Timing Effects on Fragmentation by Blasting

Transcription

1 Timing Effects on Fragmentation by Blasting by Omid Omidi A thesis submitted to the Department of Mining In conformity with the requirements for the degree of Masters of Applied Science Queen s University Kingston, Ontario, Canada (September, 2015) Copyright Omid Omidi, 2015

2 Abstract Rock fragmentation has always been an indicator for the efficiency of blasts in mines. Several suggestions have been made by blast operators and specialists to improve the rock fracturing mechanisms in order to obtain smaller fragments. The order of blast hole initiations together with the timing interval between holes has been observed to affect the blast results. In this study, a series of small-scale tests, simulating bench blasting have been made to establish the effect of delays in the sequence of blast initiation. The blasts were performed in high strength grout blocks, which were cast to provide a similar to rock condition excluding the possible structural weaknesses of the rock material. Homogeneity of the grout also helped to create a proper testing environment, ensuring the comparability of the blast results from different specimens. The grout used in the experiments had a strength of 50 MPa, density of 2.2 g/m 3, and P-wave velocity of 4000 m/s. Unwanted reflections of blast generated stress waves were eliminated by confining the blocks using a yoke. The tests were made with a range of inter hole delays from 0 to 2000 µs. The fragments achieved after each blast were collected and screened to analyze fragmentation from different delays. In general, coarse fragmentation was obtained from short delays while back break was minimal. Relatively longer delays resulted in better fragmentation with more damage to the back of the blocks. The work continued to investigate the effect of blast gas on fragmentation by conducting similar tests in blocks but by placing copper pipes in the blast holes to control the gas propagation through the blast zone. Although gas penetration did not seem to be fully inhibited by the copper pipes, the new design yielded different results in the delay range which had produced optimum fragmentation. The current study is considered to be a start point for more investigations in the field of efficient blasting with regard to the requirements for the mine to mill process. ii

3 Acknowledgements This research was accomplished with the financial support of the Centre for Excellence in Mining Innovation (CEMI). I would like to sincerely thank my supervisor, Dr. P.D. Katsabanis, for the guidance he has provided over the past few years. I am very grateful for his kindness, encouraging comments, and patience throughout the course of this project. I would also like to thank Mr. Perry Ross, Mr. Oscar Rielo, and Mr. Larry Steele for assisting me with my experiments and sharing their valuable views during this research. Finally, I would like to express my gratitude to my parents for understanding and extraordinary sacrifices made this thesis possible. iii

4 Table of Contents Abstract... ii Acknowledgements... iii List of Figures... vi List of Tables... vi List of Abbreviations... x Chapter 1 Introduction... 1 Chapter 2 Literature Review... 4 Chapter 3 Material Properties Physical and Mechanical Properties of the Selected Material UCS P-wave Velocity Determination of Shock wave Pressure and Duration Chapter 4 Fragmentation Test Set-up Testing Blocks Charging, Initiation, and Test Environment Analysis of the Blasted Material Chapter 5 Fragmentation Results from Powder Factor of 1.2 kg/m Experimental Work Initial Results Chapter 6 Fragmentation Results from Powder Factor of 2.4 kg/m Experimental Work Results Chapter 7 Fragmentation Results from Copper-lined Blast-holes Effect of Gas on Fragmentation Experimental Work Results Chapter 8 Fragmentation Results from a Medium-scale Granite Bench Experimental Work Results iv

5 Chapter 9 Analysis of Results and Discussion Blasting and Fragmentation New Approach toward Optimization of Fragmentation Effect of Timing on Fragmentation Uniformity and Delay Effect of Stress Wave on Creating Fragmentation Effect of Gas Pressure on Fragmentation of x 10, x 50, and x 80 Under Lined and Unlined Conditions Fragmentation Distribution Curves Under Lined and Unlined Conditions Back-break Analysis of the Granite Bench Experiments Chapter 10 Conclusion References Appendices Appendix A: P-wave Velocities Appendix B: Shock wave Records Appendix C: Fragmentation Distribution Curves, Powder Factor = 1.2 kg/m Appendix D: Fragmentation Distribution Curves, Powder Factor = 2.4 kg/m Appendix E: Fragmentation Distribution Curves, Copper lined Blast holes Appendix F: Fragmentation Distribution Curves, Granite Bench v

6 List of Figures Figure 1.1 Structure of the Thesis... 3 Figure 2.1 Effect of Degree of Fragmentation on Different Operating Costs and Overall Mining Costs (after Mackenzie, 1967) 6 Figure 2.2 Overall Mining Costs for Various Powder Factors (Eloranta, 1997)..7 Figure 2.3 Effect of Delay on Fragmentation Reduced Scale Tests (USBM Data) Figure 2.4 Effect of Delay on Fragmentation Full Scale Tests (USBM) Figure 2.5 Effect of Delay on Fragmentation (Katsabanis and Liu, 1996) Figure 2.6 Lagrange Diagram Representing the One Dimensional Propagation of Longitudinal (P) Wave, Shear (S) Wave, and a Crack (C), Rossmanith (2002) Figure 2.7 Representation of a One Dimensional Stress wave /Pulse in the Space and Time Domain, Rossmanith (2002) Figure 2.8 Representation of Fronts and Ends of a P-wave (P F, P E) and an S-wave (S F, S E) for a Short Pulse; After Rossmanith (2002)...19 Figure 2.9 Lagrange Diagram of the Interaction Patterns of the Waves from Two Simultaneously-Initiated Blastholes; After Rossmanith (2002)...20 Figure 2.10 Sequence of Events from Two Detonating Blast Holes (McKinstry, 2004).23 Figure 2.11 Desired Case for Overlap of Tensile (Negative) Pulses (Modified from Vanbrabant and Espinosa, Johansson, 2011) Figure 2.12 Average Fragmentation as a Function of Delay in Granodiorite Blocks (Katsabanis et al. 2006) Figure 2.13 Average Fragmentation from Short Delay Experiments (Katsabanis et al. 2006) Figure 2.14 Fragmentation Data from Johansson (2012) and Petropoulos (2013) Figure 2.15 Geometry of the Two Blast Hole Model Figure 2.16 Non Reflecting Boundaries and Free Surfaces Figure 2.17 Vertical Cuts used in the Results Presentation Figure 2.18 The Overall Crack Pattern Resulted from Finite Element Simulation (Delay = 0 ms). 33 Figure 3.1 Cylindrical Samples Before and After Strength Testing Figure 3.2 A General Representation of the Stress waves Travelling through Rock (Richards, 2009) vi

7 Figure 3.3 Experimental Set-up used to Measure P-wave Velocity Figure 3.4 Picking the Arrival Time of P-wave Figure 3.5 A Plan View of the Grout Block Specimens Used for P-wave Velocity Measurements Figure 3.6 Locations of the Carbon Resistor Gauges Figure 3.7 Typical Record for a Pressure-Time Pulse (Ghorbani, 1997) Figure 3.8 Pressure as a Function of Distance Figure 3.9 Pulse Duration as a Function of Distance Figure 3.10 Lagrangian Diagram to Show the Interaction of Stress Waves Figure 4.1 Dimensions of the Grout Blocks (main design) Figure 4.2 Dimensions of the Yoke Figure 4.3 Lagrangian Diagram to Show Interaction of a Shock Wave with an Arrested Crack from a Previously Detonated Hole Figure 4.4 The Components of the Electronic Initiation System Figure 4.5 Set-up for a Block Placed in the Yoke Figure 5.1 Block #1 after Successive Blasts Figure 5.2 An Example of Two Distribution Curves Figure 6.1 Distribution Curves from the 10 µs Test Figure 6.2 Distribution Curves from 40 µs Test Figure 6.3 Average Fragment Size as a Function of Delay Figure 6.4 Effect of Powder Factor on Fragmentation Figure 7.1 A Schematic of Crushed Zone, Fracture Zone, and Fragment Formation Zone Figure 7.2 Average Fragmentation, Lined and Unlined Conditions Figure 8.1 The Granite Bench Figure 8.2 Results from Granite Bench Figure 8.3 Fragmentation Districbution Curves from Bench Blasting Figure 8.4 Granite Bench, Before and After Blasts Fig 9.1 The Variation of Different Fragment Sizes with Time Figure 9.2 Fines Below 1 mm Figure 9.3 Slope at x Figure 9.4 Slope through x 80 and x Figure 9.5 Slope through x 60 and x vii

8 Figure 9.6 Rosin-Rammler Uniformity Index vs. Delay Figure 9.7 Uniformity Expressed as x 60/x 10 vs. Delay Figure 9.8 Interaction of Tensile Waves Figure 9.9 Lagrangian Diagrams to Create Zones of Maximum Tension (Katsabanis et al., 2014) Figure 9.10 Effect of Copper lining on x 10, x 50, and x Figure 9.11 Effect of Copper lining on Distribution Curves, Delay = 100 µs Figure 9.12 Effect of Copper lining on Distribution Curves, Delay = 200 µs Figure 9.13 Effect of Copper lining on Distribution Curves, Delay = 700 µs Figure 9.14 Effect of Copper lining on Distribution Curves, Delay = 800 µs Figure 9.15 Block Condition After a Simultaneous Initiation Shot Figure 9.16 Block Condition After 40 µs and 1000 µs Shots Figure 9.17 Block Condition After 200 µs Shots, Lined and Unlined Figure 9.18 Two Frames from Granite Bench 6 ms and 30 ms After Initiation, Delay= 0 ms Figure 9.19 Two Frames from Granite Bench 0 ms and 8 ms After Initiation, Delay= 500 ms Figure 9.20 Six Frames from Granite Bench, Delay = 2 ms viii

9 List of Tables Table 2.1 Description of Simulations Table 3.1 Some Physical and Mechanical Properties of the Grout Samples Table 3.2 Pressure Results Table 5.1 Initial Blasts Table 5.2 Swebrec Parameters from First Block Table 5.3 Swebrec Parameters from Second Block Table 6.1 Swebrec Results for Main Design Tests Table 7.1 Swebrec Results for Copper-lined Blasts Table 8.1 Swebrec Parameters for the Granite Bench ix

10 List of Abbreviations, Symbols, and Definitions A.Rock mass factor B.Burden (m) b.undulation parameter C p..p-wave velocity (m/s) PETN Pentaerythritol tetranitrate n.rosin-rammler uniformity index Rosin-Rammler..Particle size distribution function (Rosin et al.) Swebrec function.curve fitting function (after Ouchterlony) UCS.Uniaxial Compressive Strength (kg/m 3 ) VOD Velocity of detonation (m/s) x 50 Average fragment size (mm) x 80 80% fragment size (mm) x max.maximum fragment size (mm) x

11 Chapter 1 Introduction Any mining operation consists of different processes from blasting, loading, hauling, and mineral recovery. Although these phases seem to be independent from each other, in a comprehensive mine design plan they all relate to each other from the cost reduction viewpoint. Since any phase provides the next with the required feed, outcome of each unit directly affects the subsequent operation as well as the overall efficiency. Blasting, for example, is designed to reduce the size of material to a degree that it can be transported to the mineral processing plant. Several studies show that any attempt to modify blasting results, mainly fragmentation, could benefit the overall mining operation in terms of haulage requirements and reducing energy consumption in the mill. Researchers at the Julius Kruttschnitt Mineral Research Centre (JKMRC) have identified the following as the parameters influencing the blast fragmentation (Scott, 1996). - Strength parameters: the static compressive, tensile, and shear strength of the rock mass - Mechanical parameters: Young s Modulus and Poisson s ratio of the rock - Structural parameters: intactness of the rock, natural discontinuities including joint spacing and joint orientation 1

12 - Absorption parameters: the ability of the rock to absorb or transmit the energy liberated by the blast which influences the type of explosives required as well as its quantity, and firing sequence of the charges and timing. Monitoring several surface blasts in mines and small-scale tests, Singh et al., (2012) presented a number of factors to design blasts in order to optimize fragmentation. These factors included the spacing to burden ratio, burden to hole diameter ratio, explosives quantity, stemming length, the ratio between the total weight of explosives and the amount of rock broken (powder factor), and the ratio of bench height and burden (stiffness). Fragmentation improvement recommendations made based on these factors are typically limited to using smaller burdens in the blast design, higher quantity of explosives, shorter stemming (in hard rocks with fewer discontinuities), and choosing larger powder factors. To implement these recommendations, one could assume that higher amounts of explosives would be required implying larger powder factors. Although large powder factors could deliver better fragmentation results, they could be associated with some disadvantages such as increased blasting costs, excessive and undesired damage to the surrounding rock, higher vibration levels, larger risk of fly-rock, etc. Research on another effective factor on fragmentation, timing, has been conducted since the 1980 s. Distinguished researchers have published their results in many papers. Examples of some studies can be found in the literature (Stagg, M.S. and Nutting, M.J., 1987; Otterness et al., 1991, Katsabanis et al., 2006). A large variety of suggestions on the application of delay timing have been made; however, many unanswered questions have remained regarding the selection of appropriate timing in order to maximize fragmentation while negative outcomes of blasts (flyrock, vibration, etc) are controlled. 2

