Weathering, Erosion, and Deposition

5.3 Mass Wasting

Mass wasting, which is synonymous with slope failure” is the failure and downslope movement of rock or unconsolidated materials in response to gravity. The term landslide is almost synonymous with mass wasting, but not entirely because some people reserve “landslide” for relatively rapid slope failures, while others do not. Other than the video below, this textbook will avoid using the term “landslide.”

Factors That Control Slope Stability

Mass wasting happens because tectonic processes have created uplift. Erosion, driven by gravity, is the inevitable response to that uplift, and various types of erosion, including mass wasting, have created slopes in the uplifted regions. Slope stability is ultimately determined by two factors: the angle of the slope and the strength of the materials on it.

A block of rock is typically situated on a rock slope that is being pulled toward Earth’s center (vertically down) by gravity. The vertical gravitational force can be split into two components relative to the slope: one pushing the block down the slope, called the shear force, and the other pushing into the slope, called the normal force. The shear force, which wants to push the block down the slope, has to overcome the strength of the connection between the block and the slope, which may be quite weak if the block has split away from the main body of rock, or may be very strong if the block is still a part of the rock. If the shear strength is greater than the shear force, the block should not move. However, if the shear force becomes stronger than the shear strength, the block of rock will slide down the slope.

As already noted, slopes are created by uplift, followed by erosion. In areas with relatively recent uplift, slopes tend to be quite steep. This is especially true where glaciation has taken place because glaciers in mountainous terrain create steep-sided valleys. In areas without recent uplift, slopes are less steep because hundreds of millions of years of erosion (including mass wasting) have made them that way. However, as we will see, some mass wasting can happen even on relatively gentle slopes.

The strength of the materials on slopes can vary widely. Solid rocks tend to be strong, but there is an extensive range of rock strength. If we consider just the strength of the rocks and ignore issues like fracturing and layering, then most crystalline rocks, like granite, basalt, or gneiss, are very strong, while some metamorphic rocks, like schist, are moderately strong. Sedimentary rocks have variable strength. Dolostone and some limestone are strong, most sandstone and conglomerate are moderately strong, and some sandstone and all mudstones are quite weak.

Fractures, metamorphic foliation, or bedding can significantly reduce the strength of a body of rock, and in the context of mass wasting, this is most critical if the planes of weakness are parallel to the slope and least critical if they are perpendicular to the slope.

Internal variations in the composition and structure of rocks can significantly affect their strength. Schist, for example, may have layers rich in sheet silicates (mica or chlorite), and these will tend to be weaker than other layers. Some minerals tend to be more susceptible to weathering than others, and the weathered products are commonly quite weak (e.g., the clay formed from feldspar).

Unconsolidated sediments are generally weaker than sedimentary rocks because they are not cemented and, in most cases, have not been significantly compressed by overlying materials. This binding property of sediment is sometimes referred to as cohesion. Sand and silt tend to be particularly weak, clay is generally a little stronger, and sand mixed with clay can be stronger still. Finer deposits are relatively strong (they maintain a steep slope), while the overlying sand is relatively weak, and has a shallower slope that has recently failed. Glacial till, typically a mixture of clay, silt, sand, gravel, and larger clasts, forms and is compressed beneath tens to thousands of meters of glacial ice so it can be as strong as some sedimentary rock.

Apart from the type of material on a slope, the amount of water that the material contains is the most critical factor controlling its strength. This is especially true for unconsolidated materials, but it also applies to bodies of rock. Granular sediments, like the sand at Point Grey, have lots of spaces between the grains. Those spaces may be arid (filled only with air); or moist (often meaning that some spaces are water-filled, some grains have a film of water around them, and small amounts of water are present where grains are touching each other); or completely saturated. Unconsolidated sediments tend to be strongest when they are moist because the small amounts of water at the grain boundaries hold the grains together with surface tension. Dry sediments are held together only by the friction between grains, and if they are well sorted or well rounded, or both, that cohesion is weak. Saturated sediments tend to be the weakest of all because the large amount of water pushes the grains apart, reducing the mount friction between grains. This is especially true if the water is under pressure.

Water will also reduce the strength of solid rock, especially if it has fractures, bedding planes, or clay-bearing zones. This effect is even more significant when the water is under pressure, which is why holes are drilled into rocks on road cuts to relieve this pressure.

Water also has a particular effect on clay-bearing materials. All clay minerals will absorb a little bit of water, and this reduces their strength. The smectite clays, such as the bentonite used in cat litter, can absorb much water, and that water pushes the sheets apart at a molecular level and makes the mineral swell. Smectite that has expanded in this way has almost no strength; it is incredibly slippery.

Moreover, finally, water can significantly increase the mass of the material on a slope, which increases the gravitational force pushing it down. A body of sediment that has 25% porosity and is saturated with water weighs approximately 13% more than it does when it is completely dry, so the gravitational shear force is also 13% higher.

