Engineering geology

Translation Slides along Bedding Planes

Rockslides and other types of slides involve the displacement of material along one or more discrete shearing surfaces. The sliding can extend downward and outward along a broadly planar surface (a translational slide), or it can be rotational along with a concave-upward set of shear surfaces (a slump). A Translation Slides typically takes place along structural features; such as a bedding plane or the interface between resistant bedrock and weaker overlying material. If the overlying material moves as a single, little-deformed mass, it is called a block slide.

Translational Slides – Bedding plane

If the slip surface is straight then it is termed translational or planar.

Bedding planes are potential slip planes for landslides when they are inclined downslope and “daylight” that is; are exposed on the surface of a slope. A slope of a sea cliff with daylighting bedding planes in shale is diagrammatically shown in Figure 1. – Translation Slides

Figure 1 – Daylighting beds – Bedding planes that intersect the surface of the land on a slope are said to daylight. Such beds are potential slip planes.

Several months later in 2003 the slope failed; perhaps as a result of water added to the top of the slope where grass was planted and watered. The landslide deposits cover part of the sandy beach (Figure 2); and a catastrophe was narrowly avoided as a beach party was happening a short distance away. – Translation Slides

Translation Slides along Bedding Planes
Figure 2 – Translation slides – This slide occurred in late 2003. Failure was along a daylight bedding plane. Slide deposits cover part of the beach. (Edward A. Keller)

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. High probability of occurring repetitively in areas where they have occurred in the past, including areas subject to frequent strong earth-quakes.

Edward A. Keller

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