13 With the uncertainties over finding a proper timing to increase fragmentation, this study presents a series of results from experimental tests using high strength grout blocks which were blasted using a vast range of delays from 0 to 2000 µs. The results from small-scale blocks are also compared with a few data points obtained from a medium-scale granite bench. The findings of this investigation are presented to address some of the main issues regarding blast fragmentation such as: the role of short and long delays in producing fragmentation, the effect of fracturing mechanisms (gas and stress waves) in different ranges of delay, the influence of timing on uniformity of the fragments, and the role of the delay on the formation of back break. The structure of the thesis is shown in Figure 1.1. Chapter 1: Introduction Chapter 2: Literature Review Chapter 3: Material Properties Chapter 4: Fragmentation Test Set-up Chapter 5: Fragmentation Results from Powder Factor of 1.2 kg/m 3 Chapter 6: Fragmentation Results from Powder Factor of 2.4 kg/m 3 Chapter 7: Fragmentaion Results from Copper-lined Blast-holes Chapter 8: Fragmentation Results from a Medium-scale Granite Bench Chapter 9: Analysis of Results and Discussion Chapter 10: Conclusion Figure 1.1 Structure of the Thesis 3

14 Chapter 2 Literature Review Historically, in the mining industry, blasts are designed to break in-situ rock and prepare it for excavation and transport. In the cycle of mining, blast fragmentation is usually considered good enough when it is in agreement with instructions for proper digging, loading, transportation, and also mill requirements. These requirements are based on the capacity of the loading and transportation equipment and the maximum fragment size that can be fed to the rock crushers. Each division in mining is responsible for enhancing its productivity in order to lower the costs associated with their operation. In some cases, this has led to lack of a comprehensive view on the overall mine cost and profit planning. As an example, the mine processing plant may work with close to optimal efficiency when it is provided with a certain size and quality of the raw material as its feed, while other parts such as the mining department may decide to keep their costs down by lowering the powder factor or using traditional blast initiation systems. This attitude is typically due to the fairly high price of newer initiation systems while their advantages in increasing fragmentation are ignored. This decision will unwantedly affect the subsequent processes that need to be done on the broken rock. This is one common issue with many of the mines around the world. In recent years, progressive mines have begun considering plant productivity as one of the major requirements for cost and profit estimations. Due to this, a revision on the use of low powder factor and traditional blast initiation systems has been put in place to ensure that proper feed is provided to the mill (Rorke, 2012). The Mine-to-Mill concept was introduced to the mining industry first by the Julius Kruttschnitt Mineral Research Centre (JKMRC) in the recent years (Grundstorm et.al, 2001). The goal of such 4

15 studies is to find the parameters involved in reducing the overall mining costs, while the increased mill throughput is accounted for. A long list of parameters has been investigated in the Mine-to-Mill (M2M) studies. Among all, blasting and fragmentation play an important role as the initial phase of comminution. In this literature review, some of the attempts made to show the importance of a good fragmentation in the entire mining operation will be discussed, as well as some of the findings on optimization of fragmentation using theories of shockwaves and gas pressurization Blasting, First Phase of Rock Comminution Blasting is the first phase in fragmenting rock. Fragmentation is usually defined as breaking rock into suitable sizes in accordance with mill requirements such as feed size and grindability. Influence of blasting on the fragmented rock size and the rock resistance to crushing has been observed in several mine plants. Examples of such cases will be reviewed with focus on reductions in overall mining costs. Benefits of proper fragmentation are energy savings due to decreasing feed size of the primary crusher and productivity improvements in the subsequent breakage operations. The largest potential for energy savings in the mill occurs as the amount of undersize that bypasses the crushing stage increases (Ouchterlony, 2003). Also, some mine-to-mill studies suggest that remarkable grinding improvements can be achieved through optimization of fragmentation (Kim, 2010). The latter improvement depends on the micro-fractures that can survive in the primary crushing stage; however, such fractures emerge as they are influential in weakening the rock when it is processed in the grinding phase. Since mechanical crushing and grinding are expensive operations at a mine plant, any possible energy saving and cost reduction is 5

16 recommended in those operations. This goal seems to be reachable through good fragmentation despite of higher costs associated with blasting (Katsabanis, et al., 2004). Another saving in the costs is achieved in operations such as excavation of the broken rock, loading, and hauling. The chance of increasing mill throughput and subsequent higher revenues should also be considered as a result of improved fragmentation (Ouchterlony, 2003). In 1967, Mackenzie proposed simple graphs to show the importance of improved blasting and fragmentation in the overall mining costs (Mackenzie, 1967). His model was based on the effect of variation of the mean fragment size on different operating costs. Figure 2.1 illustrates how costs associated with loading, hauling, drilling, and crushing are decreasing with increasing fragmentation while drilling and blasting costs are increasing in this model. Figure 2.1 Effect of Degree of Fragmentation on Different Operating Costs and Overall Mining Costs (after Mackenzie, 1967) 6

17 Although Mackenzie s model was a primitive attempt to explain the reduction of costs by improving fragmentation, one could assume that by combining all the costs from individual mining units an optimum range of fragmentation size can be achieved with minimum fragmentation cost, and thus minimum overall mining costs. Mackenzie s studies concentrated on the direct mining costs. In order to further quantify processing costs with optimization of fragmentation, Eloranta (1997) conducted a research project with focus on cost information from the Minntac iron mine in Minnesota. Optimization of fragmentation was achieved through increase in powder factor in the blasts. As shown in Figure 2.2, processing costs have a declining trend steeper than the rise in blasting costs as the powder factor is increased. This means that total costs of mining can be reduced using improvements in fragmentation. Clearly, as shown by Mackenzie, a range of optimum fragmentation and cost reduction can be found in Eloranta s work. Figure 2.2 Overall Mining Costs for Various Powder Factors (Eloranta, 1997) 7

18 Many other people investigated the effect of blast optimization on downstream processes. Grundstrom et al. (2001) made an attempt to find the possibility of increasing mill throughput using high powder factors in the blasting operation at the Porgera Gold Mine. This research which was conducted with cooperation of the Porgera joint venture and Dyno Nobel, showed a 25% increase in mill throughput from 673 tph (standard blast design with powder factor of 0.24 kg/t) to 840 tph (modified blast design with powder factor of 0.31 kg/t). In the attempts mentioned so far, benefits of improved fragmentation were studied from increase in powder factor viewpoint. Powder factor is calculated based on the amount of explosive required to break the rock and the pattern of the blast holes including burden, spacing, and the length of the blast holes. Another parameter used to design a blast is the sequence of firing of the blast holes. This sequence is normally achieved through pre-determined timing delays between charges. Initiating explosives are designed to safely activate larger explosive charges according to the pre-determined sequence. Initiating explosives can generally be classified into electric and nonelectric types. In electric systems, a device that can generate or store electrical energy transmits the energy to the initiating explosives via a circuit of insulated conductors. Blast sequences can be controlled by means of electric timing systems but delay timing is usually achieved through pyrotechnic delay elements incorporated inside detonators. Non-electric initiating systems use reactive chemicals to store and transmit energy by controlled burning, detonation, or shock waves (ICI 1, 1997). Detonators are compact devices that are designed to safely initiate and control the performance of larger explosive charges. In recent years, electronic detonators have 1 Imperial Chemical Industries 8

19 been widely used in mines to initiate the charges in accordance with their defined timing sequence at high precisions. The idea of using electronic detonators was raised in 1973 at the Kentucky Blasters Conference (Miller et al., 2007); however, the new technology was first used in Australia in the mid 1980 s (Miller et al., 2007). Paul Worsey presented a paper on the commercial use of electronic initiation at the ISEE 1 meeting in 1983 (Cunningham, 2005). Subsequently, in 1987, the ICI Group introduced a pre-programmed electronic detonator system with 64 delays available. The system was used to determine the potential of precise timing as well as the ability to identify changes in blast results under controlled timing condition (Beattie et al., 1989). In 1990, Expert Explosives (ExEx) and Altech Technologies independently began to develop programmable electronic detonators for the mining industry 2. Finally, as the demand for this technology grew in many countries including South Africa and Australia, Orica Mining Services began manufacturing the electronic detonators in large-scale productions in 1999 (Miller et al., 2007). During 1980 s and 1990 s, mine managers did not welcome the use of electronic detonators. One reason was that the cost ratio of these detonators to traditional blasting caps was initially 10 to 1 in favor of the old systems (McKinstry, 2004). This attitude gradually changed as the advantages of the new initiation system were understood in price reduction of the overall mining activities and safety enhancements in mines around the world. Miller et al., (2007) presented examples of his observations at quarries in the north, south, and west of Australia 1 International Society of Explosives Engineers 2 History of Programmable Electronic Detonators, available on DetNet Website: 9

20 regarding the use of electronic detonators. He concluded that the following was achieved as a result of using electronic detonators: - 23% increase in loading and hauling rates - 18% increase in crusher throughput - An overall 13% decrease in operational costs despite the increased blasting costs In 2001, an evaluation was performed at the Betze Post open pit mine in Nevada aiming to assess the potential benefits of precise timing delays to the downstream processes (McKinstry et al., 2004). Precision in delays between holes was obtained by using electronic detonators. Two types of tests were made to study the effect of the electronic delayed blasts on fragmentation, excavator productivity, and mill throughput. In the first test, the pyrotechnic blast method was used whereas in the second test electronic detonators initiated the charges. In both tests 40 blast holes were drilled in the same geology and ore type. Results from the two tests showed an 11% increase in excavator productivity in the electronic trial which fully compensated for the additional costs of the new initiation system. Estimates showed a 6.5% increase in excavator productivity would offset these additional costs. Mill throughput increased from 152 to 167 St/hr while the plant operating work index experienced a 1 kwh/st decrease, which meant less energy was required to process the same amount of rock. Image analysis of the muck pile from both tests also showed a 44% improvement in fragmentation in the electronic blast. Additional evaluations were also conducted in 2002 (McKinstry et al., 2004) with focus on mill throughput benefits using electronic detonators in blasting. The outcome of the new evaluations also confirmed the results from the initial evaluation. The studies altogether convinced the management of the mine to adopt the new technology, electronic detonators, as their initiation system. In 2003, the net value added to mill throughput by the new technology at the Betze Post open pit was in excess of 2 million dollars. 10

21 2.2. Evaluation of Fragmentation Fragmentation of a blast is usually evaluated through analysis of the fragmentation distribution curves. Such curves are constructed using sieved broken fragments. The broken fragments form a wide range of fragment sizes from fine to coarse which is graphed to obtain some characteristics of the blasted material. Average fragment size (x 50), eighty percent fragment size (x 80), and maximum fragment size (x max) are some the typical indicators of fragmentation. These indictors can also be obtained using image analysis of a blast muck pile. Image analysis is usually recommended for large scale tests in mines or quarries due to high costs and difficulties associated with muck pile screening in such cases. Many attempts have been made to describe blast-induced fragmentation by introducing prediction models. Among these, the Kuz-Ram model (Kuznetsov-Rammler) is considered to be a popular tool for such predictions. The Kuz-Ram model (Cunningham, 1987) is based on expression of fragmentation by Kuznetsov s average fragment size model (1973) and the Rosin-Ramler distribution (Rosin et al, 1933). In Kuznetsov s model, the average fragment size is calculated through some parameters that define the rock and blast characteristics (Equation 2.1). where, A = rock mass factor x 50 = A. Q 1 6. ( 115 S ANFO ) 19/30 q 0.8 (Equation 2.1) Q = mass of explosive per blast hole (kg) S ANFO = weight strength of explosive relative to ANFO 11

22 q = powder factor (kg/m 3 ) The Rosin-Ramler distribution utilizes the calculated average fragment size and uniformity index (n) to predict fragmentation at any fragment size (Equation 2.2). x ( ) P RR (x) = 1 2 n x 50 (Equation 2.2) Cunningham (1987) presented an equation to calculate uniformity index based on geometry of the blast (Equation 2.3). Where, B = burden d = diameter of boreholes n = ( B d ) (1 W B ) (1 + W = standard deviation of drilling precision S = spacing L = length of borehole H = height of bench S B 1 ) 2 (L ) (Equation 2.3) H Ouchterlony (2005) presented a prediction model for fragmentation. His model uses the Swebrec function (Equation 2.4), to describe the fragmentation distribution together with the Kuznetsov s model (Equation 2.1) to achieve the average fragment size. The Swebrec function takes into account three major parameters including x 50, x max, and the undulation parameter (b). 12

23 P(x) = 1 b [1+[ ln(x max x ) ln( x ] ] max ) x 50 (Equation 2.4) The Swebrec fragmentation function was introduced as a result of work conducted by the Swedish Blasting Research Centre. The undulation parameter is a function of the average and maximum fragment sizes (Equation 2.5) b~0.5. x ln [ x max x 50 ] (Equation 2.5) It can be derived from the proposed fragmentation prediction models that these models are sensitive to the powder factor, geometry of the blast, the mass of explosive per borehole, and the type of rock and explosives used. In none of the models, timing intervals between boreholes and distribution of blast-generated energy are considered despite their role in creating fragmentation (Chung and Katsabanis, 2001). Chung et al. (2000) examined some of the experimental fragmentation data published by the USBM. They applied non-linear regression analysis to the USBM data and suggested that the Kuznetsov s equation can be re-written as (Equation 2.6). x 50 = A. Q B (SBR) H t (Equation 2.6) 13

