Stick-slip motion and high speed ejecta in granular avalanches detected through a multi-sensors flume

An oscillatory motion of dense, dry granular flows is experimentally observed by means of a 5.5. m-long densely instrumented flume. An array of 35 sensing devices allowed recording kinematic and dynamic data of the entire flow with high spatial and temporal resolution. Dynamic data, in addition to the kinematics, provide valuable information to unveil stick-slip motion that is proposed as the triggering mechanism for the appearance of periodic changes in compaction of the granular material called density waves. Periodic reverse graded strata after deposition of the flow accounts for such density waves. Besides, internal interactions between different parts of the avalanche, lead to the ejection of material and a high peak of the avalanche front velocity - or an abnormal acceleration - at the break slope and ejection of rocks from the flow was systematically observed, evidencing internal interactions among different parts of the avalanche body while descending the inclined plane. The velocity peak registered at the break in slope is interpreted as complementary evidence of these internal collisions and a non perfect inelastic-collision simple model is assumed in order to explain the counter-intuitive acceleration observed during the ejection process. High speed ejection at the break in slope, stick-slip, and expulsion from the flow are robust, ubiquitous phenomena which do not strongly depend on the granulometric distribution used. © 2015 Elsevier B.V.

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