What You Need to Know About Groundwater—A Guide for Property Owners
Reprinted and updated from NABB News, Autumn, 2002
During the 19th century, more than 2,000 acres of Boston were built on landfill. If you live in the Back Bay, there is a 95% chance your building was constructed on wood pile foundations.
Throughout the 20th century, as Boston continued to grow and change, there was concern that groundwater levels could drop too far to protect wood-pile foundations, allowing them to dry out and rot. Eventually, buildings supported on compromised foundations require extensive and costly repair or possibly demolition.
For the past few years, the Boston Groundwater Trust (BGwT) has been monitoring 150 wells throughout the city and has started to drill another 850 wells for data collection. Since summer of 2003, the BGwT’s observation well network has expanded to about 250 monitored locations. Current readings indicate that groundwater levels now may be too low in many locations in the city. In the Back Bay, in addition to the area near the Storrow Drive underpass, it appears that evidence of other “hot spots” may be emerging.
This is why property owners in Back Bay must become vigilant about groundwater conditions in their neighborhood, take preventive or remedial measures to protect their buildings, and demand immediate repair of defective underground infrastructure (e.g., sewers, tunnels, underpasses, retaining walls, vaults and piping).
A Checklist for Property Owners With Wood-Pile Foundations
Watch for visible indications of deterioration.
Damaged foundations and resulting settlement is likely to create stresses, visible from the exterior, in exposed front and rear walls of a typical brick row house. Step cracks in brick work at the corners of window and door openings and out-of-level stone lintels and sills are some indications of potential damage, which should be verified through physical inspection by a building professional–a builder, engineer, or architect–with experience in this area.
Verify groundwater elevations for comparison to top elevation of your wood piles.
The Boston Groundwater Trust monitors groundwater elevations in some locations and posts them on its Website at www.bostongroundwater.org. Some individuals have installed private monitoring wells of their own at an estimated cost of $2000. Others have had a test pit dug to provide access for physical examination of some of the piles, verification of the existence and condition of the wood and a visual sighting of the groundwater level. Estimated cost is $3,000-$4,000.
Recharge water into the ground.
An effective method of mitigating potential foundation damage in the presence of marginal or apprehended groundwater depression is the manual addition of water to the ground in the vicinity of the wood piles. Prevailing conditions in the Back Bay suggest this as an attractive, normal provision of long-term property maintenance. Concern for the quality of water being added to the aquifer limits acceptable sources, but rainwater from rooftops and paved areas generally works well. Several options are available, some requiring city permits:
- Disconnect your rainwater leader piping, install a dry well and reconnect to the sewer system for overflow protection from flooding.
- Use rooftop rainwater as an irrigation source for plantings.
- Install porous parking surfaces.
- In some extreme conditions, an underground slurry wall has been installed to conserve water and block dissipation of recharged water away from foundations.
Tap water has also been used as a more predictably available recharge source, usually connected to a simple perforated distribution pipe. This can, however, become very expensive.
Repair deteriorated wood piles.
Sometimes repair of deteriorated wood piles is referred to as underpinning. It involves excavating a building ‘s perimeter walls, removing the rotted wood and introducing steel pipe supports that are subsequently encased in concrete. A typical row use might have 150 individual piles, all of which must be treated in small groups so as to not to destabilize the building overhead. It is dirty, inconvenient and can be expensive-as much as $250,000, exclusive of possible interior-finish damage to occupied spaces in the basement. It can also involve complicated coordination with neighbors, who likely share foundations under party walls.
Preventive measures and groundwater-management policies are cost-effective measures that can help homeowners avoid this extreme treatment.