Contour blasting is commonly performed by employing linear charges, decoupled from the boreholes. This method is common in surface and underground excavations, either for civil or mining purposes. To achieve the best results in terms of rock breakage and respect of the excavation profile, blasting theory suggests that charges should be inserted coaxial to the holes to grant uniform distribution of the explosive energy and therefore obtaining a uniform Radius of Damage. Nonetheless, due to readiness of operations or lack of availability of specific products on the market, non-coaxial charges are often employed in blasting practice. Non-coaxial charging methods include the employ of high-power detonating cord (40 to 100 g/m), low-power detonating cord connecting small-diameter cartridges (commonly 10 g/m detonating cord priming 1 cartridges) or string loading (a thin layer of bulk emulsion pumped with controlled flow and controlled extraction of the injecting rod). This research focuses on evaluating the effects of the first two charging methods on the quality of final walls in open-pit and underground operations. Different drilling geometries and charging configurations were applied to both quarrying and tunneling blasts. The Half-Cast Factor (HCF), the Over-break (OB) and the Under-Break (UB) were evaluated as control indicators. Rock Quality Designation (RQD) and Rock Mass Rating (RMR) were used to classify the rock mass. The research was aimed to push contour blasts to their limits, observing for which geometry and charge configuration the blast lost its design threshold with respect to the final wall for every given rock mass. Results show the operational limits of non-coaxial charges encountered in the rock masses object of this study. In good-quality rock, smooth blasting with decoupled linear charge of 40 g/m can be extended to a spacing S = 22Of with little or no detectable drawbacks in terms of final wall quality, in contrast with theoretical formulae for the determination of the radius of damage. On the other hand, when the rock is poor, any quality of the final wall is hardly achieved at all, in spite of any care in the details of execution of smooth blasting. It is concluded that any design criterion and theoretical approach modeling the effects of contour blasting cannot ignore the features of the rock mass
In the Mining Industry, Rock Blasting can be considered as the first phase of comminution. The downstream phases of comminution are mechanical, and consist in consecutive stages of crushing and milling for grain size reduction. The total energy of mechanical comminution depends on grain size and on the resistance of the material to grindability. Usually, the first is quantified by the grains size distribution or one of its KPIs (such as the diameters X50 or X80), while the latter by the Working Index (WI), representing the amount of energy necessary to reduce the size of grains of a determined amount of material to the desired equivalent diameter (usually expressed in kWh/t). It is widely recognized that it is possible to manage the output of a blast in terms of grain size distribution varying specific charge (Powder Factor, P.F.) and charge distribution; specific algorithms such as the KUZ-RAM or the SWEBREC have been created to predict such output. Also, Katzabanis et al. have demonstrated how higher P.Fs. weaken the blasted material by creating micro-fractures that reduce its WI. This paper focuses on a related aspect: the influence of charge distribution on the specific comminution energy in crushing and milling. Small-scale blasts have been performed on 14 limestone blocks with different P.Fs. and charge distribution. For every P.F., charges have been designed to simulate concentraded and distributed geometries. Concentrated charges simulate open-cast blasts with large-diameter holes, with large burdens and spacings. Distributed charges simulate bench blasts with small-diameter holes and reduced burden and spacing. Three control blocks have been fragmented by mechanical means for comparison. For every sample has been determined the grain size distribution, the Crushing Specific Energy and the WI in ball mill. Results confirm the existence of precise correlations between charge distributions and the specific energy of mechanical comminution in crushing and milling. It is finally discussed the opportunity of adjusting charge diameters and distributions in open-cast blasts to improve the output of the comminutions circuits
Small-scale mining operations are characterized by a large variety of equipment availability and a high level of operational flexibility. Mine planning is usually scarce or absent in small quarries in Brazil, and mine management generally focuses on daily operations. As a result, analysis of the effects of unit operations, such as blasting, over the whole mining process is often neglected. The goal of the following project was to study the effect of production blasting on loading and hauling times, secondary breakage times and primary crushing times at the Experimental Mine of the Research Center for Responsible Mining of the University of Sao Paulo, Brazil. The operations analyzed included blasts that performed: under the average operational standard, defined in this paper as Case 1; according to quarry standards, defined in this paper as Case 2. In order to establish the effects of blasting on the downstream process, three key performance indictors (KPIs) were used: - the time taken to load the truck, when there is the need for material selection (tt); due to the fact that we do not have an operational baseline, the increase in time can't be established, meaning that we must content ourselves with comparing the mean time in each scenario; - the average time used for secondary breaking machinery (tf); - the time required for the primary crusher to fully process the load of a single truck (tc). The results obtained indicate correlations between the coarseness of the blasted material and theses operational times. Solutions are suggested and evaluated for the improvement of the small-scale mining process
