Read Engineering Geology Field Manual - Volume II - 2nd Ed. Chapter 19 text version Chapter 19 BLAST DESIGN Introduction This chapter is an introduction to blasting techniques based primarily on the Explosives and Blasting Procedures Manual (Dick et al., 1987) and the Blaster's Handbook (E.I. Du Pont de Nemours & Co., Inc., 1978). Blast design is not a precise science.
Because of widely varying properties of rock, geologic structure, and explosives, design of a blasting program requires field testing. Tradeoffs frequently must be made when designing the best blast for a given geologic situation. This chapter provides the fundamental concepts of blast design. These concepts are useful as a first approximation for blast design and also in troubleshooting the cause of a bad blast. Field testing is the best tool to refine individual blast designs. Throughout the blast design process, two overriding principles must be kept in mind: (1) Explosives function best when there is a free face approximately parallel to the explosive column at the time of detonation. (2) There must be adequate space for the broken rock to move and expand.
Excessive confinement of explosives is the leading cause of poor blasting results such as backbreak, ground vibrations, airblast, unbroken toe, flyrock, and poor fragmentation. Properties and Geology of the Rock Mass The rock mass properties are the single most critical variable affecting the design and results of a blast.
The FIELD MANUAL rock properties are very qualitative and cannot be sufficiently quantified numerically when applied to blast design. Rock properties often vary greatly from one end of a construction job to another. Explosive selection, blast design, and delay pattern must consider the specific rock mass being blasted. Characterizing the Rock Mass The keys to characterizing the rock mass are a good geologist and a good blasting driller.
The geologist must concentrate on detailed mapping of the rock surface for blast design. Jointing probably has the most significant effect on blasting design. The geologist should document the direction, density, and spacing between the joint sets. At least three joint sets-one dominant and two less pronounced-are in most sedimentary rocks. The strike and dip of bedding planes, foliation, and schistosity are also important to blast design and should be documented by the geologist. The presence of major zones of weakness such as faults, open joints, open beds, solution cavities, or zones of less competent rock or unconsolidated material are also important to blast design and must be considered. Samples of freshly broken rock can be used to determine the hardness and density of the rock.
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An observant blasting driller can be of great help in assessing rock variations that are not apparent from the surface. Slow penetration, excessive drill noise, and vibration indicate a hard rock that will be difficult to break. Fast penetration and a quiet drill indicate a softer, more easily broken zone of rock. Total lack of resistance to penetration, accompanied by a lack of cuttings or return water or air, means that the drill has hit a void. Lack of cuttings or return water may also indicate the presence of an open bedding plane or other crack. A detailed drill log indicating the depth of these various 210 BLAST DESIGN conditions can be very helpful in designing and loading the blast. The log should be kept by the driller.
The driller should also document changes in the color or nature of the drill cuttings which will tell the geologist and blaster the location of various beds in the formation. Rock Density and Hardness Some displacement is required to prepare a muckpile for efficient excavation. The density of the rock is a major factor in determining how much explosive is needed to displace a given volume of rock (powder factor). The burden-to-charge diameter ratio varies with rock density, changing the powder factor. The average burden-tocharge diameter ratio of 25 to 30 is for average density rocks similar to the typical rocks listed in table 19-1. Denser rocks such as basalt require smaller ratios (higher powder factors). Lighter materials such as some sandstone or bituminous coal can be blasted with higher ratios (lower powder factors).
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The hardness or brittleness of rock can have a significant effect on blasting results. If soft rock is slightly underblasted, the rock probably will still be excavatable. If soft rock is slightly overblasted, excessive violence will not usually occur. On the other hand, slight underblasting of hard rock will often result in a tight muckpile that is difficult to excavate.
Overblasting of hard rock is likely to cause excessive flyrock and airblast. Blast designs for hard rock require closer control and tighter tolerances than those for soft rock.
