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Brunswick, Maine Patriots' Day 2007 Landslide
On Monday, April 16, 2007, a landslide (Figure 1) and adjacent slope subsidence (Figure 2) occurred along the east shore of Buttermilk Cove in Brunswick, Maine. The slide occurred along a steep bluff (Bryant and others, 2002) adjacent to Route 24 (Gurnet Road) (Figure 3). The bluff top was about 30 feet above sea level (Figure 4). This shoreline has been mapped by the Maine Geological Survey as a "Potential Landslide Area" (Dickson, 2001; Figure 5).
This site location has evidence of prior slope failure and subsidence. The basement and foundation walls show previous cracking due to ground subsidence (Figure 6 and Figure 7). The most prominent crack runs along the entire length of the basement floor and resulted in a slight separation in the cement. The floor section on the west side of the basement towards the water shows subsidence and also drastic sloping down toward the shoreline. The back yard, which is located west of the house towards the water, exhibits extensive subsidence and sloping from prior slope failure events (Figure 8). There is evidence of 2 to 3 old landslide scarps in this same area between the building and the bluff face. A cement patio, built in the early 1970's level to the ground, now exhibits a 20-degree slant towards the water (Figure 9). The patio slab is now completely broken up and cracked by previous ground movements.
During a site visit by Maine Geological Survey geologists, two distinct areas of slope failures were observed (Figure 10). One, a large rotational landslide, occurred 90 feet north of the house in an area where runoff from both a cross-road culvert and the leach field drainage were focused. This rotational slide produced a distinct 30-foot long concave scarp with a tree-covered toe at the base. The landslide toe extended into the intertidal zone of Buttermilk Cove (Figure 11). The second area of slope movement extended from the southern edge of the main scarp, southward for approximately 90 feet. This is in the same area as the previous subsidence and slope failure on the west side of the house described above. This slope movement and subsidence consisted of fresh tension cracks and minor parallel scarps in the lawn. This movement caused the ground behind the house to slope more steeply toward the top of the bluff and water.
Upon observing the exterior and interior of the house foundation, extensive floor and wall cracks (fissures) were noted, along with a drastic sloping of the basement floor, as described above. Though some of the foundation cracks are relic in nature, many new cracks and foundation offsets appeared to be due to recent failure. Most of these cracks occur in the basement walls and corners along the waterside (west side) of the house. These features suggest an ongoing history of slope movement, subsidence, and site instability.
The normal stratigraphic sequence (layering of earth materials) in this area is:
Vulnerability of the Brunswick Landslide Site for Further Slope Failure
There are two basic principles to note in determining areas at risk for landslides:
This location fits both of these tenets. A landslide has occurred at this site, and there have been other coastal landslides in Maine that have occurred within similar geological settings.
Landslide Precursors Found at this Location
Future Site Stability
This property exhibits all of the above factors of previous slope failure (e.g., oversteepened slope, overburden loading, unstable soils, and previous subsidence). The only change in the conditions since the landslide of April 16th is in the area of the rotational slide where the slide slope has stabilized and become less steep. Although we can say with certainty that another slide or slope failure will occur at this site, it is not possible to predict when that failure would occur. It is our judgment that this site is unstable and prime for further slope failure, possibly up to Route 24.
Bryant, M., Barnhardt, W. A., Dickson, S. M., and Kelley, J. T., 2002, Coastal bluffs in the Orrs Island quadrangle, Maine: Maine Geological Survey, Open-File Map 02-201 (4.4 Mg pdf format), scale 1:24,000.
Dickson, Stephen M., 2001, Coastal landslide hazards in the Orrs Island quadrangle, Maine: Maine Geological Survey, Open-File Map 01-530 (2.6 Mg pdf format), scale 1:24000.
Text and photos by Michael E. Foley.
Originally published on the web as the November 2007 Site of the Month.
Last updated on November 5, 2007
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