tests. However, it is generally true that both the design of blasting 

 rounds and the chances of their success largely depend on the previous 

 experience and skill of the blaster. 



In many cases, a very hard, brittle rock will break with less difficulty 

 than a soft, spongy rock. This is dramatically true if the hard rock is 

 closely jointed and the soft rock is massive. The orientation of the 

 primary joint system in a formation is a very important factor in the blast 

 design. When the primary jointing is dipping at a steep angle, it is 

 usually advantageous to develop the quarry face at no less than 45 (and 

 preferably 90 ) to this jointing angle; however, this is not universally 

 true. In determining the angle of the quarry face, consideration should 

 also be given to the desired end product (e.g., crusher feed versus riprap), 

 slope stability questions, ease of development, traffic flow, and equipment. 

 Frequently, the direction of development is not a controllable factor. In 

 such cases, it is important to make certain that blast designs and exca- 

 vating procedures take into account the geometric relationships between the 

 jointing, explosives action, excavating sequences, and final surfaces. 



(2) Fragmentation . The degree of fragmentation desired depends on 

 the end use of the product mined. In quarrying, where the stone will be 

 sized for construction use, it is usually undesirable to produce a large 

 percentage of stone less than 5 centimeters (2 inches) in size. Even under 

 the best conditions, it should be anticipated that up to 5- percent fines 

 (fine sand, silt, and clay) will be generated by the blasting operation. 



Because many of the various parameters involved in blast design are 

 strongly interrelated, it is difficult to isolate each factor and discuss 

 its relationship to the final product. However, it is possible to make 

 certain statements which are generally true about some of these parameters. 

 For example, a high powder factor (quantity of explosive per unit volume of 

 rock) will produce a greater degree of fragmentation than a lower powder 

 factor if other factors are kept the same. Similarly, greater fragmenta- 

 tion will be achieved in a massive rock by using a larger number of smaller 

 diameter holes at closer spacing, rather than larger diameter holes at 

 greater spacings. Also, it appears that the best fragmentation is achieved 

 when holes are detonated individually rather than simultaneously. 



(3) Hole Diameter . The proper hole diameter depends largely on the 

 physical properties of the formation, the fragmentation required, and the 

 height of the quarry face. The hole diameter should be selected to be 

 compatible with the geological and physical characteristics of the forma- 

 tion, since it is the only factor in the overall blast design that cannot 

 be altered. Unfortunately, this selection is sometimes made on the basis 

 of the total volume of rock to be mined, the duration of the project, 

 production rates, capital costs, and depreciation rates, with no considera- 

 tion given to geology. 



(4) Type of Explosive . On large-scale projects, it can be reason- 

 ably assumed that the first choice of explosive will be made for economical 

 rather than technical reasons; for dry quarry work, explosives will usually 

 be ammonium nitrate fuel oil (ANFO) . If water is encountered, the choice 

 would normally be a specially packaged ANFO or some form of slurry or water 

 gel. Despite its relatively low price, ANFO is a good, general-purpose 



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