Mass-Wasting Triggers

The shear force and the shear strength of materials on slopes, and about factors that can reduce the shear strength. Shear force is primarily related to the slope angle, and this does not change quickly. However, shear strength can change quickly for various reasons, and events leading to a rapid reduction in shear strength are considered to be triggers for mass waste.

An increase in water content is the most common mass-wasting trigger. This can result from the rapid melting of snow or ice, heavy rain, or some event that changes the pattern of water flow on the surface. Rapid melting can be caused by a dramatic increase in temperature (e.g., in spring or early summer) or by a volcanic eruption. Heavy rains are typically related to storms. Earthquakes can cause changes in water flow patterns, previous slope failures that dam up streams, or human structures that interfere with runoff (e.g., buildings, roads, or parking lots).

In some cases, a decrease in water content can lead to failure. This is most common with clean sand deposits, which lose strength when there is no more water around the grains. Freezing and thawing can also trigger some forms of mass wasting. More specifically, the thawing can release a block of rock attached to a slope by a film of ice. One other process that can weaken a body of rock or sediment is shaking. The most apparent source of shaking is an earthquake, but shaking from highway traffic, construction, or mining will also do the job. Several deadly mass-wasting events (including snow avalanches) were triggered by the M7.8 earthquake in Nepal in April 2015.

Classification of Mass Wasting

It is crucial to classify slope failures so that we can understand what causes them and how to mitigate them. The three criteria used to describe slope failures are:

  • The type of material that failed (typically either bedrock or unconsolidated sediment).
  • The mechanism of the failure (how the material moved).
  • The rate at which it moved.

The type of motion is the essential characteristic of slope failure, and there are three different types of motion:

  • If the material drops through the air, vertically or nearly vertically, it is known as a fall.
  • If the material moves as a mass along a sloping surface (without internal motion within the mass), it is a slide.
  • If the material has internal motion, like a fluid, it is a flow.
image
Mass Wasting Types” by the United States Geologic Survey is licensed under Public Domain.

Unfortunately, it is not typically that simple. Many slope failures involve two of these types of motion, some involve all three, and in many cases, it is not easy to tell how the material moved. The types of slope failure are summarized below.

Failure Type Type of Material Type of Motion Rate of Motion
Rock fall Rock fragments Vertical or near-vertical fall (plus bouncing in many cases) Very fast (>10s m/s)
Rock slide A large rock body Motion as a unit along a planar surface (translational sliding) Typically very slow (mm/y to cm/y), some can be faster
Rock avalanche A large rock body that slides and then breaks into smaller fragments Flow (at high speeds, the mass of rock fragments is suspended on a cushion of air) Very fast (>10s m/s)
Creep or solification Soil or other overburden in some small cases, mixed with ice Flow (although sliding motion may also occur) Very slow (mm/y to cm/y)
Slump Thick deposits of unconsolidated sediment Motion as a unit along a curved surface (rotational sliding) Slow (cm/y to m/y)
Mudflow Loose sediment with a significant component of silt and clay Flow (a mixture of sediment and water moves down a channel) Moderate to fast (cm/s to m/s)
Debris flow Sand, gravel, and larger fragments Flow (similar to a mudflow, but typically faster) Fast (m/s)

Rock Fall

A rock fall is when fragments or rock break off relatively easily from steep bedrock slopes, most commonly due to frost-wedging in areas where there are many freeze-thaw cycles per year. When hiking along a steep mountain trail on a cool morning, one might have heard the occasional fall of rock fragments onto a talus slope. This happens because the water between cracks freezes and expands overnight. When that same water thaws in the morning sun, the fragments that had been pushed beyond their limit by the ice fall to the slope below.

image
Jalalabad Rock Fall” by Wien Sven Dirks is licensed under Creative Commons Attribution-ShareAlike 4.0 International.

Rock Slide

A rock slide is the sliding motion of rock along a sloping surface. In most cases, the movement is parallel to a fracture, bedding, or metamorphic foliation plane, and it can range from very slow to moderately fast. The word sackung describes a slope’s very slow motion of a block of rock (mm/y to cm/y).

Rock Avalanche

If a rock slides and then starts moving quickly (m/s), the rock is likely to break into many small pieces, and at that point, it turns into a rock avalanche, in which the large and small fragments of rock move in a fluid manner supported by a cushion of air within and beneath the moving mass.

image
Goodell Creek Debris Avalanche” is licensed under Public Domain.

Creep or Solifluction

The very slow, millimeters per year to centimeters per year, movement of soil or other unconsolidated material on a slope is known as creep. Creep, which generally only affects the upper several centimeters of loose material, is typically a very slow flow, but in some cases, sliding may occur. Creep can be facilitated by freezing and thawing because particles are lifted perpendicular to the surface by the growth of ice crystals within the soil, and then let down vertically by gravity when the ice melts. The same effect can be produced by frequent wetting and drying of the soil. In cold environments, solifluction is a more intense form of freeze-thaw-triggered creep.

