1. MASS WASTING/ MASS MOVEMENT

2. MASS WASTING/MASS MOVEMENT
The term mass movement describes all downhill movements of weathered material, including soil, loose stones and rocks, in response to gravity
It excludes movements where the material is carried by ice, water, or wind.
When gravitational forces exceed forces of resistance, slope failure occurs and material starts to move downwards
A slope is an dynamic open system affected by biotic, climatic, gravitational, groundwater, and tectonic inputs which vary in scale and time

3. MASS WASTING/MASS MOVEMENT
The amount, rate and type of movement depend upon the degree of slope failure (see fig 2.12 of handout A given to you with this lecture)

4. MASS WASTING/MASS MOVEMENT
Two ways of classifying mass movements:
The first type is the Varnes (1978) classification system which is self explanatory (see handout B figure 5.2
The second type is the Carson and Kirby classification of mass movement which is also self explanatory (see handout B figure 5.1)
Their diagram shows, each type of mass movement can be plotted on the triangle according to its rate, measured along the base of the triangle, and its moisture content, shown along the right-hand side of the triangle, thus the HEAVES are invariably slow process, flows are wet processes, and slides are dry.

5. MASS WASTING/MASS MOVEMENT
Three things happen:
Initial failure of slope
Transportation of slide materials
Deposition of slide materials

6. MASS WASTING/MASS MOVEMENT: causes
The operation of mass movement processes relies on the development of instability on a slope
If instability occurs then mass movement will begin
Some mass movements are fast while others can be very slow which depends on whether the stress placed on the material on the slope exceeds its shear strength greatly or minimaly
When a slope is unstable it is less resistant to gravity then and the following factors will determine the slope’s ability to withstand gravity

7. Mass wasting/mass movement causes
the energy that is exerted by gravity on a load is determined by the following factors:
Steepness of a slope
Shape and size of particles
Nature of material that forms a slope
Depth of the weathered material
Nature of the ground cover
Stability of the ground
Water content of the soil
A word I will use often this lesson is regolith  the weathered bedrock from which soils are formed

8. Mass wasting/mass movement causes
STEEPNESS
The steeper a slope is the greater the effect gravity has on it which creates a higher level of instability

9. Mass wasting/mass movement causes
SHAPE AND SIZE OF PARTICLES–
A slope made up of large angular rocks is more stable than a slope made up of small circular rocks
This is because larger rocks have a greater ANGLE OF REPOSE

10. Mass wasting/mass movement causes
NATURE OF MATERIAL THAT FORMS A SLOPE
This refers to whether the material is clay or snow for example
Snow is very unstable and is composed of air and water in both solid and liquid form, it can go from stable to unstable in seconds
Clay can hold water which allows it to be heavy and cause it to collapse at once

11. Mass wasting/mass movement causes
DEPTH OF WEATHERED MATERIAL:
The deeper the soil the higher the chance it will slide down slope
Volcanic debris is layered so deep in Hawaii a massive chunk of the Island can slide into the ocean with just a small earthquake

12. Mass wasting/mass movement causes
GROUND COVER:
The roots of vegetation hold loose material together
Means transport is less likely around areas where there is a lot of plants
Clear cutting slopes increases the chance of slope failure while grass is planted along the sloping gradient along highways

13. Mass wasting/mass movement causes
GROUND STABILTY:
The lack of ground or tectonic stability can cause slope failure to begin
Earthquakes, tremors, natural vibrations, explosions, loud noises, even heavy traffic

14. Mass wasting/mass movement causes
WATR CONTENT OF SOIL:
Water fills in the spaces between particles and lubricates them thus allowing it to slide better
Heavy storms will cause landlsides
Water that freezes in the ground casues SOLIFLUCTION – upper section of ground thaws while underneath stays frozen causing the upper layer to slide on the lower layer

15. Mass wasting/mass movement causes
HUMAN CASUES:
Humans quite often cause slope failure
Clear cut logging removes vegetation cover allowing raindrops to hit bare soil and weaken it along with the removal of roots holding loose soil
Roads undermine slopes causeing them to fail
Humans building on slopes add weight on them
Urban development increases run-off

16. Mass wasting/mass movement: prevention/stabilization
Rock bolts are fastened deep into the rock to keep the slope intact
Thin layer of concrete sprayed on the surface of the rock to hold it together

