15.2 Classification of Mass Wasting
- There are three criteria used to classify slope failures:
- The type of material that failed (e.g., bedrock or unconsolidated sediment),
- The mechanism of the failure (how the material moved as it failed), and
- The rate of movement (how quickly the material moved).
- The mechanism of the failure is the most important characteristic of a slope failure. There are three main mechanisms of slope failure:
- Fall: in falls, the material falls through the air, either vertically or nearly vertically,
- Slide: in slides, the material moves as a coherent mass along a sloping surface (with little to no internal motion within the mass), and
- Flow: in flows the failing material has internal motion (like a fluid) as it moves.
While we do not classify slope failures by the shape of the rupture surface, it is nevertheless an important feature used to describe mass wasting processes. The rupture surface is the boundary between the slope and the sliding material. Slope failures with curved rupture surfaces are called rotational slope failures, and slope failures with planar rupture surfaces are called translational slope failures.
Unfortunately, classifying slope failure is not as simple as the classification scheme above suggests. Many slope failures involve more than one type of motion, and often it is not easy to tell how the material moved. The types of slope failure that are covered in this chapter are summarized in Table 15.1.
|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), but some can be faster|
|Rock avalanche||A large rock body that slides and then breaks into small fragments||Flow (at high speeds, the mass of rock fragments is suspended on a cushion of air)||Very fast (>10s m/s)|
|Creep or solifluction||Soil or other overburden; in some cases, mixed with ice||Flow (although sliding motion may also occur)||Very slow (mm/y to cm/y)|
|Slump||Thick deposits (m to 10s of m) 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)|
Table 15.1 | Classification of slope failures based on type of material and motion. Source: Steven Earle (2015) CC BY 4.0. View source
Rock fragments can break off relatively easily from steep bedrock slopes, most commonly due to frost-wedging. If you ever hike along a steep mountain trail on a cool morning, you will probably hear the occasional fall of rock fragments onto a talus slope. Water in the cracks in the rock freezes and expands overnight, and this can break the rock apart. When the ice thaws in the morning sun, some of these broken fragments will fall to the slope below (Figure 15.7). Talus slopes form from this fallen rock, forming slopes at the angle of repose below high rock walls (Figure 15. 8, left). They are also known as scree slopes.
A typical talus slope, near Keremeos in southern BC, is shown in Figure 15.8. In December 2014, a large block of rock split away from a cliff in this same area. It broke into smaller pieces that tumbled down the slope and crashed into the road, smashing the concrete barriers and gouging out large parts of the pavement. Luckily no one was hurt.
A rock slide is a large body of rock that is sliding along a sloping surface. In most cases, the movement is parallel to a fracture, bedding plane, or metamorphic foliation plane; thus, most rock slides are translational slope failures.
The speed of the movement can range from very slow to moderately fast. The word sackung describes the very slow motion of a block of rock (mm/y to cm/y) on a slope. A good example is the Downie Slide, a translational slide north of Revelstoke, BC, which is shown in Figure 15.9. In this case, a massive body of rock is very slowly sliding down a steep slope along a plane of weakness that is caused by the foliation in the rock. The foliation is approximately parallel to the slope.
The Downie Slide was recognized prior to the construction of the Revelstoke Dam, and was moving very slowly at the time of dam construction (a few cm/year). Geological engineers were concerned that the presence of water in the reservoir (visible in Figure 15.9) could further weaken the plane of failure, leading to an acceleration of the motion. The result would have been a catastrophic failure into the reservoir that would have sent a wall of water over the dam and into the community of Revelstoke.
During the construction of the dam, the engineers tunnelled into the rock at the base of the slide and drilled hundreds of drainage holes upward into the plane of failure. This allowed water to drain out more efficiently so that the hydrostatic pressure was decreased, which decreased the rate of movement of the sliding block. BC Hydro monitors this site continuously. The slide block is currently moving more slowly than it was prior to the construction of the dam.
In the summer of 2008, a large block of rock slid rapidly from a steep slope above Highway 99 near Porteau Cove, BC (between Horseshoe Bay and Squamish). The block crashed into the highway and adjacent railway and broke into many pieces, and the highway was closed for several days. Engineers and geoscientists took steps to stabilize the slope to decrease the risk of future rock falls. Rock bolts (long metal rods) were installed to anchor the blocks of rocks and prevent them falling Drainage holes were installed to drain water from the slope and decrease the water pressure. As shown in Figure 15.10, the rock along the slope is fractured parallel to the slope, and this almost certainly contributed to the failure. However, it is not actually known what triggered this event as the weather was dry and warm during the preceding weeks, and there was no significant earthquake in the region.
If a rock slides and then starts moving quickly (m/s), the rock is likely to break into many small pieces. At this point it is considered to be 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. The 1965 Hope Slide (Figure 15.1) was a rock avalanche, as was the famous 1903 Frank Slide in southwestern Alberta. The 2010 slide at Mt. Meager (west of Lillooet) was also a rock avalanche and rivals the Hope Slide as the largest slope failure in Canada during historical times (Figure 15.11).
Creep or Solifluction
The very slow — mm/y to cm/y — movement of soil or other unconsolidated material down slope is known as creep. Creep, which normally only affects the upper several centimetres of loose material, is typically a type of very slow flow. In some cases, sliding may take place too.
Creep can be facilitated by freezing and thawing because, as shown in Figure 15.12, particles are lifted perpendicular to the surface by the growth of ice crystals within the soil, and then move downwards vertically due to 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 or fence posts 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.” Creep can also generate terracettes, horizontal and repeating ridges on slopes (Figure 15.13). Historically, people thought terracettes formed where livestock or wild animals regularly travelled along slopes. While animals can accentuate terracettes, the primary reason terracettes form is creep.
A slide is a mass movement where the material moves as a coherent mass. A slump is a type of slide that takes place within thick unconsolidated deposits (typically thicker than 10 m). Slumps involve movement along one or more curved failure surfaces, and are thus rotational slope failures. Slumps have downward motion near the top and outward motion toward the bottom (Figure 15.14). They are typically caused by high water pressure within these materials on a steep slope.
An example of a slump in the Lethbridge area of Alberta is shown in Figure 15.15. This feature has likely been active for many decades and moves a little more whenever there are heavy spring rains and snowmelt runoff. The toe of the slump is being eroded by the small stream at the bottom. The erosion contributes to continued slumping. The basal material (material at the toe of the slope) forms the support for the overlying mass of material in the slope and if this support is eroded away slumping will likely continue.
Mudflows and Debris Flows
As you saw in Exercise 15.1, when a mass of sediment becomes completely saturated with water, the mass loses strength, to the extent that the grains may be pushed apart and may 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 rapid melting of snow and ice. If the material involved is primarily sand-sized or smaller, it is known as a mudflow, such as the one shown in Figure 15.16. A mudflow or debris flow on a volcano or during a volcanic eruption is called a lahar.
If the material involved is gravel sized or larger, it is known as a debris flow. Since it takes more gravitational force to overcome friction and move larger particles, debris flows typically form in areas with steeper slopes and higher water pressure. 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 may create a temporary dam followed by a major flow of water and debris when the dam finally bursts. This is the situation that led to the fatal debris flow at Johnsons Landing, BC, in 2012.
A typical west-coast debris flow is shown in Figure 15.17. This event took place in November 2006 in response to very heavy rainfall. There was enough energy in the flow to move large boulders and to knock over large trees.
Exercise 15.2 Classifying Slope Failures
These four photos show some of the different types of slope failures described above. Try to identify each and provide some criteria to support your choice.
Source of images: Steven Earle (2015) CC BY 4.0. View source