24 where, SBR is the spacing to burden ratio. Since very little effect of timing was observed from the experimental data especially at short delays, it was decided to eliminate this factor from the equation. Thus, the final average fragment size predictor was presented as (Equation 2.7). x 50 = A. Q B (SBR) H (Equation 2.7) As shown in the presented equations, none of the models considered timing as a contributing factor to fragmentation. A simple explanation for this could be the large scatter existing in the experimental results even when accurate timings were used. Since this made it difficult to draw a certain conclusion on the effect of delay, researchers preferred to ignore this factor History of Attempts on Improving Fragmentation Early attempts on improving fragmentation were made by Stagg and Nutting (1987) in 18 cm (45 inch) high limestone benches. Their work is known as Reduced-scale Tests. Stagg and Rholl (1987) also conducted a series of Full-scale Tests in 6.6 m (22 ft) high benches of limestone. In continuation of these attempts, Otterness et al., (1991) made similar reduced scale experiments in dolomite benches. Results of the mentioned tests were based on muck pile screening of the blasted materials. These attempts are together known as the USBM Fragmentation Data. Katsabanis et al. (2014) examined the USBM data through normalization of the previously mentioned Kuznetsov s equation in order to investigate the effect of timing on the average passing size. Since powder factor and the amount of charge per hole are known in Kuznetsov s equation, the ratio x 50 q 0.8 Q1/6 can be considered as the normalized 50% passing size which 14

25 represents the effect of lithology and timing. Figure 2.3 displays the variation of the normalized passing size with delay per meter of burden derived from the reduced scale tests. Normalized x Delay time (ms/m of burden) reduced scale Figure 2.3 Effect of Delay on Fragmentation Reduced Scale Tests (Katsabanis et al., 2014) A general trend line with an optimum delay can be obtained from the reduced scale tests. Full scale tests, on the other hand, did not yield any tangible change in fragmentation as a result of using different delays in the experiments. Figure 2.4 shows the results of full scale tests in which no optimum delay was found. Neither of the tests showed any improvement in fragmentation at short delays (<3 ms) between blast holes. 15

26 Normalized x 50 Average size (cm) Delay time (ms/m of burden) Full Scale Tests Figure 2.4 Effect of Delay on Fragmentation Full Scale Tests Katsabanis et al., 2014) Katsabanis and Liu (1996) tried to establish the effect of delay in a 2m bench of granite. High speed filming of the blasts was used to analyze fragmentation. Despite inaccuracies of this technique, they managed to develop the X 50-Delay curve which answered some important questions about the role of delay in the outcome of the blasts. Results of their work are shown in Figure 2.5. At 0 ms delay the coarsest fragmentation was observed, while the optimum delay appeared to be close to 8 ms Delay time (ms/m of burden) Figure 2.5 Effect of Delay on Fragmentation (Katsabanis and Liu, 1996) 16

27 Rossmanith (2003) tried to explain the mechanical fracturing process of the rock material using the theory of stress waves and Lagrange diagrams. According to this theory, two types of stress waves arise from detonation of an explosive charge: P-wave (longitudinal or primary wave) and S-wave (shear or secondary wave). The waves are assumed to be planar and propagate in a one dimensional fashion and a Lagrange diagram can be used to describe the waves along with a crack generated by detonation. The three dimensional propagation of the waves is ignored in this model (Figure 2.6). Figure 2.6 Lagrange Diagram Representing the One Dimensional Propagation of Longitudinal (P) Wave, Shear (S) Wave, and a Crack (C), Rossmanith (2002) The tangents of the associated lines are the inverses of the speeds of the waves and the crack. Any stress wave of pulse type with finite length and finite duration consists of a leading (compressive) and a tailing (tensile) part. The leading compressive part is characterized by the index +, whereas the tailing part is shown by the index -. Hustrulid (1999) states that the positive pressure of the produced wave rapidly falls into negative values, which implies a change from compression to tension. A stress pulse can usually be described in two ways: in space or in 17

28 time. This is shown in Figure 2.7 (Rossmanith, 2002), where Ʌ W and τ W are the wave length and wave duration, respectively. Figure 2.7 Representation of a One Dimensional Stress wave /Pulse in the Space and Time Domain, Rossmanith (2002) For a single hole detonation, each wave is represented by its front and end as shown in Figure 2.8. Within the close vicinity of the blast hole the two waves overlap; however, they separate as the waves travel further away from the hole with different speeds. Rossmanith considers parallel lines for the front and the end of the P and S pulses, but he states that in reality the two lines slightly diverge as they travel along the positive x-axis. 18

29 Figure 2.8 Representation of Fronts and Ends of a P-wave (PF, PE) and an S-wave (SF, SE) for a Short Pulse; After Rossmanith (2002). As explained by Rossmanith and shown in Figure 2.9, several types of stress wave interaction can be identified for two adjacent blast holes separated by the spacing s. These interactions include the following categories. - PP interaction of the leading compressive parts of the P-waves - PP interaction of the tailing tensile parts of the P-waves - SS interaction of the S-waves - and a range of mixed wave interactions, such as the overlap of the P-wave from blast hole #1 and the S-wave from blast hole #2. 19

30 Figure 2.9 Lagrange Diagram of the Interaction Patterns of the Waves from Two Simultaneously-Initiated Blastholes; After Rossmanith (2002). Location and size of the interaction zones depend on the spacing distance between the blast holes, the duration of the pulses, and the delay between the two charges. Longer stress waves provide wider areas of stress wave overlap and increase the chance of superimposed interactions. On the other hand, if the second blast hole is delayed the regime of stress wave interaction becomes closer to the delayed blast hole. Rossmanith (2002) claimed that enhanced fracturing can occur in the areas between blast holes where maximum interaction of stress waves is obtained. Stress waves are known to propagate at large velocities; therefore, short delay times are preferred to create zones of interactions between blast holes. Since the tensile strength of rock is much lower than its compressive strength, Rossmanith s theory focuses on possible interaction and superposition of tensile tailings of stress waves. If the delay between blast holes is longer that the time required for the P-wave to travel the spacing 20

31 distance, PP interaction is eliminated. In the same manner, if the delay is chosen shorter than the P-wave arrival time at the second blast hole, PP interactions (compressional and tensional) are achieved; thus, improved fragmentation is expected. This type of analysis supports the idea of short delay and precise firings using electronic detonators in order to achieve better fragmentation results. Rossmanith (2002) also examined other types of interactions between stress waves and running cracks. According to this model, cracks are created and driven by stress waves; however, when the wave outdistances the crack, the initial crack comes to arrest. Further assistance from another stress wave is required to re-initiate the first crack. The model suggests that there are four possible interactions between stress waves and cracks as follows. - P-wave interacts with a running crack - Arrested crack interacts with P-wave and is re-initiated - Re-initiated crack interacts with S-wave, and - Re-arrested crack interacts with S-wave As a result of above, one can assume that the inter hole delay, if chosen properly, would be a good means to assist the stopped crack to re-initiate. For this purpose, the second hole should be programmed to fire in a way that its P-wave arrives at the arrested crack at the moment the crack stops and not prior to it. McKinstry et al., (2004) conducted a drill-to-mill study to evaluate the potential benefits of electronic detonators and precise timing at the Betz Post open pit gold mine. A part of this study focused on maximizing fragmentation in a full-scale rock bench using proper delay timing 21

32 between blast holes. Among 8 drilled holes in a row of blast holes, 2 of them were selected to measure P-wave arrival at the free face and the neighboring blast hole as well as the time between the firings of the holes and the face displacement. The holes were 22 cm in diameter, 14 meter deep, and altogether contained 335 kg of explosive. According to the published study, high speed cameras were used to define the time of the rock mass movement in order to measure the face displacement velocity. Fragmentation was also evaluated using high standard video cameras and image analysis. Piezoelectric accelerometers were placed in front of the blast holes to record P-wave arrival and initial movement time of the rock mass. The P-wave readings were later used to determine the allowable time to promote inner row wave collisions. Figure 2.10 shows the sequence of the post-blast events within the 36 ms of the first hole initiation. The P-wave from the first blast hole was recorded to reach its free face at 3.7 ms with the velocity of 1994 m/s, while it arrived at the next hole face at 5.56 ms with the velocity of 1935 m/s. The analysis showed an average P-wave velocity of 1981 m/s, suggesting the P-wave arrival at the neighboring hole to be around 3.5 ms. The accelerometer data showed that the face began to move at 11.7 ms; however, no visible face movement was observed by the video cameras within 18 ms of initiation of the first hole. Since the second blast hole was set to initiate with the delay of 25 ms from the first hole, no P-wave interaction between the two holes occurred in the experiment. The video cameras also captured the displacement of the second hole face 11 ms after its initiation implying that the key events took place in a timeframe of 36 ms. Based on the sequence of the events in the test and the analysis obtained from the captured images, McKinstry recommended that using an inter-hole delay of 3 ms fragmentation would be optimum with programmable electronic detonators. The reason for this delay selection is not clear given the observations on the movement of the free face. 22

33 Figure 2.10 Sequence of Events from Two Detonating Blast Holes (McKinstry et al., 2004) 23

34 In continuation of Rossmanith s work (2002), Vanbrabant and Espinosa (2006) made a series of full-scale tests to determine proper delay time between blast holes. Unlike Rossmanith s work, delays were chosen to create a time window for negative tails of P-waves to overlap in this study. The principle of overlapping is shown in Figure In this approach delays are longer than the time required for the P-wave to travel the spacing distance thus eliminating the superposition of waves between blast holes. The desired superposition of tensile waves takes place beyond the second blast hole in this model. In order to achieve the desired negative pulse superposition, a few key parameters should be take into account as follows: Figure 2.11 Desired Case for Overlap of Tensile (Negative) Pulses (Modified from Vanbrabant and Espinosa, 2006, Johansson, 2011) T Desired=T d + T 1 T 0 24

35 where T d = shock wave traveling time between the first hole and the neighboring hole. T 1 = duration of the first half wave (compressive) at the neighboring blast hole (at distance D = spacing). T 0 = duration of the first half wave at the first hole when it detonates (at distance D = 0) The idea of negative pulse superposition was tested by Vanbrabant and Espinosa (2006). Fragmentation achieved in their experiments was evaulated by the use of image analysis and it was claimed that a decrease of 45% in the average particle size was observed through their approach. Electronic blasting was recommended as a precise and reliable tool to enhance blasting results. However, no information on the actual delays was reported by them to provide a comparison between electronic and non electronic fragmentation results. Katsabanis et al (2006) made another effort to explain fragmentation results of blast holes with a range of delays between 0 to 4000 μs, in a smaller scale. Delays between 0 to 100 μs were achieved by different lengths of detonating cord between holes while longer delays were generated using a sequential blasting machine and seismic detonators. Granodiorite blocks with dimensions of 92 cm 36 cm 21 cm were chosen in the new study. Multiple rows of 11 mm diameter blast holes were drilled in each block. The holes were in an equilateral triangular pattern with the burden and spacing of 8.8 and 10.2 cm, respectively. The powder factor used in the tests was constant to investigate the effect of timing. Eight tests were made with delays under 1000 μs. In this range, as shown in Figure 2.12, instantaneous initiation had the coarsest fragmentation whereas longer delays led to better results. The short delays, if examined closely, show a horizontal line with little change in x 50 in the range of 0 to 1.1 ms/m of burden (Figure 25

36 Average Fragmentation (mm) Average particle size (mm) 2.13). The 1000 μs experiment, which corresponds to an actual delay of 11 ms/m of burden seems to have produced very close fragmentation to the 100 μs (1.1 ms/m) test. Unfortunately, it was impossible to generate a delay between 100 μs and 1000 μs, since delays were achieved by a sequential blasting machine, which had a resolution of 1 ms. Observing the results at longer delays, it appears that there was no drastic change of fragmentation between the experiments conducted at 22 and 45 ms/m delays Delay time (ms/m of burden) Figure 2.12 Average Fragmentation as a Function of Delay in Granodiorite Blocks (Katsabanis et al. 2006) Delay (ms/m of burden) Figure 2.13 Average Fragmentation from Short Delay Experiments (Katsabanis et al. 2006) 26

37 Examining the results, Katsabanis concluded that delays between 0.11 to 11 ms/m of burden would result in improved fragmentation. Based on the fact that at long delays (above 22 ms) fragmentation becomes coarse, time delay does not seem to be effective on fragmentation after a certain point. This could be due to no transmission of energy from the detonating charges to the earlier detonated holes as cracks are fully open. This type of blasting was very inefficient due to lack of stemming and decoupled blast holes. In addition, the blasted blocks had six free faces which led to reflection of the blast generated stress waves. This could further complicate the analysis of results. As explained by Blair (2009), in experiments such as this, the outgoing compressive wave from each hole will have six secondary reflected tensile wave stress, one off each face. Any reflection of the waves would either promote fragmentation or be transformed into kinetic energy of fragments. The location of each hole relative to the free faces determines the time interval between the primary and the secondary (reflected) waves. Blair (2006) also believed that in this particular case, this time interval could lie in the range of 15 to 225 µs (0.17 to 2.5 ms/m). Katsabanis results clearly contradict this claim for the delays up to 100 µs. However, since there is no experimental data available between 100 and 225 µs, no firm conclusion can be made regarding the time interval suggested by Blair. If Blair s assumption about the time span of 100 to 225 µs is true, the fracturing mechanism and the resulting fragmentation also involve a series of events such as the effect of reflected waves in addition to the action of the primary waves interacting within the blast hole initiation delays. Since in fullscale tests the rock mass is typically a continuous medium with no reflecting waves, generalizing the results from small-scale tests to larger scales should be done with caution. Also, due to the role of the reflected waves, drawing any definite conclusion regarding optimum delays may not be of enough validity. 27

