Engineering geology

Rotational landslide and Translational landslide

Slides

A slide is a downslope movement of a soil or rock mass occurring on surfaces of rupture or on relatively thin zones of intense shear strain. The movement does not initially occur simultaneously over the whole of what eventually becomes the surface of rupture; the volume of displacing material enlarges from an area of local failure.

Rotational Landslide

Rotational landslide

A landslide on which the surface of rupture is curved upward (spoon-shaped) and the slide movement is more or less rotational about an axis that is parallel to the contour of the slope. The displaced mass may, under certain circumstances, move as a relatively coherent mass along the rupture surface with little internal deformation.

The head of the displaced material may move almost vertically downward, and the upper surface of the displaced material may tilt backwards toward the scarp. If the slide is rotational and has several parallel curved planes of movement, it is called a slump.

Occurrence

Because rotational slides occur most frequently in homogeneous materials,
they are the most common landslide occurring in “fill” materials.


Relative size/range

Associated with slopes ranging from about 20 to 40 degrees. In soils, the surface of rupture generally has a depth-to-length ratio between 0.3 to 0.1.


Velocity of travel (rate of movement)

Extremely slow (less than 0.3 meter or 1 foot every 5 years) to moderately fast (1.5 meters or 5 feet per month) to rapid.


Triggering mechanism

Intense and (or) sustained rainfall or rapid snowmelt can lead to the saturation of slopes and increased groundwater levels within the mass; rapid drops in river level following floods, ground-water levels rising as a result of filling reservoirs, or the rise in the level of streams, lakes, and rivers, which cause erosion at the base of slopes. These types of slides can also be earthquake-induced.


Effects (direct/indirect)

Can be extremely damaging to structures, roads, and lifelines but are not usually life-threatening if the movement is slow. Structures situated on the moving mass also can be severely damaged as the mass tilts and deforms. The large volume of material that is displaced is difficult to permanently stabilize. Such failures can dam rivers, causing flooding.


Mitigation measures

Instrumental monitoring to detect movement and the rate of movement can be implemented. Disrupted drainage pathways should be restored or reengineered to prevent future water buildup in the slide mass.

Proper grading and engineering of slopes, where possible, will reduce the hazard considerably. Construction of retaining walls at the toe may be effective to slow or deflect the moving soil; however, the slide may overtop such retaining structures despite good construction.

Predictability

Historical slides can be reactivated; cracks at tops (heads) of slopes are good indicators of the initiation of failure. Figures 1 and 2 show a schematic and an image of a rotational landslide.

Rotational landslide
Figure 1. Schematic of a rotational landslide
Rotational landslide
Figure 2. Photograph of a rotational landslide which occurred in New Zealand. The green curve at center left is the scarp (the area where the ground has failed). The hummocky ground at bottom right (in shadow) is the toe of the landslide (red line). This is called a rotational landslide as the earth has moved from left to right on a curved sliding surface. The direction and axis of rotation are also depicted. (Photograph by Michael J. Crozier, Encyclopedia of New Zealand, updated September 21, 2007.)

Translational Landslide

landslide


The mass in a translational landslide moves out, or down and outward, along a relatively planar surface with little rotational movement or backward tilting. This type of slide may progress over considerable distances if the surface of rupture is sufficiently inclined, in contrast to rotational slides, which tend to restore the slide equilibrium.

The material in the slide may range from loose, unconsolidated soils to extensive slabs of rock, or both. Translational slides commonly fail along geologic discontinuities such as faults, joints, bedding surfaces, or the contact between rock and soil. In northern environments, the slide may also move along the permafrost layer.

Occurrence

One of the most common types of landslides, worldwide. They are found globally in all types of environments and conditions.


Relative size/range

Generally shallower than rotational slides. The surface of rupture has a distance-to-length ratio of less than 0.1 and can range from small (residential lot size) failures to very large, regional landslides that are kilometers wide.


Velocity of travel

Movement may initially be slow (5 feet per month or 1.5 meters per month) but many are moderate in velocity (5 feet per day or 1.5 meters per day) to extremely rapid. With increased velocity, the landslide mass of translational failures may disintegrate and develop into a debris flow.


Triggering mechanism

Primarily intense rainfall, rise in groundwater within the slide due to rainfall, snowmelt, flooding, or other inundation of water resulting from irrigation, or leakage from pipes or human-related disturbances such as undercutting. These types of landslides can be earthquake-induced.


Effects (direct/indirect)

Translational slides may initially be slow, damaging property and (or)
lifelines; in some cases, they can gain speed and become life-threatening.
They also can dam rivers, causing flooding.


Mitigation measures

Adequate drainage is necessary to prevent sliding or, in the case of an existing failure, to prevent a reactivation of the movement. Common corrective measures include levelling, proper grading and drainage, and retaining walls.

More sophisticated remedies in rock include anchors, bolts, and dowels, which in all situations are best implemented by professionals. Translational slides on moderate to steep slopes are very difficult to stabilize permanently.

Predictability

High probability of occurring repetitively in areas where they have occurred in the past, including areas subject to frequent strong earth-quakes. Widening cracks at the head or toe bulge may be an indicator of imminent failure. Figures 3. and 4. show a schematic and an image of a translational landslide.

Schematic of a translational landslide
A translational landslide that occurred in 2001 in the Beatton River Valley, British Columbia, Canada. (Photograph by Réjean Couture, Canada Geological Survey.)

Adapted of “The Landslide Handbook” By L. M. Highland, U.S. Geological Survey, and P. Bobrowsky, Geological Survey of Canada

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