3. Mass Wasting
What is mass wasting?
Mass wasting refers to several processes that have the following in
common:
1. Downslope movement of rock or weathered material
2. Movement is due to pull of gravity
3. There is no flowing medium (water, air or ice) that carries the
material
Why is mass wasting important?
1.
Processes represent a significant hazard to property and people
2.
Need to identify where and under what conditions these
processes occur
3.
Avoid construction in areas prone to mass wasting or attempt to
prevent mass wasting
*Generally, mass wasting occurs when gravitational forces exceed
frictional or shear forces (strength) of material.
4. Shear stress is the downslope pulling causing mass movement related to the mass
of the material and the angle of the slope.
Shear strength is the counteraction of the force of shear stress (i.e., friction).
If stress is greater than strength, then mass movement occurs.
Possibly triggered by:
1. Earthquake
2. Construction work (blasting)
3. Flooding
Driving force behind Mass Wasting:
1. Downslope pull of gravity
2. Depends on weight (amount) of material on slope.
3. Increasing amount of material can lead to mass wasting.
4. Depends on steepness of slope.
5. Over steepening slopes can lead to mass wasting.
Clays provide cohesion to unconsolidated materials.
Mineral cements hold rock together
5. Role of Water in Mass Wasting
1. Small amount of water can increase strength.
2. Surface tension of water helps hold material together.
3. Saturation with water weakens material.
4. Hydrostatic pressure can act to push grains apart.
Role of Vegetation in Mass Wasting
1.
Roots add strength to material by binding loose material together.
2.
In semi-arid and arid regions, forest fires can remove vegetation from hill
slopes, leaving surface materials vulnerable to mass wasting.
Planes of Weakness in Material
Planes of weakness in the material can facilitate mass wasting if the planes are
oriented parallel to the slope.
Planes of weakness include bedding planes in sedimentary rocks and foliation
planes in metamorphic rocks.
Mass Wasting Processes
Processes can be classified based on
1. Type of material that moves
2. Nature of movement (flow or slide)
3. How fast material moves; e.g. rock falls/slides, slump, debris flows/earth
flows/mud flows, creep
6. Slope Stability is influenced by:
1. Steepness
2. Angle of repose
3. Fluid effects
4. Expansive clays presence
5. Vegetation present
6. Earthquakes
7. Quick clays presence
Types of mass movement:
1. Fall -- rocks fall from great distances as a result of gravity
2. Slide -- mostly coherent unit slides on a slope or bedding plane
3. Slump -- rock unit falling a short distance due to rotation
4. Flow -- landslides involving an unconsolidated mass that moves in a
chaotic fashion
5. Solifluction -- soil component moving over permafrost layers, analogous
to a hockey puck sliding on ice
6. Creep -- extremely slow movement due to gravity
7. Slumps
Movement of coherent block of material along a curved surface.
More likely to occur when slope is undercut from below, material is saturated.
Common on river banks where stream erosion over steepens banks.
Example of slump
8. Creep
Extremely slow movements downslope
Rates of creep are 1-10 mm/year
Bent tree trunks, fence posts, and utility poles; gravestones tilted downslope
indicate that creep has occurred.
Creep damages buildings, roads
Debris flows, earth flows, mud flows
Flows are movements in which the material deforms chaotically as it moves.
Debris flows -- contains large boulders, gravels
Earth flows -- sandy material
Mudflow -- mud with considerable water
11. Deposits of Mass Wasting
Mass wasting produces sediment deposits.
These deposits commonly contain a wide range of sizes of particles (they
are "unsorted").
The deposits do not show layering (they are "unstratified")
How do we prevent landslides and mass wasting? This is NOT possible.
A better question is “how do we lessen the effects” of mass wasting?
1.
Remove weight from slope
2.
Engineering controls
3.
Vegetation and GeofabricTM
4.
Cables and anchoring systems
5.
Tunnels built over highways thus reducing weight