38 Johansson (2011) conducted a series of small-scale tests using confined samples to eliminate surface reflections. The set-up of the tests included 15 magnetic mortar blocks with dimensions of mm. Magnetite powder, cement, quartz sand, and water were mixed to create the blocks. The synthetic blocks had similar properties to the magnetite ore body extracted from the Kiruna Mine in north Sweden. Two rows of blast holes, row 1 and 2, were drilled with five holes in each row to examine the effect of delay on blasting products. In order to avoid unwanted reflection of stress waves from free faces, the blocks were confined in a U shape yoke. This confinement also helped to create situations similar to the Sub Level Caving method (SLC) in underground mines. The spacing and burden were 110 and 70 mm, respectively. The yoke was made from high strength concrete. The distance between the yoke and the placed blocks was filled with a fine-grained expanding grout with similar density to the yoke to prevent impedance mismatch. The first row of blast holes in each block was fired with a pre-determined inter-hole delay while the same delay was used in the second row of holes to investigate fragmentation in intact and pre-damaged blocks. A variety of delays between 0 to 146 µs were chosen to examine different possibilities of shock wave interactions. Petropoulos et al., (2013) completed Johansson s work by conducting two more tests using the same set up and with a longer delay (290 μs). The results of the two experiments are displayed in Figure

39 Average particle size (mm) Row 1 - Johansson Row 2 - Johansson Row 1 - Petropoulos Row 2 - Petropoulos Delay time (μs) Figure 2.14 Fragmentation Data from Johansson et al., (2012) and Petropoulos et al., (2013) The two works proved that the use of simultaneous initiation would not improve fragmentation. Results of the first row showed a large scatter in the delay interval 0<Δt<86 µs. This suggests that no trend can be assumed for row 1, thus making it impossible to identify any minimum x 50, optimum fragmentation, in this range. The longest delay however, 146 µs, resulted in the best fragmentation among the tested delays. In row 2, a remarkably finer fragmentation with less scatter was obtained as opposed to the first row in all shots. A general tendency toward improved fragmentation was observed with increasing delay time over the interval 0<Δt<146 µs. The data published by Petropoulos extended this range to 290 µs. No more experiments were made beyond this point to reach a possible optimum fragmentation. The improvement of fragmentation in the second row was attributed to the accumulation of damage from the previous shot in the same block. The influence of the first row also appeared in minimizing the dust and boulders behaviour associated with blast products of row 1. By increasing the delay to 290 μs, Petropoulos observed better fragmentation in both rows. He claimed that no further 29

40 crack growth and consequently enhanced fragmentation can be achieved as each hole acts independently at longer delays without influencing the neighboring blast holes. These two studies created serious doubts on the role of stress wave interactions in generating fragmentation and the recommendations made by Rossmanith (2002) on the use of short delays. Nevertheless, the scatter in the experiments on intact blocks does not provide a clear picture on the effect of delay on fragmentation. In addition, this investigation seems to be incomplete since no definite conclusion can be drawn regarding the existence of an optimum delay in either intact or pre-damaged material. Schill et.al (2012) carried out an investigation to examine Rossmanith s idea of improving fragmentation by the use of 3D Finite Element simulations. The LS-DYNA 1 computer code was chosen to simulate fragmentation. In this work, a 2 blast hole model was designed to resemble the Aitik Open Pit Mine in Sweden (Figure 2.15). The blast model was assumed to be an infinite continuum; therefore, non-reflecting boundaries were predicted in the model (Figure 2.16). Figure 2.15 Geometry of the Two Blast hole Model (Dimensions in m), Schill et.al (2012) 1 For more information refer to the LSTC Corporation website: 30

41 Figure 2.16 Non Reflecting Boundaries and Free Surfaces, (Schill et.al 2012) The Finite Element discretization was performed with a total of 20 million hexahedron elements. The rock material in the case of this study was Westerly granite with compressive, shear, and uni-axial tensile strength of 200, 36, and 10 Mpa, respectively. The explosive type used in the experiment was an emulsion with the density of 1180 kg/m 3 and detonation velocity of 5850 m/s. This emulsion (known as Emulsion 682-b) was modeled with the explosive material in LS- DYNA combined with the JWL (Jones-Wilkins-Lee) equation of state 1. The stemming material (gravel) was modeled by MAT_SOIL_CONCRETE as well as the granite part. The study was made to determine the effect of initiation delay, the amount of explosives, and the distance between blast holes on the subsequent fragmentation. Table 2.1 shows the parameters used in design of the blasts. 1 Review of Jones-Wilkins-Lee equation of state, Baudin et al. (2010) 31

42 Table 2.1 Description of Simulations Initiation time Amount of explosives # BH1 BH2 BH1 BH2 Distance between blast holes 1 0 ms 0 ms 11 m 11 m 8.7 m 2 0 ms 1.5 ms 11 m 11 m 8.7 m 3 0 ms 5 ms 11 m 11 m 8.7 m 4 0 ms 0 ms 11 m 11 m 12.3 m 5 0 ms 0 ms 8 m 8 m 8.7 m 6 0 ms 0 ms 8 m 11 m 8.7 m In order to observe fragmentation in different sections of the model, the entire model was divided into 7 cuts as displayed in Figure The fragmentation results of all cuts were evaluated at 15 ms which by then, the tension waves are assumed to have passed. Figure 2.18 presents the overall crack pattern of the model at t = 15 ms. Figure 2.17 Vertical Cuts used in the Results Presentation, Schill et.al (2012) 32

43 Figure 2.18 The Overall Crack Pattern Resulted from Finite Element Simulation (Delay = 0 ms), Schill et.al (2012) In all tests, fragmentation was higher in areas around the blast holes. An effect of stress wave interaction was observed at the top of the model around the symmetry line (V4). Also, around this line, some interesting results can be found. This line is located where the primary stress waves meet and interact in the model. If blast holes are initiated simultaneously, unlike Rossmanith s theory, fragmentation does not seem to be higher in comparison with delayed tests. In addition, the adverse influence of increased blast hole distance and decreased amount of explosives on fragmentation was seen in the experiments particularly around the symmetry line. According to the simulations, it was also found that the highest fragmentation was achieved at fairly long delay times. Since at such delays, the primary stress wave from the first hole has already passed the second hole, the role of stress waves in improving fragmentation was questioned by this research. 33

44 Johansson et al. (2013) conducted a series of numerical simulations to further determine the effect of timing on blast performance. The previously performed small-scale test pattern (Johansson, 2011) was employed with the same methodology as Schill (2012). The Finite Element discretization was performed using hexahedron elements. The mesh size of mm was chosen for the concrete part. The yoke was discretized with coarser mesh. The detonation initiation point of the holes was the top of the blocks. The concrete part was modeled with the Riedel-Hiermaier-Thoma (RHT) concrete model (Borrvall, 2011) which is a plasticity model for brittle materials such as concrete. The parameters used to model the explosives (PETN cords) in the experiments were extracted from the AUTODYN material library (ANSYS, 2010). Crack formation and propagation modeling was not directly possible in this work. Therefore, a threshold damage value was considered for each element. An algorithm was developed to identify fragments that exceeded the determined damage threshold (60%) and the area of each fragment. The fragmentation area was then specified by measuring the fragments in a number of vertical and horizontal cuts through the medium. The damage levels were evaluated at 1000 µs for all simulations. The remaining area and remaining volume of the model were also studied to evaluate the blast effect. These two concepts were the residual area and volume of the model which were determined after the fragments with damage level of above 60% were blanked out. The three parameter (fragment area, remaining area, and remaining volume) method was used to review the previous findings from Johansson s experiments on small-scale concrete blocks. Based on the simulations, it was concluded that: - Simultaneous initiation always results in coarse fragmentation. - The effect of delay timing on fragmentation is clear. Short delays improve fragmentation as opposed to instantaneous firing; however, the best fragmentation was found at a relatively long delay intervals where stress wave superposition was not possible. 34

45 - The optimal delay time could be in the interval of 73 to 86 µs. It was also noted that, the row 1 simulations resulted in coarser fragmentation compared to row 2 shots. The initial damage induced by row 1 in the blocks seems to have significant influence on the fragmentation. 35

46 Chapter 3 Material Properties 3.1. Material Selection Fragmentation studies have usually been made on different rock types including granite, limestone, dolomite, etc. One can assume that testing rock specimens may encounter structural flaws such as pre-existing discontinuities in the specimens and lack of homogeneity of the rock materials. In such cases, the pre-existing fractures dampen the energy of the blast-generated wave and block its propagation once a blast-induced crack reaches a discontinuity. Also, gaseous products of a blast will leak off through the passages existing in the rock, such as pre-blast cracks, thus reducing the influence of gas pressure on developing more cracks in the rock. Therefore, it seems reasonable that propagation of detonation products (stress waves and gas) in rock will not be similar to media without pre-existing discontinuities. Consequently, a true interpretation of blast results only with respect to effects of detonation products requires a medium without pre-blast fractures. To provide such a medium and in order to create consistent and similar test conditions in all the samples, it was decided to produce a series of small-scale test blocks using a commercial high strength grout (Sika Grout 212 SR) 1 with the following composition. Ingredient Weight % kg 2 SikaGrout 212 SR Water The amount of grout and water required to cast one block with the main design (Refer to Chapter 4). 36

47 The grout mixture was poured into a rectangular mold (wooden box) and the blast holes for placing detonating cords were inserted by using wooden dowels based on the determined burden and spacing pattern. The poured mixture required 28 days to reach the maximum strength of 60 MPa (SikaGrout 212 SR Product Data Sheet, 2012) Physical and Mechanical Properties of the Selected Material UCS Laboratory tests were conducted to determine physical and mechanical properties of the grout in this research. 18 cylindrical samples were produced to measure density, Young s Modulus, and uniaxial Compressive Strength (UCS) of the samples. The length and diameter of the samples were 12 cm and 5.2 cm, respectively. Samples were mounted in a 500 kn Material Testing System (MTS) and were subjected to uniaxial pressure. All the tests were made in the Rock Mechanics Lab at the Department of Mining, Queen s University. Figure 3.1 shows some of the samples before and after strength testing. Figure 3.1 Cylindrical Samples Before and After Strength Testing 37

48 Table 3.1 illustrates the UCS results as well as more information on the grout specimens. According to the results, the average UCS of the Grout SR212 samples is 50 MPa, which represents a material with strength of medium to high as classified by ISRM (1978) 1. Also, the average density of the grout is 2.2 g/cm 3. Table 3.1 Some Physical and Mechanical Properties of the Grout Samples Sample # Length (cm) Diameter (cm) Weight (g) Density (g/cm 3 ) Young s Modulus (Gpa) UCS (Mpa) International Society for Rock Mechanics 38

49 P-wave Velocity When an explosive charge detonates in a blast hole, the rock surrounding the charge is fractured and if the fracturing mechanism is strong enough, fractures extend to a level that where rock is fully disintegrated. The energy liberated from a detonation propagates in the form of stress waves. The most important types of stress waves are the P-wave, the S-wave, and the R-wave. P-wave stands for primary wave. It is also called pressure wave or compressive wave. In this type of wave, which is known as the longitudinal wave, particle motion is parallel to the direction of wave propagation. A P-wave travels in all directions at velocities proportional to mechanical characteristics of the material being travelled through. S-wave stands for secondary wave or the shear wave which is known to travel at 50-60% of the velocity of the P-wave (Richards, 2009). Particle motion in this type of wave is perpendicular to the wave propagation. R-wave stands for Rayleigh wave and only travels on surfaces. This type of wave is of importance in earthquake studies. Figure 3.2 demonstrates how the generated waves travel through the rock material after detonation. Figure 3.2 A General Representation of the Stress waves Travelling through Rock (Richards, 2009) 39

50 In general, wave velocity measurement is done by sending a mechanical pulse through the sample and receving it at the other end. Typically a piezoelectric transducer is used to convert voltage to a mechanical pulse and vice versa. As shown in Figure 3.3, a pulse generator sends a square wave of short duration in the form of voltage to the transducer and simultaneosuly a trigger is sent to the recording device to notify it when the pulse was sent. This is when the transducer is excited and sends a mechanical pulse across the sample which is received by the other transducer at the other end. The mechanical pulse is converted into voltage and the oscilloscope records the received pulse as well as the start time from the trigger. Figure 3.3 Experimental Set-up used to Measure P-wave Velocity Velocity of the pressure wave can then be measured using the distance of the wave traveling across the specimen and the time it requires for the second transducer to receive the pulse. The wave recorded on the oscilloscope screen and the P-wave arrival time is shown in Figure

51 Figure 3.4 Picking the Arrival Time of P-wave For the purpose of this thesis, p-wave velocity measurement was performed to furtherdetermine the characteristics of the grout material used in this research. Eight blocks of SikaGrout 212SR were cast with dimensions of 25 cm 20 cm 25 cm. P-wave velocity was measured every 1 cm along the length of blocks, providing 25 readings for each block (Figure 3.5). Figure 3.5 A Plan View of the Grout Block Specimens Used for P-wave Velocity Measurements 41

52 The results of the measurements are shown in Appendix A. According to the readings from the eight blocks, the p-wave velocity for Grout 212SR is 4075±45 m/s Determination of Shock wave Pressure and Duration The eight grout blocks mentioned in the previous section were also used to examine blast shock wave characteristics such as shock wave pressure at different distances from the explosives charge and duration of the pulses. In order to do so, the eight samples were divided into two sets of four blocks. In each block, one hole was drilled in the middle of the block and two holes were drilled in the two sides of the center hole (blast hole). The diameter of the blast hole was 1 cm and different strengths of detonating cords were used to charge the blast holes. Table 3.2 shows the strength of the detonating cord in each experiment. The volume between the wall of the blast hole and the detonating cord was filled with water to provide a continuous medium for the propagation of the shock wave. The two side holes were drilled to place carbon resistor gauges to record shock wave pressure at the different distances from the charge. The gauges in this work were 120 Ω carbon composition resistors made by the Allen Bradley Corporation. The carbon resistors were embedded and protected at the bottom of the pressure holes by polyethylene shrink tubing and epoxy. Figure 3.6 shows the locations of the resistors in the two block sets. 42

