At the Dzhezkazgan deposit the increasing depth of the mine workings has led to several cases of unjustified high losses of ore left in the pillars.This has made it necessary to determine the optimum parameters of the room-and-pillar system of working.A successful and practical solution to this problem is assisted by the use of mining geophysics to make a detailed investigation of the state of stress and the elastic properties of the rocks in the pillars, and of the disturbed zones induced by shaping of the pillars by drilling and blasting.This article gives certain results obtained by the use of ultrasonic sounding to investigate the elastic properties in a circular interchamber pillar (diameter 9 m, height 7 m) composed of strong flne-gralned sandstone and located in a horizontal bed of mine No. 45 (+235 horizon).A prel~m~nary inspection of the exposed surface of this pillar showed that its rocks are broken up by a network of cracks and mlcrocracks running in all directions due to drilling and blasting.

To ensure efficient and safe mining operations, it is important to know the stress in the rock mass around mine workings and its distribution pattern.It is known that mining operations in any rock mass disturb its internal extuilibrium, At the points in the rock mass adjoining the generatrix of the cavity the stresses are redistributed and concentrated around the working, This is the principal physical cause of the change in the properties of the rock around the working.Depending on the rock strength, two types of mechanical state of the rock mass around a working are most frequently observed; corresponding to these, two types of distribution of the acoustic characteristics of the rocks are noted as we go deeper into the rock mass and further from the periphery of the working [1].Rocks with higher stability, i.e., capable of withstanding the total stresses arising at the periphery of tim working and not in need of any support, are characterized by the elastic wave velocity distribution shown in Fig, la:

No abstract

The bed of an apron converyer designed for a load of large heavy fragments such as copper ore is subjected to heavy dynamic stresses at the loading point. It is therefore very important to allow for dynamic action in designing the plates of the bed. The large fragments of ore in failing cause heavy stresses in these plates. It is difficult to calculate the dynamic forces involved, because of the difficulty of determining the mass of the bodies participating in the collision. Accounting for the dynamic actions of the bed is especially difficult in designing new types of conveyers because preliminary calculations must be performed to select the initial structural characteristics of the bed.The development of methods of design calculations for machines and their components for percussive loads is a complex problem and is far from completely solved at present: it demands much theoretical and experimental work. For this reason we can only approximately estimate the shock loads on the conveyer bed plates. For this purpose we make the following assumptions. 1) The impact of the load on the plate is inelastic; after the impact the Ioad coalesces with the plate at the point of collision, and oscillates together with it.2) The rigidity of the masses participating in the impact and the local deformations of the colliding surfaces are allowed for by a coefficient which must be found experimentally.Let us consider the impact of a load P on a load P0 with rigidity C. At the moment of impact the velocity of load P iswhere H is the height from which P falls.After impact, the common velocity is found from the law of conservation of momentum, which gives

In coal mining, operative monitoring of the ash content is necessary during the actual winning process, e.g., on the conveyor belt.The traditional method of igniting the specimen and weighing the unburnt residue is fairly accurate, but laborious and slow. Promise is shown by nuclear methods based on the absorption and scattering of soft y rays. A number of authors have proposed that the ash content should be measured by means of the scattering of 8 rays or by neutron or density Y--7 methods.However, these methods are difficult to apply when the ash concentration is high and the requirements for accuracy are stringent.The intensity of the scattered radiation and the degree of absorption of y rays by the specimen depend on a number of factors, including the effective atomic number Zef f and the density p of the medium.If there is a unique relation between z_ff or p and the ash content of the coal, then we can estimate the ash content from the measured intensity of scattered radiation or the degree of absorption of y rays with constant measurement geometry. Coal is a complex geological material consisting of combustibles and ashes.It can be regarded as a two-component mixture of carbon (z~) and ash (z2) and then the variation of Zef f with the ash content (for low-energy ~ quanta photoelectric absorption predominates) can be written [i] in the form D ~~\ J'a where the Pi are the contents of the components with atomic numbers z i in the mixture.Calculations for coals from Ekibastuz reveal that the atomic numbers of coals with various ash contents range from 6.0 (pure carbon) to 13.1 for an ash content C of 90%, so that a 10% increase in ash content causes a 1.7% increase in Zef f.From the fundamental law of attenuation of y rays [2] we can determine the concentration,For such a medium, the error of the determination is governed by the relative error of the change in intensity AI/I: 1 AI 1 AI AC = m (~q-~L~)'7 = ~'7 ; (3)it decreases as the relative sensitivity S increases. In turn, the latter depends on the thickness m of the absorber and the mass attenuation coefficients ~ of the elements of the first and second media.V. I.