5.2 Classification of Mass Wasting 

It is important to classify slope failures so that we can understand what caused them, learn how to mitigate their effects, and communicate clearly.  The three criteria used to describe slope failures are:

The type of motion is the most important characteristic of a slope failure, and there are three different types of motion: if the material drops through the air, vertically or nearly vertically, it’s known as a fall, if the material moves as a mass (without internal motion within the mass), it’s a slide, and if the material has internal motion, like a fluid, it’s a flow. Unfortunately, it’s not normally that simple.  Many slope failures involve two of these types of motion, some involve all three, and in many cases it’s not that easy to tell how the material moved. The types of slope failure that we’ll cover here are summarized in Table 15.1.

Table 5.1.1  Classification of Slope Failures Based on Type of Material and Type of Motion

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 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)
Mud flow	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 mud flow, but typically faster)	Fast (m/s)

Rock fragments can 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.  If you’ve ever hiked along a steep mountain trail on a cool morning you might have heard the occasional fall of rock fragments onto a talus slope as the sun melts the ice, releasing rock fragments that had been wedged out the night before.  This process is illustrated in Figure 5.1.6 above.

A typical talus slope, near to Keremeos in southern BC, is shown on Figure 5.2.1.  In December 2014 a large block of rock split away from a cliff in this same area.  It broke into smaller pieces, which fell and tumbled down the slope and crashed into the road, smashing the concrete barriers and gouging out large parts of the pavement.

A rock slide is the sliding motion of rock along a sloping surface. In most cases the movement is parallel to a fracture, bedding plane or metamorphic foliation plane, and it 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 steep slope.  A good example is the Downie Slide north of Revelstoke BC, which is illustrated on Figure 5.2.2.   In this case a massive body of rock is very slowly sliding down a steep slope along a plane of weakness that is parallel to the slope.^[1] The Downie Slide, which was recognized prior to the construction of the Revelstoke Dam, was moving very slowly at the time (a few cm/year). Geological engineers were concerned that the presence of water in the reservoir (visible on Figure 5.2.2) could further weaken the plane of failure, leading to an acceleration of the motion.  The result could have been a catastrophic failure into the reservoir that sent a wall of water over the dam and into the community of Revelstoke.  During the construction of the dam, they tunneled 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 so that the pressure was reduced, and that stabilized the slide block.  BC Hydro monitors this site continuously; the slide block is currently moving slower 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 to Porteau Cove (40 km north of Vancouver).  The block slammed into the highway and the adjacent railway and broke into many pieces.  The highway was closed for several days, and the slope was subsequently stabilized with rock bolts and drainage holes.  As shown on Figure 5.2.3, the bedrock at this location is fractured parallel to the slope, and this almost certainly contributed to the failure. It is not actually known what trigged this event as the weather was dry and warm during the preceding weeks and there was no significant earthquake in the region.

A rock slide is typically a “translational slide”, meaning that the part of the rock that is moving down the slope without rotating—it is translating.  Please don’t confuse “translational” with “transitional”. As we’ll see below, a “rotational slide” (or slump) is when the material (typically unconsolidated sediments) move as a single rotating unit along a curved surface.

If a rock slide starts moving quickly (m/s) the rock is likely to break into many small pieces, and at that point it can become a rock avalanche, in which the large and small fragments of rock move in a fluid manner supported by cushion of air within and beneath the moving mass.  The 1965 Hope Slide (Figure 5.0.1) was a rock avalanche, as was the famous 1903 Frank Slide in southwestern Alberta.  The 2010 slide at Mt Meager (west of Lillooet), also a rock avalanche (Figure 5.2.4).^[2]

The very slow—mm/y to cm/y—movement of soil or other unconsolidated material on a 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, but in some cases sliding may take place.  Creep can be facilitated by freezing and thawing, because, as shown on Figure 5.2.5, particles get lifted perpendicular to the surface by the growth of ice crystals within the soil, and are then let down vertically by gravity when the ice melts.  The same effect can be produced by frequent wetting and drying of the soil.

  Figure 5.2.5  A depiction of the contribution of freeze-thaw to creep.  The blue arrows represent uplift caused by freezing in the wet soil underneath, while the red arrows represent subsidence by gravity during thawing.  The uplift is perpendicular to the slope, while the subsidence is vertical.   (Author drawing, CC BY 4.0)

Creep is most noticeable on moderate to steep slopes where trees, fence posts or gravestones are consistently leaning in a downhill direction (Figure 5.2.6).  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” (or “j-shaped tree trunk”).  Creep can take place on nearly flat surfaces.

Slump is a type of slide (movement as a mass), that takes place within thick unconsolidated deposits (typically greater than 10 m).  Slumps involve movement along a curved surface, with downward motion near to the top and outward motion towards the bottom (Figure 5.2.7).  They are typically caused by an excess of water within the materials on a steep slope.

An example of a slump in the Lethbridge area, Alberta, is shown on Figure 5.2.8.  This feature has likely been active for many decades, and moves a little more whenever there are heavy spring rains and significant snow-melt runoff.  The toe of the slump is failing because it has been eroded by the small stream at the bottom.

As you will have seen from completing Exercise 5.1, when a mass of sediment becomes completely saturated with water, to the extent that the grains are pushed apart, the mass will lose strength and flow, even on a gentle slope.  This can happen during rapid spring snow melt or heavy rains, and is also relatively common during volcanic eruptions because of the rapid melting of snow and ice.  If the material involved is primarily sand-sized and smaller it is known as a mud flow, such as the one shown on Figure 5.2.9.  If the material involved is a mixture of sizes, including gravel-sized and larger, it is known as a debris flow.  Because it takes more gravitational energy to move larger particles, a debris flow typically forms in an area with a steeper slope and more water than does a mudflow.  A typical debris flow is shown on Figure 5.2.10.  This event took place in November 2006 in response to very heavy rainfall.  There was enough energy to move large boulders and to knock over large trees.

As already noted, to understand a slope failure we need to be able to determine what type of material moved, what type (or types) of motion were involved, and how quickly it moved.  The type of motion is the most important of these, and so Figure 5.2.11 is provided here to help you clearly understand how things moved in different types of slope failure.