Experiments On The Fluidization Of Rock Avalanche Under The Effect Of Entrapped AIR

For the study of the flow behaviours of rock avalanche under the effect of entrapped air, the Xiejiadianzi rock avalanche is studied with a series of experiments, equipped with a fluidized bed, a 3D terrain model, and a ring shear test equipment, designed by authors, carried out. In these tests, observations for the fluidization of rock avalanche debris from quasi-static state to dynamic state are realized with abundant of data on rock avalanche fluidization obtained. Based on these experimental data, the variations of debris movement (including velocity, travel distance, and movement form of debris), deposit characteristics (including accululation form and deposit thickness), and dynamic shearing stress between debris and underlying travel path under varying air supply conditions, are discussed in detail. The main results reached in these tests are as follows:(1) For materials having wide grain size distribution (ranging from 0.1 to 7 mm), sharp irregular shapes, and high fine-grain content at its lowest part, bridging phenomenon is easy to form, which can strengthen the stability of the deposit and prevent the escape of entrapped air. When air quantity is low, such deposits present a high stability with limited particle motion displayed. As the increase of air quantity, air entrapped in debris begins to gather with some nearly horizontal bubbles generated and forming local high pressure zones in debris. Due to the high stability of such debris, when air quantity is high enough, an "air cushion layer" will present at the bottom of the deposit, originating an air born system.(2) In the 3D model tests, the travel distance and velocity of debris display an increasing trend as the increase of air supply, which indicates that the entrapment of air during rock avalanche motion can lubricate the movement of debris in some degree. However, due to the high fragmentation of material used in the tests, the lubrication effect induced by air entrapment just existed in a short section as the rapid escape of entrapped air.(3) In the 3D model tests, debris shows obvious movement characteristics of fluid. Under the control of topography, some fluidized phenomena, such as ramp turn supperelcvation and climbing accumulation, can be observed. During the high-speed travel of such debris, a "shunting-convergence movement" was observed under the control of the topography along the travel path, which induced the formation of longitudinal ridges and grooves. In addition, the unconfined motion of debris along the travel path caused the formation of conjugated "X" type accumulation form of debris.(4) During the movement of debris along complex terrain, the velocity of debris decreases so fast, inducing a rapid accumulation of travel debris with short travel distance presented. The display of such deposit characteristic indicates that the layout of the topography along the travel path has obvious energy consumption effect during rock avalanche motion. Hence, for the realization of long runout travel of rock avalanche, some lubrication mechanisms, such as entrapment of air or water, must be existed in the transport of rock avalanche.(5) The results of the ring shear tests exhibit that the entrapment of air can lubricate the motion of rock avalanche obviously. Under the air supply conditions, the shear stress between the loading plate and debris ranges from 120 to 200 Pa that is only half of the corresponding values under the conditions without air supply.(6) No matter with or without the supply of air, the increase of the shear stress between the loading plate and debris is not clear as the increase of shearing rate. This indicates that the variation of the travel velocity of debris has no obvious influence on the shear stress between the loading plate and debris.