The city of Boston has expanded its land area both by annexing other towns, and by using reclaimed land to expand the city boundaries.
The original topography and shoreline of greater Boston has been altered significantly since the 1600s. Boston was originally known as Trimount, for the three hills of the city, but an astute observer will notice that there are not three defining hills in downtown Boston. Two of these have been almost fully leveled, and the surviving one, Beacon Hill, is almost 60 feet lower than it had been originally. The rubble and dirt from these hills was used to expand the Shawmut peninsula by filling marshes and waterfront to make more buildable land.
Faneuil Hall was once waterfront, before that part of the harbor was filled in with gravel. A plaque showing the original 1630 shoreline is just past the Samuel Adams statue, and the brick sidewalks are marked with wavy lines and fish designs indicate where today's land is built over the water. Similarly, Griffin's Wharf – where the Boston Tea Party occurred in 1775 – is now considerably inland, and is marked with a plaque at the corner of Congress and Purchase streets.
Many neighborhoods of Boston were expanded using, or built entirely upon, reclaimed land. These include Back Bay, Bay Village, Chinatown, East Boston, Fenway-Kenmore, Flat of the Hill (on Beacon Hill), Fort Point, Leather District, South End, and the Waterfront.
However, this approach can result in some challenges about which homeowners and renters should be aware.
Reclaimed land can be prone to flooding. Reclaimed land can settle over time, and thus can be more exposed to the risk horizontal flooding from storm surges or rising sea levels. In addition, because reclaimed land could not support heavy structures – including tall brick rowhouses – nearly every building constructed on this land in the 1800s to early 1900s is supported on a system of wood pilings. This method has been successfully used in Europe for centuries. The pilings (basically trees stripped of their branches) were driven through the new land to the underlying hard clay, typically 30-40 feet below the ground. These pilings create a foundation that will last for centuries – if they remain submerged in groundwater. However, if groundwater levels drop because of drought, construction, or changes in the water table, the tops of the pilings can be exposed to air and begin to rot. This can cause severe foundation problems for the building and homeowners.
While builders arranged for the tops of pilings to remain submerged, the groundwater levels have changed since these buildings were erected. On one hand, more of Boston is covered by impermeable surfaces such as concrete and asphalt, which causes rainwater to run into sewers rather than into the soil. There has also been considerable underground construction, including major infrastructure projects like highway tunnels, sewers, and subways. Water often leaks into these structures, and is drained or pumped away, causing the surrounding groundwater levels to fall – and putting the system of pilings at risk.
How can owners or buyers tell if the pilings are failing? Because the pilings are a key part of the foundation, a building with rotting pilings will gradually settle unevenly. A few cracks will appear on walls. Eventually, windows and doors become inoperable, and floors will be out of level. However, rotted pilings can be repaired through a labor-intensive and expensive process called underpinning. Contractors need to dig beneath the building, cut away the rotted section of piling to below the lowest expected future groundwater level, replace it with steel, and encase the steel in concrete. The process involves a lot of manual labor. The cost to underpin a typical three or four story rowhouse can be over $400,000.
Can the problem be solved? Boston will always be vulnerable to groundwater-related problems. However, the problem can be managed. Groundwater levels rose in areas where infrastructure was repaired and water recharged. That said, successful outcomes require homeowners to follow best practices - if there is groundwater entering property, the best option is to repair the leak rather than installing a sump pump, which can lead to lowering of groundwater levels. Homeowners can also install 'recharge systems' which capture rainwater from the roof and direct it into the ground. Boston also established a Groundwater Conservation Overlay District that requires people building or renovating properties in the affected area to make sure that their projects will not cause any reduction in groundwater levels and to incorporate systems that will recharge rainwater into the ground instead of sending it all down the sewer. With more of these efforts, groundwater levels can be increased in areas where they are low, thus averting the expense and disruption of foundation repairs.
The Boston Groundwater Trust posts about groundwater levels across neighborhoods.