PILE DENSITY IS A CONTROL PARAMETER OF SAND AVALANCHES

Centrifuge experimentation (10-1000 m/s2) on sand avalanches has shown that (i) grain cohesion and elasticity are negligible; (ii) the avalanche size deltatheta and the maximum angle of repose theta(m) depend on the initial pile density d; and (iii) the pile surface evolves spontaneously towards an asymptotic state after a few avalanches, this state being characterized by a mean surface angle equal to the macroscopic friction angle phi and a mean avalanche size equal to 1.7-degrees. This demonstrates that the density d is a controlling parameter of the avalanche process. These results are compatible with a unique internal friction angle phi with phi = 34-degrees; phi corresponds to the value measured with other soil-mechanics devices; however, phi fluctuates within 2-degrees from pile to pile or test to test. These data are also consistent with a dilatancy effect which depends on density, but the existence of at least one other parameter controlling the avalanche size has been also proved.