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Gates HQ

May 12, 2011

More than meets the eye — below ground

  • Hidden civil and site structural work provide the backbone for the new campus.
    KPFF Consulting Engineers


    As construction of the first phase of the Bill & Melinda Gates Foundation’s new headquarters winds down, Seattle is beginning to see the end-product of years of collaborative design and construction: 12 acres of inspiring architecture and landscaping that define the Seattle Center-area campus.

    What’s less apparent to the casual observer, however, are the infrastructure efforts put forth below the surface, creating a true foundation of collaboration for this world-class project.

    KPFF Consulting Engineers is the civil and site structural engineer for the project, responsible for the sitework design and permitting, design of utility relocations, stormwater management, and planning and design for surrounding roadways. KPFF also worked with the design and construction team to create a complex below-grade structural system to support the campus’ entry court and central plaza.

    Remove, reroute, rebuild

    KPFF’s work began in earnest more than six years ago, well before the initial phase of the campus — the five-story parking garage — broke ground. The site is bisected with vacated rights of way containing operating local and regional utility systems; a legacy to a time when the parcel was five separate city blocks. Before construction could begin on the garage — or anywhere else on campus — a number of these above- and below-grade utility systems had to be relocated, removed or reconstructed.

    Photo by Bob Bowlin/Sellen Construction [enlarge]
    This 50-foot-diameter, 60-foot-deep thermal energy storage tank was designed to accommodate top-down construction in a constrained area of the campus.

    Seattle Public Utilities operates several 40-foot-deep combined sewers that cross the site. KPFF coordinated the reconstruction of an oval-shaped, brick sewer built in the early 1900s; removing the aging system that crossed the campus and replacing it with a 42-inch sewer in a steel casing. This work, which involved open-cut and jack-and-bore construction, required careful coordination with the future foundations of the garage and campus basement.

    The campus is next to Seattle City Light’s Broad Street substation, which resulted in the site and adjacent streets being filled with above- and below-grade electrical circuits leaving the substation. The project required temporary and permanent relocation of both overhead and underground distribution and transmission circuits around the campus.

    By the time the entire project is complete, more than 6,000 feet of 26-kilovolt and 115-kilovolt power circuits will have been installed underground in multiple phases of construction.

    The team also worked with King County Wastewater Treatment Division to monitor the stability of the Lake Union Sewer Tunnel — a brick sewer built in 1891 that serves the majority of the South Lake Union neighborhood and parts of Capitol Hill. Bisecting the property 70 feet below the campus, the aging tunnel was monitored for “heave” as soil loads were excavated from above.

    A vertical access shaft for the tunnel was partially reconstructed within the campus parking structure, detailed to maintain the integrity of the remaining brick structure below.


    Photo courtesy of KPFF [enlarge]
    The central plaza includes trees, water features and boardwalks.

    Central plaza: the heart of the campus

    Below the central plaza’s collection of stone paving, boardwalks and water features is a structural system designed to support the serene landscaping above.

    In addition to performing the Gates Foundation’s civil work, KPFF was the project’s site structural engineer, creating the supporting elements of the campus heart and entry plaza.

    KPFF worked closely with the building structural engineer, landscape designer and contractors to create a detailed system above the campus’ basement and below the plaza.

    The fillings of this “sandwich” included a combination of carefully placed backfill and EPS foam layers to offset the weight of the soil and allow sufficient space for tree roots, site drainage systems, electrical conduits and water feature piping.

    Sustainability and awareness of the site’s environment were primary considerations of the overall campus design.

    One goal of the site design was to minimize the amount of stormwater leaving the campus and to honor the site’s natural history as a wet meadow. A key part of this strategy was the reduction of impervious area.

    Before its transformation, the campus property was a surface parking lot with only a half-acre of pervious surfaces. Through site planning efforts, the site’s softscapes have been increased to 5 acres, thereby reducing the quantity of runoff.

    Green roofs, which cover a number of the project’s buildings, make up 2 acres of the campus. The largest green roof is on the garage — an area measuring 1.4 acres. Other softscape areas include landscaping around the campus perimeter and areas within the central plaza.

    At campus completion, less than 1 acre of the site will be subject to pollution-generating vehicular traffic.

    The stormwater runoff from the remaining rooftop and hardscape areas is collected in a 1 million-gallon cistern — one of the largest such tanks in the state — located in the basement of the campus. The cistern provides both storm detention as well as rainwater storage for re-use for site irrigation, water features, building gray water and cooling tower makeup supply.

    A below-grade thermal energy storage tank features a structural design that maximized efficiency for both design and construction. The tank — 50 feet in diameter and 60 feet deep — enhances the efficiency of the campus’ mechanical heating and cooling systems.

    The tank’s structure was designed to accommodate top-down construction in a constrained area of the campus. Designers created a ring of piles to form the perimeter of the tank, removed the soil from the middle and cast the concrete tank’s wall as excavation progressed. The resulting tank wall acts in both compression and tension, depending on loading conditions.

    Photo courtesy of KPFF [enlarge]
    An oval-shaped brick sewer built in the early 1900s was exposed with campus excavation.

    While many tanks of this type require the construction of temporary shoring within which a separate tank is built, KPFF’s design allowed for the single concrete structure to serve as both.

    Surrounding roadwork

    Few projects of this size and duration are unaffected by changes in the surrounding cityscape, and the Gates Foundation campus is no exception. Since initial planning efforts started years ago, a number of nearby infrastructure projects have progressed, affecting the design of the campus and requiring creativity to keep the project on track.

    When campus design began, the fate of the WSDOT Alaskan Way Viaduct was far from decided, and the SDOT Mercer Street’s reconstruction had not been designed. Since then, both projects have moved forward considerably.

    To accommodate the resulting changes, designers have planned the campus’ north margins to account for the widening of Mercer, which runs along the north perimeter of the property. The north portal of the viaduct replacement tunnel has since been planned immediately east of the campus. Both projects required significant coordination between the design team, Washington State Department of Transportation and Seattle Department of Transportation.

    Team effort

    In the case of the Bill & Melinda Gates Foundation’s new campus, collaboration formed the basis for the team’s work. This culture of collaboration, respect and mindful design was a reflection of the foundation’s leadership.

    Occupancy of the first phase of the campus is a testimony to the efforts and commitment of many talented project team members and individuals at the local public agencies.

    Mark Veldee, PE, is a principal at KPFF Consulting Engineers, a civil and structural engineering firm in Seattle with a local staff of more than 220. Veldee leads the civil engineering group, which specializes in sustainable site development and infrastructure projects for public and private clients throughout the Pacific Northwest.

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