Rockslides and Their Motion

The motion of landslides sourced from mostly bedrock (called rockslides) is controlled by the phenomenon of grain flow, and the frictional resistance of the constituent rock grains and their interstitial fluids. Modern understanding of grain-flow dynamics recognises that the important interactions between grains are irregularly distributed within the grain mass, with fortuitous alignments of grains carrying most of the stress in “force chains”, while other grains are only weakly stressed. In rapidly shearing grain flows, under substantial confining stress, force-chain stresses rise high enough to crush grains. Such comminuting grain flows develop a distinctive grain-size distribution that is fractal over many orders of magnitude of grain size down to sub-micron sizes. In the moment of crushing, grains are not solids, and behave as high-pressure fluids. As the grain fragments are injected into lower pressure surroundings, they behave as would any other fluid, lowering the effective stress on other grains, and thereby lowering frictional resistance to flow. We show how this affected the blockslide component of New Zealand’s prehistoric giant Waikaremoana rockslide; New Zealand’s Falling Mountain rock avalanche triggered by an earthquake in March 1929; and a small prehistoric New Zealand rockslide that was too small to be a comminuting grain flow, but which fell on and mobilized a fine, saturated substrate. We use grain-flow dynamics to explain the motion of these rockslides determined through field studies and physical and numerical modeling.