Creep is most noticeable on moderate-to-steep slopes where trees, fence posts, or grave markers are consistently leaning in a downhill direction. In the case of trees, they try to correct their lean by growing upright, and this leads to a curved lower trunk known as a “pistol butt.”

image
Soil Creep” by Derek Harper is licensed under Creative Commons Share-Alike 2.0 Unported

Slump is a type of slide (movement as a mass) that takes place within thick unconsolidated deposits (typically thicker than 10 m). Slumps involve movement along one or more curved failure surfaces, with downward motion near the top and outward motion toward the bottom. They are typically caused by an excess of water within these materials on a steep slope.

Mudflows and Debris Flows

When a mass of sediment becomes wholly saturated with water, the mass loses strength, to the extent that the grains are pushed apart, and it will flow, even on a gentle slope. This can happen during rapid spring snowmelt or heavy rains and is also relatively common during volcanic eruptions because of the rapid melting of snow and ice. (A mudflow or debris flow on a volcano or during a volcanic eruption is a lahar.) If the material involved is primarily sand-sized or smaller, it is known as a mudflow.

If the material involved is gravel-sized or larger, it is known as a debris flow. Because it takes more gravitational energy to move more massive particles, a debris flow typically forms in an area with steeper slopes and more water than a mudflow. In many cases, a debris flow takes place within a steep stream channel and is triggered by the collapse of bank material into the stream. This creates a temporary dam and a significant flow of water and debris when the dam breaks.

Preventing, Delaying, Monitoring, and Mitigating Mass Wasting

As already noted, we cannot prevent mass wasting in the long term as it is a natural and ongoing process; however, in many situations, there are actions that we can take to reduce or mitigate its damaging effects on people and infrastructure. Where we can neither delay nor mitigate mass wasting, we should consider moving out of the way.

Preventing and Delaying Mass Wasting

Delaying mass wasting is a worthy endeavor, of course, because during the time that the measures are still effective, they can save lives and reduce damage to property and infrastructure. The other side of the coin is that we must be careful to avoid activities that could make mass wasting more likely. One of the most common anthropogenic causes of mass waste is road construction, and this applies to both remote gravel roads built for forestry and mining and large urban and regional highways. Road construction is a potential problem for two reasons. First, creating a flat road surface on a slope inevitably involves creating a cut bank that is steeper than the original slope. This might also involve creating a filled bank that is both steeper and weaker than the original slope. Second, roadways typically cut across natural drainage features, and unless great care is taken to reroute the runoff water and prevent it from forming concentrated flows, oversaturating fill of materials can result — a specific example of the contribution of construction-related impeded drainage to slope instability.

Apart from water issues, engineers building roads and other infrastructure on bedrock slopes must be acutely aware of the geology, especially of any weaknesses or discontinuities in the rock related to bedding, fracturing, or foliation.

It is widely believed that the construction of buildings on the tops of steep slopes can contribute to the instability of the slope. This is probably true, but not because of the weight of the building. A typical house is not usually heavier than the fill that was removed from the hole in the ground made to build it. A more likely contributor to the instability of the slope around a building is the effect that it and the changes made to the surrounding area have on drainage.

Monitoring Mass Wasting

In some areas, it is necessary to establish warning systems so that we know if conditions have changed at a known slide area, or if a rapid failure, such as a debris flow, is actually on its way downslope.

Mt. Rainier, a glacier-covered volcano in Washington State, has the potential to produce massive mudflows or debris flows (lahars) with or without a volcanic eruption. Over 100,000 people in the Tacoma, Puyallup, and Sumner areas are in harm’s way because they currently reside on deposits from past lahars. In 1998, a network of acoustic monitors was established around Mt. Rainier. The monitors are embedded in the ground adjacent to expected lahar paths. They are intended to provide warnings to emergency officials, and when a lahar is detected, the residents of the area will have anywhere from 40 minutes to three hours to get to safe ground.

Mitigating the Impacts of Mass Wasting

In situations where we cannot predict, prevent, or delay mass-wasting hazards, some effective measures can be taken to minimize the associated risk. In some parts of the world, similar features have been built to protect infrastructure from other types of mass wasting. Debris flows are inevitable, unpreventable, and unpredictable. The results have been deadly and expensive many times in the past. It would be costly to develop a new route in this region, so provincial authorities have taken steps to protect residents and traffic on the highway and the railway. Debris-flow defensive structures have been constructed in several drainage basins. One strategy is to allow the debris to flow quickly through to the ocean along a smooth channel. Another is to capture the debris within a constructed basin that allows the excess water to continue through but catches the debris materials.

The United States Geologic Survey and the Utah Geologic Survey are excellent sources for more information regarding mass wasting.

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Physical Geography and Natural Disasters by R. Adam Dastrup, MA, GISP is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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