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Sismiscs Effects of Blasting

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Sismiscs Effects of Blasting
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  RESEARCH ON THE SEISMIC IN THE MINING INDUSTRY EFFECTS OF BLASTING V. N. Moslnets and E. I. Shemyakin UDC 622.235:344.3 Contemporary ore mining is one of the most active fields in the use of blasting in the national economy, in which theoretical solution and practical realization have been effected for two very important scientific-technical problems - to attain a given degree of crashing of rocks by blasting and to protect engineering constructions and exposed rocks from file seismic action of the blast. The solution of problems in the seismic effects of mine blasts is playing an important part in the successful development of contemporary mining production. 1. The importance and industrial necessity of solving problems in the seismic effects of mining blasts in contemporary conditions is due to the fact that at the beginning of the nineteen sixties there were perfected very efficient high-output roller-bit drills for open-cut mining and pneumatic-percussive and percussive-rotary drilling rigs for underground rocks. As a consequence, in the mines there arose the possibility of drilling large volumes of rock, and this entailed the necessity of reconsidering the accepted, and creating fundamentally new, technological methods of working mineral deposits. Thus, in open-cut mines there appeared systems of working with multirow blasting of benches, blasting in enclosed media, and blasting with preservation of the geological structure of the ore masses. In underground mining, wide acceptance was obtained for systems with massive breaking-down of ore - sublevel and level forced caving, subdrifts and crosscuts, and caving of strata. The fundamental distinction of these systems of working for open-cut underground mine workings is that their effective action is possible only under the condition that there is a sharp rise in the amount of simultaneously exploded charges. In a short time, the weight of simultaneously exploded charges necessary to attain an effective mining technology rose from 20-30 to 200-300 tons, and in some cases to 500-1000 tons in open-cut mining [1-3]. and from 5-6 to 20-50 tons, in some cases to 50-200 tons. in undergound mining [4, 5]. The sharp rise in the amount of explosives simultaneously detonated placed before the blasting theoreticians and practical workers a number of fundamentally new problems in the as- sessment of the seismic action of the blast; though the Unified Safety Rules might ensure safety, they did not permit effective blasting under these conditions. Th_us, practical problems of estimating the seismic effect of a blast, arising as a consequence of the develop- ment of production, made a powerful impact on the broad development of research on the seismic effects of blast- ing in many institutes of the Soviet Union. The problem of the seismic action of blasting in ore mines has been a continual object of attention by scientists, engineers, and miners. Together with the problem of attaining a given degree of crushing of the rock by blasting, there has been a common front of research on the national-economic use of blasting, in which a good deal of success has recently been achieved. A great part in the development of research on the seismic effects of mine blasting has been played by the 9 Institute of Physics of the Earth of the Academy of Sciences of the US~t rIPE A S USSR) where scientists have systematically directed the inception and continuation of many investigations [6]. At the First All-Union Seminar on the Seismic Effects of Mine Blasting, organized by the IPE AS USSR in February 196,3. M. A. Sadovskii. S. V. Medvedev, D. A. Kharin, and others formulated for the first time those important problems in the study of the seismic pattern of blasting to the solution of which many investigations have later been directed at the institutes of the Academies of Sciences of the USSR. the Ukrainian SSR. the Kazakh SSR. and the Klrgiz SSR, together with branch institutes of mining and teaching institutes. An important role in the formulation of problems in research on the seismic action of blasting has been played by the production undertakings themselves, which have been led by the necessity of Institute f Mining Moscow Novosibirsk. Translated from Fiziko-Tekhnicheskie Problemy Razrabotki Polez- nykh Iskopaemykh No. 4 pp. 58-65 July-August 1974. Original article submitted Aprll 22 1974. 9 19 75 Plenum Publishing Corporation, 22 7 West 17th Street, New York, N. Y. 10011. No part of this publication may be repro- duced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, micro- filming, recording or otherwise, without written permission of the publisher. A copy of this article is available from the publisher for $15.00. 