Abstract
Large landslides exhibit surprisingly long runout distances compared to a rigid body sliding from the same slope, and the mechanism of this phenomena has been studied for decades. This paper shows that the observed long runouts can be explained quite simply via a granular pile flowing downhill, while collapsing and spreading, without the need for frictional weakening that has traditionally been suggested to cause long runouts. Kinematics of the granular flow is divided into center of mass motion and spreading due to flattening of the flowing mass. We solve the center of mass motion analytically based on a frictional law valid for granular flow, and find that center of mass runout is similar to that of a rigid body. Based on the shape of deposits observed in experiments with collapsing granular columns and numerical simulations of landslides, we derive a spreading length Rf~V^1/3. Spreading of a granular pile, leading to a deposit angle much lower than the angle of repose or the dynamic friction angle, is shown to be an important, often dominating, contribution to the total runout distance, accounting for the long runouts observed for natural landslides.
Highlights
Landslides are natural hazards responsible for serious damage to life and property
We study kinematics of granular materials flowing down an inclined plane and decelerating on a flat plane thereafter
Large landslides (V > 108) exhibit low Heim’s ratio and corresponding long runout distance compared to sliding of a rigid body
Summary
Landslides are natural hazards responsible for serious damage to life and property. Understanding their flow would allow us to predict their runout (horizontal travel distance), estimate their energy and reduce the associated risk. From the mechanical point of view, landslides are granular mass flowing down a hill slope and on a flat plane while spreading due to internal shear. The mechanical problem of evolution of a granular flow in the configuration depicted in Figure 1 have not been fully solved yet. We give an analytical solution for the case when the slide can be modeled as a granular flow driven by gravity and mechanical interaction between grains. We predict the runout distance L and compare the prediction to natural landslides
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