Before venturing onto a slope, identifying whether the mass of earth, rock, or snow is unstable is a foundational safety practice. An unstable slope is any material prone to sudden failure, such as a landslide, rockfall, or avalanche. Once a slope fails, the speed and force involved make escape nearly impossible. Understanding the static characteristics and dynamic warning signs of instability is the only way to minimize risk. This assessment must be completed before a person’s presence adds the triggering weight that leads to a collapse.
Structural Features and Slope Angle
Slope angle is the most fundamental static characteristic determining a slope’s stability, as it directly relates to the gravitational shear stress acting on the material. For snow, most slab avalanches occur on slopes angled between 35 and 45 degrees, with instability peaking around 38 to 40 degrees. Slopes below 30 degrees generally lack the steepness to overcome the snowpack’s internal friction, making them less prone to sliding. For loose earth, the natural angle of repose is typically between 30 and 37 degrees, meaning any steeper incline is inherently stressed.
The direction a slope faces, known as its aspect, also influences stability by affecting the temperature and moisture content. Sun-exposed slopes may experience rapid freeze-thaw cycles that weaken the structure, while shady slopes retain colder, more fragile layers of snow. The terrain’s shape plays a distinct role. Concave slopes, which resemble a bowl, are generally more stable due to lateral confinement. Conversely, convex slopes, which bulge outward, are often less stable because they lack lateral support and can act as trigger points where the angle abruptly increases.
Immediate Visual Warning Signs
Observing dynamic, visible clues is a direct way to gauge the slope’s current level of stress and potential for immediate failure. Tension cracks, or fissures, are a primary indicator of material separation, often forming near the crest of an earth slope as the mass below begins to move. In a snowpack, these are known as “shooting cracks,” which propagate outward from a weight source and signal that an unstable slab layer is fracturing. Slumping or subsidence is another crucial sign, appearing as a hummocky, uneven surface or a bulge near the toe, indicating the ground is visibly sagging or displacing.
Audible warnings are also present, particularly in a snowpack, where a “whumpfing” sound occurs when a deep weak layer fractures and collapses under weight. This sound is a muffled thunderclap, accompanied by the ground settling slightly, and clearly signals that the entire snow column is unstable. Look for evidence of recent activity, which confirms the slope is actively unstable and should be avoided:
- Fresh debris.
- New crown lines.
- Small, previous slides within the last 48 hours.
External Triggers and Consequence Zones
Understanding external forces that influence stability is as important as recognizing the slope’s inherent characteristics. Weather history, especially recent precipitation, can rapidly destabilize a slope by adding significant weight and increasing the water content. Rain on a snowpack is particularly dangerous because water percolates down and weakens the bonds between snow layers. On earth slopes, rain increases the pore-water pressure, which dramatically reduces the soil’s internal shear strength. Rapid temperature fluctuations, such as a quick thaw, can also destabilize the material by melting the ice bonds or causing permafrost to thaw.
Triggers are the final factors that initiate movement in an already unstable slope, and they can be human or natural. Before committing to a route, one must identify the consequence zone, or runout zone, which is the area at the bottom of the slope where debris will accumulate if failure occurs. For avalanches, this zone can be estimated by looking for evidence of past slides, such as broken trees or open swaths of terrain.
Triggers
Human factors include:
- Weight from people or vehicles.
- Vibration from construction activity.
Natural triggers include:
- New snowfall or wind-loading.
- A seismic event.
- Falling ice.