New methods of calculating load on supports

Self Cite

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.

mentioning

confidence: 85%

The second form of flow for loose materials in bunkers

Stazhevskii¹

1985

Self Cite

“…Experiments performed by the method in [3] for seems dipping at ~ < 60 = have revealed that for certain displacements of the support, the material (dry sand) beyond the guard is separated into blocks by global sllp planes (lines) (Fig. i), As a result, the guard is acted on not by the whole column of loose material but only by part of it, AxAsA3, intersected by some sllp plane and directly linked ~rith the support.…”

mentioning

confidence: 97%

“…2 and Table l) differs from the value widely used In mechanics to determine the active pressure of the rocks on the supporting walls of the coal (~/4 + r The above classlflcatlon can be used as a criterion for choosing the expressions for calculating the load on the supports: Loads on supports with a > 75-80 ~ can be calculated by mean~ of the expressions derived for ~ = 90 ~ [3]; loads in seams dipping at 60 ~ < a < 75-80 ~ can be calculated uslng the expressions for steep dip [3].…”

mentioning

confidence: 99%

“…If the dip is steep (a Z 60 = = ~/4 + @/2), the deformation process begins to involve not only friction of the loose material on the floor and roof (the latter increasing with a) but also the dilation properties of the material [3].…”

mentioning

confidence: 99%

“…In the range a = 50-60 ~ , the loads on supports of the guard type can also be calculated using the expressions in [3] provided that we take account of the load increment due to the malt roof.…”

mentioning

confidence: 99%

See 2 more Smart Citations

Calculating the load on movable guard-type supports in flat-lying or inclined seams

Stazhevskii

1977

Self Cite

An urgent problem in mining practice is to determine the loads on guard-type supports in flat-lylng and inclined (semlsteep) seams.In this article we will discuss one approach to the construction of load calculation schemes based on investigations of the deformation mechanisms of loose materials beyond the support and of the surrounding rocks [i, 2].i. Experiments performed by the method in [3] for seems dipping at ~ < 60 = have revealed that for certain displacements of the support, the material (dry sand) beyond the guard is separated into blocks by global sllp planes (lines) (Fig. i), As a result, the guard is acted on not by the whole column of loose material but only by part of it, AxAsA3, intersected by some sllp plane and directly linked ~rith the support. This fact forms the basis for our scheme of calculations.Thus, one of the principal parameters determining the pressure on the support is the angle of inclination of the slip plane to the floor of the seam, which we call B. The angle ~ is found experimentally and depends on the angle of dip a of the seem and the angle ~ between the guard part of the support and the floor. We will not consider how B depends on the properties of the material.Figure 2 is a plot of 8 and 8 vs a (where e is the angle of inclination of the slip plane to the horizontal) for ~ = 90 = . In the construction of both curves the sllp curves were approximated by straight lines. We see that the curves of 8 and ~ vs r have characteristic points A, B, C, and D. The slope of the sllp lines is a consequence of all the conditlons of deformation of the material in its limiting state. Therefore we can expect that the singular points on the diagrams plotting the slopes of the sllp lines vs the angle of dip of the seam will mark changes in the conditions of deformations of the material beyond the support. These changes can be followed by various experiments.Suppose that the angle of dip of the seam is 0 < a < 30 ~ " ~@ (where @o = 32_34 @ is the angle of internal friction of the material).In this case the roof of the seam exerts no influence on the formation of the region AxA=As, because from the onset of movement of the guard the material loses contact with it (see Fig. la-d). The slope of llne A,As depends on the properties of the material, the slope of the free surface of the loose material (the surface "unstuck" from the roof) to the horizontal, and the friction conditions at the floor of the seam.On further increase in ~, the picture changes (see Fig. le-f).The free space formed between the roof and the block AxA=A4 is continually filled with loose material (the roof comes down) and the formation of the sllp planes involves friction of the material both on the floor and on the roof. However, in this case also the floor continues to play the principal role.If the dip is steep (a Z 60 = = ~/4 + @/2), the deformation process begins to involve not only friction of the loose material on the floor and roof (the latter increasing with a) but also the dilation properties of the material [3].Instit...

Results of modeling of the operational conditions of a pneumatic cylinder mobile support system in the mining of thick steep beds

Kulakov¹,

Ogorelkov²

1984

The pneumatic cylinder mobile support system serves to protect the near-face area from rocks being ejected into the worked-out chamber and prevents the roof from collapsing when miming to the dip in thick steep beds using pillars, with the collapse or self-filling of mined-out spaces. At the first stage in support system development, laboratory tests were done to study the following aspects:the interaction between the support system and the lateral and overhead rocks; investigate and develop methods for support control; and evaluate the parameters of the stope face and support system stability in respect of fold formation.The support model and the testing installation were made to the scale of 1:20. The similarity between the model and natural conditions is provided by geometric s~m~larlty and also by the fact that all loads applied to the support model (distribution on the surface and flexing moments) should be in the same relative ratios as the respective loads on the prototype.We consider the conditions and similarity criteria in detail.The pattern of motion of the support system as the bed is worked and its sagging over the stope space are determined by the direction and magnitude of the loads from the overhead rocks. The model scale should be selected such that the mechanism of movement and deformation of the overhead rocks in the model correspond to the natural conditions. Studies [I, 2] have shown that granular material retains its properties when particle sizes relate to the smallest opemingdimenslon as 1:30 or less. For coal seams thicker than 6 m, this condition is met, because the particle size of filling material is not larger than 200 mm. When filling material of 0-6 --,was used in the model, the effective model width (bed thickness) should be at least 200 mm. The mechanism of deformation of granular material in the model will then correspond to the real processes.Meeting the first sdmdlarity condition implies that the second condition is also met to some extent. Basically, this similarity requires that the loads on the support from the transported rocks are determined by the mechanism of rock deformation.If the deformation mechanism in the model corresponds to natural processes, the load on the mobile support in the model would be determined as in situ according to [2] as or in nondimensional parameterswhere P(l~m~) is the ~ pressure on the mobile support system; a. bed dip; 8, shear angle of the transported rock from the bed floor; and C, a nondimensional coefficient determ~-ed by the internal friction and lateral pressure.Therefore, P(HLax) is the principal criterion for the model's s~m~larity to the natural conditions.The rocks transported in the worked-out space must also have equal internal friction coefficients tan 9 -idem, in addition to the adequate ratios of granular sizes. Because the sag of the support system in the near-face area is determined by the bending Institute of Mining, Siberian Branch, Academy of Sciences of the USSR, Novosibirsk.