Energy components in rock blasting

Abstract

Ten production blasts and one single-hole confined blast have been monitored in two quarries in order to assess the measurable forms of energy in which the energy delivered by the explosive is transformed in rock blasting. The seismic field from seismographs readings, the initial velocity of the blasted rock face obtained from high-speed video camera records, and the fragment size distributions from image analysis of the muckpile material are used to determine the seismic wave energy, the kinetic energy and the fracture energy, respectively, transferred in the blasting process. The blasting data and the methods of calculation of the energy terms from those are described in detail. Heat of explosion and useful work to 100 MPa have been used as descriptions of the energy of explosives. The maximum total energy measured accounts for not more than 26% of the available explosive energy if this is rated as the heat of explosion, though lower figures are usually obtained. The values measured for each of the energy components range from 2% to 6% of the total energy available for the fragmentation energy, 1–3% for the seismic energy and 3–21% for the kinetic energy. For the confined shothole, the seismic energy was 9% of the heat of explosion. The uncertainty of the calculated energies is analyzed from the variability of the measured data. Particularly important influential parameters are the treatment of the fines tail of the fragment size distribution in the determination of the fragmentation energy, and the use of P or S wave velocity values, and whether these are determined from in situ or from laboratory measurements, in the calculation of the seismic energy.

Introduction

Explosives are the primary source of energy for rock breaking in the mining, quarrying and construction industries. The work into which the energy is converted transforms rock into a distribution of fragments and displaces them so that they can be conveniently loaded and hauled for further comminution and processing. Although the energetic qualification of explosives is not particularly high (any fuel/oxygen mixture used in the power industry delivers more energy per unit mass than do explosives), they are compact sources, which are able to deliver their energy in an autonomous form at a very fast rate. This results in reaction products at high pressure that can perform mechanical work in deforming and breaking the material in their vicinity. This is what makes explosives useful and in many cases irreplaceable for rock excavation. This fast energy delivery, in the form of a large amount of reaction products at high pressure and high temperature, is inseparable of a number of transformations other than the desired fragmentation and throw, such as the seismic wave into the rock.

Any explosive data sheet or commercial brochure quotes some type of energetic description. Explosives energy is rated in a variety of ways, obtained either from calculation or from experimental tests. However, the questions of what amount of that explosive energy is transferred to the rock and what fraction of it is converted into efficient work in the usual civil application of rock blasting remains largely undefined. Although the measurement of some of the effects of the explosive in rock is customary (vibration, fragmentation and, to a minor extent, rock movement), they are usually conducted for blast control purpose and the results are rarely cast in terms of their energy content. The reason for this may be that it is not the energy consumption in this or that phenomenon that matters, but rather the end effects, i.e., degree of fragmentation, throw and vibration levels. Data and estimations on energy components in rock blasting are thus limited to a few researchers. Berta [1], Spathis [2] and Ouchterlony et al. [3] calculate the amounts of energy transformed in kinetic energy of the rock, fracture generation and seismic wave. Seismic energy has received special attention since earlier times; calculations of seismic energy and its comparison with explosive energy have been reported by Howell and Budenstein [4], Fogelson et al. [5], Berg and Cook [6], Nicholls [7], Atchinson [8], and more recently by Hinzen [9].

Berta [1] attempted to use some of the energy concepts in his principles of blast design, though this is seldom used in practice. Spathis [2] suggested, as a recommendation for future work, the practical use of the energy balance to enable blast designs which direct the available energy into the desired work and hence control the energy split between fracture energy, kinetic energy and radiated seismic energy, resulting in a more efficient use of the explosive energy. The present work assesses to some extent the feasibility of this energy approach, aiming at establishing, through new data and a thorough revision of the published work on the matter, the fraction of explosive energy transferred to the rock in its various components, with particular attention to their variability and the reasonable ranges that could be expected in quarry blasting.

The basic theory and experimental background for the determination of some of the energy components in rock blasting are described first. These are then applied to ten production blasts and one single, confined (without rock movement) blasthole. Eight of the production blasts and the confined shot were conducted in a limestone quarry (El Alto, Spain); two more production blasts were monitored in an amphibolite quarry (Eibenstein, Austria). Seismographs, high-speed video camera and fragmentation monitoring systems were used to measure the seismic field, the initial velocity of the blasted rock face and the fragment size distribution curve, respectively, from which the various energy terms are calculated. Vibrations were measured in the single confined hole.