53 Figure 3.6 Locations of the Carbon Resistor Gauges When a carbon resistor is subjected to shock waves from detonation, it undergoes compression which leads to a change in resistance of the carbon composition resistor. This change in resistance can be derived from the voltage readings that can be recorded on any suitable data capture device (Mencacci and Chavez, 2005). In the case of this study, the setup of the experiments consists of detonating cords of different strengths, electric detonators to initiate the event, two 120 Ω Allen Bradley carbon resistors embedded in the blocks, a PCB signal conditioner (Model 482C) to supply constant current of 4 ma to the resistors, and a Data Trap II manufactured by MREL to record voltage variations of the sensors. The data acquisition device (Data Trap) was set to initiate recording by a trigger line connected to the detonator in each experiment. Typically, an experimental setup such as above is used to measure relative resistance change of the sensors, caused by detonation, in order to calibrate the pressure response of the gauges. Many researchers have conducted experiments using resistor gauges to produce calibration 43

54 equations for shock wave pressures of different amplitudes. Wieland (1987 and 1993) proposed a calibration equation relating relative conductance (inverse of resistance) change to pressure for amplitudes below 1.0 kbar (Equation 3.1). P (kbar) = R 0 R R (Equation 3.1) for 0 < P < 1 kbar where, R 0 and R are the original (initial) and instantaneous resistances of the gauge respectively, expressed in ohms. Katsabanis (2013, personal communications) presented another equation to calculate shock wave pressure which was suggested to be reliable for amplitudes up to 3000 bar (Equation 3.2). P (bar) = e ( ln(R 0 R R ) for 0 < P < 3000 bar (Equation 3.2) In this study, the two mentioned equations were utilized in order to examine the shock waves at different distances from the charge. The wave duration was then determined as well as the shock wave peak amplitude using the pressure pulses in each experiment. Figure 3.7 demonstrates a typical recording of a pressure-time pulse obtained from carbon resistor gauges in one of the measurements. 44

55 Pressure (kbar) Time (ms) Figure 3.7 Typical Record for a Pressure-Time Pulse Using the two relationships, the pressure-time profiles were graphed (Appendix B) and the characteristics of the pulses were as displayed in Table 3.2. Block # Charge loading (grams/m) Table 3.2 Pressure Results Distance between gauge and blast hole (cm) Pressure (Kbar) Wieland Calibration 45 Pressure (bar) Katsabanis Calibration Pulse Duration (µs)

56 Pressure (bar) Distance (cm) Pressure-Distance (3.15 g/m) Pressure-Distance (5.25 g/m) Pressure-Distance (21 g/m) Figure 3.8 Pressure as a Function of Distance One conclusion from the pressure-distance graph (Figure 3.8) is that the pressure attenuates as the wave moves away from its generating source. Another characteristic of the pressure waves as shown in Figure 3.9 is the pulse duration obtained at different distances from the borehole. Although different loadings were used in the experiments, it can be claimed that an average pulse duration of 19 µs was achieved 2 to 4 cm from the charge. It is worthy of noting that in the case of 21 g/m detonating cords (similar to the strength used in fragmentation tests in this study) 30 µs duration pulses were produced at 3 to 4 centimeters from the borehole. This is of importance, if one intends to draw Lagrangian diagrams to examine areas of overlap between stress waves to modify fragmentation as discussed in the literature. Assuming this pulse duration (30 µs), a P-wave velocity of 4075, an S-wave velocity of 2500 m/s, and a spacing between boreholes of 10 cm, the Lagrangian diagram can be shown for two boreholes initiating simultaneously, as shown in Figure It is evident that a large area between the two boreholes is covered by the interactions of the stress waves arising from each hole. According to Rossmanith s theory (Rossmanith, 2002), these interactions could strongly contribute to the 46

57 Duration (µs) process of fragmentation, while others have suggested to use time delay in order to move the wave interactions closer to the delayed hole or even beyond the spacing distance. In any case, such diagrams, using the velocity and duration of the pulses, are helpful to identify the areas of stress wave interactions under various delayed and non-delayed blast conditions. Based on the type of interactions, their locations, and the fragmentations achieved, positive impacts of stress waves could be examined more accurately Distance (cm) Distance-Pulse Duration (3.15 g/m) Distance-Pulse Duration (5.25 g/m) Distance-Pulse Duration (21 g/m) Figure 3.9 Pulse Duration as a Function of Distance 47

58 Time (µs) Distance (m) PCF1 PCE1 SF1 SE1 PCF2 PCE2 SF2 SE2 PTE1 PTE2 Figure 3.10 Lagrangian Diagram to Show the Interaction of Stress waves. PCF1: Front of Compressive Wave from 1 st Borehole; PCE1: End of Compressive Wave from 1 st Borehole; SF1: Front of S-wave from 1 st Borehole; SE1: End of S-wave from 1 st Borehole; PCF2: Front of Compressive Wave from 2 nd Borehole; PCE2: End of Compressive Wave from 2 nd Borehole; SF2: Front of S-wave from 2 nd Borehole; SE2: End of S- wave from 2 nd Borehole; PTE1: End of Tensile Wave from 1 st Borehole; PTE2: End of Tensile Wave from 2 nd Borehole. 48

59 Chapter 4 Fragmentation Test Set-up A series of small-scale blasts were conducted to study the effect of timing sequence on fragmentation. Using the idea of confinement by Johansson and Ouchterlony (2013) it was decided to place the testing blocks in a concrete yoke to minimize the impacts of free face reflections. The yoke simulated a condition similar to bench blasting in open pit mines, resulting in attenuation of the stress waves as they traveled inside the yoke without reflecting at the boundaries to interact with the blocks Testing Blocks As shown in Figure 4.1, the dimensions of the testing blocks (main design) were cm (L H W). The burden and spacing were 7.5 and 10.5 cm, respectively. The length and diameter of the blast holes were chosen to be 23 and 1 cm. Wooden dowels were used to create the holes in their specified locations. The shape and dimensions of the yoke are demonstrated in Figure 4.2. A fast-curing concrete with a density of 2.3 g/m 3 was used to fill the gap between the blocks and the yoke. This density is close to the density of the grout, thus minimizing the possibility of impedance mismatch between the two mediums. 49

60 Figure 4.1 Dimensions of the Grout Blocks (main design) Figure 4.2 Dimensions of the Yoke 50

61 4.2. Charging, Initiation, and Test Environment The blast holes were loaded with two strands of detonating cord, with the exception of the first six tests where single strands were used. Each detonating cord consisted of 10.5 g/m (50 grain/ft) of PETN, providing a powder factor of 2.4 kg/m 3 based on the pattern of the blast. The powder factor for the first six tests was 1.2 kg/m 3. The blast holes were coupled with water prior to the blasts. A wide range of delays was chosen from 0 to 2000 μs (0 to 26.6 ms/m of burden) between blast holes to fully investigate the effect of initiation delay on blast fragmentation. As demonstrated earlier in Figure 3.18, the P-wave front arrives at the neighboring blast hole 24.5 μs after initiation. Assuming the pulse duration of 30 µs, the end of the compressional wave reaches the second hole at t = 54.5 μs. If the two blast holes are simultaneously initiated, a wide area of PP interaction is generated covering the spacing distance between blast holes. Lagrangian diagrams also suggest that other interactions of waves such as overlap of tensional pulses or the overlap of tensional pulses and S-waves can occur between two adjacent blast holes. Using shorter duration pulses, one can assume that the wave interaction zones would become smaller. In the same manner, initiation delay can change the areas of interaction bringing them closer to the delayed blast hole with narrower zones of overlap. Other interactions of shock waves may take place beyond the spacing distance. Johansson (2011) selected a number of inter-hole delays to investigate such interactions. Some delays were also chosen slightly longer than the arrival time of the shock wave from the first blast hole at the last hole to ensure no possible wave interaction would occur. A different type of interaction takes place when a wave arrives at an outgoing crack which is initially driven by another stress wave and is later assisted by gas. The cracks come to arrest 51

62 Time (µs) immediately after the stress intensity factor at the crack tip falls below the critical value (Rossmanith, 2002). Lagrangian diagrams can be helpful in describing how waves and cracks interact and thus lead to re-initiation of the arrested crack. As displayed in Figure 4.3 a crack propagating at the velocity of (680 m/s) 1 reaches the free face (Burden = 7.5 cm) at t 130 µs. This will cause the arrest of outgoing cracks due to the venting and loss of gas as a driving factor in developing fractures. Under such circumstance, if a tensional pulse from a second blast hole arrives at the moment the crack stops, it can be concluded that the arrested crack will resume its movement. In order to do so, the second blast hole has to detonate 124 µs after the first hole. This delay provides the required time for a tensional pulse from the second hole to arrive at the moment the mentioned crack stops to re-initiate it. Unlike stress wave interaction assumptions, this model suggests that a certain delay is needed between blast holes, if fragmentation optimization is desired Distance (m) PCF1 PCE1 SF1 SE1 PTE1 Crack PCF2 PCE2 SF2 SE2 PTE1 Figure 4.3 Lagrangian Diagram to Show Interaction of a Shock Wave with an Arrested Crack from a Previously Detonated Hole. 1 Crack Velocity (1/6) P-wave Velocity, Katsabanis et al., (2006) 52

63 Considering the velocity of detonation of 7000 m/s for the detonating cord, short delays (< 100 μs) were achieved by using appropriate lengths of detonating cord between successive holes. An upgraded version of the Orica electronic initiation system (I-kon I) was used to obtain delays longer than 100 μs (Figure 4.4). The I-kon I initiation system showed an accuracy of ± 20 µs (Katsabanis et al., 2014) which seems to be adequate for the range of delays in which electronic detonators were used (>100 µs). The sub-millisecond system is composed of the following components: - A logging and testing device (Logger A3) with the capability of programming up to 400 detonators, - A blasting device (Blaster 400 A3) to issue the fire command according to the timing sequence information stored in the logger, and - Sub-millisecond electronic detonators connected to each charge to initiate them based on the signal received from the blaster. This initiation technology delivers benefits such as: high timing accuracy with regard to delays assigned for each detonator, programmability of detonators in increments/multiples of 0.1 ms between 0 to 250 ms, independence of detonators in receiving their fire signal which minimizes the risk of possible cut-offs and concerns with respect to early initiations, and capability of detonators to communicate with the logger at any time prior to firing which allows the user to identify possible problems associated with the detonators (I-kon II Electronic Blasting Manual, 2013). 53

64 Figure 4.4 The Components of the Electronic Initiation System (i-kontm II Technical Data Sheet, 2013) Considering the closeness of the holes, a lot of care was taken to separate the detonating cords and detonators of the neighboring holes using wooden boards to avoid cut-offs or unwanted initiation of charges due to possible contacts between charges (Figure 4.5). 54

65 Figure 4.5 Set-up for a Block Placed in the Yoke Experiments were done in a 76 m 3 chamber at the Alan Bauer Explosives Laboratory. The chamber was used to contain the flying fragments after each blast. The walls and floor of the chamber were covered with 2.5 cm thick rubber mats to minimize further breakage of the fragments due to impact Analysis of the Blasted Material The blasted material was collected after each blast and was transported to the Mineral Processing Laboratory in the Mining Department at Queen s University. Sieving analysis was done to achieve the particle distribution curves as well as the average fragment size and several other fragment sizes for further examination of the results. Fragments were sieved in two steps. 55

66 The larger particles were size-measured individually using a measuring tape. The finer material was then passed through the following standard size sieves: 63; 53; 45; 37.5; 31.5; 26.5; 22.4; 18.85; 13.33; 9.5; 6.73; 4.76; 3.36; 2.38; 1.7; 1.18; 0.84; mm. Fragmentation can be evaluated using different methods. One of the measures for evaluation of fragmentation is the average fragment size (x 50) that can be achieved using the particle distribution results from the sieved material. A simple interpolation between the two particle sizes with passing percentages close to 50% can be used to predict the average fragment size. However, the best fragmentation model is the Swebrec function that enables one to construct a comprehensive size distribution fit from the sieved material with a high accuracy to approximate the desired fragment sizes other than x 50 (x 10, x 60, x 80, etc). In this study, the Swebrec function (Equation 2.4) was used as the main fragmentation model. This function uses the 50% passing size particle as the central parameter while introducing an upper limit, x max, to the fragmentation distribution and a curve undulation parameter (b). It has been found that this fragmentation distribution gives excellent fits to hundreds of fragmentation data with correlation coefficients of (r 2 > 0.995) or better (Ouchterlony, 2005). In addition to the Swebrec curves, the Rosin-Rammler function was used to describe the cumulative distribution of fragments. The results are given in Appendix C. The Swebrec and Rosin-Rammler parameters (x max, x 50, b, n, and x c) were calculated through least squares fitting. 56

