Prudential Tower
The Prudential Tower, commonly referred to as “The Pru,” was topped off in 1964 at 749 ft (228 m) or 52 stories. Today it stands as the second tallest building in Boston. Located in the Back Bay area, this office tower is perhaps the most iconic building in the city skyline. The Back Bay area was once not included in Boston’s coastline (hence its name), but was filled in during the late 19th century. The soil profile for the Back Bay according to A City Upon a Hill: The Geology of the City of Boston is as follows: 20-30 ft of fill over organic soil, over 40-80 ft of outwash sand that overlays overconsolidated marine clay, known as Boston Blue Clay. All of this then sits upon lower outwash, till, and deep lying Cambridge Argillite. A building the size of The Pru cannot be supported by ordinary foundation piles, and thus requires drilled-in caissons to anchor itself to the argillite under the fill. The process for constructing this type of foundation begins with drilling deep holes into which hollow cylinders, called caissons, are inserted. The Pru utilizes steel caissons 30 inches in diameter extending down to 200 ft below the surface. Excess soil was removed from the hollow caissons using compressed air, and replaced with H-piles anchored 15-25 feet into the bedrock reaching up to the surface. Finally the rest of the space was filled with concrete, creating a deep sturdy foundation capable of supporting such a large building.
Boston's Back Bay: The Story of America's Greatest 19th Century Landfill Project-- by William Newman and Wilfred Holton
|