Rockfalls occur frequently on steep slopes due to the effect of gravity causing the movement of unstable materials downwards. Steep slopes tend to accumulate a larger amount of potentially unstable materials, thus increasing the risk of rockfalls.
On steep slopes, everything is a matter of balance. Gravity, this natural force that pulls materials downward, plays a key role here by constantly pushing rocks, soil, and everything resting on them down the slope. As long as the friction between these materials and the ground holds, it stays in place. But as soon as this resistance decreases, due to weight, rain, or vibrations, gravity pulls everything down, and that's when it starts to go awry. The steeper the slope, the more easily gravitational attraction acts, thereby increasing the risk that entire chunks will tumble down without warning.
The climate greatly affects the stability of sloped soils, mainly through the action of water. Prolonged periods of rain saturate the soil, making it heavy and slippery. This reduces the cohesion of the ground, thus promoting landslides or collapses. Conversely, drought episodes lead to cracks and a loss of elasticity in the earth. The result? At the first rains, it gives way. Sudden temperature fluctuations do not help either: repeated freezing and thawing gradually weaken already fragile rocks, facilitating their fall.
Human activities such as earthworks or construction sites significantly alter the natural balance of soils. When roads are dug in the mountains or large rocks are blasted to create tunnels, the slopes are clearly weakened. The rock then becomes more unstable, and the risks of landslides increase significantly. The same goes for deforestation, which removes trees that often help stabilize the ground with their roots. Without trees, the soil holds much less firmly, making landslides more frequent, especially after rainfall. Finally, massive urbanization adds pressure to already sensitive areas by adding weight with buildings, roads, or facilities that further destabilize steep slopes.
Plants play an essential natural role in stabilizing sloping terrain, particularly due to their roots that act like a true underground net. The denser and deeper the roots, the more effectively they hold the soil, preventing it from sliding. Trees and shrubs protect the soil surface by cushioning the direct impact of raindrops, which limits soil erosion. A vegetated soil absorbs water better than a bare soil. Conversely, a deforested slope quickly tends to lose stability, as it is directly exposed to the elements without natural protection. This is why we often see reforestation efforts after landslides or significant collapses.
Under the action of wind and rain, rocks gradually wear away, slowly weakening the terrain, particularly on steep slopes. Water erosion creates cracks, widens faults, and quietly erodes rocky surfaces and loose soils. Meanwhile, frost can play a nasty trick: water that infiltrates the cracks freezes, takes up more space as it expands, and causes the rocks to burst from the inside—this is known as the phenomenon of frost shattering. The sun also contributes to wear; it heats during the day and cools at night, causing repeated expansions and contractions of the stones, making these terrains significantly more unstable in the long term. After enduring all these seemingly harmless little damages, everything inevitably gives way: that’s when major landslides occur.
Vegetation can reduce the risk of landslides by up to 80%. Indeed, the roots of trees and plants firmly hold the soil, providing valuable stability.
Some animals, such as mountain goats, indirectly play a role in the occurrence of landslides. Their frequent movements on steep slopes contribute to the displacement of small rock fragments, sometimes increasing the risks.
Did you know that the vibrations caused by heavy truck traffic can trigger landslides on certain mountain roads? Specific measures are necessary to secure these sections.
According to scientific studies, a rocky slope that has experienced a forest fire is up to 5 times more prone to landslides than a well-vegetated slope.
Yes, climate changes can exacerbate erosion and soil destabilization phenomena, for example by increasing the intensity of rainfall or causing prolonged drought episodes. These extreme weather conditions have the effect of multiplying the risks of landslides.
If you find yourself near an imminent or ongoing landslide, quickly move sideways, away from the direct path of the falling rocks. Seek shelter behind a solid obstacle and immediately report the situation to emergency services.
Several techniques exist to prevent rockfalls on roads and pathways: installation of nets or rockfall barriers, reinforcement of rock faces through anchoring or drainage systems, and vegetation of slopes to stabilize the soil.
Although vegetation significantly contributes to enhancing the stability of steep terrains by anchoring the soil with its roots, it is not always sufficient, especially in cases of extremely steep slopes or unstable subsoil. A combined approach that also includes soil stabilization works is often necessary.
Some visible signs include cracks in the ground or rocks, tilted or uprooted trees, and small, frequent movements of stones or soil on the slope. It is important to regularly monitor these indicators to anticipate potential landslides.
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