Geology Reference
In-Depth Information
Figure 11.8 Minimizing Damage from Rock Falls
A wire mesh fence along the base of this hillside of Highway 44
in California has caught many boulders and prevented them from
rolling onto the highway.
b
Two types of slides are generally recognized: (1) slumps
or rotational slides, in which movement occurs along a curved
surface; and (2) rock or block slides, which move along a more
or less planar surface.
A slump involves the downward movement of material
along a curved surface of rupture and is characterized by the
backward rotation of the slump block (
Figure 11.9). Slumps
usually occur in unconsolidated or weakly consolidated
material and range in size from small individual sets, such
as occur along stream banks, to massive, multiple sets that
affect large areas and cause considerable damage.
Slumps can be caused by a variety of factors, but the most
common is erosion along the base of a slope, which removes
support for the overlying material. This local steepening may be
caused naturally by stream erosion along its banks (Figure 11.2c)
or by wave action at the base of a coastal cliff (
Wire mesh has been used to cover this steep slope near Narvik
in northern Norway. This is a common practice in mountainous
areas to prevent rocks from falling on the road.
a
Figure 11.10).
TABLE 11.2 Classifi cation of Mass Movements and Their Characteristics
Type of Movement
Subdivision
Characteristics
Rate of Movement
Falls
Rockfall
Rocks of any size fall through the air from steep cliffs, canyons,
and road cuts
Extremely rapid
Slides
Slump
Movement occurs along a curved surface of rupture; most
commonly involves unconsolidated or weakly consolidated
material
Extremely slow to
moderate
Flows
Rock slide
Movement occurs along a generally planar surface
Rapid to very rapid
Mudfl ow
Consists of at least 50% silt- and clay-sized particles and up to
30% water
Very rapid
Debris fl ow
Contains larger-sized particles and less water than mudfl ows
Rapid to very rapid
Earthfl ow
Thick, viscous, tongue-shaped mass of wet regolith
Slow to moderate
Quick clays
Composed of fi ne silt and clay particles saturated with water;
when disturbed by a sudden shock, lose their cohesiveness and
fl ow like a liquid
Rapid to very rapid
Solifl uction
Water-saturated surface sediment
Slow
Creep
Downslope movement of soil and rock
Extremely slow
Complex movements
Combination of different movement types
Slow to extremely
rapid
 
Search WWH ::




Custom Search