67 Chapter 5 Fragmentation Results from Powder Factor of 1.2 kg/m Experimental Work In the beginning of the project, it was decided to use one strand of 10.5 g/m detonating cord with the pattern mentioned in Section 4.1. This provided a powder factor of 1.2 kg/m 3 which appears to be close to practical blasts. The first three test blocks were cast with a slight difference from the main design. The dimensions of the initial blocks were cm (L H W), thus enabling us to perform more than one blast in the blocks. The blast holes were drilled manually in the first three blocks using masonry drill bits with a diameter of 1 cm. The burden, spacing, and length of the holes were as discussed in Chapter 4. In the first block, the first row of blast holes was drilled and the holes initiated with no delay using an electric detonator to initiate all detonating cords in order to study fragmentation under a simultaneous firing condition. After blasting the first row and collecting the fragments, another row of blast holes was drilled with 4 holes in the block under the same burden and spacing. The blasts continued in the same manner with but different inter-hole delays and number of blast holes in the remainders of the block. Table 5.1 shows the number of blasts performed in the first block along with the applied delay between the blast holes. Table 5.1 Initial Blasts Row # Number of blast holes Delay (μs) Powder factor (kg/m 3 )

68 5.2. Initial Results The fit of the Swebrec function and its parameters were obtained for the four shots (Table 4.2). The coefficient of determination obtained by the Swebrec fit was with one exception ( t = 40 μs) above Table 5.2 Swebrec Parameters from First Block Block and Row # Delay (μs) x 50 (mm) x max (mm) b r 2 B1R B1R B1R B1R The first test with 0 μs delay between blast holes produced a very large fragment constituting approximately 75% of the total weight of the obtained material, which made it very difficult to describe fragmentation by a continuous sieving curve and predict the fragment sizes of interest including x 50, x 80, and x max (See Appendix C). However, the Swebrec function gave a very good fit for fragments up to 75 mm. Figure 5.1 displays the condition of the first block after several blasts until the block fully disintegrated. 58

69 Figure 5.1 Block #1 After Successive Blasts As shown in Table 5.2, the average fragmentation improved as the delay time increased; however, it is not clear if this was merely due to the influence of timing. It seems necessary to note that the blast in row #1 was carried out under an intact material condition as opposed to the next rows in which possible radial cracks existed before blasting. Petropoulos et al., (2013) described this as a preconditioned material caused by previous blasting in the block. The next set of fragmentation data using a powder factor of 1.2 kg/m 3 was achieved from two blasts in separate blocks with the same blasting pattern. The delays chosen in these two experiments were 40 and 200 μs. The first delay provided a comparison between the two experiments with the same timing factor under two different material conditions (damaged and intact). Interestingly, the two major fragment sizes (x 50 and x 80) became coarser by 255% and 59

70 Passing (%) 216%, respectively in the repeated 40 μs experiment. Table 5.3 illustrates the Swebrec parameters from the two tests. Table 5.3 Swebrec Parameters for Second Block Block and Row # Delay (μs) x 50 (mm) x max (mm) b r 2 B2R B3R The Swebrec fits from the blasts in the two blocks were plotted together (Figure 5.2). The distribution curve of the 200 μs experiment has moved toward the finer range of fragments, if compared to the 40 μs test. In the 40 µs experiment, the fit of the Swebrec function is rather poor, especially for fragments larger than 100 mm (boulders). However, in the longer delay test (200 µs), the Swebrec model seems to have predicted fragments of different sizes with better accuracy to their experimental values. 100 Particle Size Distribution Size (mm) 40 μs - Swebrec 200 μs - Swebrec 40 μs - Experiment 200 μs - Experiment Figure 5.2 An Example of Two Distribution Curves 60

71 Another point worthy of noting is that the initial powder factor of 1.2 kg/m 3 produced average fragments larger than half of the burden. In blast practice, this type of fragmentation is known as dust and boulders (Johansson, 2012) which is indicative of poor blasting. Producing oversize fragments (boulders) suggests that the blast has not been successful in breaking the rock into suitable sizes for the cycle of excavation, loading, hauling, and finally the crushing plant. Also, very fine particles (dusts) resulting from blasting can become both an environmental issue and an economical loss to the producers (Parihar et al., 2012). The remedy to this situation can be achieved by increasing powder factor or making changes to the blast design including burden and spacing (Singh et al., 2012). For the purpose of this project, powder factor was doubled, although it is much higher than the powder factors normally used in bench blasting. Significant losses of energy and gas, released by blasting, due to lack of stemming as well as decoupling of the blast holes justifies this decision. In addition, increasing powder factor makes it possible to compare results from similar delays under different loadings of blast holes. 61

72 Chapter 6 Fragmentation Results from Powder Factor of 2.4 kg/m Experimental Work The blast holes in this research project were top initiated and did not allow for using any stemming material. Under such blast conditions, there will be premature escape of blastgenerated-gases which can lead to poor fragmentation qualities (Rajpot, 2009). To compensate for the energy lost due to the escape of gas, two strands of PETN cord were used in each hole to double the powder factor of the blasts (2.4 kg/m 3 ). Twenty blocks of grout were cast with dimensions of the main design, cm (L H W), and they were blasted with a variety of delays from 10 to 2000 microseconds. This choice of delay generated more data points in the range of interest and could provide a better answer to questions about fragmentation and delay. In the shots with delays below 100 μs (10, 40, and 80 μs), pieces of detonating cord with the lengths that corresponded to the VOD of 7000 m/s and the desired delays (7, 28, and 56 cm of PETN cord between holes) were cut to achieve the delays. These pieces of detonating cord connected the charges inside the blast holes to an electric detonator and the firing line. In longer delays, it was difficult to use the same method due to the short distance between the holes. Two tests were implemented with a delay of 2000 μs. A sequential blasting machine (REO model BM ST-M) was used to create this delay for one of the tests. The sequential blasting board failed to initiate all of the charges. The last two holes did not fire which was assumed to be due to a cut-off in the circuit. A new technology (sub-millisecond electronic detonators, ikon detonators manufactured by Orica) was used to obtain delays in all other 16 blocks. 62

73 6.2. Results Results of the failed test did not seem to be a true representation of the parameters used in evaluating fragmentation; therefore, this shot was repeated with the new electronic initiation system. Table 6.1 presents the results from the Swebrec fits of the 20 tests. The Swebrec function fit runs very well through the experimental data with r 2 of above at delays equal to 80 μs and longer. At delays t = 10 and 40 μs, discrepancy between the sieved data and the Swebrec curves can be observed in the range of fine particles, coarse fragments, and around average fragment sizes (Figures 6.1 and 6.2). Table 6.1 Swebrec Results for Main Design Tests Block # Delay (μs) x 50 (mm) x max (mm) b r

74 Passing (%) Passing (%) Size (mm) Experiment Swebrec Figure 6.1 Distribution Curves from the 10 µs Test Size (mm) Experiment Swebrec Figure 6.2 Distribution Curves from 40 µs Test Considering x 50 as an indicator for fragmentation, a general tendency toward size reduction can be seen with an increase in the inter-hole-delays. Regardless of the small scatter, this constant size reduction continues to t = 200 μs rapidly, where the trend seems to reach a plateau. At 64

75 x 50 (mm) this point forward, a slight variation in the x 50 values can be observed with x 50 (mm) = 26.1 ± 8.7 mm (mean ± std-dev). Within the delay interval of 200 to 2000 μs an optimum fragmentation appears to be obtained at t = 700μs Delay - x 50 PF=2.4 kg/m3 PF=1.2 kg/m t=700 μs Delay (μs) Figure 6.3 Average Fragment Size as a Function of Delay Reviewing the results from the two different powder factors, a comparison can be made in terms of the role of this factor in improving fragmentation (Figure 6.3). Using a similar delay (200 μs), doubling the powder factor has reduced the average fragment size by more than 50%. Figure 6.4 provides a one-by-one comparison between two tests with the same delay but where different powder factors were used. It is clear that fragmentation enhances as the powder factor is increased. In addition, the figure shows that different fragmentation results can be expected even with the same blast pattern and powder factor but different initiation delays. Therefore, despite the importance of powder factor, it is evident that powder factor is not the 65

76 x 50 (mm) only parameter contributing to fragmentation. In other words, parameters such as delay can be utilized to improve fragmentation as desired in this study. The higher powder factor (PF=2.4 kg/m 3 ) appears to have produced very few boulders, thus reducing the average fragment sizes below half of the burden. At delays shorter than 200 μs, however, a number of large fragments were still obtained suggesting that fragmentation was poor even using fairly large powder factors. 120 Powder Factor - x Delay = 40 µs Delay =200 µs PF=2.4 kg/m3 PF=1.2 kg/m Powder Factor (kg/m 3 ) Figure 6.4 Effect of Powder Factor on Fragmentation 66

77 Chapter 7 Fragmentation Results from Copper-lined Blast-holes 7.1. Effect of Gas on Fragmentation The process of fragmentation is usually attributed to the combined effect of stress and gas. Upon detonation, a large amount of energy is released which immediately pressurizes the wall of the blast hole, generating radial compressive stress around the hole. This stress is much higher than the compressive strength of the rock which breaks the rock and creates a thin crushed zone around the blast (Esen et al., 2003). Depending on the compressive strength of the rock, the extent of the crushed zone can reach twice the diameter of the borehole (Sharma, 2009). The stress pulse propagates beyond the crushed zone in the form of radial compression and circumferential tension. As the tensile stress exceeds the tensile strength of the rock, it creates a radial pattern of fractures. The amplitude of the stress wave rapidly attenuates such that after a distance no further fracturing and propagation of the cracks can occur. Depending on the size of the burden, if the blast-induced stress wave reaches a free face, it will be reflected and the compressive component of the pulse travels back as a tensile wave. The newly generated tensile wave may be of sufficient amplitude to create surface spalling in the rock. The stress waves are normally followed by a quasi-static gas pressurization generated by gas products of detonation. The liberated gas travels through the widened borehole, crushed zone, and finds its way through the stress wave induced cracks, resulting in further extension of the fracturing into a wider area (Bozic, 1998). This is the zone where most of the fragmentation process takes place (Figure 7.1) and produces the coarsest fraction of rock whereas, the fine fraction of 67

78 fragmentation normally originates from the process of compressive crushing within the crushed zone (Sharma, 2009 and Iqbal, 2013). Figure 7.1 A Schematic of Crushed Zone, Fracture Zone, and Fragment Formation Zone, (Sharma, 2009) Multiple interactions between the components of stress waves, generated cracks, and reflections of the stress waves from free faces can be assumed as discussed by (Rossmanith, 2002). Many researchers (Rossmanith, 2000; Rossmanith and Kouzniak, 2004) have tried to explain positive effects of stress wave interactions on fragmentation; however, this idea has been challenged by others such as (Blair, 2006), who claimed that stress wave interactions are very localized in distances between the blast holes and the stress waves from neighboring blast holes are not necessarily similar in amplitude to interact constructively to maximize damage and fragmentation. Blair also indicates that, a very comprehensive model is required to describe the role of stress wave propagation and the subsequent gas interactions with the stressed rock to clarify both aspects of rock fragmentation. 68

79 7.2. Experimental Work In order to investigate the effect of gas penetration on blast-induced fragmentation, it was decided to make a series of small-scale tests using the same pattern as discussed in the earlier chapters. The new tests were carried out in blocks using the same or very close delays as in Chapter 6, but with copper lined holes. Blasting under such conditions, with a copper lining, is assumed to prevent or reduce gas penetration into the blast stressed zone and the radial cracks, thus changing the process of fracturing in the blocks. To achieve this goal, copper pipes with a diameter of 11 mm and thickness of 0.7 mm were purchased and annealed in an oven at the temperature of 932 (ᵒF) for 45 minutes. Annealing 1 the copper pipes increases their ductility and strengthens them against gas pressures. Eight blocks of grout were prepared and copper pipes were placed in the blast holes. To create consistency between the blast conditions and to obtain comparable results, blast holes were loaded in similar fashion as explained in Chapter 4. Different delays were chosen among the range of 40 to 1200 μs Results After each shot, the fragments were sieved and the fragmentation distribution was described by the Swebrec function as shown in Table 7.1. The new set of data was plotted together with the identical unlined blocks to examine if the new set up would make a difference in the average fragment size (Figure 7.2). 1 For more information refer to file/13.pdf 69

80 Average particle size (mm) Table 7.1 Swebrec Results for Copper-lined Blasts Block # Delay (μs) x 50 (mm) x max (mm) b r For delays as long as 200 μs no firm conculsion can be reached regarding the influence of gas on the average fragmentation. While at t = 80 μs a considerable coarser fragmentation was achieved than the unlined block with the same delay, no such behaviour was seen at t = 40 or 100 μs. In fact, at these two delays fragmentation unexpectedly became fine or resulted in no different x 50 values from the unlined condition Δt 1 = 200 µs Δt 2 = 200 µs Delay time (μs) unlined Cu lined Figure 7.2 Average Fragmentation, Lined and Unlined Conditions 70

