Landslides and other downslope movements (Chapter 8)

Mass wasting: any downslope movement of solid earth materials under the influence of gravity.

Angle of repose: The angle of a slope for a loose (unconsolidated) material where the downslope force of gravity exactly matches forces resisting downslope movement (i.e. Cohesion, friction).

Consider dry sand: If sand is poured from a bucket, it falls into a pile with slopes of about 30 degrees. With steeper slopes, the sand naturally move downslope.

Physics of landslides.

On a slope, the pull of gravity on an object is broken down into a normal force (perpendicular to the slope) and a downslope force (F) parallel to the slope. The downslope force pulls the object downhill. Friction, cohesion, etc. resist downslope motion and comprise a resisting force (f). If f > F, the slope is stable. If f < F, a slide will occur.

Factors that can cause landslides

1. Increase slope (increases F): Examples, excavations, undercutting of banks by streams.

2. Decrease friction or cohesion (decreases f): Heavy rain, shaking during earthquake, etc.

3. Increase load: (add weight, increases F) Saturate soil with water, construction of heavy buildings, etc.

Type of slides (p. 198-220):

Debris: > 2 mm, earth: < 2 mm

Most slides are complex and are a combination of different types.

Rock Fall: Falls are characterized by objects in free fall, where they do not impact a cliff face as they fall. Objects may be rocks (rock fall) debris (debris fall) or soil (earth fall). In the case of rock, rocks accumulate as talus at the base of the cliff,

Rock Topple: In topples, blocks of material fall over as a unit, akin to falling dominos.

Rotational Slide: A type of slide where a coherent block of material (acting as a unit) slides and rotates along a curved rupture (or slip) surface. Material may consist of rock (slump) debris (debris slide) or soil (earth slide).

Translational Slide: Translational slides occur where a coherent block of material slides along a flat surface. Material may consist of rock (rock slide), debris (debris slide), or soil (earth slide)

Rock or debris avalanche: Avalanches are dry flows where an incoherent mass of material moves down a slope. Here numerous individual rocks or smaller particles flow in a chaotic fashion, and act as a fluid (i.e. Like water) flowing down a slope.

Earth flow: Slow or moderately fast movement of soil or debris as an incoherent mass. Often form as the tow of a rotational slide. Undersaturated in water.

Debris flow: Rapid movement of water saturated soil, debris or rock. Often associated with flash floods.

Triggering mechanisms and other factors enhancing risk of slides:

1. Rock structure: Planes of weakness characteristic of certain rocks – Bedding planes in sedimentary rocks, foliation in metamorphic rocks.

2. Materials: Does a slope consist of rock, sand, clay etc.

1. Clay: Loss of cohesion when wet, Quick clays

2. Sand: Subject to collapse as sand becomes water saturated, Liquefaction with strong shaking

3. Addition of water:

1. Adds weight (increases load)

2. Decreases friction and cohesion in clays

3. Overpressure in sands

4. Shaking

1. Liquefaction of sand

2. Collapse of quick clays

5. Excavation: Increasing slope by natural (erosion) means, or by man during mining, construction, etc.

Mitigating risks of landslides.

1. Drains and drain lines

2. Bolts

3. Retaining walls / wire mesh

4. Slope control