Geology Issue Submitted to the PZC by Seven Petsch
To the Members of The Ashford Planning and Zoning Commission c/o: Michael D’Amato, Zoning Officer: zeo@ashfordtownhall.org cc: Katie Connolly, Land Use Clerk: kconnolly@ashfordtownhall.org
I am writing to inform the Ashford Planning and Zoning Commission about an aspect of the underlying geology of our town that may cause extreme challenges for any very large building constructed on properties in the IID Zone. In short, the IID zone sits on the rock formation that is implicated in crumbling home foundations in Ashford and many surrounding towns in northeast Connecticut.
I write this as an Ashford resident and also as the holder of a B.S. in Geosciences (Penn State, 1994) and Ph.D. in Geochemistry (Yale, 2000) with 20+ years’ experience in academia with research and teaching focused on rock weathering and geochemistry.
The issue of crumbling foundations results from the presence of a mineral called pyrrhotite in the building’s foundation concrete. Pyrrhotite is an iron sulfide mineral that slowly reacts to form rusty deposits and generate sulfuric acid when exposed to air and water. Although the process takes several decades, this weakens and destroys the concrete resulting in structurally-unsound foundations that often must be replaced. The number of impacted homes is conservatively 30,000 across towns in Tolland and Windham Counties, including many in Ashford; the true number will not be known for decades as pyrrhotite decomposition take years to develop.
Pyrrhotite in concrete is associated with a quarry located in Willington, Connecticut that produced crushed rock aggregate from a geologic rock formation known as the Brimfield Formation. This formation extends in a northeast-trending belt from Willington, across the northwest corner of Ashford (including the IID zone), and north through Stafford and Union into Massachusetts. Any number of geologic maps show the locations and mineralogy of this formation, from Emerson’s 1917 description of the Brimfield Schist as a “sulfidic mica schist”, to Yale’s John Rodgers 1985 Bedrock Geological Map of Connecticut that describes the formation’s members as “thinly layered fissile sulfidic and graphitic sillimanite schist and rusty-weathering gneiss of similar mineralogy”, “rusty-weathering semipelitic gneiss and pelitic schists”, “Moderate-reddish-brown weathering medium- to coarse-grained ... gneiss and schists” [emphasis mine to highlight the long-standing awareness of iron sulfide minerals in this formation,] to several Connecticut bedrock geology maps available online including the CT DEEP website on pyrrhotite and crumbling foundations: (https://portal.ct.gov/DEEP/Geology/Pyrrhotite-and-Crumbling-Concrete).
The presence of pyrrhotite in bedrock by itself does not necessarily preclude development and construction; many small buildings throughout Ashford and surrounding towns are underlain by the Brimfield
Formation. The problem arises for construction of buildings with very large footprints on sites of steep topography building (such as proposed in this zoning amendment). To create a level site for a building of the size allowed in this proposed zoning change (1.1 million square feet) will require substantial excavation intothe underlying bedrock. Examination of an elevation map of the IID zone and proposed site reveals an approximately 100’ elevation difference from the lowest to the highest point; as such many feet from the top elevation of the site will be removed and the bottom portions will need to be filled in and held back by high retaining walls. Several smaller buildings, even if of equal total footprint, would not require such substantial excavation into the underlying bedrock. If constructed as a balanced site, this pyrrhotite-bearing excavated rock will be retained on site, building up lower elevation sections of the property and (according to the proposal) used to create a level base for development. Any issues resulting from excavating this bedrock and using it in construction at this site may not be revealed for a decade or more, but could include buckling and settling of the site’s surface, retaining walls and building foundation; iron and other metal contamination in the soil; and acid released into surface and groundwater.
I strongly suggest that the Ashford Planning and Zoning Commission reject this proposed zoning amendment on the grounds that construction of a very large building on the pyrrhotite-bearing Brimfield Schist will prove problematic in the future, as its use in home foundation construction has already proven devastating for many local homeowners.
Sincerely,
Steven Petsch, Ph.D.
148 Seckar Road, Ashford CT, 06278