each below-grade floor.
Maintaining this horizontal pressure during the upward build-out of the concrete train box also required unusually complex engineering solutions. Braces were removed and reinstalled in a tightly coordinated sequence as the horizontal concrete floor layers were constructed.
Today, the massive hole in the ground is gone, replaced with reinforced-concrete walls that will soon house the train station. A steel structure that will contain commercial spaces, a huge bus and rail transit center, and the rooftop garden is now in construction.
When the facility opens in December 2017, it will serve an estimated 100,000 people a day.
McLandrich believes that the geotechnical solutions developed for the TTC will help push the industry forward, enabling similar projects to be built around the world.
“Hopefully the work done on this project shows transportation policy makers that these kind of projects are not only feasible, but entirely safe, and will provide benefits for their citizens.”
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