Fragmentation prediction using improved engineering formulae

In the last decade, fragmentation prediction has been attempted by many researchers in the field of blasting. Kuznetsov developed an equation for the estimation of average fragment size, x 50 , based on explosive energy and powder factors. Cunningham introduced a uniformity index n as a function of drilling accuracy, blast geometry and a rock factor A associated with a “blastability index”, which can be calculated from the jointing, density and hardness of the blasted rock mass. Knowing the mean size and the uniformity index, a Rosin-Rammler distribution equation can then be derived for calculating the fragment size distribution in a blasted muckpile. Analysis of existing data has revealed serious discrepancies between actual and calculated uniformity indices. The current integrated approach combines the Kuznetsov or similar equation and a comminution concept like the Bond Index equation to enable the estimation of both the 50% and 80% passing sizes ( k 50 and k 80 ). By substituting these two passing sizes into the Rosin-Rammler equation, the characteristic size x c and the uniformity index n can be obtained to allow the calculation of various fragment sizes in a given blast. The effectiveness of this new fragmentation prediction approach has been tested using sieved data from small-scale bench blasts, available in the literature. This paper will cover all tested results and a discussion on the discrepancy between measurement and prediction due to possible energy loss during blasting.     

Fragmentation Energy in Rock Blasting

The theoretical explosive energy used in blasting is a common issue in many recent research works (Spathis 1999; Sanchidrian 2003). It is currently admitted that the theoretical available energy of the explosives is split into several parts during a blast: seismic, kinetic, backbreaks, heave, heat and fragmentation energies. Concerning this last one, the energy devoted to the breakage and to the creation of blocks within the muckpile can be separated from the microcracking energy which is devoted to developing new and/or extending existing micro cracks within the blocks (Hamdi et al. 2001; López et al. 2002). In order to investigate these two types of energy, a first and important task is to precisely study the main parameters characterising the two constitutive elements of the rock mass (rock matrix and discontinuity system). This should provide useful guidelines for the choice of the blasting parameters (type of explosive, blasting pattern, etc.), in order to finally control the comminution process. Within the frame of the EU LESS FINES research project, devoted to the control of fines production, the methodology was developed in order to: (1) characterize the in situ rock mass, by evaluating the density, anisotropy, interconnectivity and fractal dimension of the discontinuity system and (2) evaluate fragmentation (both micro and macro) energy spent during the blasting operation. The methodology was applied to three production blasts performed in the Klinthagen quarry (Sweden) allowing to estimate the part of the fragmentation energy devoted to the formation of muck pile blocks on one side and to the muckpile blocks microcracking on the other side.     

Efficiency of standardized image processing in the fragmentation prediction in the case of Sungun open-pit mine

The paper proposes a standardized image-processing procedure with the use of sieve analysis results for calibration which is utilized to measure the size distribution of fragmentation at Sungun mine. Through this procedure, a number of 19 bench blasting in various levels have been initially selected as the target of the study for each, multiple photos were taken immediately after blast from suitable perspectives and locations of the muckpiles surfaces. The number of image sampling was chosen adequately high to achieve further reliability of the whole photography procedure. Then fragments of each muckpile were separately mixed by a loader, where another image sampling from these new muckpiles, bucket of loaders, and haulage trucks was performed. For the purpose of sieve analysis, seven sieves with the mesh sizes between 1.27 cm (0.5 in) and 25.4 cm (10 in) were designed, manufactured, and then installed at Sungun semi-industrial laboratory. Additionally, three mass samples of the mixed fragments were randomly chosen among the 19 muckpiles for sieving. During image analysis stage, “sieve shift” and “mass power” factors, required to obtain standardized size distribution, were precisely assigned when the results obtained by the image analysis software was in accordance with the sieving results. In order to validate the reliability of the image processing, a comparative analysis of the achieved results was made with the results of the original Kuz–Ram model [Cunningham (1983) The Kuz–Ram model for prediction of fragmentation from blasting. In: Proceedings of the first international symposium on rock fragmentation by blasting, Lulea, Sweden, pp 439–454]. Finally, the image-processing procedure was found to be more efficient, with results close-matched to the real results of the sieve analysis.     