Voids and Zones of Weakness Undetected voids and zones of weakness such as solution cavities, 'mud' seams, and shears are serious problems in 211 FIELD MANUAL Table 19-1.-Typical rocks, densities, and unit weights Range of unit weights Density range (g/cm3)1 U.S. Customary (lb/ft3)2 Metric (kg/m3)3 2,500 to 2,800 2,600 to 2,800 2,200 to 2,700 2,700 to 2,900 2,000 to 2,600 1,200 to 1,500 Rock type Limestone Schist Rhyolite Basalt Sandstone 2.5 to 2.8 156 to 174.7 2.6 to 2.8 162.2 to 174.7 2.2 to 2.7 137.2 to 168.5 2.7 to 2.9 168.5 to 181 2/0 to 2.6 124.8 to 162.2 Bituminous 1.2 to 1.5 74.9 to 93.6 coal 1 2 3 Grams per cubic centimeter. Pounds per cubic foot. Kilograms per cubic meter. Explosive energy always seeks the path of least resistance.
Where the rock burden is composed of alternate zones of hard material, weak zones, or voids, the explosive energy will be vented through the weak zones and voids resulting in poor fragmentation. Depending on the orientation of zones of weakness with respect to free faces, excessive violence in the form of airblast and flyrock may occur. When the blasthole intersects a void, particular care must be taken in loading the charge, or the void will be loaded with a heavy concentration of explosive resulting in excessive air-blast and flyrock. 212 BLAST DESIGN If these voids and zones of weakness can be identified and logged, steps can be taken during borehole loading to improve fragmentation and avoid violence. The best tool for this is a good drill log.
The depths of voids and zones of weakness encountered by the drill should be documented. The geologist can help by plotting the trends of 'mud' seams and shears. When charging the blasthole, inert stemming materials rather than explosives should be loaded through these weak zones. Voids should be filled with stemming. Where this is impractical because of the size of the void, it may be necessary to block the hole just above the void before continuing the explosive column. If the condition of the borehole is in doubt, the top of the powder column should be checked frequently as loading proceeds. A void probably exists if the column fails to rise as expected.
At this point, a deck of inert stemming material should be loaded before powder loading continues. If the column rises more rapidly than expected, frequent checking will ensure that adequate space is left for stemming. Alternate zones of hard and soft rock usually result in unacceptably blocky fragmentation. A higher powder factor seldom will correct this problem; it will merely cause the blocks to be displaced farther. Usually, the best way to alleviate this situation is to use smaller blastholes with smaller blast pattern dimensions to get a better powder distribution. The explosive charges should be concentrated in the hard rock. Jointing Jointing can have a pronounced effect on both fragmentation and the stability of the perimeter of the excavation.
Close jointing usually results in good fragmentation. 213 FIELD MANUAL Widely spaced jointing, especially where the jointing is pronounced, often results in a very blocky muckpile because the joint planes tend to isolate large blocks in place. Where the fragmentation is unacceptable, the best solution is to use smaller blast holes with smaller blast pattern dimensions. This extra drilling and blasting expense will be more than justified by the savings in loading, hauling, and crushing costs and the savings in secondary blasting. Where possible, the perimeter holes of a blast should be aligned with the principal joint sets.
This produces a more stable excavation, whereas rows of holes perpendicular to a primary joint set produces a more ragged, unstable perimeter (figure 19-1). The jointing will generally determine how the corners at the back of the blast will break out. To minimize backbreak and flyrock, tight corners, as shown in figure 19-2, should be avoided.
Figure 19-1.-Effect of jointing on the stability of an excavation (plan view). 214 BLAST DESIGN Figure 19-2.-Tight and open corners caused by jointing (plan view). The open corner at the left of figure 19-2 is preferable. Given the dominant jointing in figure 19-2, more stable conditions will result if the first blast is opened at the far right and designed so that the hole in the rear inside corner contains the highest numbered delay.
Bedding/Foliation Bedding has a pronounced effect on both the fragmentation and the stability of the excavation perimeter. Open bedding planes, open joints, or beds of weaker materials should be treated as zones of weakness.
