Ground improvement approach saves mixed-use development millions
A waterfront view with underground challenges
A 66-acre property in the inner ring of urban Boston, on the Mystic River in Somerville, Massachusetts, has a rich history. Following its early use as a train yard, it became home to a busy Ford Motor Company assembly plant in the 1920s – hence the name “Assembly Square.” After decades of producing domestic and military vehicles, the Ford facility closed in 1958 as the Edsel fiasco prompted the automaker to retrench.
For the past 55 years, the prime waterfront site has been under-utilized as a food warehousing and distribution center and for limited industrial and retail uses. In 2005, Federal Realty Investment Trust purchased the Ford parcel and other abutting properties that it planned to develop in phases into a vibrant mixed-use community combining up to 2,100 residential units, a major retail center and nearly 2 million square feet of office space. While the site enjoyed a wonderful waterfront setting, proximity to Boston and ready access to major transportation corridors, the development faced construction challenges due to the presence of very poor soil conditions. The property is underlain by urban fill, compressible organic deposits and clay soil – residuals from its waterfront and geologic heritage – which meant new ideas were needed to help reduce premium building construction costs resulting from the adverse underground conditions.
Thinking beyond the conventional
Haley & Aldrich shared some of the site’s history as one of the company's earliest projects in the late 1950s, converting the Ford plant into a food distribution facility. Additional work enabling other uses followed in later years. Our experience on this and many other challenging sites, and our focus on efficient, practical solutions, prepared us for the special needs of the Assembly Square development.
Long-standing industry solutions to similar site subsurface conditions usually involved use of long piles driven into the ground to reach dependable bearing soils or bedrock, bypassing the weak, compressible layers above. Ground floors were typically constructed of thick, heavily-reinforced concrete slabs connected to the pile foundations. While this approach was common and reliable, it came with a steep price tag which would result in millions of dollars for each of the many planned Assembly Square buildings covering the equivalent of eight city blocks. The significant financial stakes provided a need and opportunity for a new foundation approach.
After considering a number of ideas, Haley & Aldrich devised a way to use ground improvement that would eliminate the need for expensive pilings and structural slabs for the first buildings of the newly named Assembly Row development. Ground improvement, consisting of columns of crushed stone installed through the fill and organic deposits to stiffen these otherwise unsuitable soils, enabled “floating” of the buildings directly above thick, soft organic soils using shallow spread footings and soil-supported ground floor slabs. Creative design, special details and testing allowed ground improvement and shallow foundations to be used in ways that were previously considered unthinkable.
Banking on the design
Although aggregate column ground improvement has been used to limit settlements of low-rise buildings on poor ground for several years, it had not been used to provide support for such heavy buildings underlain at very shallow depth by soft organic soils - the new way the technology was applied at Assembly Row was truly groundbreaking. The foundation design pushed the state of practice, dramatically reduced impacts of noise and vibrations on neighbors during construction, and the owners realized cost savings in excess of $1 million per building.
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