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 {δ}{θ} and the maximum angle of repose θₘ 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 {φ} and a mean avalanche size equal to 1.7^∘{}. This demonstrates that the density d is a controlling parameter of the avalanche process. These results are compatible with a unique internal friction angle {φ} with {φ}=34^∘{}; {φ} corresponds to the value measured with other soil-mechanics devices; however, {φ} fluctuates within 2^∘{} 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.