81 As demonstrated in Lagrangian diagrams (Figure 3.10) stress wave generated cracks travel at velocities around 680 m/s. This implies a minimum required time of around 155 µs for the cracks to arrive at the neighboring blast hole. As stated by Daehnke et al. (1997), this is a phase where the fracturing mechanism is mostly dominated by the effect of stress waves. Then, in the second phase, the gases become the main driving force in extending the initial fractures which are generated by the stress waves. The experiments show that in the short delays stress waves from subsequent holes form a combined system of cracks and the gases vent through them. On the contrary, in the long delays, there is no cooperation of stress waves reulting in formation of independent networks of cracks due to the action of stress and gas. The two phase approach appears to be in agreement with the results of the short delay tests from the copper lined holes. At short delays below 200 µs, the effect of gas is not significant; therefore, the average fragmentations appears not to differ drastically from the unlined condition. For longer delays a change in fragmentation can be observed as a result of eliminating or reducing the effect of gas pressure in the process of fragmentation by copper lining the holes. With one exception ( t 1 = 200 μs), all other delays gave coarser x 50 values under the lined condition. The unlined borehole test at the delay of 200 μs was repeated and gave an x 50 smaller than the x 50 with copper lined holes. Therefore, it can be claimed that a general trend of increase in fragmentation occurred for delays > 200 μs, when gas penetration was inhibited by the copper pipes. This also proves the importance of the effect of gas pressure at long delays. At such delays, each hole creates a damage zone and radial fractures around it under the influence of stress waves. Since there is sufficient delay time between blast holes, the damage will expand by the action of gases, resulting in better fragmentation. Under such conditions, if 71

82 gas is controlled by copper pipes, less damage will be created as observed in the long delay tests with lined blast holes. The average fragmentation was in the range of 27.4 ± 4.9 mm (mean ± std-dev). Although the blocks were blasted under a different rock fracturing process in the new tests, the minimum average fragmentation was again achieved at the delay of t = 700 μs between holes. 72

83 Chapter 8 Fragmentation Results from a Medium-scale Granite Bench 8.1. Experimental Work In previous chapters, fragmentation was studied with different delays in small-scale blocks. The medium in which the tests were made was homogeneous, ensuring the repeatability of blast conditions. Typically, the delay timing results from different burden sizes are scaled up to real dimensions and are expressed as ms/m of burden. For the purpose of this study, it was decided to conduct a few tests in a medium-scale granite bench to reach a better understanding of fragmentation in rock materials existing in nature. In addition to the chosen scale, heterogeneity of the granite bench, altered rock type due to weathering, and the pre-existing fractures in the rock were also influential to create a different sample condition from the previous tests. However, the bench shape and size as well as its confined condition seemed to be appropriate for blast experiments (Figure 8.1). Figure 8.1 The Granite Bench 73

84 The diameter of the holes, length of the holes, burden, and spacing were 12 mm, 56 cm, 20 cm, and 30 cm respectively. The blast holes were loaded using 85 g/m detonating cords with a length of 30 cm, resulting in a powder factor of 0.76 kg/m 3 of burden based on the blast pattern. Multiple single row blasts were performed in the bench. At first, a 0 μs delay test was made to create a proper and smooth face. As predicted, very large fragments were produced in this test, making it impossible to easily move them in order to measure their size. Three main shots were carried out with 500, 1000, and 2000 μs interhole delays. Similar to the first main test, clean-up shots, t = 0 μs, were implemented prior to each main shot to obtain an appropriate new face. The generated fragments were collected for sieving analysis after the three tests Results Table 3.1 shows summary of the Swebrec parameters obtained from the experiments. The x 50 values were also plotted as a function of time (Figure 8.2). Only three data points were achieved from the granite bench which do not show a clear pattern; however, the longest delay, 2000 μs, gave the finest fragmentation which seems to be in agreement with the general trend from the small-scale tests. Table 8.1 Swebrec Parameters for the Granite Bench Row # Delay (μs) x 50 (mm) x max (mm) b r

85 Passing (%) x 50 (mm) Delay - x Delay (μs) Average Fragment Size Figure 8.2 Results from Granite Bench As displayed in Figure 8.3, among the results from the bench, the 2000 μs test clearly yielded the finest particle distribution curve with good consistency between the experimental data and the predicted distribution from the Swebrec fit Size (mm) 500 μs - Swebrec 2000 μs - Swebrec 500 μs - Experiment 2000 μs - Experiment Figure 8.3 Fragmentation Distribution Curves from Bench Blasting 75

86 As mentioned earlier, fragments from the instantaneous shots were not included in the analysis due to their very coarse sizes (a few times of that burden size). All observations showed that the zero microsecond blasts gave the coarsest fragmentation which, together with other data, empowers the assumption regarding the gradual improvement in fragmentation by increasing interhole delays. Figure 8.4 shows the condition of the bench before the experiments (a) and after a few tests. It also demonstrates a very large fragment was produced after an instantaneous test (b), and large back-break occurred after a delayed shot (c). The powder factor used in the experiments was good enough to produce fragments smaller than half of the burden in the delayed shots. Figure 8.4 Granite Bench, Before and After Blasts 76

87 Chapter 9 Analysis of Results and Discussion 9.1. Blasting and Fragmentation The importance of blast results to the efficiency of the downstream processes has been of interest to many blast designers and mill specialists in the past few decades. Mackenzie (1967) presented a series of conceptual curves to determine the cost dependence of different mining operations on fragmentation at the Quebec Cartier Iron Mine. The study suggested that costs associated with loading, hauling, and crushing are directly influenced by the degree of fragmentation; therefore, the overall mining costs can ultimately reduce under an optimum fragmentation condition. More studies by Nielsen and Kristiansen (1996) focused on the effect of blasting on the subsequent crushing and grinding aspects of mining. The results indicated that blasting as the first phase of size reduction could have positive impacts on the optimization of mine operations. Eloranta (1997) compared energy requirements between blasting, crushing, and grinding. Using the Bond equation as the indictor of energy usage, he concluded that blasting could have a cost advantage of 3:1 over grinding. Another investigation by Grundstrom et al. (2001) showed that achieving a finer fragmentation delivers many benefits to the downstream process of mineral recovery. In addition to cost reductions in crushing and grinding, he concluded that the following can be obtained through improved fragmentation: - Improvements in excavator productivity through increase in muckpile digability and increased bucket factor - Increase in crusher throughput due to reduction of feed size distribution - Reduction in ore dilution and potential for increased liberation of valuable minerals leading to more mill recoveries. 77

88 The main concentration in the mentioned investigations was on optimizing blast results through increasing powder factor. Other studies including Rossmanith (2002), Rosenstock (2004), and Miller et al. (2007) selected a newer approach toward optimization of fragmentation. Their research mainly targeted the results of blasts performed by electronic detonators in open pit and quarry industries. Not only did all these researchers confirm the improvements mentioned earlier in downstream processes, but also some of them addressed other advantages of using electronic detonators as follows: - Programmability, flexibility, and accuracy of electronic detonators compared to electric and non-electric detonators - A potential for minimizing vibration levels where possible damage to the neighboring structures are concerned as well as environmental control - Higher safety of blast operators since the risk of unintended detonation is minimized with electronic initiation system. Using the means of precision timing (electronic detonators), increased powder factor, or both, any optimization in fragmentation typically involves the following aspects (Wuchi, 2010): Minimizing oversize boulders to reduce secondary breaking costs Minimizing ultra-fine products Maximizing lump products with proper degree of uniformity Ensuring efficient digging and loading through decreasing fragment sizes. In the following, the results from recent experiments will be discussed with emphasis on size reductions obtained when fragmentation appears to be optimal. 78

89 9.2. New Approach toward Optimization of Fragmentation Various researchers have long tried to establish a relationship between fragmentation and the parameters used in blast design. The quantity of explosives, burden, spacing, hole diamater, sequence of firing and the associated timings, etc have been discussed in several works by different blast specialists. Among all the different parameters, the role of delay has always been a challenging topic with several unanswered questions. A variety of approaches have been proposed from experimental methods to numerical modelling aiming at defining a time window between blast holes in which the blast damage on the surrounding rock is maximized. In this research project, an attempt was made to find a proper delay time with respect to results from actual blasts. In order to do so, several blocks of grout were cast with mechanical properties close to rock to generate blast fragmentation. The influence of reflective boundaries, normally observed in free blocks, were minimized using a new experimental set up, simulating open pit blasting. For this purpose, a yoke was designed to confine the blocks from three free faces to absorb the blast generated stress waves when they reached the face Effect of Timing on Fragmentation The current study has examined the possible effects of timing factor on maximizing fragmentation by the use of electronic initiation in the grout samples. The produced blocks were blasted with a wide range of delay timing from 0 to 2000 μs between holes to ensure that all rock fracturing mechanisms were taken into account. This choice of delay helped to evaluate suggestions regarding fast initiation benefits of blasting made by Rossmanith (2004) and Vanbrabant and Espinosa (2006) as well as assumptions about fracturing mechanisms of blasts. 79

90 The results of the new work appear to contain less experimental scatter compared to the previous studies by Katsabanis et al., (2006), Johansson (2012), and others. The most scatter seems to appear in the range of 0 to 200 μs (0 to 2.7 ms/m of burden); however, a continuous reduction of the 50% passing size is obtained with increase in delay time. At delays above t = 200 μs, the x 50 Delay plot reaches a steady zone which continues up to 1000 μs. This trend indicates that in the scaled up delay interval of 2.7 to 13.3 ms/m of burden no substantial change in the value of x 50 occurs while the finest mean fragment size (x 50 = 16.8 at t = 9.3 ms/m) falls in this range. This can be considered as the optimum range of delay, knowing that beyond the delay of 1000 μs fragmentation becomes slowly coarser again. The extent of this optimum range appears to be somewhat in agreement with the previous experiments in granodirite blocks (Katsabanis et al., 2006) and the USBM data (Stagg and Rholl, 1987; Otterness et al, 1991) but certainly is not compatible with the idea of improving fragmentation through simultaneous initiation or the use of very short delays. Moreover, if the results are compared to suggestions made by Cunningham in South Africa s mines (Cunningham, 2005) and Johansson s fragmentation data, some very interesting similarities and differences can be found. Cunningham suggested that the proper interhole delay time to obtain maximum fragmentation can be calculated by the use of the following formula. T best (ms) = 15.6 C P B where, C P is the compressional stress wave velocity (km/s) and B is the burden (m). The T best in this formula corresponds to a time window where the fracture network around the blast holes evolves optimally due to the action of stress waves. In support of the T best concept, Cunningham compared some previously found fragmentation data by Bergmann et al. (1974) from full-scale tests, where delays longer than T best led to coarser fragmentation. He also claimed that if delays 80

91 shorter than T best are selected, stress waves from the delayed blast hole would destructively interfere with the fractures from the earlier hole, thus suppressing the growth of the fracture network. As a result such delays would not be assisting in improving fragmentation. If Cunningham s recommendation is used, the appropriate delay time to enhance fragmentation would be equal to 3.8 ms/m of burden. This lies inside the range of delays where relatively fine fragmentation was achieved, according to the findings from the small-scale tests. However, at longer delays, blasts produced even finer fragmentation. Johansson et al. (2013) re-examined their small-scale experimental data (Johansson et al., 2012) using a numerical FEM code. The laboratory tests were modeled to identify the influence of superposition of stress waves at different inter hole delays as tested in the experimental work. The numerical simulations of the concrete blocks demonstrated that simultaneous initiation or very short delays are useless in improving fragmentation and the optimum delay is in the range of 73 to 86 μs for the burden size of 7 cm. Comparing these two results with the findings of this research, the first conclusion is clearly in agreement with the current data. The optimum fragmentation, however, seems to have been produced at very longer delays, thus questioning the hypothesis that stress waves and their interactions are the only cause for creating fragmentation. The change in the 80% passing sizes with delay time is another topic of value to discuss if fragmentation is evaluated from the comminution design viewpoint in mills. The importance of x 80 becomes evident since the energy required for the primary crusher is dependent on the muckpile 80% passing size (Bond, 1961). The current study shows that the x 80 reaches its minimum value (33.83 mm) at the delay of t = 700 μs (Figure 9.1). 81

92 Fragment size, (mm) Assuming that the results can be converted to real scale blasts, there is a potential for energy cost saving through providing the mill with a feed size produced by a delay in the range of 2.7 to 13.3 ms/m of burden. The behaviour of the 10% passing size is also shown in the figure with the least variation compared to other particle sizes. This is rather expected as the 10% fragments are typically products of the crushed zone in which the fragmentation is under the influence of compression from detonation and the the finest fraction is formed within this region. In all passing sizes, the Swebrec values are very close to the experimental observations, thus proving the capability of this model to predict different fragment sizes with acceptable accuracy. The very fine fraction, below 1 mm, varies between 1% to 8% of the total collected fragments in the experiments (Figure 9.2). Due to the large scatter in the distribution of the very fine fragments, it is impossible to draw any reasonable conclusion regarding the effect of timing on the formation of fines Fragmentation - PF = 2.4 kg/m 3 50% Passing Size - Swebrec Delay (μs) 80% Passing Size - Swebrec 10% Passing Size - Swebrec 50% Passing Size - Experimental 80% Passing Size - Experimental 10% Passing Size - Experimental Fig 9.1 The Variation of Different Fragment Sizes with Time 82

93 % passing Fines 1mm - PF = 2.4 kg/m Delay time, μs Figure 9.2 Fines Below 1 mm Uniformity and Delay Another interesting point is the effect of timing on the slope at the 50% passing sizes obtained from the fragmentation distribution curves. It seems logical that a steep slope at the average size, in the distrbution curves, is indicative of the closeness of the fragments around x 50. Similarly, if the slope of the line that passes through two desired fragment sizes becomes steeper, it can be concluded that those two fragments are closer to each other in size. The closeness of different fragment sizes is typically described as the uniformity of fragmentation. Higher uniformity suggests fewer coarse fragments, less fines, and more efficient blast products for mill purposes. Therefore, uniformity can be considered as another measure to evaluate fragmentation along with x 50. Interestingly, at some delays used in this research, it was found that uniformity was optimum in the same manner as the average fragment size. As displayed in Figure 9.3, the increasing pattern of the slope values at x 50 starts from short delays and continues toward the upper limit of the delay range that was previously found to be optimum in 83

