Geology Reference
In-Depth Information
Mass movements
Liquid
Plastic
solid
Mass movements may be classified in many ways.
Table 3.2 summarizes a scheme recognizing six basic
types and several subtypes, according to the chief mech-
anisms involved (creep, flow, slide, heave, fall, and
subsidence) and the water content of the moving body
(very low, low, moderate, high, very high, and extremely
high):
Elastic
solid
Brittle
solid
AIR
WATER
1
Rock creep
and
continuous creep
are the very slow
plastic deformation of soil or rock. They result from
stress applied by the weight of the soil or rock body
and usually occur at depth, below the weathered
mantle. They should not be confused with soil creep,
which is a form of heave (see below).
2
Flow
involves shear through the soil, rock, or snow
and ice debris. The rate of flow is slow at the
base of the flowing body and increases towards the
surface. Most movement occurs as turbulent motion.
Flows are classed as
avalanches
(the rapid down-
slope movement of earth, rock, ice, or snow),
debris
flows
,
earthflows
,or
mudflows
, according to the
predominant materials - snow and ice, rock debris,
sandy material, or clay. Dry flows may also occur;
water and ice flow.
Solifluction
and
gelifluction
-
the downslope movement of saturated soil, the lat-
ter over permanently frozen subsoil - are the slowest
flows. A
debris flow
is a fast-moving body of sedi-
ment particles with water or air or both that often has
the consistency of wet cement. Debris flows occur as
a series of surges lasting from a few seconds to sev-
eral hours that move at 1 to 20 m/s. They may flow
several kilometres beyond their source areas (Figure
3.6a). Some are powerful enough to destroy buildings
and snap off trees that lie in their path.
Mudflows
triggered by water saturating the debris on the sides of
volcanoes are called
lahars
. When Mount St Helens,
USA, exploded on 18 May 1980 a huge debris
avalanche mobilized a huge body of sediment into
a remarkable lahar that ran 60 km from the volcano
down the north and south forks of the Toutle River,
damaging 300 km of road and 48 road bridges in the
process.
SOLIDS
0
100
Moisture content (per cent)
Figure 3.5
The composition of soil, ranging from air-filled
pores, to water-filled pores, to a liquid. The Atterberg
or soil limits are shown.
Source:
Adapted from Selby (1982, 76)
moisture content, the soil becomes a suspension of par-
ticles in water and will flow under its own weight. The
three limits separating different kinds of soil behaviour -
shrinkage limit, plastic limit, and fluid limit - are known
as
Atterberg limits
, after the Swedish soil scientist
who first investigated them (Figure 3.5). The
plastic-
ity index
, defined as the liquid limit minus the plastic
limit, is an important indicator of potential slope insta-
bility. It shows the moisture range over which a soil will
behave as a plastic. The higher the index, the less stable
the slope.
Some soils, which are referred to as
quick clays
or
sensitive soils
, have a honeycomb structure that allows
water content to go above the liquid limit. If such soils are
subject to high shear stresses, perhaps owing to an earth-
quake or to burial, they may suddenly collapse, squeezing
out water and turning the soil into a fluid. Quick clays
are commonly associated with large and swift flows of
slope materials. A violent shaking, as given by a seismic
shock, may also liquefy a saturated mass of sand.
