Slope Stability and Colluvial Landforms

Definition of Landslide

The force of gravity shifting regolith (weathered rock and soil) down a natural or man-made slope as a result of natural geologic processes or human activity.

What is a Scarp Line

Fault line where mass movement begin

Possible causes of Landslides

• Erosion at toe

• Addition of load at top of slope

• Steepening of slope angle

• Increase in groundwater pressures

• Weak layers in rock/soi

Categories of Slope Instability

• Soil Creep

• Earth Flow

• Debris Flow/Mud Flow

• Slump

• Rock Slide

• Rock Fall

Soil Creep

Downslope movement of soil that is slow as to be imperceptible (very slow, impossible to perceive) in the short term. Accumulated movement results in damage to or destruction of shallow structures and vegetation

Indication of Soil Creep

Titled and displaced items

What is Earth Flow?

Stepped slope where mass of material has slumped downhill and a bulging lobe-shaped landform that results at the toe of the slope

Rate of movement of earth flow

Hours on saturated slopes

Rate of movement of soil creep

Typical several centimeters per year

What is Debris Flow?

Turbulent liquid that can transport large quantities of sediments over long distances. Soil is highly saturated.

Rate of Movement of Debris Flow

Can reach speed of 160 km/hr

What are the causes of Debris Flow?

• Abundant Loose Debris

• Deforestation (Man-Made)

• Moisture/Precipitation

• Sparse Vegetation (Natural)

• Steep Slope

Formation of Debris Flow

• Formed when heavy rainfall increases the water flow in a stream

• Slump slides into stream increasing volume of flow

• Debris flow picks up sediment and vegetation increasing volume of flow

• Emerges from canyon and damages/overwhelms in path

What is a Jkulhaup?

Glacier generated debris flow caused by release of liquefied clay from base of glacier (terminal moraine)

What is Lahar?

Where mud holds so much water that it flows as slurry even down a gentle slope. Also known as a mud flow.

Rate of movement of Mud Flow

10mph to 20 mph

What is a Slump?

Rotational or translational failure of slope along a surface

What is a Graben?

Depression at top of slope

Rotational Slump Block

Downward movement of a block of earth material along a curved surface of failure. Rock masses tilt back as they slide from a cliff or escarpment.

How can you identify a Rotational Slump?

Tilting of trees back towards the slope

What is a Translational Slump Block

Downward movement of a block of earth material along an angular surface of failure.

What are Rock Slides?

Masses of bedrock slipping down a sloping fault or bedding plane

What is a Rock Fall?

Free fall of fragments of any size from a cliff or the toppling of large block-like fragments.

Toppling of large block-like fragments

Rock Topple