94 terms of the average fragment size. It is then followed by a declining trend in uniformity, particularly beyond Δt = 1000 µs. In agreement with the x 50 evaluation, the optimum uniformity (peak slope value) was found at Δt = 700 µs. A similar trend is observed if the slopes through different fragment sizes of interest are drawn. For instance, Figures 9.4 and 9.5 illustrate the slopes of the lines connecting the x 80, x 20 and x 60, x 40 passing sizes. All the peak slope values lie inside the range within which fragmentation was optimum from the average size analysis viewpoint. This suggests that as the delay increases, the produced fragments from different sizes become finer and form a distribution curve with a higher uniformity Slope at x Slope at x unlined Cu lined Delay time (μs) Figure 9.3 Slope at x50 84

95 Slope through x 80 and x Slope through x 80, x unlined Cu lined Delay time (μs) Figure 9.4 Slope through x80 and x20 Slope through x 60 and x Slope through x 60, x Unlined Cu lined Delay time (μs) Figure 9.5 Slope through x60 and x40 85

96 Uniformity index (n) The figures also reveal that, the slopes through the average fragment size as well as the slopes through the x 80, x 20 and x 60, x 40 passing sizes are lower when blast holes were lined with copper tubes at delays above 200 µs. As a result, uniformity appears to be generally poorer in the case of the lined blasts where the effect of gas is reduced. The poorer uniformity is even more noticebale in the case of longer delays where fragmentation is highly dependent on the action of gases. The traditional Rosin-Rammler uniformity index (n) obtained from the small-scale tests (Figure 9.6) does not appear to show an obvious trend with delay time. Therefore, no comment can be made on the impact of timing on this type of uniformity. Another type of expression of uniformity (x 60/x 10), commonly used in soil mechanics studies, is plotted in Figure 9.7. The large scatter in this graph also suggests no tangible effect of timing. Given the results of the two known uniformity indices, the author suggests using other methods such as assessment of the slopes obtained from distribution curves in order to perform fragmentation uniformity analysis Uniformity index - Rosin-Rammler Delay time (μs) Figure 9.6 Rosin-Rammler Uniformity Index vs. Delay 86

97 x60/x10 Uniformity Delay time (μs) Figure 9.7 Uniformity Expressed as x60/x10 vs. Delay Effect of Stress waves on Creating Fragmentation Considering the compressive wave velocity of 4075 m/s, the arrival time of the P-wave at the neighboring blast hole is assumed to be (105 mm) / (4 mm/μs) 26 μs. If the second hole detonates at a delay equal to this or later, no P-wave interaction between the spacing distance can be achieved and any PP-wave interactions will occur only at the second hole and distances beyond that. Theoretically, a five times longer delay (130 μs) would lie outside the range of PPinteraction between the waves arising from the blast holes in the block. This logic was chosen by Johansson (2011) to determine the shortest and longest delays in his work. Apart from one experiment at a delay of 0 μs, all other tests were made at a delay range from P-wave interaction at the neighboring hole to no P-wave interaction. Johansson also used the idea of superposition of pulses beyond the hole detonating later. At first, Johansson initiated the second charge at the delay of 37 µs which coincided with the stress wave arrival from the first hole. He then increased the delay to 46, 56, 73, and 86 µs to obtain different tensile pulse interactions near neighboring holes. This attempt is simplified in Figure

98 Figure 9.8 Interaction of Tensile Waves The shaded area in this figure represents the zone where tensile pulses from two neighboring holes interact. Clearly, the extent of this zone is controlled by the delay used between the holes. Johansson then chose a delay time of 146 µs between blast holes to investigate fragmentation when no stress wave interaction was possible. Fragmentation improvements were seen as the inter hole delays increased; however, this study remains restricted to the range of stress wave interactions. Johansson s approach was continued by Petropoulos et al., (2013) to study the effect of longer delays (218 and 290 μs) on fragmentation. More improvements were observed as the delay increased. Using these two delays, he concluded that with a crack propagation velocity of m/s, the generated crack moved past the neighboring hole before it is initiated and no more improvement in fragmentation is possible as the burden starts moving 250 μs after initiation of the first hole. Katsabanis et al. (2014) also presented Lagrangian diagrams (Figure 9.9) to display tensile wave interaction in the case of a blast with the P-wave velocity of 4200 m/s, the compressional tail velocity of 2000 m/s, and the tensional tail velocity of 1000 m/s. The pulse duration was also assumed to be 50 µs in compression and tension, close to what was previously measured using carbon resistor gauges. The diagram suggests that in a case such as this, if the second hole initiates at the delay of 50 µs, the tension pulses from the 88

99 Time (µs) two holes fully overlap with each other, creating maximum tension in the areas past the delayed charge. This approach along with other findings from Johansson (2011) recommends the use of short delays but certainly does not explain the observations in this study where much longer delays yielded better fragmentation results Tensile tail from 1st and 2nd hole Superposition of tensile pulses from 1st and 2nd Compression from 2nd hole Distance (m) Compression from 1st hole PF1 PE1 PF2 PE2 PF2R PE2R TT1 Figure 9.9 Lagrangian Diagrams to Create Zones of Maximum Tension (Katsabanis et al., 2014) The current study agrees with the improving trend of fragmentation with time from Johansson s work; however, it shows that results can still change toward the formation of finer fragments as observed in the delay span of 200 to 1000 μs, where average fragment size has reached a plateau. This longer range of improvement could be due to the accumulation of damage caused by stress waves when they have passed the consecutive blast holes and subsequently the role of gas pressurization in expanding the damage into a wider extent before venting. According to Stagg and Rholl (1987), the blast generated gas travels at the rate of 6% of the P-wave velocity. In the present work, this would be equal to 240 m/s suggesting that gas pressurization continues for about 430 μs before venting. Since the blast holes were left 89

100 unstemmed, this velocity can be even lower. As mentioned in Chapter 7, upon detonation of the explosives the stress waves are initially the dominant mechanism to create fractures. This type of fracturing could take up to 130 µs (refer to 9.2.3) in the case of these experiments. Subsequently, as the fractures develop, the gas entering the fractures becomes the driving force to generate further fragmentation. This seems to account for the long delay window required for the action of fracturing mechanisms to create close to optimum fragmentation Effect of Gas Pressure on Formation of x 10, x 50, and x 80 under Lined and Unlined Conditions A new approach toward determining the effect of gas pressure was tested by placing copper pipes in the blast holes. The pipes were assumed to act as a trap to minimize penetration of the blast gases into the stress wave-created cracks. Comparing the copper lined experiments with the non-lined blasts,the average fragmentation showed an increase at delays between 200 to 1000 µs. This increase in the average fragment size was in the order of 25% for the 200 and 700 µs tests, whereas, at Δt = 800 µs, the x 50 increased by 52%. However, at delays below 200 µs no such increase was obtained. This could be attributed to the role of gas only at long delays where the blast gases have sufficient time to further pressurize the stress-created damage zone and develop the fractures before venting. On the contrary, in the short delay tests, the superposition of stress waves most likely creates a large crack along the plane connecting the axes of the boreholes. This major crack provides a path for the gases to escape through reducing the effect of gas on pressurizing the block. The copper lined blast holes produced larger 80% passing sizes of fragmentation in the range of 200 µs 800 µs compared to the unlined tests in the same range. The 80% passing size showed an increase of 36%, 52%, and 58% at delays equal to 200, 700, and 800 µs, respectively when gas penetration was inhibited by copper pipes 90

101 Average particle size (mm) (Figure 9.10). This seems to be of importance considering that large fragments, such as x 80, are likely produced in the zones with the most distance from the stress wave fracturing zone and their formation is mainly under the influence of gas pressure released from a blast. 160 Effect of Copper lining on Fragmentation - PF = 2.4 kg/m % unlined 50% Cu lined 80% unlined 80% Cu lined 10% unlined 10% Cu lined Delay time (μs) Figure 9.10 Effect of Copper Lining on x10, x50, and x80 The figure also indicates the non-sensitivity of the 10% passing sizes to gas pressurization since the results of the lined holes did not change significantly from that of the unlined condition. This is rather expected since the small particles such as the 10% passing size are most likely originated from the thin crushed zone around the blast holes as shown in Chapter 7. This zone is believed to be the product of the initial compressive stress, within which the influence of other fracturing mechanisms such as gas is expected to be minimum Fragmentation Distribution Curves under Lined and Unlined Conditions The effect of copper lining the blast holes on the fragmentation distribution curves is displayed in Figures 9.11 to The impact of reducing the gas flow on the overall fragmentation 91

102 Passing (%) Passing (%) becomes apparent as explained earlier at long delays. The distribution curves clearly suggest that fragmentation deteriorates remarkably under lined hole condition in terms of obtaining much larger particles in a wider size range (lower uniformity). Delay = 100 μs Size (mm) No Copper Copper Figure 9.11 Effect of Copper Lining on Distribution Curves, Delay = 100 µs Delay = 200 μs Size (mm) No Copper Copper Figure 9.12 Effect of Copper Lining on Distribution Curves, Delay = 200 µs 92

103 Passing (%) Passing (%) Delay = 700 μs Size (mm) No Copper Copper Figure 9.13 Effect of Copper Lining on Distribution Curves, Delay = 700 µs Delay = 800 μs Size (mm) No Copper Copper Figure 9.14 Effect of Copper Lining on Distribution Curves, Delay = 800 µs 93

104 9.3. Back-break Any blasting operation is associated with damage around the blast holes. A portion of this damage, occurring in the burden area, is the desired fragmentation produced by the effect of the blast. Typically, blast researchers investigate the possibilties of maximizing this type of damage in order to gain benefits in the downstream processes as discussed in Chapter 2. Another type of damage in the blasted material is known as back break, which is an undesirable phenomenon and may cause instability of walls in surface mine benches. In this research, this phenomenon was seen in most of the small-scale blocks as well as in the granite bench shots. According to the observations in the post blast blocks, there seems to be a direct relationship between increase in the interhole delay and the extent of the back break. Figure 9.15 shows a block condition after a zero delay test. The experiment resulted in formation of very large fragments with the minimum damage to the left-overs of the block. Figure 9.15 Block Condition After a Simultaneous Initiation Shot As the delay was increased to Δt = 40 µs, breakage emerged in the back of the block. The effect became larger at longer delays. For example, at Δt = 1000 µs, although a better fragmentation 94

105 was obtained, the back break in the block was large (Figure 9.16). In this experiment, not only was the entire block damaged, but the yoke seemed to have moved due to exposure to the energy liberated from the blast. A simple explanation for this could be the effect of gas that can remain inside the block and pressurize the blast zone for a longer period of time; whereas, at very short delays gas is escaping through the opening created by interaction of stress waves on the plane where the blast holes are located. This is in agreement with observations from two blocks blasted at the same delay (200 µs) but with copper lined and unlined blast holes (Figure 9.17). In the unlined block, more back break is visible after the blast, while the copper lined block, which is not as much affected by gas penetration, appears to be less influenced by detonation. Figure 9.16 Block Condition After 40 µs and 1000 µs Shots 95

106 Figure 9.17 Block Condition After 200 µs Shots, Lined and Unlined 9.4. Analysis of the Granite Bench Experiments In order to examine the effect of delay time on fragmentation in a larger scale, a series tests were conducted in a granite bench located in the explosives laboratory. Fragments obtained from three of the tests were collected and analyzed as in the small-scale grout blocks. At the longest delay experiment (2 ms), the best fragmentation was achieved while shorter delays (0.5 ms and 1 ms) resulted in coarser fragmentations. Since too few data points were achieved, and also due to the unusual coarse fragmentation obtained in the 1 ms shot, it is not possible to draw a firm conclusion regarding the existence of a trend in the variation of delay-x 50 in the medium scale experiments. However, very interesting facts were revealed from frame-by-frame review of the high-speed videos captured in the three experiments. To start, the first blast in the bench was performed with no delay between the holes in order to create a clean and even free face. This test was filmed by a high-speed video camera. As shown 96

107 in Figure 9.18, it is obvious that the burden starts to move with formation of very large fragments as expected where simultaneous initiation is used. Figure 9.18 Two Frames from Granite Bench 6 ms and 30 ms After Initiation, Delay = 0 ms The 0.5 ms test was also analyzed by reviewing some frames obtained with intervals of 2 ms. Burden movement and gas venting started less than 2 ms after initiation of the first hole. Detonation gases appeared to vent initially from the collar region rather than the free face, thus indicating that cracks developed between the holes provide a proper passage for gases to flow. The simultaneous shot further confirmed this gas venting process as burden displacement coincided with the escape of a considerable volume of gas and dust from the top of the holes along the new generated free face (Figure 9.19) 97

108 Figure 9.19 Two Frames from Granite Bench 0 ms and 8 ms After Initiation, Delay= 500 ms The sequential frames captured in the 2 ms delay experiment are also displayed in Figure Venting of gas and dust occurred in less than 2 ms after initiation. Similar to the 0.5 ms test, burden movement started less than 2 ms after initiation of each hole. This time corresponds to the 700 µs delay test in the small-scale blocks. Therefore, it can be assumed that no interaction of gas with stress waves takes place at longer delay detonations. As a result, fragmentaion will not be benefited by the action of gas in blasts with delays longer than 700 µs. 98

109 Figure 9.20 Six Frames from Granite Bench, Delay = 2 ms 99