Surface waves play a fundamental role in the propagation of vibrations caused by blasting. Indeed, active and passive surface wave tests, which are widely used techniques in engineering and geophysical exploration to infer model parameters for soils and rocks, can be profitably applied for the prediction of ground motion from blasting supplying relevant information on subsurface mechanical properties and source parameters (patterns of propagation and dominant wavelengths). In this respect, monitoring systems for ground vibrations can be conceived in order to use the collected data also for ground characterization purposes. Moreover, the interpretation of surface wave data can lead to an estimate of shear modulus and damping ratio (quality factor) as a function of depth. This information can be used to build a numerical model to simulate wave propagation and predict ground vibrations evaluating the performances of vibration mitigation systems. For preliminary assessments of ground vibrations induced by blasting, the information from surface wave tests can be used to improve the predictive capability of attenuation relationships. In particular in the present work reference is made to a recently proposed relationship based on energy spreading in the form of Rayleigh wave propagation. Data from a case history will be presented to exemplify the relevant aspects in the proposed methodology.
Predicting the generation of dust from open pit mining operations is traditionally based on the use of that describe the mass of dust expected to be generated from individual mining and materials handling processes. These emission factors are based on field measurements from typical operations. Estimates of dust generation would have much greater credibility if they could be related directly to the characteristics of the rock at the particular site and the actual blast designs used. Available blast fragmentation models predict breakage down to perhaps 1 mm in size. An approach is described that utilizes laboratory crusher test results to extend the predicted size distribution from blasting down to dust sized particles.
In the late 1970s and early 1980s in conjunction with other oil and gas well stimulation studies, personnel from the Dynamic Effects Laboratory performed model testing to demonstrate the effectiveness of utilizing an open section of borehole just before a plug. We called the process stem induced fracturing. The open section beneath the stem was used to increase the pressure magnitude and spread out the duration of the pressure pulse. This technique was later utilized by Frank Chiapetta [Chiappetta, R.F. and Mammele, M.E., 1987 Chiappetta, R. F. and Mammele, M. E. . Analytical high-speed photography to evaluate air decks, stemming retention and gas confinement in pre-splitting, reclamation and gross motion applications. Proceedings of 2nd International Symposium on Rock Fragmentation by Blasting. Keystone, Colorado, USA. pp.257–309. [Google Scholar], Analytical high-speed photography to evaluate air decks, stemming retention and gas confinement in pre-splitting, reclamation and gross motion applications. Proceedings of 2nd International Symposium on Rock Fragmentation by Blasting, Keystone, Colorado, USA, 257 – 309] in the fracture and fragmentation of rock in quarry blasting situations. He called his technique air deck blasting. In fact, Frank found that the Russians had previously discovered the same technique. There is currently interest in utilizing the same technique with an open hole beneath the explosive at the bottom of the bore hole to better remove the toe in a fragmentation shot. This paper reviews the development of stem induced fracturing and describes a series of model tests conducted to measure borehole pressure at points along a borehole when an explosive charge is detonated at the midpoint of the borehole. Tests were conducted in both stiff boreholes (aluminum) and less rigid boreholes (PMMA). Pressure time profiles were measured at the charge site, midway between the charge and the bottom of the hole, at the stemming at the top of the borehole, and at the bottom of the borehole. Crack initiation sites and crack propagation were also determined in the PMMA models. Some high speed pictures were taken of the event in the PMMA.
A series of small scale tests, simulating multi-hole blasts have been performed to establish the effect of delays on blast fragmentation. The blasts were performed in high quality granodiorite blocks, which were cut from stone prepared by dimensional stone quarry operations. The pattern used was equilateral triangular, with a distance of 10.2 cm between boreholes, which had a diameter of 11 mm, were loaded with detonating cord and the coupling medium was water. The delays used were achieved using different lengths of detonating cord for the cases of delays between 0 and 100 μs between holes and a sequential blasting machine firing seismic detonators for larger delays up to 4 ms. All fragments were collected and screened. The experiments showed that the worst fragmentation was achieved with simultaneous initiation of all charges. Fragmentation improved with the delay time between holes up to 1 ms between holes. If the experiments are scaled up, the results show that in granodiorite, fragmentation optimization requires delays of few milliseconds per metre of burden. The findings, agree with previously published work, involving larger scale experiments and other rock types.
