J. David Rogers

INTRODUCTION TO INTRODUCTION TO SLOPE STABILITY SLOPE STABILITY

ANDANDLANDSLIDESLANDSLIDES

• Landslide susceptibility map of the continental United Landslide susceptibility map of the continental United States, taken from the U.S. Geological Survey. States, taken from the U.S. Geological Survey.

• Worldwide annual landslide losses cost between Worldwide annual landslide losses cost between US$10 and 20 billion per year, causing on average, US$10 and 20 billion per year, causing on average, about 1,000 deaths per year in the CircumPacific about 1,000 deaths per year in the CircumPacific countries (from Schuster, 2003). countries (from Schuster, 2003).

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BASIC CONCEPTS BASIC CONCEPTS OF SLOPE OF SLOPE

EQUILIBRIUMEQUILIBRIUM

Slopes Slopes in equilibriumin equilibrium with their with their environment typically exhibit environment typically exhibit concave, straight, and convex concave, straight, and convex hillslope profiles shown at left. hillslope profiles shown at left.

Slopes Slopes out-of-equilibriumout-of-equilibrium typically typically exhibit anomalous, hummocky exhibit anomalous, hummocky profiles, shown at right profiles, shown at right

Concepts of EquilibriumConcepts of Equilibrium

Stepped Stepped TopographyTopography

• Stepped topographyStepped topography is not necessarily indicative of is not necessarily indicative of slope instability, but can be structurally controlled by slope instability, but can be structurally controlled by underlying stratigraphy, as shown here. underlying stratigraphy, as shown here. Shales are Shales are most often associated with landslides. most often associated with landslides.

Out-of-Out-of-Equilibrium Equilibrium

SlopesSlopes

• Slopes that are Slopes that are experiencing experiencing rampant erosion rampant erosion are typically out-of-are typically out-of-equilibrium, and equilibrium, and exhibit truncated exhibit truncated concave slope concave slope profiles, shown at profiles, shown at left. left.

• The slope profile is also The slope profile is also influenced by runoff influenced by runoff requirements and the requirements and the relative position of relative position of controlling base level, as controlling base level, as sketched here.sketched here.

• Note change in hill profile Note change in hill profile as channel moves away as channel moves away from the ridgeline from the ridgeline

Beginning of an erosive cycle. Note slight changes in hillslope profileBeginning of an erosive cycle. Note slight changes in hillslope profile

Continuation of an erosive cycle, with gradual shift of the hillslope Continuation of an erosive cycle, with gradual shift of the hillslope profile, resulting in hummocky topography. Hummocky slopes are profile, resulting in hummocky topography. Hummocky slopes are typical of slopes experiencing landslippage.typical of slopes experiencing landslippage.

EXAMPLES OF HUMOCKY TOPOGRAPHY

Hummocky topography is usually indicative of solifluction or active landslippage