Mass Movements



Landscapes in Motion Objectives

  • Understand how to classify mass movements

  • Explain the forces that affect slope stability

  • Explain the factors that affect the forces affecting slope stability can

Mass Movements In the Field


Why Study Landslides?

In the Field



Landscapes in Motion

  1. How to classify mass movements?

  2. What forces affect slope stability?

  3. What Factors Determine Slope Stability?



1. How to classify mass movements?


Distinguishing the materials in motion

  • Rocks

  • “Debris” is coarse-grained; 20–80 percent >2 mm in size

  • “Earth” is fine-grained; 80 percent or more are <2 mm in size


Distinguishing the type of motion

  • Free fall—material detaches from a slope and then free falls through the air, or bounces and rolls downslope

  • Slide—material (either rock or regolith) that slides along a surface of rupture

  • Flow—continuous movement of rock, regolith, or both that behaves as a high-viscosity liquid


Distinguishing the speed of motion

  • Slow (creep)

  • Fast (Avalanche)


Three factors make up the criteria for the classification scheme or mass movements:

  • Nature of the mixture of solids(Rock, Debris, or Earthmaterials)

  • Type of motion (Fall, slide, or flow)

  • Velocity of motion (Avalanche,flow or creep)


Flow occurs on different scales. Creep is slow and only detected by dislocation or bending of features at the surface, whereas debris avalanches are very rapid flows of rock, regolith, vegetation, and/or sometimes ice.


Falls

Rock falls are common where the rock is highly jointed and on a steep slope, such as New Hampshire’s “Old Man of the Mountain,” which collapsed in May 2003.


Talus Slopes

The loose material that piles up at the base of steep slopes is called talus. The lack of vegetation or soil on talus indicates that material is actively accumulating.


Slides: Translational slides (planar)

  • Hope slide 1965 

  • Hope slide/avalanche as seen today

  • Frank slide/avalanche Alta, 1903, as seen today

Slides: Rotational slides (slump)

  • Slump Diagram slump


Slides

  • Mass movement by sliding along a surface

    • Planar slides move downslope in contact with a surface of rupture, typically along bedding planes, foliation, or joint planes oriented parallel to the slope.

    • If the rupture surface curves, then the slide mass rotates as it moves downslope, causing rock layers and surface features to tilt. The curved rotational slide surfaces are scoop shaped, and they usually form in regolith or poorly consolidated or weak rock where bedding or joints do not influence failure. This is called a slump.


Flows

Flows are the continuous movement of rock, regolith, or both that behave like a high-viscosity liquid. Figure 6.7a illustrates the “liquid” appearance of moving flows.


Very slow flows, called creep, are detected only by dislocation or bending of features at the surface like these sedimentary layers and trees.



2. What forces affect slope stability?


Experiments to Find Out

Under the conditions below, a brick on a board will slide if tilted far enough. What does this mean? 1) Increasing slope favors motion or slope instability. Next, we may sand the board and find it moves more easily. 2) Smoother surfaces favor motion and slope instability, and if we wet the board, it moves easier still. 3) The presence of water favors motion and slope

instability.

This simple set of experiments suggests that slope, surface roughness, and water are factors affecting the balance between driving and resisting forces.



For motion to happen, the gravity force parallel to the surface (called the driving force) must be greater than the resisting strength. Figure 6.11 shows that the slope angle determines the size of the gravity force.


Friction and cohesion are the resisting forces to gravity.

Friction is the force that opposes motion between two objects that are touching one another. The friction increases as the roughness of the surfaces increases. Friction also varies with slope angle because on lower slopes the weight of fragments pulls them down in stronger contact with underlying material.

Cohesion, the other component of resisting strength, is the attraction of particles at the atomic level. Materials such as loose sand and gravel have very little cohesion between particles, but clay particles, with charged surfaces, have high cohesion.


Figure 15.13 shows the importance of slope angle in determining whether motion occurs, because driving force increases and the resisting strength decreases with increasing slope angle.



3. What Factors Determine Slope Stability?

Adding small amounts of water to sand initially increases cohesion between particles, but too much water leads to failure.

Angle of repose: the maximum angle of a stable slope as determined by the friction, cohesion, and particle shape.


The Role of Slope

Figure to the left, illustrates natural and artificial processes that increase slope angles and, therefore, enhance mass movement. As a stream cuts down, the adjacent slopes become steeper and may reach the critical angle where driving force exceeds resisting strength and failure occurs.


Vegetation

The role of vegetation

Vegetation is a very important factor in slope stability. Roots penetrate and bind together regolith and absorb water from precipitation.

This Figure shows debris flows following rainfall on wildfire- denuded slopes in southern California.


Thickness of Regolith

These figures illustrate the role of vegetation plus climate in determining the thickness of regolith on hillslopes. Here in a temperate mid-latitude forest, we have dense tree cover and thick regolith. As a result, slopes tend to be fairly stable. Mass movement occurs, but generally it is slow-velocity types such as creep, slumps, and flows.


Arid Climate

In arid regions with low vegetation abundance and slow rates of weathering, surface-water removes most regolith particles almost as quickly as they loosen from bedrock. Mass movements are typically rock falls and slides.