Mass wasting - 2, types, classification.ppt

Mass wasting refers to the downslope movement of soil, rock, and debris under the direct influence of gravity, without the primary involvement of a transporting medium such as water, wind, or ice. It is one of the most important geomorphic processes shaping the Earth’s surface, constantly working to reduce relief, modify landscapes, and transfer material from higher to lower elevations. The term encompasses a wide variety of processes that differ in terms of speed, moisture content, type of material involved, and triggering mechanisms, but all share gravity as the driving force. The stability of a slope is determined by the balance between resisting forces, which hold materials in place, and driving forces, which tend to pull them downslope. When the driving forces exceed resistance, failure occurs, leading to mass movement. Factors influencing slope stability include the angle of slope, nature and structure of the underlying materials, water content, vegetation cover, and human modifications. Water plays a dual role, as a small amount of moisture can increase cohesion in soils and provide stability, but excessive water reduces friction, adds weight, and acts as a lubricant, greatly increasing the likelihood of failure. Earthquakes, volcanic activity, heavy rainfall, rapid snowmelt, and human activities such as deforestation, road cutting, mining, and construction often act as triggers. Mass wasting processes are commonly classified based on the type of movement and the material involved. Falls involve the free fall of detached rock fragments from steep cliffs or slopes, often accumulating as talus at the base. Slides occur when coherent blocks of soil or rock move downslope along a well-defined surface of weakness; they can be rotational, forming curved slip surfaces as in slumps, or translational, where movement occurs along planar bedding or fault planes. Flows, in contrast, involve the movement of unconsolidated material that behaves like a fluid, ranging from slow soil creep to rapid and destructive events such as debris flows, mudflows, and earthflows. Creep, the slowest type, is almost imperceptible but widespread, gradually tilting trees, poles, and retaining walls downslope over long periods. Solifluction, a type of creep in periglacial regions, results from freeze-thaw cycles that cause soil and regolith to flow slowly downslope. Avalanches and landslides represent some of the fastest and most hazardous forms of mass wasting, capable of destroying infrastructure and causing significant loss of life. The rate and magnitude of these processes depend on local geology, slope angle, climate, and land use practices. Human interventions frequently exacerbate mass wasting hazards by destabilizing slopes through deforestation, quarrying, excavation, and poorly planned urban development. The consequences of mass wasting extend beyond immediate destruction; they can block rivers to form temporary lakes, disrupt transportation routes, strip soils..

1. Mass Wasting
Mass wasting is a term used
to describe the movement of
geological material (rocks,
soils, sediments) under the
effects of gravity

2. •Mass wasting is the movement of
regolith and masses of rock
downhill
•Gravity is the force which drives
mass-wasting

3. The Role of Gravity
•Shear stress is the force of gravity
acting on an object on a slope
–always pulls straight down,
perpendicular to the horizon
–objects move downhill at some angle

5. Definition of Mass Wasting
Movement of rocks and soil (debris)
down-slope due to gravity without a
flowing medium.
Controlling Factors
Gravity (g) is resolved into two
components, g
n
(normal) and g
s
(shear).
Movement caused when
g
s
> g
n
+ cohesion + friction.
A block’s movement will be
controlled by friction at its base.

6. Safety Factor
•The ratio of resisting forces to the driving
forces
– Shear resistance/Magnitude of stress
SF > 1 Slope is stable
SF < 1 Slope is unstable

7. Cohesion and friction may be reduced by:
Mass Wasting
Weathering – removes mineral constituents.
Earthquakes – disturbs grain contacts.
Water – lubricates debris by forcing grains apart.
Sand grains are actually held together by the
surface tensions of the water films – increasing
the water decreases the tensions.

8. Resistance to Movement
•Cohesive strength-the resistance
of an object to move downhill
Friction
–Cohesion, stickiness of particles
–Other forces holding a particle in
place (plant roots, cementation)

9. The Role of Water
•Water may act to increase or
decrease cohesive strength
–Dry regolith or soil has little or no
cohesion
–Damp regolith or soil is sticky
–Saturated regolith or soil flows easily

10. IMPORTANT CONCEPTS
Features include crown, head scarp, basal surface of rupture, transverse cracks,
transverse ridges and radial cracks. All created in the downward and outward
movement

11. Types of Mass Movement
•Classified by
–Type of motion
–Type of material
–Rate of movement

12. Mainly 3 types
•Flowage
•Sliding
•Subsidence

13. Flowage: downward movement of materials without any
definite surface of failure.
2 types: slow flowage and rapid flowage
Slow flowage (creep)
Movement is not easily visible
Low rate of movement a few cm/year
Talus creep: a variety of soil creep in which movement is
relatively fast and material involved is made up of
rock fragments and talus.
Solifluction: Higher water content, characteristic of
permafrost region

14. Creep
•slow movement of soil and regolith
- 1 to 10 mm/yr

15. Creep
When downslope movement is quite slow, even particle
by particle, the motion is called creep
Soil creep, which is commonly imperceptible to human
observation, is the slowest form of mass wasting

17. Creep: Imperceptibly slow down-slope movement,
<1 cm/year.
Affects the upper few meters only.
Enhanced by frost
heaving, wetting
and drying cycles,
washing away of
fine particles.