44  solving practical problems of production, namely, the seismic action of a blast near populated built-up areas or in combined open-cut and underground mining, the stability of the sides of deep open mines, etc. As a consequence of the formulation of the practical requirements of the problems, since the beginning of the nineteen sixties, in many undertakings the efforts of the research and teaching institutes have been bent towards broad seismological research. The results have been very fruitful and have made a large economic saving thanks to the increased weight of simultaneously exploded charges, the preservation of engineering objects surrounding the undertaking, and the additional bringing into production of ore resources which had previously been lost in the ground, in connection with the newly opened possibilities of solving problems in mining seismics of repeated or combined working of deposits. Important results in the seismic effects of mine blasting have been obtained by the Sverdlovsk group of inves- tigators [7-11] who have developed a number of fundamentally new recommendations on the stability of buidings, structures, and quarry sides, and have elucidated the features o f the seismic action of borehole charges spread over the front of a mine working, and also the features of the seismic action of blasts with combined open-cut and under- ground mining such as those in the Ural Ore-Mining Combines. A large contribution to the study of the seismic effects of mining blasts has been made by the Branch Labora- tory of Seismics and Acoustic Measuring Methods of Investigation of the Magnitogorsk Mining-Metallurgical Institute [12, 13], at which there have been developed new ideas on the seismic action of blasts with short-delay blasting, on the stability of buildings, structures, and underground pillars; studies have been made on blasting in frozen ground, and some theories have been developed on the control of the seismic action of blasting in relation to the amount of free surfaces and the direction of development of the detonation. Important research on the seismic action of blasting has been undertakenlby scientists at the Institute of Physicsand Mechanics of Rocks of the Academy of Sciences of the Kirgiz SSR [14-17], who ~have studied the seismic action of blasting with propagation of a seismic wave through worked-out mine workings and underground cavities, have developed methods of screening seismic waves in the building-up of limiting sides of quarries and slopes of hydro- technical constructions, have created theoretical methods of determining the parameters of seismically safe blasting with combined production in open-cut and underground mine workings over stowed and caved cavities, have created a classification of the seismic stability of structures under the maximum permissible elastic deformation and veloc- ity, and have on this basis calculated the safe sizes for the charges. Important research on the seismic effects of mine blasting has been carried out at VNIItsvetmet Institute at Ust -Kamenogorsk [18-19], where fundamental investigations have been made on the seismic action of blasting with short-delay detonation and on the propagation of seismic waves in fissured media, recommendations have been made on the normalization of the parameters of the seismic action, and the spectral characteristics of the blast have been deeply studied. Important research on the seismic effects of mine blasting in open-cut and underground mines has been done at the Scientific-Research Institute for the Problems of the Kursk Magnetic Anomaly [20, 2t], where scientifically based recommendations have been developed for estimating the seismic stability of pillars between rooms, blocks of untouched ore, mine workings, sides and slopes of quarries, buildings, and surface structures. An important result of the work of this institute is the creation of the first classification of engineering methods of controlling the seis- mic action of a blast, which is now used by many organizations. A definite contribution to the solution of production problems in mine blasting seismic effects has been made by the Institute of Mining of the Academy of Sciences of the Kazakh SSR [22], where work has been done to assess the seismic stability of pillars and blocks of untouched ore in the operation of blasting in the Dzhezkazgan Combine and various other undertakings. Interesting work on the practical estimation of the seismic action of blasting in open pits has been done in the UkrNIIproekt Institute [23] and at the Dnepropetrovsk Mining Institute. A brief survey of the work done by these collectives reveals successes in the study of the seismic effects of mine blastin~ the first results of which were reported to the All-Union Conference on the Seismics of Industrial Blasting in December 1966 at Sverdlovsk. At present we can assert that we have available practical scientifically based methods for solving complex engineering problems in assessing the seismic action of blasting, on the following bases: a) for quantitative estimation of the parameters of blasting in the general case, the corrected weight of 443  charges in one delay period, and in the case of distributed column charges, the weight of charges per unit length of the front; b) as a criterion of seismic safety, the