Development of arching and silo wall pressure distribution in storage and discharging state based on discrete element analysis

It has been widely recognized that the most dangerous condition is the phenomenon that the emptying pressure on the wall of silos vibrates and increases obviously during discharging. However, the cause of this phenomenon is currently largely unknown. Most of the available papers and reports are based on continuum mechanics, that is, the stored material is regarded as a continuous entity on macroscopic level, and its particulate property is ignored. In fact, it is the microcosmic mechanical behavior of individual particles and the interaction between the particles and the silo walls that determine the emptying pressure distribution on the silo walls. Therefore, in this paper, the characteristics of emptying pressure were studied from the point of view of granular materials and microcosmic particle mechanics. A new method combining particle mechanics with discrete element method (DEM) was introduced to explore how the mechanical behavior of particles effects the distribution of emptying pressure. Firstly, the behavior of granular material in stored state was studied by DEM method. The silo with flat bottom is 0.5 m in diameter, 1.0 m in height and 0.1 m in outlet diameter, which is filled with 20 400 spherical particles. The distribution of the static wall pressure in stored state was verified by the test results and the Jassen Formula which is widely used in engineering. Secondly, the pressure distribution on the silo bottom wall was studied by simulating the discharge process, and the statistical analysis of multiply simulation results was performed, the arching effect near the outlet were proved according to the periodic pressure profile. Thirdly, in order to study the arching effect, three time points, i.e. start of arching, completion of arching and arch collapse, were selected on the periodic pressure profile. For each time point, the behaviors, such as force chain network, vertical stress distribution, and lateral stress distribution, the direction of principal stress, velocity field of granular material and so on, were systematically studied. On this basis, the features and evolution mechanism of the arching effect were investigated from the viewpoint of particle mechanics. Finally, the static pressure under the static stored state and the emptying pressure in discharging process on the silo wall were analyzed and verified by a model test, Standards (GB50322-2011) and published results. The results indicated that the arch which is 4.0 times wide of outlet diameter and 2.5 times high of outlet diameter was produced during the discharge process. Due to the formation of arching, the vertical stress above arching was transformed into the horizontal stress in a certain range above the arch foot, thus the horizontal stress was transferred to both sides of the silo wall and to the pressure of the silo wall was increased. When the arching collapsed, the vertical and horizontal above arching decreased, then the horizontal stress transferred to both sides of the silo wall also decreased, which resulted in to the decrease of wall pressure. The coefficient of overpressure reached a peak value of 2.70 at depth ratio of 0.35, while the peak wall pressure was 3.57 kPa at depth ratio of 0.85. It was found that the arching effect near the outlet behaved dynamically, following the rule of “start of arching-completion of arching-collapse of arch”, thus the dynamic arching effect is put forward. In addition, the important connection between the dynamic arching effect and the resulting emptying pressure distribution was identified. That is, the increased emptying pressure on silo wall is caused by the formation of arch, while the vibration of emptying pressure on wall of silo is induced by the dynamic arching effect. The research provides a new method of exploring the relationship between the arching effect and the emptying pressure distribution on wall of silos from the micro and macro aspects. The findings obtained in this paper can provide references for revealing the load-transfer mechanism from particles to silo wall.