The April 1996 Rockland Landslide

Factors Contributing to Landslide Potential

Factors which contribute to landslide potential of an area may be divided into natural factors and human factors. Natural factors most often cited in the literature are material type and thickness, geomorphology, precipitation, and undercutting of slopes. Human factors include modification of slopes, overloading slope tops, modification of drainage, and removal of vegetation. Much of the information in this section has been taken from Pomeroy (1982), Schuster and Krizek (1978), Keefer and Johnson (1983), and Sidle and others (1985). Engineering aspects and case studies of geotechnical problems associated with the Presumpscot Formation in Maine are presented in Andrews and others (1987).

Natural Factors

Material type and thickness. Landslides and mass movements of all types tend to occur in geological materials that are poorly consolidated (loosely packed) and fine grained in texture. Investigations of earth flows (Keefer and Johnson, 1978) show that they occur in a variety of grain sizes: sand, silt, and clay. Silt and clay predominate, and these materials tend to be of lower strength and more easily deformable than coarser materials. Sediment thickness or overburden thickness is a significant factor in determining slide potential. With increasing thickness of the unit or overlying units, there is an increase in load. Failure occurs when this load exceeds the internal strength of the material.

It should be noted that not all sediment materials within the Presumpscot Formation have the same strength. Furthermore, once sediment has been disturbed or deformed it becomes weaker than the original, undisturbed unit, a material property called sensitivity. A study of the 1983 Gorham landslide (Amos and Sandford, 1987) found that the Presumpscot Formation at that site has a lower strength and higher sensitivity than the Presumpscot Formation found at Bunganuc Point, Brunswick, another location in Maine where landslides are common. This small difference in strength properties may account for the fact that the bluffs at Gorham that were only 20 to 25 ft (6 - 8 m) high failed in a catastrophic retrogressive landslide, while the bluffs at Bunganuc Point that are almost 40 ft (12 m) high have not failed in such a catastrophic manner. Because of the variability of natural sediments, it is difficult to predict the strength of the Presumpscot Formation from place to place without careful measurements by a soils engineer.

Geomorphology. Slides occur where there is a steep slope on an unconfined face in material of low strength. Where there is a more gradual slope, more weight at the base of the slope helps to confine the pressure at depth. In the case of the Rockland slides, failure was at the base of a relatively steep slope where pressure was not constrained by weight of slope material. However, some of the more fluid types of slides can be initiated on remarkably gentle slopes of 7% (Sidle and others, 1985). Larger rotational slides typically require at least a 12% slope for initiation. More typically, slides require slopes of at least 15% to 25% for initiation of movement (e.g. Pomeroy, 1982; Schultz and Southworth, 1987).

Precipitation. Almost universal in the literature is the recognition that precipitation can play a significant role in initiation of landslides. Heavy winter precipitation, severe late winter and early spring rains, and rapid spring thaws resulting in loss of soil strength and additional water in the system all present landslide problems in North America (see, for example Pomeroy, 1982; Schultz and Southworth, 1987; Keefer and Johnson, 1983). Precipitation from extratropical storms can be factors, even when they occur during the period in which ground conditions are usually their driest (Pomeroy, 1982; Schultz and Southworth, 1987). Additional water in the system affects the stability in two ways. First, the weight of the water is an additional load on the materials in the system: second, the pore pressure increases with the additional water and reduces the strength of the material. Once strength is reduced, gravitational forces exceed friction and slope failure begins.

Undercutting of slopes. As stream and river courses meander, slopes are undercut on curves. In the marine setting, tides, waves, and currents undercut slopes of unconsolidated material. These are normally slow, but incessant, processes, and erosion undermines the stability of slopes. Bank erosion can be extremely rapid during storms, where water level of rivers, streams, or the ocean are elevated above normal and sediment is removed rapidly.

Human Factors

Modification of slopes. Oversteepening and undercutting of slopes are common in road and facilities engineering, or bank landscaping construction. Undercutting of slopes commonly occurs to provide more flat space for buildings or roadways. This practice removes lateral support and undermines the slope, leading to landslides in many areas (Pomeroy, 1982; Schuster and Krizek, 1978). Oversteepening of slopes can occur as fill is used to extend flat areas near tops of slopes.

Overloading slope tops. The weight of fill and structures on top of a slope places additional load on the materials below. If this additional load causes the total load to exceed the strength of the material, then a slide can occur. Additional load of this nature has been identified as a contributing factor in the Gorham slide (Sanford and Amos, 1987) and in the Norridgewock landfill slide (R.G. Gerber, Inc., 1991). Modification of drainage. Factors include leaking water and sewer lines, poorly drained roads, septic systems, landscape watering, seepage from reservoirs, and others (see Terzaghi, 1960; Pomeroy, 1982; Sidle and others, 1985). These modifications can result in additional load from water weight and reduction in material strength through increased pore pressure.

Removal of vegetation. Plant roots stabilize slopes by binding soil particles. Vegetation and roots also slow runoff, thereby reducing gullying and the removal of soil. Evapotranspiration by vegetation can also reduce pore pressure at some times of year by removing ground water. Thus, removal of vegetation can promote slope instabilty. However, it is only the upper part of the section of soil that is stabilized by vegetation. Moreover, in the case of large trees, there is both reinforcement of the slope by tree roots and extra loading on the slope by the force of wind on the branches. The net effect suggests that shrubs are superior to trees for slope stabilization, or that removing large trees will actually increase slope stability somewhat. Immediately after the 1996 April landslide, the City of Rockland cut down the large trees still standing above the headscarp in an attempt to stop further retrogression of the landslide.

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

Last updated on October 6, 2005