tailieunhanh - Systematic Drilling and Blasting for Surface Excavations Part 9

Tham khảo tài liệu 'systematic drilling and blasting for surface excavations part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | EM 1110-2-3800 1 Mar 72 Fig. 5-23. Main charge delays in numerical sequence progressing parallel to the presplit reduce back pressure in wall rock requirements. Maximum fragment size gradation can be estimated by studying the spacing of joints in bedrock or size of talus produced from outcrops. a. Riprap. The degree of fragmentation in blasting for riprap must be controlled so that proper size and gradation can be obtained. Coyote blasting may be used for producing large rocks for riprap and breakwaters quickly and economically see c below . In some rocks low-velocity ammoriia dynamites are used because of their low shattering effects. ANFO while often detonat- ing at a higher velocity than many low-velocity ammonia dynamites is also used largely due to its lower price per pound. However coyote blasting seldom yields well-sorted rock for riprap and secondary blasting mudcapping or blockholing as well as screening off of fines may be necessary. Restraint should be exercised in considering the coyote method for jetty stone. Jetty stone quarries commonly contain only about 10 to 20 percent of the best grade large stones and the excessive fracturing and poor control of a coyote blast can ruin a quarry. Depending on their availability it may be advantageous to mine these stones one by one by multiple-row or irregular array. b. Aggregate. 1 Material used for concrete aggregate usually is of small sizes and therefore blasts should be designed to produce a high degree of fragmentation and thereby reduce handling and crushing costs. 2 Good fragmentation is commonly achieved by adequately charged staggered holes in a pattern utilizing the optimum spacing burden ratio and detonated by a millisecond delay system Fig. 5-24 . Staggered holes allow more of the rock to be affected by the blast and thus produce better breakage throughout. The spacing should 5-24 EM 1110-2-3800 1 Mar 72 eocr BENCH Fig. 5-24. Plan of blasthole pattern for fragmentation of rock to produce aggregate .