The second form of flow for loose materials in bunkers

Abstract

The external manifestation of the so-called second (free) form of flow for loose materials in bunkers during discharge is descent of the charge surface parallel to itself, or as it is customary to assume, material movement as a whole column alon~ the wall.It is well known that this form of flow occurs, as a rule, in high silo bins (H = H/D ~ 3-4; H is silo height, D is diameter).The second form attracts most attention both from specialists in the field of loose body mechanics, and specialists in the field of design, planning, and operating silo structures. This interest is caused by the fact that with the second form of movement horizontal pressures on the silo walls increase by a factor of 2 to 2.5 compared with static pressures, and according to some data even by a factor of 5-7 [i]. Increased pressures develop at a certain characteristic height asymmetric relative to the bunker axis of symmetry and they have a pulsating character [2,3].The increase in pressure at the~closing surface during discharge, as already noted in [4], is connected with a freely formed change in flow and even with sonic shock waves.A number of researchers are inclined to explain this increase by dynamic phenomena [5] (whence the extensively used term "dynamic" loading).There is no single view on the nature of the rest of these facts.Standards set in most countries for determining static loads on silo el~ents recommend the method proposed in 1895 by Janssen [6] in spite of the existence of many other methods. This selection is dictated by the simplicity of relationships obtained in [6], and numerous verifications of calculated results by experiments.This situation is a weighty argument in favor taking the Janssen relationships as a basis for determining dynamic loads (in future we will call them peak or maximum loads). However, the difference in views on the mechanism of load formation, and, as a consequence of this ambiguity in estimates of absolute values of the latter, leads to the fact that the load curves for the same structure calculated by standards of different countries differ markedly.This paradoxical fact is illustrated in Fig.

Application of loose materal mechanics to solving some rock mechanics problems

“…la in [3], in which the initial stage of flow of dense sand is given during discharge from a vessel, then it is possible to observe its overall qualitative similarity with the deformation picture in question in an underworked rock mass. As was shown in [4,5], the slip surface formed here has the minimum shear strength (~mob = ~, ~mob is mo-, bile friction angle ~ is contact friction angle of the particles), forms in all cases in the first instance, and makes it possible for further development of deformation in the loose rock mass.…”

“…As was noted in [8,9], almost all of the studies for discharge reveal that flow of loose materials in large vessels is accompanied by pulsating stresses which in an oscillogram are recorded in the form of steps of different amplitude and frequency. Pulsation effects are accompanied by typical sound effects very similar to sharp claps.…”

“…The reasons and mechanism for this phenomenon have been studied in detail in [8,9]. We mention its main features and dwell on some new details which may be useful for understanding phemonena occurring in rock masses, First it has been shown that the effect of stress pulsation is related to formation of slip 8-surfaces forming a "filled" converging channel, and it only develops in regions where the material has sufficiently compact packing of particles [8,9]. The main reason for development of stress steps is the noncompatibility of deformations during discharge in central and peripheral regions of the vessel.…”

“…As was shown in [4,5], the slip surface formed here has the minimum shear strength (~mob = ~, ~mob is mo-, bile friction angle ~ is contact friction angle of the particles), forms in all cases in the first instance, and makes it possible for further development of deformation in the loose rock mass. The inclination angle from the vertical of these surfaces for materials with compact particle packing is of the order of 23-35 ~ [3,5], and consequently it is 67-55 ~ from the horizontal.…”

“…A result of the noncompatability is periodic alternating advance and stopping of the deforming part of the rock mass at slip B-surfaces. A stoppage is the result of impact through an "intermediate layer" E of two blocks $ and $' moving with a certain velocity to meet each other [8,9] (Fig. 2).…”