A study of the dynamic rock fracture initiation and propagation due to explosive energy is presented through a detailed state-of-the-art review. Explosive energy dissipation in crushing and fracturing is examined and the various means to enhance the explosive energy utilization for dynamic rock fracturing are reviewed. The study highlights the need for a better understanding of the dynamic fracturing process particularly in the presence of in situ stresses in the rock mass.
Rapid vibration estimation usually relies on charge weight scaling laws. These empirical curves that fit field data are sometimes involved in linear superposition models. A scaled charge weight superposition model combines the traditional charge weight scaling law within a linear superposition framework so that the influence of timing and blast design parameters may be assessed. The model involves an extra parameter of time overlap and poses a partial explanation for the over-estimation of blast vibration levels sometimes predicted by linear superposition models. It is suggested that the time overlap parameter be determined from measured vibration data. The new model recovers the Holmberg-Persson equation for near-field vibration from a single blast hole. Three examples compare the predictions of the new model with those from a linear superposition model. The results indicate a consistent, but not compelling demonstration of the predictive power of the scaled charge weight superposition model. Further verification studies are recommended using a wide range of blast types and scaled distances to points of interest. The new model lies between a traditional charge weight scaling law and a linear superposition model. It ignores waveform superposition explicitly but assumes non-linear interaction between adjacent charges in a blast within a user-defined time window. In that sense, the scaled charge weight superposition model offers an alternative method for rapid vibration estimation.
Blast hole pressure is the starting point for many blast design calculations, but the way in which it is usually derived, from measured detonation velocity, indicates that more thought is needed as to its true meaning and implication. The general impression is given that the energy in the hole is defined by velocity of detonation (VoD), but this is rarely the case. VoD is defined by the energy released in the detonation driving zone between the shock front and the sonic (or CJ) surface, and for commercial explosives it is normal for reaction not to be complete within this zone. Reaction and energy delivery continues behind it, not reflected by VoD. Thus it would be more appropriate to use the theoretical VoD, not the measured VoD, to derive the starting pressure, since this would reflect the energy input of full reaction. In decoupled situations, the derivation of pressure at the blast hole wall using a polynomial decay concept is also of debatable value, and an alternative is offered.
This paper describes qualification of devices to measure sub micro-meter changes in crack width, which is the basis of autonomous crack monitoring for control of blasting vibrations. Performance of LVDT, eddy current and potentiometer sensors to monitor long-term and transient displacements will be described. Potentiometers are attractive for wireless measurement, which is important as future autonomous crack displacement measurement almost certainly will be wireless. Long-term performance in the laboratory and the field is described in terms of drift, hysteresis, and noise upon exposure to cyclic changes in displacement and temperature. Transient performance is described in terms of relative response of the three systems to impact induced displacements with eddy current and LVDT serving as the benchmark.
Spalling is a wave-induced dynamic fracture phenomenon. The waves can be either one: elastic, elasto-plastic, or shock waves. From a continuum mechanics point of view, fracture mechanics and wave propagation form the main ingredients in the formation of spalls. Recently, however, micro-structural effects have become important in the initial stages of spall formation in a variety of engineering materials ranging from metals to rock and concrete. From a structural geology point of view, the rock mass cannot be modelled as a continuum. In this case, a discontinuum approach has to be taken where the individual features of the rock mass such as joints and faults need to be taken into account. From an application point of view, spallation is important where rapid loading by explosives, impact, or energy deposition, occurs. The range of applications stretches from blasting in mining engineering to damage prevention to structures under explosive excitation. This contribution offers a multi-faceted and multi-disciplinary approach to the study of spalling with special attention to analytical and experimental work. The reader is assumed to be somewhat familiar with the basics of continuum mechanics, fracture mechanics, and propagation of elastic, plastic and shock waves. The application to rock and concrete will show the effects of structural geological discontinuities such as open and closed joints – and to some degree also faulting – in rock, as well as the micro-structure of concrete on the (shock) wave field. Extensive use will be made of time-space diagrams which proved very useful in practical applications to blasting problems [Rossmanith, H.P., 2002, The use of Lagrange diagrams in precise initiation blasting. Part I: two interacting blastholes, Fragblast 6, 104–136].
