Mass Wasting

The Origins of Landslides

There are many possible reasons behind why a slope might fail, but the number one cause is gravity.

A bit of physics:

Resisting forces = Material strength. So, the stronger the material strength, the stronger the resisting force. However, cohesion (the ability of a substance to stick to itself) and friction (the force resisting movement on a solid surface) also play a role in slope stability. For mass wasting, the steeper the slope, the larger these forces need to be in order to successfully resist gravity.

But there are other factors, which include:

Water - The buildup of water within a slope can cause slope instability. Weather comes in cycles with dry periods of 15 years and wet of 12 years. The greater the storm frequency, intensity, and duration the greater the hillside damage. During periods of heavy rainfall, hills can become become pocked with soil failures.

Drainage systems are needed for some slopes in construction areas or new developments. Impermeable surfaces can cause excess runoff, which can lead to erosion and slope failure.

Excess water in the slope can cause:

• A increase in the slope weight and pore pressure as water is added to the slope.
• Clay minerals will also expand, causing inflation of the slope.
• Water will also dissolve the minerals holding grains together, decreasing slope stability.
• Fractures within the rock comprising the slope can form. A condition called piping, where water flows through the rocks via these conduits, which can physically erode away (remove) loose material, can develop as a result.
• Pressure on water in pore spaces of rocks increases with increasing weight of sediment piled on top of rocks. If pore space water becomes over-pressurized, gives ‘lift’ to overlying sediments making them unstable.

Steep Slopes

Slope failure occurs when gravitational forces exceed the strength of the rock or soils that comprise the slope. Sediments piling up at the base of a slope will continue to do so until the slope angle becomes too great (the angle of repose). This angle, measured in degrees from the horizontal, varies depending upon the size and shape of the material. In general, the larger and more angular the particle, the steeper the slope.

Slopes can become over-steepened when:

• A natural slope is over steepened during construction
• A heavy load is placed at the top of the slope
• Material at the base of the slope is removed (either by nature during flooding or other means)

Vegetation

The type of vegetation present (or not) on a slope can affect slope stability. Plants with deep roots hold bedrock and loose materials together. Shallow-rooted plants provide stability for soils and other surficial deposits. Slope stability can be affected if vegetation is removed. Reseeding or replanting slopes (aka "hyrdoseeding") of questionable stability is often recommended.

Geology

The geology of a region greatly affects slope stability.

Adverse Geological Structure (aka "bad bedding")
Bedding surfaces of weak rocks dip downslope and are unsupported at the lower end . Similar problems can occur with faults, joints, foliation etc.

The Palos Verdes Penisula is being uplifted and folded into a large anticlinal structure by the Palos Verdes Fault Zone. The result is that the beds on the southern side of the fold are dipping in the same direction as the slope of the hillside - straight into the Pacific Ocean. Constant wave action along the cliffs undermines the bedding, causing the beds to slide downslope towards the ocean. (see image below).

Weak Rocks
Many Southern California rocks are rich in clay or silt. Clays like montmorillonite swells in water & disintegrates, undermining the stability of the rocks. Clays may cause instability on a 5º slope.

Interbedding of sandstone and clay is a common problem in some areas.

Serpentine, the state rock, causes many landslides

Quick clays are the most mobile of all deposits – fine rock flour scoured by glaciers, deposited in seas and later exposed above water. The salts that are helping to hold the clay together dissolve in water, causing the slope to fail.

 

Earthquakes

Earthquakes, or any other type of strong vibrations, can trigger landsides on slopes which are already unstable. In general, only earthquakes that are a magnitude 4.0 or greater will be strong enough to cause a slide.

 

Humans

We humans usually just can't leave something that is perfectly fine alone. Slopes that were once stable can be made unstable during mass grading activities. Cut slopes result in the removal of hill support, uncontrolled fills may settle, erode, slough away or slip in mass, and so on.

Four ways to make a stable slope unstable

1. Steepen slope angle
2. Increase the height of the slope
3. Saturate with water
4. Place an extra load on the slope