What causes landslides?
Landslides happen when the pull from gravity exceeds the strength of the geomaterial forming the slope of a hill or mountain.
Geomaterials can be as varied as rocks, sand, silt and clays.
Then, part of this slope starts sliding downhill.
Most natural landslides are triggered by earthquakes or rainfall, or a combination of both.
Earthquakes shake the ground, stress it and weaken it over time.
Rainwater can seep through the ground and soak it – the ground is often porous like a sponge – and add weight to the slope.
Another adverse effect of water is erosion: the constant action of waves undercuts coastal slopes, causing them to fail.
Humans can (and do) cause landslides in several ways, too.
For example, deforestation has a negative impact on slope stability, as tree roots naturally reinforce the ground and drain water out.
Also, mine blasts produce small earthquake-like ground vibrations that shake slopes nearby.
Why can’t we predict landslides?
It’s very difficult to predict and mitigate landslide risk effectively.
Under our feet, geomaterials may include multiple, entangled layers of various kinds of rocks and particulate materials, such as sand, silt and clays.
Their strength varies from a factor of one to 1,000, and their spatial distribution dictates where the slope is likely to fail.
To accurately assess the stability of the slope, a three-dimensional mapping of these materials and their strengths is needed.
No sensor can provide this information, so geologists and geotechnical engineers must deal with partial information obtained at a few selected locations and extrapolate this data to the rest of the slope.
The weakest link of the chain – such as an existing fracture in a rock mass – is easily missed.
This is an inevitable source of uncertainty when trying to predict how much material might slip.
We do know that the larger the volume of a landslide, the farther its runout distance.
But it’s hard to gauge the exact size of a landslide, making predictions of runout distances and safe zones uncertain.
Mechanical analysis enables us to estimate the vulnerability of a slope in a particular scenario, including earthquake magnitude and distribution of groundwater.
But predicting if and when these triggers will happen is a difficult task.
Unfortunately, all the money in the world can’t buy accurate landslide predictions – especially in remote parts of the world.
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