Features of the stress-strain state of loose materials in converging channels and bunkers

Abstract

624.074.7~042.8 I. We concentrate first on the strength properties of loose materials which are well described by three typical curves plotted on the coordinates "ratio of tangential to normal stresses-shear" (Fig. i). Materials with compact particle packing have peak and residual "strength" (~ = ~max, ~ " @in' respectively, curve i), and the limiting and residual "strength" of the same materials with the loosest packing coincide and they are determined by the internal friction angle ~in (curve 2) [i]. Curve 3 characterizes the material being considered independent of the original quality of packing with shears of relatively smooth surfaces. In this case "strength" is governed by the material friction angle ~w at the wall a whose value is alwasy less than the angle Smax, as also for angle ~in" The latter is easily explained if this property of loose materials is considered as dilation. Shear of loose materials for relatively smooth surfaces proceeds without dilation effects. Such changes in volume which may be observed here are only related to reorientation of individual particles and a changeover to a more stable state (shear at stress levels which lead to particle failure at points of contact are not considered). With loose material shear for a relatively smooth surface made of the same material as the particles themselves, angle ~wa takes on the sense of angle ~ ' for the contact friction of a group (aggregate) of particles. The latter differs from the contact friction angle for an individual particle ~ and it may be considered as the friction angle for a nondilating (u = 0, where ~ is dilation ~ngle) loose material (strictly speaking the angle ~' should possibly be called the internal friction angle).Taking account of this, the peak value of the friction angle (traditional terminology) 9 @max " ~' + ~max, and the internal friction angle ~in = ~min = @~' + @min"It follows from Fig. 1 that the contribution of dilation to the overall strength of loose materials and materials with a structure as a whole is greater, the more compact the original particle packing. As was shown in [2][3][4], dilation makes a decisive contribution to the stress-strained state of loose materials with loading under constrained conditions. 2. We now consider the flow of loose materials whose "stength" is characterized by curve 1 in Fig. 1 in plane converging channels with a rigid wall inclination to the vertical 8 < 8* (8* is the angle atwhich in statics the condition T n > o n tan ~wa will be fulfilled alSng the walls, where T and o n are tangential and normal stresses at the wall). The discharge from converging c~annels has been considered previously [5][6][7]. We return to it only since it is possible to explain certain details of the process. It was shown in [5] that flow in channels proceeds with formation of slip surfaces separating masses into individual "rigid" blocks, and it is characterized by deformation asymmetry. As in other cases of gravitational flow [3,4,8], the initial deformation paths during discharge from channel...

Selecting the shape and bracing for excavations

“…Experience has shown that discharge hegins with a column motion in this case. active family [5], and are analogous to Ill slip surfaces and surfaces of the active family beyond the retaining walls. For the case in question, the gate or discharge opening AA: plays the role of a wall for small initial displacements.…”

Application of loose materal mechanics to solving some rock mechanics problems

“…It was demonstrated in [12] that in =he transition zone during discharge of loose materials from bunkers equipped with a converging section, slip surfaces form which are members of a passive family. Formation of these surfaces is related to noncompatibility of deformations in the transition and it causes an intense increase in stresses below it and a drop above.…”