Stemming, rather than explosive, should be loaded into the borehole at the location of these zones as shown in figure 19-3. In a bed of hard material (greater than 3 feet 1 m thick), it is often beneficial to load an explosive of higher density than is used in the remainder of the borehole. To break an isolated bed or zone of hard rock (3 feet 1 m thick or greater) near the collar of the blasthole, a deck charge is recommended, as shown in figure 19-4, with the deck being fired on the same delay as the main charge or one delay later. Occasionally, satellite holes are used to help break a hard zone in the upper part of the burden. Satellite holes (figure 19-4) are short holes, usually smaller in diameter than the main blastholes drilled between the main blastholes. 215 FIELD MANUAL Figure 19-3.-Stemming through weak material and open beds. 216 BLAST DESIGN Figure 19-4.-Two methods of breaking a hard collar zone.
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A pronounced foliation, bedding plane, or joint is frequently a convenient location for the bench floor. It not only gives a smoother floor but also may reduce subdrilling requirements.
Dipping beds frequently cause stability problems and difficulty in breaking the toe of the excavation. When bedding or foliation dip into the excavation wall, the stability of the slope is enhanced. When the dip is out of the wall, slip planes exist that increase the likelihood of slope deterioration or failure. Blasthole cutoffs (part of a column of explosives not fired) caused by differential bed movement are also more likely. Beds dipping out of the final slope should be avoided wherever possible. Although beds dipping into the face improve slope stability, the beds can create toe problems because the toe rock tends to break along the bedding or foliation planes. 217 FIELD MANUAL Dipping beds such as these require a tradeoff, depending upon which is the more serious problem-a somewhat unstable slope or an uneven toe.
In some cases, advancing the opening perpendicular to dipping beds may be a compromise. Many blasting jobs encounter site-specific geologic conditions not covered in this general discussion. Good blasting techniques require constant study of the geology to make every effort to advantageously use the geology, or at least minimize its unfavorable effects in blast designs. Surface Blasting Blast Hole Diameter The blast hole size is the first consideration of any blast design.
The blast hole diameter, along with the type of explosive being used and the type of rock being blasted, determines the burden (distance from the blast hole to the nearest free face). All other blast dimensions are a function of the burden. This discussion assumes that the blaster has the freedom to select the borehole size.
Many operations limit borehole size based on available drilling equipment. Practical blasthole diameters for surface construction excavations range from 3 (75 mm) to approximately 15 inches (38 cm). Large blasthole diameters generally yield low drilling and blasting costs because large holes are cheaper to drill per unit volume, and less sensitive, cheaper blasting agents can be used in larger diameter holes. Larger diameter blastholes also allow large burdens and spacings and can give coarser fragmentation. Figure 19-5 illustrates this comparison using 2- (50-mm) and 20-inch (500-mm)-diameter blastholes as an example.
218 BLAST DESIGN Figure 19-5.-The effect of large and small blast holes on unit costs. Pattern A contains four 20-inch (500-mm) blast boles, and pattern B contains 400 2-inch (5-mm) blast holes.
In all bench blasting operations, some compromise between these two extremes is chosen. Each pattern represents the same area of excavation-15,000 square feet (1,400 m2)-each involves approximately the same volume of blast holes, and each can be loaded with about the same weight of explosive. As a general rule, large diameter blast holes (6 to 15 inches 15 to 38 cm) have limited applications on most construction projects because of the requirements for fine fragmentation and the use of relatively shallow cuts. However, borehole size depends primarily on local conditions. Large holes are most efficient in deep cuts where a free face has already been developed.
219 FIELD MANUAL In most construction projects, small diameter drilling with high-speed equipment provides relatively low unit costs and permits fairly close spacing of holes. This close spacing provides better distribution of explosives throughout the rock mass, which in turn produces better breakage. An additional advantage of small diameter blast holes is that the reduction in the amount of each explosive used in each hole reduces ground vibrations. Construction blast hole diameters usually vary from 3.5 to 4.5 inches (90 to 114 mm), and the normal drilling depth is less than 40 feet (12 m).