17. Mass wasting/mass movement: prevention/stabilization
Pipes inserted deep into slopes to drain excess water to stop saturation
Put a wired mesh to stop rocks from falling

18. Mass wasting/mass movement: prevention/stabilization
Retaining walls stop rock from falling and reduce debris
Terracing reduces slope

19. Mass wasting/mass movement: prevention/stabilization
Snow sheds are built in areas where there is large amounts of snow fall
Tunnels are made in places where gradient is too steep

22. Mass wasting/mass movement: types
A great graph showing moisture and speed for all types of slides listed in this powerpoint is shown on page 145 of This Earth fig

23. Mass wasting/mass movement: types
CREEP
This is the slowest of the downhill movements and difficult to measure since it moves at a rate of less than 1cm per year
Almost a contineous process
Occurs mainly in humid climates where there is a vegetation cover
Often reffered to as SOIL CREEP
ussually occurs on a 5 degree slope
Produces TERRACETTES

24. Mass wasting/mass movement: types
there are 2 major causes of soil creep, both resulting from repeat expansion and contraction
WET-DRY PERIODS – during times of heavy rainfall moisture increases the volume and weight of soil, causing expansion and allowing the regolith to move down hill under gravity. Drying occurs which will then cause the soil (especially clay) to contract
FREEZE-THAW – when the regolith freezes the presence of ice crystals increases the volume of the soil by 9%. As soil expands, particles are lifted at right angles the slope in a process called HEAVE. When the ground thaws and the regolith contract, less particles fall back vertically under the influence of gravity and so move down slope

25. Mass wasting/mass movement: types
terracettes

26. Soil creep

27. Mass wasting/mass movement: types
Solifluction:
Means soil flow
Slightly faster movement between 5cm and 1m per year
Usually takes place under periglacial conditions where vegetation is minimal
During winter the bedrock and regolith are frozen
In the summer the surface layer thaws but underlying layer remains frozen and acts like impermiable rock
Surface melt water cannot infiltrate downwards which will saturate the surface layer and will flow above the frozen layer

28. Mass wasting/mass movement: types
solifluction

29. Mass wasting/mass movement: types
FLOW MOVEMENTS:
Flows occur in three main forms  mudflows, earthflows, debris flow
MUDFLOWS – are rapid movements, occurring on steeper slopes. Most likely to occur during times of heavy rainfall, when both volume and weight are added to the soil (can contain 60% water)
EARTHFLOWS – when a regolith on slopes of degrees becomes saturated with water (not as much as mudflows), begins to flow downhill at a rate varying between 1 and 15 km per year

30. Mass wasting/mass movement: types
FLOW MOVEMENTS:
DEBRIS FLOW – is the rapid movement of relatively course material. They commonly occur in mountain regions where the surface is made of debris unconsolidated by any layers of vegetation. Heavy rain can mobalize it as a slurry that looks like cement. They move VERY fast and can carry cars and boulders

31. Mass wasting/mass movement: types
mudflow

32. Mass wasting/mass movement: types
earthflow

33. Mass wasting/mass movement: types
Debris flow

34. Mass wasting/mass movement: types
RAPID MOVEMENTS:
Slides are different from flows because they are a dry mass movement
They rely on a weakness already present in their slope as their causative mechanism
In a pure slide, failure occurs along a well defined SHEAR PLANE (which is a plane that occurs when to parts of the slope slide past each other (see handout D)
A TRANSITIONAL SLIDE is a slide where the shear plane is parallel to the surface of the slope (handout D fig 5.3b)

35. Mass wasting/mass movement: types
RAPID MOVEMENTS:
ROTATIONAL SLIDES or SLUMPS (handout D fig 5.3c) have a curved concave shear plane
Material moves from the higher parts of the slope to the lower and is pushed outward

36. Mass wasting/mass movement: types

37. Mass wasting/mass movement: types
Very Rapid Movements:
Rockfalls are spontaneous and rare debris movements on slopes which exceed 40 degrees
They may result from extreme physical or chemical weathering in mountains, pressure release, storm wave action on sea cliffs, or earthquakes
Material once broken from the surface will either bounce or fall vertically from the scree at the foot of a slope
(handout C helps you visulaize all of this)

38. Mass wasting/mass movement: types

39. Links
c&feature=related (Japanese landslide)
kY&feature=related (Japanese Landslide)
HA&feature=related k

40. homework Characteristics of mass wastage worksheets
Case studies (in class)