Application of Air Decks in Production Blasting to Improve Fragmentation and Economics of an Open Pit Mine

In blasting with air decks, repeated oscillation of shock waves within the air gap increases the time over which it acts on the surrounding rock mass by a factor at between 2 and 5. The ultimate effect lies in increasing the crack network in the surrounding rock and reducing the burden movement. Trials of air deck blasting in the structurally unfavourable footwall side of an open pit manganese mine has resulted in substantial improvements in fragmentation and blast economics. Better fragmentation resulted in improved shovel loading efficiency by 50–60%. Secondary blasting was almost eliminated. Use of ANFO explosive with this technique reduced explosive cost by 31.6%. Other benefits included reductions in overbreak, throw and ground vibration of the order of 60–70, 65–85 and 44% respectively. This paper reviews the theory of air deck blasting and describes in detail the air deck blast trials conducted in a manganese open pit mine in India. The blast performance data have been analysed to evaluate the benefits of air decking over conventional blasting.     

Experimental study of the influence of loading rate on tensile mechanical behavior of sandstone damaged by blasting

The drill and blast method (D&B) is perhaps the most common excavation method for rock mass, and intense blasting vibrations would induce an excavation damage zone (EDZ) around the excavated space. The tensile failure of rock mass in EDZ at diverse rupture velocities results in various geological disasters in engineering practices. The objective of this paper is to investigate the effect of blasting on the tensile strength of sandstone rock and the influence of loading rate on the disk specimens affected by blasting. We firstly performed a D&B exercise on a sandstone block with a size of 600 mm × 600 mm × 120 mm. Then, a total number of 49 standard disk specimens were prepared from large fragments of this blasting sandstone block and an undamaged block. A series of Brazilian split tests was carried out using these specimens to determine their indirect tensile strength, and to assess the effects of the distance from the blasting source and loading rate (varying from 1.67 × 10^?5 to 8.33 × 10^?2 mm s^?1). The results show that the tensile strength of specimens exhibits an upward trend with increasing distance from the blasting source, to approach that of undamaged rock, following a power function with a positive exponent (0~1). The loading rate affects the tensile mechanical behaviors of disks, in terms of the convergence of microscopic defects, the main load-bearing area, and the absorbed energy at the fracture moment of specimens. Both the tensile strength and absorbed energy have positive linear correlations with the natural logarithm of the loading rate. In addition, the fragmentation degree of disk specimens also increases due to an increasing brittleness of sandstone with the loading rate.     

The use of air decks in production blasting in an open pit coal mine

The influence of air deck blasting on blast performance and blast economics and its feasibility has been studied in the production blasting of soft and medium strength sandstone overburden rocks in an open pit coal mine in India. The air deck blasting technique was very effective in soft and medium strength rocks. Its main effects resulted in reducing fines, in producing more uniform fragmentation and in improving blast economics. The fines were reduced by 60–70% in homogeneous sandstones. Oversize boulders were reduced by 80% and shovel loading efficiency was improved by 20–40% in blocky sandstones. The explosive cost was reduced by 10–35% dependent on the type of rock mass. Throw, backbreak and ground vibration were reduced by 10–35%, 50–80% and 30–94% respectively. For a particular rock mass and blast design environment, air deck length (ADL) significantly influenced the fragmentation. ADL as represented by air deck factor (ADF) in the range of 0.10–0.35 times the original charge length (OCL) produced optimum results. ADF beyond 0.35 resulted in poor fragmentation and in inadequate burden movement.     

A three dimensional model of muckpile formation and grade boundary movement in open pit blasting

Summary Formulation and case studies of a three dimensional kinematic model are presented. Thein situ overburden geometry can be simulated accurately and various initiation patterns of blasts can be modelled. The overburden geometry, hole patterns and explosive distribution are all explicit model inputs. Because the effect of explosive properties, rock mass condition and inter-row delay are very difficult to measure in terms of blast performance, these are represented in the model by control parameters which are left for calibration using field data. The output of the model is a three dimensional muckpile shape of any cross section and a contour map of grade distribution within the muckpile. Two case studies are presented which have shown that the model is a valuable tool for optimizing production blasting as well as for controlling grade dilution during blasting.     

A Review of General Considerations for Assessing Rock Mass Blastability and Fragmentation

Blasting is the primary comminution process in most mining operations. This process involves the highly complex and dynamic interaction between two main components. The first is the detonating explosive and the second is the rock mass into which the explosive is loaded. The mechanical properties of the rock material (such as dynamic strength, tensile strength, dynamic modulus and fracture toughness) are important considerations in understanding the blasting process. However, it is the characteristics of the geological defects (joints, foliation planes, bedding planes) within the rock mass that ultimately determine how effectively a blast performs in terms of fragmentation, all else being equal. The defect characteristics include, but are not limited to, their orientation, spacing, and mechanical properties. During the blasting process, some of the geotechnical characteristics of the rock mass are substantially changed. From the blasting outcome point of view, the most notable and important is the change in fragment size distribution that the rock mass undergoes. The pre-blast in situ defect-bounded block size distribution is transformed into the post-blast muckpile fragment size distribution. Consequently, it is fundamental to our understanding of and ability to predict the blasting process that both the blastability of a rock mass and its transformation into the fragment size distribution can be appropriately quantified.     

