Small-scale tests to investigate the dynamics of finite-sized dry granular avalanches and forces on a wall-like obstacle

Small-scale laboratory tests investigate the force from finite-sized granular avalanches on a wall. First, the reference flows, in absence of the wall, were analysed in a wide range of slopes from a minimum angle for which no flow is possible to a critical angle for which the flow becomes very dilute. The changes in thickness and velocity over time exhibit transitions at the minimum slope angle and at intermediate slopes. Then the normal force exerted on a wall spanning the flow was measured. It is notable that the transitions detected in reference flows had a direct effect on the force. The maximum force was equal to the kinetic force of the incoming flow at high slopes, whereas it scaled like hydrostatic force at lower slopes. This is the effect of the dense-to-dilute transition. Furthermore, the maximum force at low slopes was found to be several times greater than the hydrostatic force of the incoming flow. This finding is explained by the considerable contribution of the stagnant zone formed upstream of the wall. Furthermore, the jamming transition was highlighted at the avalanche standstill by the collapse of the residual force on the wall when approaching the minimum angle for which no flow is possible. These results are useful for the design of protection dams against rapid mass movements.