Abstract
To investigate the effects of internal shear fragmentation on dry granular flow, in this study a series of ring shear tests were performed on quartz sand samples under different normal stresses (100 kPa, 200 kPa, and 300 kPa), shear displacements (3 m, 5 m, 10m, 15 m, and 20 m), and shear rates (30 deg min−1, 60 deg min−1, and 90 deg min−1). Next, the grain-size distributions, fractal dimensions, and microcharacteristics of the quartz sand before and after the experiments were compared and analyzed. The study results show that grain breakage under shearing preferentially occurs at the edges of the particles and forms a bimodal distribution in frequency grain-size distribution curves, which is consistent with observations of rock avalanches. The fine particles prevent the coarse particles from breaking, in turn leading to the ultimate grain-size distribution and stable fractal dimension (2.61) of quartz sand at relatively small shear displacements compared with the travel distance of rock avalanches. The results of this study suggest that the fragmentation of rock avalanches during the shear spread stage may be far less significant than previously believed. Therefore, the fragmentation effect is not considered to be a major factor of the hypermobility in the late stage of rock avalanches.
Highlights
Since Heim conducted a series of field investigations and described the hypermobility of the debris flow of the Elm landslide which occurred in Switzerland in 1881 [3], various theoretical models have been proposed to explain the highspeed and long-runout movement mechanism of rock avalanches [4], including the cushion of trapped air [5, 6], frictional melting [7,8,9], substrate liquefaction [10, 11], dynamic fragmentation [12,13,14], acoustic fluidization [15, 16], and momentum transfer motion [17]
Based on the fact that granular flow is influenced by multiple factors, dynamic fragmentation is more of a complex process controlled by a nonsingle mechanism [24,25,26,27,28]
Quartz sand can provide more microscopic information to reveal the dynamic processes of fragmentation [48,49]
Summary
Rock avalanches are distinguished by their massive scale, rapid speed, and extensive travel distances, all of which are extremely dangerous and destructive [1, 2]. erefore, it is of great significance to clarify the kinematic characteristics and mechanism of rock avalanches for their monitoring and early warning. Since Heim conducted a series of field investigations and described the hypermobility of the debris flow of the Elm landslide which occurred in Switzerland in 1881 [3], various theoretical models have been proposed to explain the highspeed and long-runout movement mechanism of rock avalanches [4], including the cushion of trapped air [5, 6], frictional melting [7,8,9], substrate liquefaction [10, 11], dynamic fragmentation [12,13,14], acoustic fluidization [15, 16], and momentum transfer motion [17]. We systematically analyzed the results of long-displacement ring shear tests in terms of shear stress variation, grain-size distribution and fractal dimension variation, and particle microscopic characteristics, so as to provide insights regarding the shear fragmentation effect of rock avalanches
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