Large amounts of soil and rock enter streams as a result of landslide activity, thus reducing the potability of the water and its ability to support fish and aquatic plants. Biotic destruction by landslides also is common.

Soil creep is a slow and long term mass movement. The combination of small movements of soil or rock in different directions over time is directed by gravity gradually downslope.

Mass wasting, which is sometimes called mass movement or slope movement, is defined as the large movement of rock, soil and debris downward due to the force of gravity. The causes of mass wasting include an increased slope steepness, increased water, decreased vegetation and earthquakes.

Increased water content within the slope is the most common mass-wasting trigger. Water content can increase due to rapidly melting snow or ice or an intense rain event. Mass-wasting events are classified by type of movement and type of material, and there are several ways to classify these events.

rockfall

Humans can contribute to mass wasting in a few different ways: Excavation of slope or its toe. Loading of slope or its crest. Drawdown (of reservoirs)

47 Cards in this Set

Mass wasting occurs more readily in rainy seasons for this reason. It turns out that a small amount of water in the soil can aid slope stability because of increased surface tension. Too much water, of course, eliminates any effects of surface tension.

How does water affect mass wasting processes? Water in pores and cracks displaces air, so water adds to the mass of soil and broken rock on a slope. If pores and cracks are saturated (filled with water), the pore pressure tends to push the material particles apart, further promoting failure and downslope movements.

The types of mass movements caused by the above factors include: the abrupt movement and free fall of loosened blocks of solid rock, known as rockfalls; several types of almost imperceptible downslope movement of surficial soil particles and rock debris, collectively called creep; the subsurface creep of rock material.

Erosion is the physical removal and transportation of weathered material by water, wind, ice, or gravity. Mass wasting is the transfer or movement of rock or soil down slope primarily by gravity.

e) Spreads: A spread is an extension of a cohesive soil or rock mass combined with a general subsidence of the fractured mass of cohesive material into softer underlying material. The rupture surface is not a surface of intense shear. Spreads may result from liquefaction or flow (and extrusion) of the softer material.

Trees with curved trunks are often signs that the hillside is slowly creeping downhill. Creep is the imperceptibly slow, steady, downward movement of slope-forming soil or rock. Creep is indicated by curved tree trunks, bent fences or retaining walls, tilted poles or fences, and small soil ripples or ridges.

There are three stages of fatigue fracture: initiation, propagation, and final rupture.

A quick analysis of the fracture surface of a fatigue failure will often show features casually referred to as “beach marks”. These indicate the propagation of the failure from the initial cracks. Once the crack size has reached a critical level, it will propagate very rapidly until the fracture is complete.