permissible velocity on the wave front, with additional information on the period of seismic vibrations; c) the weight of the charge for one delay period with subsequent brief-delay blasting is chosen by the two- thirds role from an isolated charge; on condition that the delay interval exceeds the duration of the positive phase of the seismic vibrations, the number of such su essively exploded charges is unlimited; when the delay interval is less than the duration of the positive phase, the total number of charges must be limited to 8-9 delay steps; d) the permissible charge weight is linearly reduced as the potential energy of the explosives increases in relation to Ammonite No. 6, and vice versa; e) in multiple blasting, the permitted charge weight is reduced in proportion to the number of explosions; t9 as the amount of free surface on which the explosive charge acts is increased, the permissible weight increases in proportion to one quarter of the square of the amount of free surfaces; g) when seismic waves are propagated through screening interlayers, cavities, or underground workings, the permissible charge weight can be increased by a factor of 2-4, according to the screening capacity of the obstacle. In these cases it is difficult to determine the permissible velocity; to establish this, use is mainly made of statistical data from long-term observations. 2. Some progress in the solution of many practical problems in the seismics of mine blasting has recently furthered the formulation of important theoretical problems, solution of which may prove extremely important for further development of methods of controlling the seismic action of blasting, development of technologies for indus- trial blasting, and creation of means of monitoring the development of an explosion. One of the conclusions which has been fundamentally confirmed is the presence of two main wave groups in the overall seismic motion due to blasting near the free surface [24]. These groups have different velocities of propagation and different rates of amplitude attenuation and spectral compositions. Waves in the first group are longitudinal (direct in a homogeneous rock mass) or refracted (in a stratified rock) with reflection from the surface. This group has the highest spectral frequencies and the most rapid attenuation [24-26]~ Waves in the second group are transverse or surface waves of various types (including interference of waves in the surface layer). The apparent periods w of vibration in this group are several times greamr than those for the first group, and the amplitude attenuation is less. It is important that these waves be comparable at the boundary of the range of safe distances (1-5-10 cm/sec for the maximum particle velocity amplitude): at closer distances the amplitudes of the first group predominate, and at more remote distances, those of the second. Among the most important problems are those concerning the sources of seismic vibrations. The accepted concept of the wave source as a spherical symmetrical radiator generating only a compression-tension wave has in recent years been expanded and supplemented. This is of special importance in: research on the seismic effects of blasts in quarries and open pits. Thus, ithas been shown [24, 25] that in blasting by charges near the free surface, the radiator of seismic waves should rather be regarded as a two-stage source. In the first stage there is the usual formation of a cavity and radia- tion of the longitudinal wave, and in the second stage there is a dome-shaped heave of the ground at the epicenter of the blast and generation of surface waves, mnsion waves, and shear waves. A further development is found in [29], * in which it is shown that when direct compression and reflected tension waves pass through a medium, causing breakup of the medium, at the boundary of fragmentation of the medium there must arise,and unavoidably do arise, new sources of vibrations which generate waves of various types. For this reason, the following three principal theo- ries of the nature of the source and the possibility of its utilization have been put forward: 1) A source of seismic wave can be represented as a chain of successively acting sources generating waves of various types. 2) Waves of various types emitted by the same source are characterized by a constant vibration period for *As in Russian srcinal; there is no [29] for this article - Publisher. 444  this source. At the same rime, waves of the same type generated by different sources have characteristic individual velocities of propagation and absorption gradients. 