The April 1996 Rockland Landslide

Introduction

This report describes the results of geological, geophysical, and geotechnical investigations made at the site of the April 1996 Rockland landslide. A team of geologists from the Maine Geological Survey, the University of Maine Department of Geological Sciences, and the geological consulting firm Robert G. Gerber, Inc., made repeated visits to the site in order to determine the following:

the potential for further slope failures at the immediate site,
the need to evacuate additional homes,
the relationship of this event to previous landslides nearby,
the importance of more gradual bluff erosion caused by the sea, and
the cause(s) of the landslide

so that the public and government officials would have better information to make decisions about public safety, infrastructure, and remedial actions.

Overview of the Landslide Event

Early in the morning of April 16, 1996, there was a landslide along the north shore of Rockland Harbor (Figure 1). The slide occurred along a steep bluff adjacent to Samoset Road. The bluff top was about 50 feet (15 m) above sea level and the base of the bluff slope ended at the high water mark. Slumping of the bluff occurred progressively in a series of discrete landslide events. This style of landslide is called a retrogressive landslide because it works gradually backward from the slope into the bluff. Most of the initial motion occurred between 1:15 AM and about 4:00 AM during a falling tide immediately preceding a new moon the following day, and prior to heavy rains in the afternoon of April 16. Maine Geological Survey geologists surveyed the site for signs of additional imminent failure at 1:00 PM on April 16 and consulted with city officials and emergency personnel.

The initial slide motion on April 16 awoke Douglas and Susan Gerrish, residents of 23 Samoset Road. They notified local authorities and removed two vehicles from their garage. The Rockland Fire Department evacuated Dorothy Smalley from the house next door and alerted the neighboring residents. Portions of the Gerrish home and the Smalley residence and garage fell into the main body of the landslide. These structures were about 75 feet (23 m) landward of the top edge of the bluff prior to the landslide (Figure 2A). A topographic survey by geologists from the Maine Geological Survey and University of Maine determined that the slide displaced more than half an acre of backyard from the Smalley and Gerrish lots (Figure 2B, Figure 2C, and Figure 2D). When it was over, the slide had moved horizontally over 400 feet (120 m) onto the mudflats of the intertidal zone and had disturbed a total area of 3.5 acres (1.4 hectares). Nonetheless, even a month after the slide, some of the Gerrish home remained in place with a portion overhanging the main scarp.

For a month following the major slide event, smaller blocks continued to fall from the steep landward slopes. These continued slope failures resulted in enlargement of the affected area by landward progression of the vertical scarp. By landward retreat, the main scarp moved to within 15 feet (5 m) of a sewer main on the seaward side of Samoset Road. Possible disruption of a city water main buried beneath the landward shoulder of Samoset Road was also of public concern. In order to measure continued movement and to assess the potential for additional failures, the City of Rockland hired a geotechnical consulting firm, R.G. Gerber, Inc., that installed three inclinometers at the head of the slide near Samoset Road. The inclinometers were installed after drilling through the surficial deposits down to solid bedrock, and gave direct information about the subsurface geology at the landslide site. R. G. Gerber, Inc., also engineered the stabilization of the area during the Fall of 1996. As of the writing of this report, the total cost in terms of lost property value and cost to the city for emergency activities, evaluation of the two homes destroyed and loss of land, clean up, and engineered stabilization was approximately $710,000 and is expected to exceed that when work is completed.

This field geology study and the analysis of other events for similar areas in coastal Maine led the authors to conclude that several factors in combination contributed to the April 1996 Rockland landslide. The necessary preconditions in this case appear to be the following:

a thick section of poorly-drained, fine-grained sediment,
a high water table (thick saturated zone) near the land surface , and
a high and steep-sloped (unconfined) bluff toward open water.

Contents Introduction Description Other slides Factors Potential Action Summary References Appendix A Appendix B Appendix C Plate 1

Last updated on October 6, 2005