OVERVIEW

At 5:15 A.M. on December 18, 1986, the residents of a townhouse on Otis Place, on the Beacon Hill Flat in Boston, were suddenly awakened by a loud rumbling and the house shaking. On inspecting the house, they found many fresh cracks in the walls and ceilings and the exterior doors jammed shut, their locks out of line. ln fact, in order to get out, the residents had to pry open a basement door with a crowbar. Had this damage been caused by an earthquake, as the residents originally thought? No, only their house was affected, although the next-door neighbors had also been awakened by the loud rumbling. Structural engineers soon ascertained that the house had sunk several inches. But why had it sunk so precipitously? Because the tops of the wooden foundation pilings supporting it had rotted. The house was built on fill and, like most nineteenth-century masonry houses in filled areas of Boston, was set on a Foundation of wood pilings driven through the fill. Wood pilings are preserved if they remain submerged in water, but the groundwater level had been dropping on Otis Place in the mid-1980s, exposing the tops of the foundation piles to air and thus making them susceptible to rot. In this case, although the owners had no idea their house might have rotted foundation piles, they did know it was built on fill. But most people are unaware that the Beacon Hill Flat and many other parts of Boston are filled.

-Excerpt from: Gaining Ground A History of Landmaking in Boston By Nancy S. Seasholes

What is the problem and where does this occur?
Boston, being limited in size, faced challenges that prompted early residents to undertake efforts to expand the city by creating additional buildable land as depicted in the video above. This "filled land" was meticulously crafted by layering sand and gravel over the existing mud flats. Despite the transformation, this new terrain proved inadequate to support substantial structures like multi-story brick row houses. The map provided below offers a visual representation of the filled land, distinguished by black and white sections, in contrast to the original land marked in red. Today, many Bostonians reside or work in older buildings located within neighborhoods built on filled land. These areas include the Fenway, Back Bay, South End, Bay Village, parts of Beacon Hill, Chinatown, the Leather District, the Bulfinch Triangle, the North End, Downtown waterfronts, the Fort Point Channel section of South Boston, and sections of East Boston

“....wooden piles must be driven down into mud or waterlogged ground and buried completely in order to create a stable foundation for buildings.”

— Excerpt from: Vitruvius: The Ten Books On Architecture

Wood pilings were strategically driven through the filled land and the underlying organic silt layer, usually located 15 to 25 feet beneath the current ground surface. The majority of buildings erected on filled land prior to the 20th century rest upon these wooden piles. When kept submerged in groundwater, these pilings have the potential to endure for centuries. Nonetheless, should the groundwater levels diminish, the upper sections of the wood piles become vulnerable to microbial degradation when exposed to air. Subsequently, this decay can lead to settling issues in the supported structure, as depicted in the accompanying video below.

Water levels declining, exposing wood piles to air and subsequent decay from Wood Piles: Preserving Boston's First Deep Foundations

“As soon as the water table drops and stays down for a certain period of time, that wood becomes exposed to oxygen, and the oxygen creates an environment for bacteria and bugs to basically grow. And when those bugs grow they eat the wood.”

-Michael J. Atwood, PE (MA), Haley & Aldrich, Inc.

Because of the extensive history of using this type of building foundation system for centuries in Europe, builders working on Boston’s filled land were already aware of the potential problem. They made sure to cut off the tops of pilings at a level low enough to keep them submerged in groundwater, preventing rot and deterioration. However, in 1929, the Boston Public Library’s main building in Copley Square faced significant foundation repairs totaling over $200,000, equivalent to approximately $3,500,000 in today's currency. The root cause was traced back to the decayed wood pilings, worsened by a significant drop in groundwater levels compared to those during the foundation's original installation in 1888. Since the Library's repairs in 1929, numerous buildings across Boston have been forced to undergo costly and disruptive piling repairs due to similar groundwater-related issues.

When wood piles decay what happens to the building?
When wood piles decay (see images above), the crucial connection between the granite block and the top of the wood pile deteriorates. This phenomenon initiates the building's gradual settling process, leading to visible cracks as shown in the images below. The resulting uneven settlement, termed "differential settlement," poses various detrimental effects. In severe instances, windows and doors may cease to function properly, floors warp, and ultimately, the building's safety for occupancy is compromised.

How can I check the conditions of the piles under my building?
A Civil or Geo-technical Engineer is specifically brought in to excavate a test pit for analysis purposes. In alignment with the visuals provided below, the test pit is carefully dug in close proximity to the building's perimeter, specifically targeting the wooden piles' location. Through this method, the Engineer can uncover crucial insights into the condition of the piles beneath the structure. When the test pit is excavated, three key pieces of information come to light:

  • The evaluation of whether the piles are in a sound state or if they have succumbed to rot

  • The cut-off elevation of the piles

  • The comparison of this elevation with both the groundwater levels observed on-site and the data collected from the surrounding area's groundwater monitoring wells

A detailed report containing drawings similar to those displayed below will be generated for the property owner. The report aims to provide comprehensive insights and visual representations of the relevant information.

Can failed pilings be repaired?
Yes, the typical procedure to reinforce a structure on its wooden pilings is known as underpinning. The contractors are required to excavate beneath the building, remove the decayed segment of the piling to a level below the anticipated future groundwater, substitute it with steel, and then encase the steel within concrete, as depicted in the video and image below. This method necessitates significant manual work and can span over several months based on the number of wood piles necessitating underpinning and the scale of the building. For a standard three or four-story row-house, underpinning costs can start at $300,000 and may surpass that figure. Restoration expenses for larger structures are notably higher.

“We want to show people this is unpleasant. This is expensive. If we weren’t monitoring groundwater levels this would be done on a much larger scale, much more frequently.”

-Christian Simonelli, BGwT Executive Director

Rotted wood piles being underpinned from Wood Piles: The Process of Underpinning

So what's the good news?
The positive development is that, should groundwater levels increase to cover the pilings once more, the decay halts once the air within the wood escapes. Initially, the pilings were set with a significant safety buffer. If there has been no building subsidence, there's a strong chance the pilings can still uphold it. Yet, in certain instances, though pilings may have decayed, the building may not have shown any evident signs of settling.

What can a property owner do?
If you have groundwater coming into your property, it is advisable to promptly repair the leak instead of opting to pump the water away. This is crucial because even the operation of a small sump pump can significantly lower groundwater levels not just on your property but also for your neighbors.

You have the option to integrate a recharge system on your property to collect rainwater from the roof and direct it into the ground. It's crucial to ensure these systems are designed and installed to effectively facilitate water infiltration into the ground, preventing any potential issues related to flooding.

Over the years, much of the unpaved surface in the affected neighborhoods has been covered with buildings and impervious blacktop, leading to decreased groundwater infiltration. Despite the significant annual rainfall in Boston, only a fraction is able to percolate into the soil naturally. The situation took a positive turn in 2005 with the establishment of the Groundwater Conservation Overly District (GCOD) and in October 2022 when the City unveiled a fresh directive mandating specific municipal endeavors to incorporate green infrastructure for environmental advantages and improved stormwater management utilizing Green Infrastructure (GI).

Can the problem be solved?
Due to the presence of man-made land and numerous historic structures, Boston's susceptibility to groundwater issues remains persistent. Collaborative efforts aimed at repairing leaks, implementing recharge systems, and promptly addressing emerging concerns are essential in effectively managing this challenge. Encouragingly, investments in infrastructure repair and water recharge have led to notable increases in groundwater levels throughout the affected areas. By expanding such initiatives, it becomes feasible to elevate groundwater levels in areas currently experiencing depletion, preventing costly and disruptive foundation repairs linked to groundwater issues.