Boston Groundwater Trust Executive Director Christian Simonelli presented an update on the Trust’s work monitoring the city’s groundwater levels at the November Neighborhood Council (NEWNC) meeting.
Boston Groundwater Trust (BGwT) was established in 1986 to monitor groundwater levels in the city where there are wood piling supported buildings to make sure water levels remain above these pilings. If the water level drops below the pilings they begin to rot, compromising the building’s foundation and causing a building to settle unevenly.
The BGwT has a network of 800 observation wells throughout the city that they use to monitor groundwater levels, 31 of which are in the North End. They also work with the Boston Planning & Development Agency to be sure any new developments don’t negatively impact the groundwater system.
Simonelli traced the establishment of the BGwT back to sinking flats in Beacon Hill in the 1980s (3:06). At that time, engineers came in and discovered these buildings were supported by wood pilings that had been exposed to air for 6-7 years and become rotted.
Why wood pilings? About 5,000 acres of Boston is filled land that was once a bay. Between the mid 1800s and early 1900s, brick buildings constructed on filled land were built on wood pilings. The land was made using unconsolidated fill, which could not support heavy structures. Wood pilings can last indefinitely, as long as the groundwater level remains above the pilings.
Groundwater levels can drop due to leaking pipes, sewers, tunnel work or an extensive drought. Now that the BGwT is monitoring levels under the city, they can identify and correct low water levels before wood pilings begin to rot. This is important because the cost to repair rotted pilings, a process known as underpinning, is quite expensive. It can range anywhere from $250,000 to $3,000,000. The process must be done by hand and takes many months.
Classic signs of distress include uneven floors, cracks above and below windows and doorways, and bricks pulling away from the building (10:36). This is because wood pilings rot at different rates, causing the building to sink unevenly.
How does BGwT monitor groundwater levels? Engineers go out and read all 800 wells every 5-6 weeks (12:24). A public report is generated showing the measurements and any hot spots. The BGwT looks at the well water level and the wood pile cut-off elevations of that building, and then determine how many feet of water are above the piles. If they determine there is not enough water, they begin to investigate why.
Simonelli shared wood pile cut-off data for the North End along Thacher, Cooper and North Washington Streets (18:50). These buildings have high pile cut-off elevations, but the water elevation in this area is also high, so it’s not a problem. BGwT also works on groundwater recharge – getting rainwater back into the ground. One way to do this is through porous pavement, like the North End bike lane (26:09).
Q&A from the Neighborhood Council and the audience begin at 32:20.