Reclamation generally limits blast hole diameters for structural excavation drilling to 3.5 inches (90 mm). Blasting patterns usually range from 6 by 8 feet (1.8 by 2.4 meters) to 8 by 15 feet (2.4 by 4.6 m) and are usually rectangular with the burden being less than the spacing. In a given rock type, a four-hole pattern will give relatively low drilling and blasting costs.
Drilling costs for large blastholes will be low, a low-cost blasting agent will be used, and the cost of detonators will be minimal. In a difficult blasting situation, the broken material will be blocky and nonuniform in size, resulting in higher loading, hauling, and crushing costs as well as requiring more secondary breakage. Insufficient breakage at the toe may also result. The 400-hole pattern will yield high drilling and blasting costs. Small holes cost more to drill per unit volume, powder for small diameter blastholes is usually more expensive, and the cost of detonators will be higher. The fragmentation will be finer and more uniform, resulting in lower loading, hauling, and crushing costs. Secondary blasting and toe problems will be minimized.
Size of equipment, subsequent processing required for the blasted material, and economics will dictate the type of fragmentation needed and the size of blast hole to be used. 220 BLAST DESIGN Geologic structure is a major factor in determining the blast hole diameter. Planes of weakness (i.e., joints, shears, or zones of soft rock) tend to isolate large blocks of rock in the burden. The larger the blast pattern, the more likely these blocks are to be thrown unbroken into the muckpile.
Note that in the top pattern in figure 19-6, some of the blocks are not penetrated by a blast hole. In the smaller bottom pattern, all the blocks contain at least one blast hole. Because of the better explosives distribution, the bottom pattern will give better fragmentation. Figure 19-6.-The effects of jointing on selection of blast hole size. 221 FIELD MANUAL Airblast and flyrock often occur because of an insufficient collar distance (stemming column) above the explosive charge.
As the blast hole diameter increases, the collar distance required to prevent violence increases. The ratio of collar distance to blast hole diameter required to prevent violence varies from 14:1 to 28:1, depending on the relative densities and velocities of the explosive and rock, the physical condition of the rock, the type of stemming used, and the point of initiation. A larger collar distance is required where the sonic velocity of the rock exceeds the detonation velocity of the explosive or where the rock is heavily fractured or low density. A topinitiated charge requires a larger collar distance than a bottom-initiated charge. As the collar distance increases, the powder distribution becomes poorer, resulting in poorer fragmentation of the rock in the upper part of the bench. Ground vibrations are controlled by reducing the weight of explosive fired per delay interval.
This is done more easily with small blast holes than with large blast holes. In many situations where large diameter blast holes are used near populated areas, several delays, along with decking, must be used within each hole to control vibrations.
Large holes with large blast patterns are best suited to an operation with: (1) a large volume of material to be moved, (2) large loading, hauling, and crushing equipment, (3) no requirement for fine, uniform fragmentation, (4) an easily broken toe, (5) few ground vibration or airblast problems (few nearby buildings), and (6) a relatively homogeneous, easily fragmented rock without many planes of weakness or voids. Many blasting jobs have constraints that require smaller blast holes.
222 BLAST DESIGN The final selection of blast hole size is based on economics. Savings realized through inappropriate cost cutting in the drilling and blasting program may well be lost through increased loading, hauling, or crushing costs. Blast Patterns The three drill patterns commonly used are square, rectangular, and staggered. The square drill pattern (figure 19-7) has equal burdens and spacing, and the rectangular pattern has a larger spacing than burden. In both the square and rectangular patterns, the holes of each row are lined up directly behind the holes in the preceding row. In the staggered pattern (figure 19-8), the holes in each are positioned in the middle of the spacings of the holes in the preceding row. In the staggered pattern, the spacings should be larger than the burden.