A cell powder factor defines the local powder factor in a simple geometric procedure that accounts for the charge in each blast hole and the triangulation formed by blast hole collars in a blast pattern. In the case of asymmetric blast patterns, the cell powder factor deviates from the global powder factor. Poor drilling of blast hole collars produces significant variation in the cell powder factor, more so, than does poor blast hole charging. The cell powder factor does not account for timing within a blast and does not easily handle non-parallel blast hole configurations. The cell powder factor is best used as a design or audit tool before blast holes are loaded. It gives the opportunity to correct for poor drilling and/or poor charging practices prior to firing a blast.
Shallow buried explosives pose a significant threat to lightweight vehicles and their onboard personnel. To date, designers of lightweight vehicles are limited in their knowledge of what occurs during the blast. The high intensity, short term loading imparted by the explosion is enormously complex and can be significantly affected by a number of parameters including the size, shape, type, detonation point and depth of burial (DOB) of the explosive and the type, density and water content of the soil. Recent advancements in numerical simulations have enabled the complex blast event to be accurately modelled by coupling Eulerian and Lagrangian analyses: the former is well suited to modelling the blast and while the latter, the structural response. Further validation of the modelling technique is considered in the current paper, which details simulations performed utilising the coupled Eulerian-Lagrangian analysis to study the blast output of explosives buried in saturated sand. These experiments varied explosive charge size, its depth of burial, the target stand-off (SO) distance and the dimensions of the target plate. The investigation concludes with a discussion of the accuracy of the numerical simulations when compared with the experimental observations.
A set of experiments were conducted at the Aberdeen Proving Grounds 1 Taylor, L. C., Skaggs, R. R. and Gault, W. . Vertical impulse measurements of mines buried in saturated sand. Proceedings of the 31st Annual Conference on Explosives and Blasting Technique. February6 – 9, Orlando, FL. Vol. 2, [Google Scholar] (Taylor, L.C., Skaggs, R.R. and Gault, W., 2005. Vertical impulse measurements of mines buried in saturated sand. Proceedings of the 31st Annual Conference on Explosives and Blasting Technique, Orlando, FL, 6 – 9 February, 2.) in which explosive charges were buried in saturated sand beneath a suspended rigid platform. The goal of these experiments was to measure the dependence of the impulse transmitted to the platform on the standoff distance and the charge burial depth. Simulations of these experiments were performed using the BUB2D axi-symmetric code using a frictional-cohesive visco-plastic model to describe the response of the saturated sand. This code solves a constrained set of conservation laws in which the liquid region (in this case saturated sand) is assumed incompressible. The explosion is initialized as a high pressure gas bubble (void) within the fluid. Comparisons of the simulations to the experiments are presented together with a study of the physical phenomenology associated with the loading process. In particular, it is shown that the force imparted to the platform is a combination of the impact of the sand on top of the explosion gas bubble and the pressure of the bubble as it expands before venting into the atmosphere. Under certain conditions, when the platform standoff is sufficiently small and the platform is sufficiently large, the bubble can over-expand before venting and pull the platform downward. This phenomenon was studied further through carefully measured and photographed small-scale experiments performed at the University of Maryland 2 Fourney, W., Leiste, U., Bonenberger, R. and Goodings, D. . Predicting explosive impulse by means of small scale tests. Proceedings of the 31st Annual Conference on Explosives and Blasting Technique. February6 – 9, Orlando, FL. Vol. 2, [Google Scholar] (Fourney, W., Leiste, U., Bonenberger, R. and Goodings, D., 2005, Predicting explosive impulse by means of small scale tests. Proceedings of the 31st Annual Conference on Explosives and Blasting Technique, Orlando, FL, 6 – 9 February, 2.). The small-scale experiments provide additional important validation benchmarks for our model.
This paper investigates the various mechanisms and parameters that are responsible for delivering impulse to a vehicle that is unfortunate enough to detonate a buried mine. Small scale tests are used to examine the effects of air blast or ejected sand in imparting impulse to a plate that is located above the surface of the saturated soil that contains the explosive. Parameters such as confinement, stand off distance, depth of burial of the explosive, density of the soil, and saturation level of the soil are also examined.