The BLAST-CODE model - A Computer-Aided Bench Blast Design and Simulation System

Blast design is a critical factor dominating fragmentation and cost of actual bench blasts. However, due to the varying nature of rock properties and geology as well as free surface conditions, reliable theoretic formulae are still unavailable at present and in most cases blast design is carried out by personal experience. As an effort to find a more scientific and reliable tool for blast design, a computer-aided bench blast design and simulation system, the BLAST-CODE model, is developed for Shuichang surface mine, Mining Industry Company of the Capital Iron and Steel Corporation Beijing. The BLAST-CODE model consists of a database representing geological and topographical conditions of the mine and the modules Frag + and Disp + for blast design and prediction of resultant fragmentation and displacement of rock mass. The two modules are established in accordance with cratering theory qualitatively and modified quantitatively by regression of the data collected from 85 bench blasting practices conducted in 3 mines of the Shuichang surface mine. Blasting parameters are selected based upon quantitative and comprehensive evaluation on the effect of the factors such as rock properties, geology, free surface conditions and detonation characteristics of the explosive products in use. In order to ensure practicality and reliability of the system, the BLAST-CODE model allows automatic adjustment to the selected parameters such as burden B and spacing S as well as explosive charge amount Q of any blasthole under irregular topographic and/or varying blastability conditions of the rock mass to be blasted. Simulation of the BLAST-CODE model includes prediction of fragmentation and displacement that are demonstrated in terms of swell factor, characteristic rock size x c and size distribution coefficient n by Rossin-Ramler's equation, and 3-dimentional muck pile profile. The BLAST-CODE model also permits interactive parameter selection based on comparison of the predicted fragmentation and displacement as well as the cost for drilling, explosives, and accessories until the most effective option can be selected.     

Use of Air-Decks to Reduce Subdrillings in Escondida Mine

This paper aims to show the results and comparison obtained from blasting using overdrilled blastholes and blasting using a bottom of the blasthole located air-deck. The effect of using a blasthole air-deck, on medium to low hardness rock found in the western area of the Escondida pit, helped maintain the required grade level even after loading with heavy equipment. Additional benefits include satisfactory fragmentation of the blasted material.     

Asymmetric Blasting: A Rock Mass Dependent Blast Design Method

Blasting has been the most frequently used method for rock breakage since black powder was first used to fragment rocks, more than two hundred years ago. This paper is an attempt to reassess standard design techniques used in blasting by providing an alternative approach to blast design. The new approach has been termed asymmetric blasting. Based on providing real time rock recognition through the capacity of measurement while drilling (MWD) techniques, asymmetric blasting is an approach to deal with rock properties as they occur in nature, i.e., randomly and asymmetrically spatially distributed. It is well accepted that performance of basic mining operations, such as excavation and crushing rely on a broken rock mass which has been pre conditioned by the blast. By pre-conditioned we mean well fragmented, sufficiently loose and with adequate muckpile profile. These muckpile characteristics affect loading and hauling [1]. The influence of blasting does not end there. Under the Mine to Mill paradigm, blasting has a significant leverage on downstream operations such as crushing and milling. There is a body of evidence that blasting affects mineral liberation [2]. Thus, the importance of blasting has increased from simply fragmenting and loosing the rock mass, to a broader role that encompasses many aspects of mining, which affects the cost of the end product. A new approach is proposed in this paper which facilitates this trend 'to treat non-homogeneous media (rock mass) in a non-homogeneous manner (an asymmetrical pattern) in order to achieve an optimal result (in terms of muckpile size distribution).' It is postulated there are no logical reasons (besides the current lack of means to infer rock mass properties in the blind zones of the bench and onsite precedents) for drilling a regular blast pattern over a rock mass that is inherently heterogeneous. Real and theoretical examples of such a method are presented.     