Figure 19-7.-Three basic types of drill patterns. Square or rectangular drilling patterns are used for firing V-cut (figure 19-9) or echelon rounds. The burdens and subsequent rock displacement are at an angle to the original free face either side of the blast round in V-cut or echelon patterns. The staggered drilling pattern is used for row-on-row firing where the holes of one row are fired before the holes in the row immediately behind them as shown in figure 19-9. Looking at figure 19-9, with the burdens developed at a 45-degree angle to the original 223 FIELD MANUAL Figure 19-8.-Corner cut staggered pattern with simultaneous initiation within rows (blast hole spacing, S, is twice the burden, B).
Figure 19-9.-V-Echelon blast round (true spacing, S, is twice the true burden, B). 224 BLAST DESIGN free face, the original square drilling pattern has been transformed to a staggered blasting pattern with a spacing twice the burden. The three simple patterns discussed here account for nearly all the surface blasting.
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Burden Figure 19-10 is an isometric view showing the relationship of the various dimensions of a bench blast. The burden is defined as the distance from a blast hole to the nearest free face at the instant of detonation. In multiple row blasts, the burden for a blast hole is not necessarily measured in the direction of the original free face. The free faces developed by blast holes fired on lower delay periods must be taken into account.
As an Figure 19-10.-Isometric view of a bench blast. 225 FIELD MANUAL example, in figure 19-8, where one entire row is blasted before the next row begins, the burden is measured perpendicular between rows. In figure 19-9, the blast progresses in a V-shape. The true burden on most of the holes is measured at an angle of 45 degrees from the original free face. It is very important that the proper burden be calculated, accounting for the blast hole diameter, relative density of the rock and explosive, and, to some degree, the depth of the blast hole.
An insufficient burden will cause excessive airblast and flyrock. Too large a burden will produce inadequate fragmentation, toe problems, and excessive ground vibrations. If it is necessary to drill a round before the previous round has been excavated, it is important to stake out the first row of the second round before the first round is fired. This will ensure a proper burden on the first row of blast holes in the second blast round. For bulk-loaded charges (the charge is poured down the hole), the charge diameter is equal to the blast hole diameter. For tamped cartridges, the charge diameter will be between the cartridge diameter and the blast hole diameter, depending on the degree of tamping. For untamped cartridges, the charge diameter is equal to the cartridge diameter.
When blasting with ANFO (ammonium nitrate/fuel oil mixture) or other low density blasting agents with densities near 53 lb/ft 3 (0.85 g/cm3), in typical rock with a density near 170 lb/ft3 (2.7 g/cm3, the normal burden is approximately 25 times the charge diameter. When using denser products such as slurries or dynamites with densities near 75 lb/ft3 (1.2 g/cm3), the normal burden is approximately 30 times the charge diameter. These are first approximations, and field testing usually results in adjustments to these values. 226 BLAST DESIGN The burden-to-charge-diameter ratio is seldom less than 20 or seldom more than 40, even in extreme cases. For instance, when blasting with a low density blasting agent such as ANFO in a dense formation such as basalt, the desired burden may be about 20 times the charge diameter. When blasting with denser slurries or dynamites in low density formations such as sandstones, the burden may approach 40 times the charge diameter.
Table 19-2 summarizes these approximations.
Author by: M. Walker Language: en Publisher by: Geological Society of London Format Available: PDF, ePub, Mobi Total Read: 43 Total Download: 779 File Size: 51,8 Mb Description: This volume provides an authoritative and comprehensive state-of-the-art review of hot desert terrains in all parts of the world, their geomaterials and influence on civil engineering site investigation, design and construction. It primarily covers conditions and materials in modern hot deserts, but there is also coverage of unmodified ancient desert soils that exhibit engineering behaviour similar to modern desert materials. Thorough and up-to-date guidance on modern field evaluation and ground investigation techniques in hot arid areas is provided, including reference to a new approach to the desert model and detailed specialized assessments of the latest methods for materials characterization and testing.
