Delving the depths of downtown Seattle

Building into the city underground before the project goes up

It seems fitting that the city known for a colossal earthwork endeavor called the Denny Regrade is continuing the tradition of large-scale excavation in one of the nation's biggest explosions of digging deep holes in a downtown environment.

From a project management perspective, it appears our forefathers had it easy. In 1910. downtown Seattle was a relatively blank slate, and their biggest challenge was one that seems simple in the 1990's-moving huge quantities of dirt. Nowadays, we move huge quantities of dirt in record time. For example, the excavation for the recently completed Pacific Place is large enough to hold the concrete of three Kingdomes, and it was excavated at a staggering pace of 225 truck loads per day.

Pacific Place, a full block excavation.

The tools - shoring 101

Any deep excavation attempted downtown uses shoring. The principle behind shoring is simple: when you dig a hole in Seattle soils, the sides of the hole want to cave in, carrying with them anything adjacent.

The most common type of shoring is called soldier pile and tieback. This system relies on vertical steel columns (soldier piles) with wood beams (lagging) between them to hold back the face of the soil. The soldier piles are then held from falling into the hole by steel cables (tiebacks) that are drilled into the adjacent soil mass and anchored in concrete. These shoring elements must be installed around the many below-grade constraints of the downtown environment.

It's a common sight in downtown to see a recently completed sidewalk obliterated with seemingly random paint markings. This pavement "graffiti" is a necessary evil for understanding the most common challenge to shoring systems: underground utilities.

Similar to the variety of buildings in downtown, a wide range of utilities exist underground. Utilities can vary in size from conduits 2 inches in diameter to sewer tunnels 8.5 feet in diameter. These systems include brick sewers built at the turn of the century, high pressure steam lines encased in hollowed-out logs and high-voltage underground power and fiber optic cables. Other common utilities include water, sewer, telephone, natural gasand storm drainage.

Since most excavations in downtown abut public streets, the challenge is to locate the project's tiebacks within that street so that none of these utilities are hit during drilling. Meeting this challenge begins with a thorough investigation of city record documents to gain a three-dimensional understanding of utility routes. Then, tiebacks are located to meet or exceed the clearance requirements dictated by the city of Seattle.

Are you walking on air?

Sometimes you may wonder, "Is there any more space left to develop in downtown?" Though the answer is "yes," most projects today are infilled immediately adjacent to existing buildings. The full-city-block development is becoming a thing of the past.

An existing building sitting on a site relies on more than just the soil beneath it to hold it up, because its soil is confined by the soil on its neighboring sides. Therefore, digging a hole next to an existing building requires a shoring system with enough stiffness and strength to take the place of that previously provided by the soil being removed.

Shoring around the monorail.

Do you know that often when you walk along a Seattle sidewalk, you are really walking on air? In years past, the city encouraged designers to extend basements beneath the sidewalk to create areaways. The intent of an areaway was to allow the city access to underground utilities so that they could be maintained without digging up the streets. But, the city soon realized that trying to maintain public utilities through private property was too much of a hassle. Now, new developments that encounter existing areaways are required to abandon and fill them. Which means that developers have to address problems caused not only by their neighbor's basement, but by their own!

So why is this a challenge to shoring systems? When areaways were allowed, they were built as part of a new structure. Therefore, they rely on the adjacent existing building for support. If the building is removed, the sidewalk would collapse, and the areaway wall would tip over due to the horizontal pressure of the soil. In turn, this would undermine the adjacent roadway and underground utilities.

To avoid this, areaway walls must be tied back or the areaway void filled with "liquid dirt" at the beginning of construction. This stabilizes the areaways and allows complete building demolition and conventional shoring installation in an adjacent site.

Downtown is blessed with numerous urban transportation facilities: the Metro Bus Tunnel, the Battery Street Tunnel, the Burlington Northern Railroad Tunnel and the monorail. Three of those facilities, however, run underground in the heart of Seattle, presenting even more shoring design obstacles. The Monorail runs in the sky, but it, too, must remain operational and untouched during any neighboring construction.

Shoring immediately adjacent to existing building at Millennium Tower site.

But "common" certainly doesn't apply to the Experience Music Project, which has the monorail running right through its center. The project is being built directly above, below and adjacent to the monorail, without benefit of setbacks. To complicate matters, EMP's basement is up to two levels deep and extends between monorail piers.

Though the excavation was never greater than 30 feet deep, the forces exerted by the monorail piers were equivalent to a basement 150 feet deep. The layout of EMP's basement dictated that shoring be placed in a U-shape around a given pier. The bottom of the "U" was restrained via conventional tiebacks, while the sides of the "U" were tied to each other via tierods. The wedge of soil contained within the "U" supported the monorail's foundation during excavation and construction.

Where we go from here

It may seem that shoring doesn't have much room for innovation. In fact, while components remain rudimentary, the function of shoring is evolving. More developers are looking at moving the shoring system inside the property lines and using it as part of the building's permanent foundation system, thereby reducing overall development costs. Permanent tiebacks are becoming more common within a private parcel to allow the proposed building to be designed free of soil effects. Finally, getting rid of tiebacks altogether is being contemplated via a method called "top-down." This method involves constructing the basement from the street level downward, utilizing each completed basement floor slab as the horizontal soldier pile restraint. Soil beneath each successive floor slab is mined and conveyed to the street.

While the evolving shape of the Seattle skyline grabs most of the headlines, it's what's happening underground that is key to the on-going development of successful projects throughout the city. Now, if we can just figure out where to put all that dirt.

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