隧道围岩爆破损伤防护的霍普金森压杆试验

为减小爆破对隧道围岩的损伤,在传统光面爆破装药结构基础上,设计一种在炮孔一侧安放PVC-U材料来保护隧道围岩的装药结构,并分析了该方法的作用机制。为分析和验证PVC-U管对隧道围岩损伤的防护效果,利用Φ100 mm的霍普金森试验装置进行隧道围岩损伤防护试验,从试件宏观破坏、应力波衰减和能量变化3方面进行了分析。结果表明：防护材料改变了岩石的受力状态,当防护材料厚度从0增加到6.24 mm时,无防护材料的试件的破坏程度比有防护材料的试件严重,且随着防护材料厚度的增加破坏程度不断减小;试件的应力峰值呈不断降低趋势;应力峰值的降低率不断增加;耗散能与入射能比值从61.08%下降到39.97%。可见,防护材料对岩石冲击损伤具有一定防护作用。该方法在某巷道掘进中得到应用,并取得了良好效果。     

A Practical Procedure for the Measurement of Fragmentation by Blasting by Image Analysis

Summary. The operation of a digital image analysis system in a limestone quarry is described. The calibration of the system, required in order to obtain moderately reliable fragmentation values, is done from muckpile sieving data by tuning the image analysis software settings so that the fragmentation curve obtained matches as close as possible the sieving. The sieving data have also been used to extend the fragment size distribution curves measured to sizes below the system’s optical resolution and to process the results in terms of fragmented rock, discounting the material coming from a loose overburden (natural fines) that is cast together with the fragmented rock. Automatic and manual operation modes of the image analysis are compared. The total fragmentation of a blast is obtained from the analysis of twenty photographs; a criterion for the elimination of outlier photographs has been adopted using a robust statistic. The limitations of the measurement system due to sampling, image processing and fines corrections are discussed and the errors estimated whenever possible. An analysis of consistency of the results based on the known amount of natural fines is made. Blasts with large differences in the amount of fines require a differentiated treatment, as the fine sizes tend to be the more underestimated in the image analysis as they become more abundant; this has been accomplished by means of a variable fines adjustment factor. Despite of the unavoidable errors and the large dispersion always associated with large-scale rock blasting data, the system is sensitive to relative changes in fragmentation.     

The application of size distribution equations to rock breakage by explosives

SummaryThe Application of Size Distribution Equations to Rock Breakage by Explosives Size distribution equations can be used to describe the degree of fragmentation produced by explosive rock breakage. This paper describes the results of small scale blasting experiments and the derivation of equations to relate size distributions to blasting design parameters. The application and relevance of these techniques to large scale blasting operations is also discussed.With 7 Figures     

Optimum Blasting? Is it Minimum Cost Per Broken Rock or Maximum Value Per Broken Rock?

In most mining operations the ore undergoes several processes such as drilling, blasting, loading, hauling, crushing, grinding and liberation to become the final salable product. Drilling and blasting is an important step in this process chain and it's results such as fragmentation, muckpile shape and looseness, dilution, damage and rock softening effect the efficiency of downstream processes. The value created per ton of broken ore is the difference between the price it commands when sold as the final product and the cost to produce it. Traditionally, the total process in the mining industry is classified into two groups as mining and milling. These are managed as separate cost centres inspite of the interdependency. Each process has a budget and production target and emphasis is usually on maximising production (tons) and minimising cost rather than the overall profitability of the whole business unit. The efficiency of each process is considered to be satisfactory as long as they are within budget and meet the production targets. The mine and mill managers usually try to optimise each process independently rather than the entire process. This paper discusses the potential pitfalls of decreasing the drilling and blasting cost per ton of broken rock without considering its impact on downstream processes. It introduces a holistic approach to blast optimisation by identifying and measuring the leverage that blast results have on different downstream processes and then optimising the blast design to achieve the results that maximise the overall profitability rather than just minimising the drilling and blasting costs. This paper demonstrates the benefits of such a holistic approach to blasting based on computer model simulations and field studies from metal and open cut coal mining.     

Application of soft computing in predicting rock fragmentation to reduce environmental blasting side effects

In the blasting operation, risk of facing with undesirable environmental phenomena such as ground vibration, air blast, and flyrock is very high. Blasting pattern should properly be designed to achieve better fragmentation to guarantee the successfulness of the process. A good fragmentation means that the explosive energy has been applied in a right direction. However, many studies indicate that only 20–30 % of the available energy is actually utilized for rock fragmentation. Involvement of various effective parameters has made the problem complicated, advocating application of new approaches such as artificial intelligence-based techniques. In this paper, artificial neural network (ANN) method is used to predict rock fragmentation in the blasting operation of the Sungun copper mine, Iran. The predictive model is developed using eight and three input and output parameters, respectively. Trying various types of the networks, it was found that a trained model with back-propagation algorithm having architecture 8-15-8-3 is the optimum network. Also, performance comparison of the ANN modeling with that of the statistical method was confirmed robustness of the neural networks to predict rock fragmentation in the blasting operation. Finally, sensitivity analysis showed that the most influential parameters on fragmentation are powder factor, burden, and bench height.     