The volume is based on world-wide experience in hot desert terrain and draws upon the knowledge and expertise of the members of a Geological Society Engineering Group Working Party comprising practising geologists, geomorphologists and civil engineers with a wealth of varied, but complementary experience of working in hot deserts. This is an essential reference book for professionals, as well as a valuable textbook for students. It is written in a style that is accessible to the non-specialist. A comprehensive glossary is also included. Author by: Steve D. Bowman Language: en Publisher by: Utah Geological Survey Format Available: PDF, ePub, Mobi Total Read: 23 Total Download: 496 File Size: 45,8 Mb Description: The purpose of these guidelines for investigating geologic hazards and preparing engineering-geology reports, is to provide recommendations for appropriate, minimum investigative techniques, standards, and report content to ensure adequate geologic site characterization and geologic-hazard investigations to protect public safety and facilitate risk reduction.
Such investigations provide important information on site geologic conditions that may affect or be affected by development, as well as the type and severity of geologic hazards at a site, and recommend solutions to mitigate the effects and the cost of the hazards, both at the time of construction and over the life of the development. The accompanying suggested approach to geologic-hazard ordinances and school-site investigation guidelines are intended as an aid for land-use planning and regulation by local Utah jurisdictions and school districts, respectively. Geologic hazards that are not accounted for in project planning and design often result in additional unforeseen construction and/or future maintenance costs, and possible injury or death. Author by: Terry R.
West Language: en Publisher by: Waveland Press Format Available: PDF, ePub, Mobi Total Read: 53 Total Download: 787 File Size: 46,5 Mb Description: West purposely developed a versatile text for bridging the gap between geology and civil engineering that can be used in engineering geology courses taught by either geologists or engineers. Mindful that students enrolled in these courses have diverse backgrounds, the author provides basic information on minerals and rocks, geological processes, and geological investigation techniques. He addresses the relationship of physical aspects of geology to engineering construction and explains how to recognize and provide for geologic factors that affect the location, design, construction, and maintenance of engineering projects. Engineering applications throughout the text emphasize the direct association of geology and engineering, while sufficient depth in geologic subjects provides a working knowledge of applied geology.
Exercises at the end of each chapter are designed for chapter review and problem solving. Some of the end-of-chapter exercises form the basis for laboratory studies on minerals, rocks, maps, geologic processes, and applied geology. Additional problem sets give students an opportunity to relate geologic detail to engineering construction.
The liberal array of photos, maps, and diagrams provide extra detail to clarify new concepts. Author by: J. Russell Boulding Language: en Publisher by: CRC Press Format Available: PDF, ePub, Mobi Total Read: 47 Total Download: 259 File Size: 55,8 Mb Description: A synthesis of years of interdisciplinary research and practice, the second edition of this bestseller continues to serve as a primary resource for information on the assessment, remediation, and control of contamination on and below the ground surface. Practical Handbook of Soil, Vadose Zone, and Ground-Water Contamination: Assessment, Prevention, and Remediation, Second Edition includes important new developments in site characterization and soil and ground water remediation that have appeared since 1995. Presented in an easy-to-read style, this book serves as a comprehensive guide for conducting complex site investigations and identifying methods for effective soil and ground water cleanup. Remediation engineers, ground water and soil scientists, regulatory personnel, researchers, and field investigators can access the latest data and summary tables to illustrate key advantages and disadvantages of various remediation methods.
Genre/Form: Handbooks and manuals Handbooks, manuals, etc Additional Physical Format: Print version: Engineering geology field manual (DLC) 99166147 (OCoLC)40662146 Material Type: Document, Government publication, National government publication, Internet resource Document Type: Internet Resource, Computer File All Authors / Contributors: OCLC Number: 643716525 Reproduction Notes: Electronic reproduction. S.l.: HathiTrust Digital Library, 2010. MiAaHDL Description: 1 online resource (2 volumes): illustrations Details: Master and use copy. Digital master created according to Benchmark for Faithful Digital Reproductions of Monographs and Serials, Version 1.
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