Slump

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The word slump is most commonly used as a colloquial description of a landslide with a markedly curved and concave-upward slip surface, which results in rotational movement of the mass above the slip surface. This stands in contrast to landslides with more nearly planar slip surfaces, above which the sliding motion is predominantly translational rather than rotational. Most landslides exhibit both kinds of motion, so the distinction is based on the predominant type of motion. Rotational slides tend to occur in slopes that are, at least mechanically speaking, relatively homogeneous whereas translational slides tend to occur in slopes that contain mechanical discontinuities such as steeply dipping bedding or soil-bedrock contacts that can evolve into slip surfaces.

Like the term mudslide, slump is frequently used but is not defined in the classification system used by most landslide specialists. Depending on the type of earth material involved, a slump can be properly described as a rock slide, debris slide, or earth slide with predominantly rotational movement.

The curved slip surface and resulting rotation of the material in a slump cause strata within a slump mass to be tilted backwards relative to undeformed strata beneath the slip surface. This back-tilting can produce a topographic depression that collects water and sediment, which can be used as a criterion to identify old slumps in the field. The material within a slump mass is in most cases remarkably undeformed, albeit rotated, which is a characteristic that distinguishes slides from flows.

A less common use of the term slump is in reference to the downslope flow of unlithified submarine sediments, which frequently occurs along topographic concavities such as submarine canyons. In this case the phenomenon would in most cases be properly described as a submarine debris or earth flow rather than a slide.

As is the case for all landslides, slumping begins when there is an imbalance between resisting and driving forces in a potentially unstable slope. If the slip surface is very nearly circular (as opposed to curved but non-circular), which is an idealized situation that does not often occur, stability can be assessed by comparing resisting and driving moments acting about a center of rotation. The resisting force (or moment) is a function of the shear strength of the soil or rock integrated over the area of the potential slip surface as well as any engineered structures put in place to increase slope stability. The primary effect of water within the slope is to reduce the normal stress acting across the potential slip surface, thereby reducing the shear strength along the surface. The driving force (or moment) is due primarily to the component of the slump block weight acting parallel to the potential slip surface, the weight of imprudently designed or constructed structures built on the slope, and seismic shaking. Movement can be triggered if the ratio of driving to resisting forces (or moments) is altered by adding water to the slope or by changing its geometry during construction projects.

Debris Flow; Mass Movement; Mass Wasting