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Norridgewock Landslide: July 9, 2009
Introduction and Backround
The normal layering of earth materials in the area of this landslide is bedrock overlain by glacial till overlain by marine clayey silt known as the Presumpscot Formation. The clay of the Presumpscot Formation is known for its susceptibility to slope failure. Between 80 and 99 percent of identified landslides in Maine occur in areas underlain by glaciomarine deposits, in particular the Presumpscot Formation.
The toe and base of the landslide extend out into the Sandy River (Figure 3). Holocene stream alluvium deposits overlie bedrock within the Sandy River. The material observed on the face (scarp) of this landslide shows approximately 2-3 feet of artificial fill (road material) overlying approximately 20 feet of Presumpscot clay (Figure 4).
The bedrock underlying the sediments at this landslide location is a Devonian age muscovite-biotite granite. These rocks intrude through Silurian age metamorphic rock of the Sangerville Formation.
The July 9, 2009 landslide occurred in the town of Norridgewock, Maine along the Sandy River Road on the east side of the Sandy River (Figure 5). The slide occurred along a sharp bend on an unpaved section of the Sandy River Road, where the road encroaches upon the Sandy River (Figure 6). The Sandy River at this site appears to have been eroding and undercutting the river bank, causing oversteepening of the slope.
Two adjacent landslides occurred where the base of the slope along the Sandy River was undercut, causing the slope to oversteepen. This oversteepening of the slope, along with the high precipitation which "lubricated" the underlying marine clay, caused the slope failure. It appears that when the larger slide began, it activated a smaller adjacent slide. The large slide is approximately 40 feet across with a relief (height) of over 20 feet (Figure 8). The smaller slide is approximately 15 feet across with a relief of only 10 feet. The large slide butts up against the unpaved Sandy River Road and slightly cuts into it (Figure 9). The near vertical scarp is approximately 10 feet high, with about 3 feet of artificial fill (road material) overlying marine clay. The toe or base of the slide extends out into the Sandy River with toppled trees and exposures of marine clay (Figure 10 and Figure 11).
Along the road above the area of the smaller slide, tension cracks were observed (Figure 12). These are strong indicators of possible future slope failure that could be easily triggered either by heavy trucks travelling along the road or a sudden high precipitation event.
These landslides appear to be caused by the erosional effects of the Sandy River and the recent long-term rains that have occurred in the area. The Sandy River at this locale has been eroding and undercutting the bank over time. Due to the high precipitation over the last few months, the water level and flow of the Sandy River has been high, which increased the erosional power of the river. Normally, high flow occurs in the spring and at times of sudden downpours. However, the constant rains of the last few months have caused higher stream flows which increased the erosion and undercutting along streams and rivers in Maine. The combination of increased stream flow and high water levels increases the chances for landslides and slope failures.
The marine clay of the Presumpscot Formation becomes less stable when water saturation increases. This decreases the shear strength of the clay and makes it more susceptible to failure. The continual rains in Maine have made areas underlain by the Presumpscot Formation more susceptible to landslides and slope failure.
Vulnerability of the Norridgewock landslide site to further slope failure
There are two basic principles to note in determining areas at risk for landslides:
The Norridgewock landslide fits both of these principles. Areas above and below (north and south) of this site along the Sandy River show evidence of past landslide activity and display similar conditions (risk factors) of high relief, steep slope, and being underlain by glaciomarine deposits (marine clay).
Landslide precursors found at the Norridgewock location
Future site stability
The Norridgewock landslide site exhibits 3 primary risk factors that are indicators of possible landslides. As the number of significant risk factors increases, the chance that a landslide will occur increases. With this site exhibiting three of these risk factors, the landslide susceptibility or risk of landslides occurring is moderate to high. The three significant risk factors observed at this location are:
The large landslide has become more stable with the slide's slope becoming less steep. The near vertical scarp along the road remains very susceptible to failure, due to the fact that the material is comprised primarily of marine clay and silt. With more rain and/or heavy trucks causing vibration along the road, failure could happen very easily.
The area above the smaller landslide shows evidence of tension cracks which are indicators of possible future slope failure.
Text and photos by Michael E. Foley.
Originally published on the web as the July 2009 Site of the Month.
Last updated on August 6, 2009
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