Landslides and avalanches
By R. David Whitby, Contributing Editor | TLT Worldwide March 2023
Friction plays a key role in why these natural or human-caused occurrences are difficult to stop.
The major driving force for landslides and avalanches is gravity, although other factors can affect slope stability.
A landslide is when a mass of earth or rock slides down a slope. Landslides include rock falls and mudslides. They can happen in a range of environments, such as either steep or gentle slopes, coastal cliffs or underwater. They occur when the slope becomes unstable, which may be due to an increase in the shear stress borne by the slope material, a decrease in the shear strength of the material or a combination of these. An avalanche is a landslide of snow and ice, and perhaps some rocks.
The major driving force for landslides and avalanches is gravity, although other factors can affect slope stability, producing specific conditions that make a slope liable to fail.
Landslides and avalanches can happen due to natural causes, including saturation by rainwater or snow and ice melting, loss of soil structure after a wildfire, erosion of the top of a slope by rivers or sea waves, earthquakes, volcanic eruptions or chemical and physical weathering from repeated freezing, thawing, heating or cooling and from salt leaking into groundwater or mineral dissolution.
They also can be aggravated by human activities, such as vibration from machinery or traffic during construction or earthworks, mining, deforestation, logging, agriculture or changes to river systems that introduce more water into soils.
Once a landslide or avalanche has started, it is very difficult to stop, and will only do so when the mass of earth, rock, snow and/or ice arrives at a relatively flat area or a natural or human-made physical barrier. This is because, before the start of the landslide, the coefficient of static friction for the mass is greater than the coefficient of sliding friction. When a trigger disturbs the mass, the static friction suddenly is much lower than the sliding friction, and the mass starts to move. Then, when the coefficient of sliding friction becomes lower than the coefficient of static friction, the landslide will come to a halt. A similar phenomenon occurs during stick/slip friction in lubricated contacts.
Another trigger for landslides was investigated recently by Lucas Pelascini and his colleagues at the University of Rennes, France.
1 They report that very few studies have focused on the effect of atmospheric pressure on slope stability. During periods of very heavy rainfall, such as with typhoons and hurricanes, both rainfall-induced groundwater level changes, and atmospheric pressure changes can generate large pore pressure changes in soil. A little more than 10 years ago it was discovered that sporadic sliding of the Slumgullion landslide in the San Juan mountains near Colorado was triggered by changes in atmospheric pressure. The researchers developed an analytical model of transient groundwater dynamics to compute slope stability for finite hillslopes. Slope stability was evaluated through a safety factor based on the Mohr-Coulomb failure criterion. Both rainfall infiltration and atmospheric pressure variations, which impact slope stability by modifying the pore pressure of the media, are described by diffusion equations.
1
Weather and landslide data from several typhoons in Taiwan were fed into the model. It was found that the typhoon events could prime hillsides by bringing heavy rain and higher pressure of water in the soil, although the triggering of landslides depended on the weather over the preceding months. Although rainfall can induce pore pressure change up to hundreds of kilopascals, its effects are delayed in time due to flow and diffusion. Conversely, atmospheric pressure change induces pore pressure changes of only a few kilopascals, which propagates instantaneously through the soil before diffusion leads to an effective decay of pore pressure.
1 Rain falling on already saturated ground doesn’t alter the pore pressure enough to trigger a landslide. However, a subsequent change in atmospheric pressure, for example, when the center of the typhoon passes over the ground, was sufficient to trigger a landslide.
The study provides a better insight of slope stability through pore pressure analysis, showing that small changes in atmospheric pressure can start a landslide in certain weather conditions, and these effects should not always be neglected.
REFERENCE
1.
Pelascini, L., Steer, P., Mouyen, M. and Longuevergne, L. (2022), “Finite-hillslope analysis of landslides triggered by excess pore water pressure: the roles of atmospheric pressure and rainfall infiltration during typhoons,”
Nat. Hazards Earth Syst. Sci., 22, pp. 3125-3141. Available
here.
David Whitby is chief executive of Pathmaster Marketing Ltd. in Surrey, England. You can reach him at pathmaster.marketing@yahoo.co.uk.