Physical models of rock avalanche spreading behaviour with dynamic fragmentation

The dynamic fragmentation of rock during avalanche motion has been postulated as a mechanism explaining the long runout of large rock avalanches or Sturzstrome. This paper investigates whether test conditions that produce dynamic fragmentation can lead to greater runout or spreading of physical model rock avalanches. Model avalanche experiments were carried out under enhanced acceleration to generate breakage in coal, a fragmentable, brittle solid. Coal blocks were released from a stationary position on a slope to run out on a plane. The motion of the ensuing fragmenting debris was captured using a high-speed camera placed above the horizontal plane. The average position of the front was tracked and the degree of fragmentation of the model avalanches was quantified. The paper presents results of the frontal velocity of the avalanches, corrected for centrifuge Coriolis effects. Comparison is made between the peak and impulsive front velocities, the final runout, and the degree of fragmentation of the model avalanches. Strong relationships are found between runout normalized by the cube root of volume, impulse velocity, and Hardin's relative breakage parameter, B-R. Results are discussed in light of the mechanics involved and are compared with field-scale events.