15.2 Classification of Mass Wasting
It’s important to classify slope failures so that we can understand what causes them and learn how to mitigate their effects. 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 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 along a sloping surface (without internal motion within the mass), it’s a slide.
- 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 easy to tell how the material moved. The types of slope failure that we’ll cover here 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 type of motion [SE]
Rock Fall
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. This happens because the water between cracks freezes and expands overnight, and then 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 (Figure 15.7).
A typical talus slope, near Keremeos in southern B.C., is shown in Figure 15.8. In December 2014, a large block of rock split away fro