Much of modern-day Boston was underwater when European settlers first arrived on the Shawmut Peninsula. From the late-1700s to the late-1800s, the city aggressively expanded, filling parts of Massachusetts Bay with soil, sand and gravel. Today, the city has about 5,250 acres of filled land, said Mr. Simonelli.
To build on the unstable surface, builders drove tree trunks into the fill until they hit firmer ground, then placed foundation stones on top of these wooden piles. This technique was used until the 1920s, when foundation-building technology changed, Mr. Simonelli said.
Wooden piles can remain intact for hundreds of years if covered by groundwater, as they were when first installed. As the city grew, construction of tunnels, sewers, basements and subways caused the groundwater level to drop in many areas, which exposed the tops of the pilings. Air causes the wood to rot, said Giuliana Zelada-Tumialan of the engineering firm Simpson Gumpertz & Heger. As the rotted wood crumbles, the foundation stones sink, and so do the structures they support…
Repair means an expensive process called “underpinning”—cutting off the rotten wood at the top of the piles and replacing it with steel. It usually involves hand-digging a series of pits in the basement floor, a laborious process that can cost more than $200,000, and another $100,000 to repair the brick damaged by settling, said Mr. Kempel of Pegasus Luxury Homes, who has bought, renovated and sold a number of Boston houses. That cost doesn’t include any repairs or renovations that would be required if that basement unit was living space, as many are in row houses.
This is a hidden dimension of many urban buildings. What exactly do they stand on? How solid is the land underneath large structures? What happens if the foundations underneath are threatened? I remember looking as a kid at diagrams of what was underground in New York City or Chicago and wondering how it all worked with subways, gas, water, and electric lines, and other items.
The focus of this article is on pretty expensive real estate in Boston, particularly residences. With this kind of money involved, property owners – who the article notes may not even know about the potential problems and/or bypass inspections – can afford to fix their foundations. What happens when this affects public buildings or property owners with fewer resources? Perhaps this has to do with the value of fill land; given its proximity to the water and the city center, such land might be more valuable on the whole across cities. But, I could also imagine where a sizable city would have to put together a significant effort to help out a range of property owners.