Mass Movement

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# Mass Movement - PowerPoint PPT Presentation

Mass Movement. Dr. R. B. Schultz. 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

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## Mass Movement

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### Mass Movement

Dr. R. B. Schultz

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?

• Processes represent a significant hazard to property and people
• Need to identify where and under what conditions these processes occur
• 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.

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. Oversteepening slopes can lead to mass wasting.

Clays provide cohesion to unconsolidated materials.

Mineral cements hold rock together

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

• Roots add strength to material by binding loose material together.
• 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

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

Most common mass wasting types

Avalanche Slump

Creep Flow Rockslide

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 oversteepens banks.

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

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

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

Key Terminology

Mass wasting Shear stress

Shear strength Cohesion

Fall Slide

Slump Flow

Solifluction Creep

Avalanche Debris flow

Mud flow Earth flow

Pertinent Web Sites

Disaster Finder (NASA)

A complete index to the best disaster Web sites on the Internet.

Geologic Hazards—Landslides (USGS)

A great site from the Landslides Hazards Group of the United States Geological Survey (USGS) that conducts research, gathers information, responds to emergencies and disasters, and produces scientific reports and other products for a broad-based user community.

Glossary of Soil Science

An extensive glossary of soil science terms from the Canadian Soil Information System.

Landslides (USGS)

Landslide information from the United States Geological Survey (USGS).

This Central Michigan University site maintained by Dr. Mark Francek lists several good links to Web sites containing information relative to topics discussed in the chapter.

Mass Wasting Links (Houghton Mifflin)

Links to several mass wasting sites, including class lecture notes, arranged by topic.

National Cooperative Soil Survey

National Cooperative Soil Survey provides a dynamic resource of soils information for a wide range of needs.

National Landslide Information Center (USGS)

The National Landslide Information Center (NLIC) is responsible for distributing information about landslides to the lay public, researchers, planners, and local, state, and federal agencies.

Recent Landslide Events (USGS)

A listing with descriptions and explanations of recent landslide events from the United States Geological Survey (USGS).