3) Waves arising at different stages of the blasring process are propagated independently, are superimposed, and thus the seismogram represents vibrations which are superimposed according to the principle of superposition and must be deciphered. This approach to the assessment of a source of seismic vibrations enables us to solve both the direct and the converse problem, i.e., to determine the parameters of the waves generated by a known source, or to find the source from the observed waves. This is important for the determination of the radii of action of charges invariouszones - plastic deformation, fissure formation, shear deformation, elastic, and surface vibration. Taken together, this work shows that further development of the elucidation of the true nature of sources of seismic vibrations may serve as a basis for the creation of a number of techniques of controlling the energy of a blast and of obtaining information on phenomena occurring in a medium at various stages of disintegration: these may be used as controlled parameters in the converse relation of the process of fragmentation with its source. Finally, an important theoretical development is the use of seismic vibrations to obtain information on the fissuring of the solid rock, the strengths of worked rocks for the purpose of choosing means and techniques for break- ing them up, and the acoustic characteristics of rocks composing a cross section of a quarry or mine side. Thanks to successes in the development of methods and devices for seismic measurements, this approach is becoming increas:- ingly popular in the solution of various engineering problems. An important theoretical trend in seismological investigations is the recent work on the spectral composition of seismic vibrations and the distribution of energy among the wave groups and vibration spectrum. The results of this research are very important for the development of methods of controlling the seismic energy of blasts and of screening seismic vibrations, and also for the study of the geological characteristics of the worked deposits, the state of str ss of the rocks as the deposits are worked, the deformation of the slopes of hydrotechnical cuttings, and the stability of quarry sides, and in estimating the absorption capacities of various structural cross sections and solving other problems. Long-term accumulation and processing of this information on the spectral characteristics of seismic waves will enable us to predict a number of phenomena which can arise at great depths in mines - shock bumps - and in predicting dangerous situations in quarries and cuttings. On the whole, refinement of our knowledge of the character of the source of seismic vibrations in open and underground workings and the composition of the wave groups according to their kinematic and dynamic charac- teristics) already enables us to indicate a unified method of examining large quarries and mines from the viewpoint of seismic safety. This examination, being based on a standard method and apparatus, has the aim of refining the parameters of seismic disturbances characteristic of given mechanical conditions, technologies, and engineering conditions adjacent buildings and structures); the cost of this extra work is usually negligible in comparison with its economic effect. This effect arises owing to refinement of the blasting parameters the weights of single charges and the total charge weight, the network parameters, and the delay) and to the development of more rational blasting technologies - the study of seismics and that of blasting have a common basis. An important theoretical aspect of work on the seismic effects of mine blasting is the determination of the critically permissible deformations, displacements, and accelerations of buildings, structures, exposed rocks, mine workings, and subquarry, interroom, and hydrotechnical pillars. On the success and validity of the results of this work largely depends the advance of general engineering seismology. At our present level of knowledge it is possible to pose questions, not on the seismic danger of blasting, which can be fairly precisely predicted, but on the economic losses due to the seismic action of blasts, which should be determined in each particular case and a decision taken on this basis - either to go ahead with large-scale blasts with high technical-economic indices of technological production but with definite economic losses on the restora- tion of buildings, structures, and workings damaged by the seismic vibrations, or to carry out smaller blasts to pre- serve the surrounding structures, but with limited technological possibilities of mining output. Suceessfulsolution of the problems o fthe seiamie effects of blastingin the mining industry, arising out of produc6on. is a basis for improvement in the technology of mining operations and for increase in the o.m.s, due to introduction of advanced technologies of short-delay blasring of large masses of explosives in a compressed medium. An espe- cially marked economic effect of positive solution of problems in mining seismic effects is achieved by securing stability of the sides of quarries and open pits, cutting for hydrotechnical constructions, subquarry rooms, and inter- room pillars, where, as well as securing safe labor conditions, we can also achieve a marked reduction in the amount of stripping or ore dressing required. If we sum the economic effects obtained by realization of these questions in the quarries and open pits of the Soviet Union alone, we get an amount of many millions of rubies. 5
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