Research

Shear Strength and the Stability of Natural Slopes in Sensitive Glaciomarine Clays of Coastal Maine

November 22, 1868 Landslide along the Presumpscot River, Westbrook, Maine

“MGS Landslide Study Draws Praise”

“Three Maine Geological Survey (MGS) open-file reports in Cumberland County are “among the best produced by any state as part of the U.S. Geological Survey-State Cooperative Landslide Program” according to the U.S. Geological Survey.”

Documented Historical Landslides from Around the World

The Great Landslide of 1868, Westbrook, Maine

Westbrook_Landslide-01

Westbrook_Landslide-02

September 1983 Gorham Landslide, Gorham, Maine

Gorham_Landslide

Failure Pattern Prediction Criteria

Stability of Natural Slopes in the Presumpscot Formation

Maine Geological Survey Open File Report No. 90-24

Steven C Devin and Thomas C Sandford (1990)

Link to Full Report

Identification of landslide prone sites in Maine is imperative in order to maintain the safety of affected developed areas and future developments. Large landslides in Gorham, Maine in 1983 and Rockland, Maine in 1973, a number of smaller landslides, and evidence of historical and prehistorical landslides indicate that some areas of Maine, including developed areas will be susceptible to future landslides. Past landslides have occurred in deposits of the sensitive glaciomarine clay known as the Presumpscot Formation which covers the populated and rapidly developing eastern and southern Maine.

This paper examines two southern Maine sites – one at Brunswick and another at Westbrook- both of which have been subject to mass movements in the past and are suspected to be vulnerable to future mass movements. The landslides in the Brunswick area are single slides, whereas past slides in the Westbrook area included major flowslides such as the one which occurred in November, 1868, affecting an area of thirty-four acres. The flow or spreading slides, such as have occurred in Westbrook, are potentially far more destructive. This paper examines the geological characteristics and soil properties at the two sites with the purpose of identifying the causes of landslides, particularly flowslides or landslides that spread by retrogression. Historical investigations, subsurface investigations, laboratory testing and engineering slope stability analyses were performed in examining the two sites.

Although the mineral constituents and pore fluid of the deposits at each site were similar, the different structure of the deposit at each site led to the different stability behavior. The material at Westbrook had a higher water content, lower undrained peak shear strength, and a higher strength loss upon remolding than the deposit at Bunganuc bluff. Higher water content, lower peak strength, and higher strength loss have been related elsewhere to flow slides or retrogressive slides.

It is recommended that sites in Maine with deposits exhibiting similar geotechnical, geological and geomorphological characteristics as Westbrook and Bunganuc bluff be identified for possible site stability problems. It is also recommended for sites with possible stability problems that considerations of disturbance, loading and erosion by construction or natural causes be integrated into maintenance, planning and development decisions for these areas.

Generalized Depositional Model

Map of November 22, 1868 Flowslide Extents, Westbrook, Maine

Flowslide Potential of Natural Slopes in the Presumpscot Formation

Steven C Devin

A Thesis Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science (in Civil Engineering); The Graduate School, University of Maine, December 1989

Identification of landslide prone sites in Maine is imperative in order to maintain the safety of affected developed areas and future developments. A large landslide in 1983 in Gorham, a number of smaller landslides, and evidence of historical and prehistorical landslides indicate that some areas of Maine, including developed areas will be susceptible to future landslides. Past landslides have occurred in deposits of the sensitive glaciomarine clay known as the Presumpscot Formation which covers the populated and rapidly developing areas of eastern and southern Maine.

This thesis examines two southern Maine sites – one at Brunswick and another at Westbrook- both of which have been subject to mass movements in the past and are suspected to be vulnerable to future mass movements. The landslides in the Brunswick area are single rotational slides, whereas past slides in the Westbrook area included major flowslides such as the one which occurred in November, 1868, affecting an area of thirty-four acres. The flowslides, such as have occurred in Westbrook, are potentially far more destructive. The results of an earlier study performed at the site of a retrogressive, or spreading, landslide in Gorham are utilized for comparison. This thesis examines the geological characteristics identified by Belknap et al. (1988) and soil properties at each site with the purpose of identifying the causes of landslides, particularly flowslides or landslides that spread by retrogression. Historical investigations, subsurface investigations, laboratory testing and engineering slope stability analyses were performed in examining the two present sites.

Although the mineral constituents and pore fluid of the deposits at each site were similar, the different structure and associated geologic interpretation of the deposit at each site led to the different stability behavior or failure pattern exhibited at each site. The material at Westbrook, interpreted as belonging to geologic unit GM-P, had a high liquidity index and a much higher strength loss upon disturbance than the deposits at Gorham or Bunganuc bluff in Brunswick. The deposit at Gorham has been interpreted as belonging to the geologic unit GM-P and has a lower liquidity index and less loss upon disturbance than does the deposit at Westbrook. The Presumpscot Formation at Bunganuc bluff has been interpreted as belonging to geologic unit GM-D and has the lowest liquidity index and loss of strength upon disturbance of the three sites considered. The results of this study indicate that the failure pattern can be correlated to the geologic interpretation, liquidity index, and measured field vane shear strength for each deposit.

Prediction criteria are presented to provide a working means by which the flowslide potential of natural slopes in the Presumpscot Formation may be evaluated. It is recommended that sites in Maine with deposits exhibiting similar geotechnical, geological and geomorphological characteristics be identified for possible slope stability problems.

Boring Log and Testing Summary – 1868 Flowslide, Westbrook, Maine

Presumpscot Glaciomarine Ponded (GM-P) Facies