A Definition

Many types of mass movement of natural material are included in the general geologic term “landslide.” However, for purposes of these guidelines the term will be restricted to mean those mass movements where there is a distinct surface of rupture or zone of weakness that separates the slide material from more stable underlying material.

Such slides involve en masse downward and outward movement of a relatively dry body of rock and/or surficial material in response to gravitational stresses. Other varieties of landslides that are treated separately in these guidelines include: 1) rockfall which involves either direct fall or forward rotation of a rock mass followed by free-fall and/or rolling, bounding, or rapid sliding motions with only intermittent contact with the ground surface; and 2) mud flows and closely related phenomena which involve movement by viscous flow of material with high water content and which may lack a distinct surface of separation between the moving mass and underlying more stable material.

Landslides as defined above include two major types: 1) Rotational slides which refer to all landslides having a concave upward, curved failure surface and involving a backward rotation of the original slide mass; and 2) translational slides in which the surface of rupture along which displacement occurs is essentially planar. Either type of landslides can involve various combinations of bedrock, broken bedrock, and unconsolidated surficial material, and the displaced material in either type of slide may be either greatly deformed or nearly intact.

Rate of movement of landslides varies from very slow to very rapid. They may be extremely small in extent or measurable in miles. Volumes of material involved may range from a few cubic feet to millions of cubic yards. Landslides result from some change in the physical condition of an unstable slope area (see section of guidelines on potentially unstable slopes). Such changes may be natural or man-induced. Some of the major mechanisms that initiate slides are: removal of the toe or lower end of a potentially unstable slope (commonly known as “day-lighting”); removal of lateral support material adjacent to an unstable area; placement of additional material on the upper portion of an unstable area (commonly referred to as “loading”); weakening of clay or other fine-grained materials by wetting; weakening of natural cohesive forces by ground water circulating along potential failure surfaces; or decrease of stability by excessive pore water pressures within the slope-forming materials or along a potential failure surface. Other mechanisms include; redistribution of mass by erosion and deposition; chemical and physical weathering, which may weaken slope materials; earthquake vibrations and release by erosion of stresses related to active faulting or past stresses “locked in” rock materials.

Many of the above-described disturbances that are capable of inducing land sliding of unstable slopes can result from activities of man. The most common activities of man that can produce land-sliding include: Excavations such as road cuts, quarries, pits, utility trenches, site grading, landfill operations, stockpiling of earth, rock, or mine waste; alteration of natural drainage which may lead to increased runoff and erosion or to local ponding and saturation of potentially unstable slopes; and vibrations from blasting or heavy vehicular traffic.

Actual landslide movement can occur in several ways. It may be rapid, and of short duration, after which natural equilibrium (stability) of landslide material is achieved. It may consist of intermittent periods of active movement, separated by relatively inactive periods. A third possibility involves slow, continuous move-slide material may involve movement that can be measured in a few feet, or it may involve displacement measurable in hundreds or thousands of yards, and in some cases even miles. Differential movement may also occur within an active slide mass. Isolated smaller slides may take place within the body of a large slide during its movement (multiple sliding), or they may occur after much of the larger slide has stabilized. Also, the reverse is true, where large parent slides include, or incorporate, smaller slides.

Permanent features that commonly aid identifying the presence of old slides are the appearance of a main scarp and a corresponding bulge of landslide deposits on hillside. These features or relict anomalous slope changes often remain for many years as evidence of past instability. It should be noted that all such breaks in the natural profile of a hillside are not necessarily remnants of landslide scarps or deposits, and that determination of slope stability requires study by an experienced engineering geologist.

Rotational slides can occur anywhere that the following conditions are present, and in the necessary combination to promote sliding: 1) slopes sufficiently steep to allow lateral downslope movement of materials in response to gravity; 2) gravitational stress sufficient to move such material; 3) presence of unstable material susceptible to sliding; 4) underlying zone of weakness as a potential surface of rupture; 5) introduction of a disturbing factor – natural or man-made – sufficient to initiate instability and movement.

A translational landslide is characterized by a planar surface of rupture, and frequently by little deformation of slide material. Physical relationships prevalent in this type of slide are the presence of relatively competent materials above and beneath a planar zone of weakness along which sliding occurs. This condition is quite common in nature and may be the result of various combinations of materials and/or physical conditions. Translational slide material may range from fairly loose unconsolidated soil to extensive slabs of hard, resistant rock. Movement of translational slide material may be initiated by a variety of conditions, which are listed under general description of factors tending to produce land sliding.

The same criteria outlined above as prerequisites for rotational sliding to occur, apply to translational gliding, with the exception of item 3. In contrast to rotational slides, the entire slide mass in a translational slide need not necessarily be weak, unstable material itself – there may be very thin zone of weakness such as thin layer; bedding, joint or foliation plane; or the surface separating weak surficial material from underlying competent material.

Rotational Slide Terminology

Main scarp: steep undisturbed ground surface above the highest part of the slide, resulting from downward movement of slide material.

Minor scarp: steep surfaces in slide material resulting from differential movement within the body of the slide.

Crown: in-place material just above the main scarp.

Head: uppermost part of slide material along the contact between the main scarp and the slide material.

Transverse cracks: tension cracks more or less perpendicular to the direction of slide movement, generally resulting from downward and outward movement of slide material over a hump in the rupture surface.

Radial cracks: tension cracks resulting from lateral spreading of unconfined slide material.

Tip: furthest forward extension of slide material.

Toe: furthest forward margin of slide material.

Foot: contact between original ground surface, and lowermost extension of surface of rupture.

Surface of rupture: projection of main scarp surface beneath the slide mass.

Right flank: right extent of slide as viewed from the crown, looking down onto the slide.

Left flank: left extent of slide as viewed from the crown, looking down onto the slide.

Prevailing slope: direction of predominant ground surface slope in undisturbed area.

Original ground surface: undisturbed ground surface surrounding disturbed slide area.

Longitudinal fault zone: faulting resulting from differential forward progress of downward moving slide material.