20. Creep

21. s
h
a
l
e
s
a
n
d
s
t
o
n
e

22. Soil Creep on a hillside [note old fence posts and tree growth]

23. Solifluction: soil creep in area of permafrost [e.g. Alaska]
-permafrost acts as impermeable barrier to warm weather melt water

25. Flows
In a flow, the material flows downslope in a chaotic
manner, more like a liquid rather than a solid
Flows of unconsolidated material are very common and
can move at high speeds
Earth flow – slow moving, low water content
Mud flows – fast moving, high water content

26. Earth Flow
Earth flows are where dry soil moves downslope,
usually at a slow pace

27. Earthflow

28. Earthflow: debris moves downslope as a viscous fluid,
slow or rapid movement typically after heavy rains.

29. Mud Flows
•Mixtures of water, mud and rock
–Liquified soil flows downhill
•Up to 50 km/hr
–Water lubricates mass of soil and rock
–Large boulders, building etc. may be carried by
viscous fluid.

30. Mud Flow
Lemhi Range, Id
Mud Flow
Coast Ranges, Ca

31. Mudflows
Dried Mudflow, Peru.
Mudflow from
Mount St. Helens.

32. Mud Flows
Mud flows occurred when soil
becomes saturated with water

33. Mud Flows
Mud flows can flow for great distances, because it is
more like a fluid than a solid

34. Avalanche
In general, the term
avalanche refers to the
rapid down slope
movement of snow
It occurs when a
buildup of snow is
released down a slope,
and is one of the major
dangers faced in the
mountains.

35. Avalanche
The worst avalanche in the U.S. history occurred in the
Cascade Mountains in Washington in 1910.
96 people were trapped when two trains became
snowbound.
An avalanche then swept them to their deaths in a gorge
150 feet below the tracks.

36. Sliding
•Mass movement along well defined slippage
plane.
•Landslide block moves as a single or group of
units.
•Rock type, orientation and water content
influence events.

37. Rockslide: rapid sliding of
a mass of bedrock along
preferred surfaces (often
bedding planes).

39. Rockslide
•Rapid movement of large blocks of
rock
•Slippage plane usually associated
with:
–Bedding plane
–Joint plane
–Structural weakness

40. Rotational slide (slump)
•Spoon shaped slippage plane
•Block moves downward and outward
•Usually have multiple slippage planes

41. Example of slumpExample of slump

44. Slump
scarp

47. Rock Falls
Rock falls frequently occur on very steep slopes and
cliffs
Weathering loosens support for rocks and boulders and
gravity does the rest
Rock falls are very common along steep road cuts

48. Rock Falls
Some rock falls are bigger than others...
Rock fall in Yosemite
National Park in 1997

50. Most common Most common
mass wasting mass wasting
typestypes
Rock fall Slump (rotational slide)Rock fall Slump (rotational slide)
Creep Flow Rockslide Creep Flow Rockslide

51. MASS MOVEMENTS
SUBSIDENCE
Slow: Mississippi Delta, New Orleans
Left) Loosely packed sand and clay have high porosities near the surface. When
buried they expel the water and contract over a period of time.
Right) As much as 100 ft of subsidence of the Mississippi Delta around New Orleans
over the past 20,000 years. Has been faster recently up to 3m in the past 50
years making city very prone to damage from hurricanes.

52. MASS MOVEMENTS
SUBSIDENCE
Catastrophic: Sink Holes
View of sinkhole formed by the collapse of
a limestone cavern on May 31 1981
in winter park Florida. Sinkhole is 100
m across by 30 m deep.
Most of southern and eastern US lies on
limestone rocks created from the
deposition under seawater of soft
shells made of calcium carbonate.
Uplifted above seawater they are prone to
dissolution from circulating acidic fresh
ground water. Creates unstable
caverns underground.

53. Stabilization
Move material from the top to the toe.
Build barriers.
Build retaining walls.
Drain the slope.
Plant vegetation.
Prevent flooding.
Prevent undercutting.
Don’t over-steepen slope.

54. Fluid Removal
Since water (and ice) play a major role in promoting
mass movements, a major strategy for reducing such
hazards is to decrease the water content
Diverting runoff above the hazard
Install subsurface drainage
This type of prevention
will only work if you
can drain off enough
water to matter

55. 0085

56. Fluid Removal
Water trapped in
wet soil causes
movement, pushing
down retaining wall
Water drains
through pipes,
allowing wall to
keep slope stabile

57. Rock Bolts
The use of rock bolts
to stabilize rocky
slopes has been
fairly successful
The bolts are steel
cables anchored in
cement
The bolt and steel
plates on the
surfaced are
tightened to pull and
hold the layers of
rock together
This works best in
thin layered rocks

58. Stabilization

60. Vegetation
Vegetation (especially fast-growing vegetation
with sturdy root systems) is natures way of
stabilizing a too steep slope
Ice plant is commonly used to stabilize road cuts
along highways in California

61. Retention Structures
Mankind has a long history of building retention
structures to try and stabilize man-made slopes
In many cases after the fact

62. Retention Structures

63. Retention Structures
(Right) The retaining wall of concrete blocks
allows vegetation to help stabilize the slope
Dry and wet spells will loosen the blocks

64. Retention Structures
(Left) Stair-step structure formed of rubble
encased in wire mesh cages can be effective

65. Terracing
A series of stair-step terracing reduces the
weight of the overburden for each step and
creates an overall slope that is not too steep

66. Fencing
Chain-link
fencing can be
draped over a
road cut that is
subject to
repeated minor
rock fall
The fencing
traps the rock
and prevents
rock falls from
ending up on
the highway