The April 1996 Rockland Landslide

Other Maine Landslides

Rockland, January 25, 1973

During the early morning of January 25, 1973, a landslide covering about three acres occurred within tens of feet of two homes on Waldo Avenue in Rockland (Figure 1). Reports of this event were published in local newspapers, and the Bangor Daily News and Portland Press Herald. There are no known technical reports describing the landslide at the time of the event. However, newspaper accounts and the general descriptions and photographs of the event by geologists who visited the site (including geologists from the Maine Geological Survey and Bowdoin College; one author of this report, Belknap, was a Bowdoin College student at the time) indicate the slide was similar in size and displacement to the 1996 slide. The Portland Press Herald (February 7, 1973) reported that weather conditions at the time of the 1973 slide were warm temperatures (January "thaw"), and that drainage was "excessive," and that in the opinion of geologists who visited the site, the slide was due in part to these conditions.

Photographs of the 1973 slide (Plate 1) show that it had the classic features of a retrogressive (headward) landslide, with a steep headscarp and slumped blocks rotated toward the headscarp. Toward the slide toe, the surface of the slumped material appears to have flowed, as though it had bulldozed across the land surface. The slide is described in an inventory and bibliography of Maine landslides as a complex lateral spread / soil flow (Novak, 1987a; Figure 10). Some newspaper reports of the event incorrectly quote Arthur M. Hussey II, geology professor of Bowdoin College who visited the site, as saying that a smooth rock surface at the base of the clay provided the glide surface. Our seismic work in the area of the January 1973 slide shows bedrock to be at least 10 feet below sea level in this area. Professor Hussey (personal communication, 1996), who visited the 1973 landslide, states that he pointed out that while bedrock was exposed nearby along the coast, it was not exposed at the base of the slide and probably was not the slip surface upon which the slide moved.

Other than newspaper articles, the only published report on the 1973 landslide is by Time and Tide Resource Conservation and Development, entitled "Rockland Shore Erosion Critical Area Measure, Final Report, April 1978" (anonymous author). This report outlines the history of local, federal, and state involvement regarding potential remediation activities of the 1973 slide. Results of findings and conclusions in the report include an outline of potential causes of the 1973 slide as well as corrective measures. The potential causes included removal of lateral support or oversteepening of the bluffs, additional surface weight by placement of fill or increased moisture, blockage of already slow drainage, introduction of excess water through septic systems, and loosely backfilled water main, storm drain, and sewer main trenches. Recommendations for several corrective measures included appropriate city ordinances preventing excess water drainage into the ground from septic systems, storm drains, and gutters; upland water surface runoff control; bank shaping and slope reduction; protection from wave erosion by rip-rap; loading of the slide toe to provide weight and increased stability; blanketing the face of the bank with free draining material; establish suitable vegetative cover; and engineering deep internal drainage systems. The recommendations were underscored by the statement that an engineering consultant with proven expertise in landslide prevention and suitable design control be obtained by the City of Rockland.

Gorham, September 28, 1983

Novak (1987b) and Sanford and Amos (1987) provide an account of the 1983 Gorham landslide. This landslide occurred in a bluff of marine sediments at the confluence of the Stroudwater River and Indian Camp Brook. Seven acres (3 hectares) of land slid to the south and southwest into the river and the brook, taking with it a house and garage, several vehicles, a tank truck, and a well drilling rig. The total area affected by the slide was about 12 acres (5 hectares). This slide is classified as a complex type, having components of translational slide (blocks move out or down along a planar surface without rotation), a rotational slump (blocks which rotate about an axis parallel to the slope and along a concave-up slip surface), and an earth flow (a fluidized failure lacking distinct blocks). The slide failure plane was calculated to be at a depth of about 42 feet (13 m), within a thick section of Presumpscot clay.

Factors contributing to this slide include the following: geomorphology - the slope was unconfined on two sides; thickness of clay - bluffs of 20-25 feet (6 - 8 m) high in a section of marine sediments up to 70 feet (21 m) thick; load from fill - there is disagreement concerning the degree to which fill contributed to this slide. For the size of the slide, the estimated 480 tons of fill used for the driveway and building site are relatively small, but it is possible that this additional load could have contributed to the initiation of a small block of the slide. Also, vibrations from the construction traffic and drilling of a nearby bedrock water-well drilling operation may have contributed as a trigger, but only because the site was already unstable. The landslide occurred shortly after the drill rig had penetrated approximately 5 feet (1.5 m) into the bedrock.

Abnormal precipitation conditions are not considered to have been a significant factor in this slide. September is a relatively dry time of year and significant rains had not fallen at the site in a week. There was some evidence of water expulsion during or after the slide, but the importance of precipitation as a slide trigger is speculative. However, liquefaction by the well-drilling operation either by introduction of water to the subsurface, or by vibrations of the drilling or construction operations may have been a factor in the cause of the slide.

Norridgewock Landfill Slide, August 14, 1989

Although this slide occurred in an artificial mound of compacted trash and cover materials (R.G. Gerber, Inc., 1991; Reynolds, 1991), it is useful to illustrate some of the factors contributing to landslides. Underlying the landfill site is 50 to 60 feet (15 - 18 m) of marine clay and silt. On top of this clay, the landfill operator had added about 70 feet (21 m) of compacted trash and cover material (sand and gravel) covering over several acres.

When the landfill gave way in the early morning of August 14, it did so as a retrogressive failure with individual blocks moving to the southwest, west, and northwest as they slid on the underlying clay. A detailed investigation followed the slide and concluded that a series of interrelated events resulted in the failure:

Excavation of stiff clay from the toe of the landfill slope. This removed some lateral support from the soft clay and silt beneath the landfill.
Storage of materials on top of the landfill. Excavated materials were stored on the crest of the landfill to await future use as capping material for an expanded portion of the landfill. This added weight to the load on the materials within and beneath the landfill.
Addition of waste. In addition to the stockpile of cover material, waste continued to be added to the landfill, further increasing the load.
Trench at toe of landfill. To intercept leachate from under the landfill, a trench was excavated at the toe of the landfill. This further reduced the lateral resistance that supported the soft clay and silt under the landfill.
Heavy rain. The failure followed a 10-day period in which a total of 5 inches of rain fell. The rain saturated soil cover on the landfill and added further load to the system.

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

Last updated on October 6, 2005