Norridgewock Landslide: July 9, 2009

Introduction and Backround

On Thursday, July 9, 2009 the Maine Geological Survey was notified by the Maine Emergency Management Agency (MEMA) that a landslide had occurred along the Sandy River in Norridgewock, Maine (Figure 1 and Figure 2). MEMA requested a site visit by Maine Geological Survey staff to evaluate this slide. This would help assist MEMA and the town of Norridgewock in their efforts to remediate the slide and to help prevent further slope failure at this location and other areas with similar conditions in the town of Norridgewock.

Geology

Surficial geology

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).

Figure 3

Figure 4

Bedrock geology

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.

Site Investigation

Site location

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.

Figure 5

Figure 6

Site condition

Figure 7

Upon inspecting the slide site, and areas both north and south of the site, it appears that slope failure has occurred in the past as evidenced by fallen trees, old scarps, and leaning and bent trees (Figure 7). These leaning and bent trees are indicators of slow and imperceptible slope movement called creep, which is a precursor of possible future slope failure.

Slide description

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:

It is likely that landslides will occur where they have in the past.
Landslides are likely to occur in similar geological, geomorphological, and hydrological conditions as they have in the past.

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

Slope oversteepening - slope along the river above and below this slide exhibits extreme slope angle, a major cause of landslides.
High relief - the area of the slide has over 20 feet of relief. This overburden causes increased stress on any underlying marine clay, increasing the risk of slope failure.
Surficial geology - the area is underlain by the highly unstable marine clay of the Presumpscot Formation. With increased precipitation, the shear strength of marine clay decreases, making it more susceptible to failure.
Precipitation/drainage - heavy precipitation can trigger landslides. We believe precipitation over time seeped into the marine clay deceasing its shear strength, leading to subsequent slope failure.
Tension cracks - landslide scarps and slope failure begin as tension cracks. Field evidence shows a tension crack in the Sandy River Road above the smaller slide.
Unstable soils - leaning and bent trees indicate prior slope creep and movement.
Bluff-toe erosion - high flow of the Sandy River eroded sediments at the base of the slope.

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:

Slope - greater than 5%
Relief (height) - greater than 20 feet
Geology - underlain by the marine clay of the Presumpscot Formation

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.

Suggestions

Riprap and fill the areas of these slides between the Sandy River Road and the Sandy River to stabilize the slope to prevent further failure. This would be similar to what Maine Department of Transportation did at the Greenbush landslide site along Route 2.
Other areas in Norridgewock where similar conditions exist should be evaluated and mapped as to their susceptibility for future landslides. This would help mitigate the landslide problem and reduce future expensive remediation efforts.

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