Grain size and mineralogy of Al Batinah beach sediments,Sultanate of Oman

The nature of Al Batinah coast beach sediments in the Sultanate of Oman was investigated by the analysis of grain size and mineralogy. The beach sediments, mostly light-medium gray green, were predominantly fine sands, with the average grain size of all samples about 200 μm. Some of the particles were gravel (2–16 mm), and some were even larger pebble-size particles (16–256 mm). Some mud (sediment <63 μm) was present, mostly in the sub-tidal sediments. The majority of the samples were skewed to the coarse size with coarse tail partly due to the presence of shell fragments. Approximately 50 % of the beach sediments were quartz with different varieties based on shape and size. The second major component of beach sediment was calcium carbonate which varied from 10 to 65 %. The other components in decreasing order consisted of microbreccia, feldspar, pyroxene, igneous rock fragments, biotite flakes, and heavy minerals. The levels of carbonate were lower in NW Al Batinah coast from Harmul to Al Khaburah but were higher in the SE from Al Khaburah to Al Ghubrah. This could be attributed to either lower carbonate production or more sediment input by wadis along the north-western part of Al Batinah coast. The unique and complex nature of these sediments is largely due to the geology of the terrestrial source area in the Hajar Mountains which contains the famous Samail ophiolite complex and the weak sorting along the shoreline in these tide-modified beaches.     

Incremental explosive energy distribution in blasting design

Summary The increasing range of explosive types and methods of initiation available to the blasting design engineer, and the possibilities of obtaining more detailed rock property data, require improvements in the precision of blasting design methods. Average design values, such as powder factor and specific charge, have little significance where rock properties vary in any lithological section of the blast. Application of the concept of incremental explosive energy distribution will increase the design sensitivity and control over blastability variations. In this paper the use of this concept is described for different levels of complexity. These range from the simple allocation of explosive energy for large rock sections, to the use of more complex energy attentuation functions to allocate incremental specific energy levels. Procedures to develop rock fragmentation predictions from such data are also outlined.     

An RES-Based Model for Risk Assessment and Prediction of Backbreak in Bench Blasting

Most blasting operations are associated with various forms of energy loss, emerging as environmental side effects of rock blasting, such as flyrock, vibration, airblast, and backbreak. Backbreak is an adverse phenomenon in rock blasting operations, which imposes risk and increases operation expenses because of safety reduction due to the instability of walls, poor fragmentation, and uneven burden in subsequent blasts. In this paper, based on the basic concepts of a rock engineering systems (RES) approach, a new model for the prediction of backbreak and the risk associated with a blast is presented. The newly suggested model involves 16 effective parameters on backbreak due to blasting, while retaining simplicity as well. The data for 30 blasts, carried out at Sungun copper mine, western Iran, were used to predict backbreak and the level of risk corresponding to each blast by the RES-based model. The results obtained were compared with the backbreak measured for each blast, which showed that the level of risk achieved is in consistence with the backbreak measured. The maximum level of risk [vulnerability index (VI) = 60] was associated with blast No. 2, for which the corresponding average backbreak was the highest achieved (9.25 m). Also, for blasts with levels of risk under 40, the minimum average backbreaks (<4 m) were observed. Furthermore, to evaluate the model performance for backbreak prediction, the coefficient of correlation (R ²) and root mean square error (RMSE) of the model were calculated (R ² = 0.8; RMSE = 1.07), indicating the good performance of the model.     

Class I and Class II Rocks: Implication of Self-sustaining Fracturing in Brittle Compression

Tests to determine the complete stress–strain curve of rocks indicate whether the rocks can be classified a Class I or Class II. Class II rocks exhibits the potential for self-sustained failure in the post-peak region. The purpose of the research described in this paper was to investigate whether or not this self-sustained failure characteristic is related to the fragmentation of the rock. The aim of the research was, therefore, to determine possible relationships between fragmentation and various properties of several rocks types, including the influence of the Class II characteristic. Fragmentation of rock depends on its self-sustaining failure behaviour and the energy available in the post-peak region to shatter the rock. The correlation of static and dynamic rock properties with size of fragments resulting from compression tests demonstrate clear relationships of Class II rocks, but the same cannot be said for Class I rocks. Analyses of test results show that fragmentation increases with an increase in rock strength, and is explosive for Class II rocks. Probability density distributions were constructed to show the overall comparison of fragment sizes produced during failure of Class II and Class rocks. The calculated probability of passing at X₅₀ and X₁₀ sieve sizes show that Class II rocks as a group are more finely fragmented. It can therefore be concluded that, when breaking rocks under the same steady loading conditions, Class II rocks will show greater fragmentation than Class I rocks.