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Summary of Surface Blasting with Comparison of Two Mitigation Techniques - Presplitting and Smooth Blasting. Web-based Class Project on Rock Mechanics. Prepared by:. Report prepared as part of course CEE 544: Rock Mechanics Winter 2015 Semester Instructor: Professor Dimitrios Zekkos
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Summary of Surface Blasting with Comparison of Two Mitigation Techniques - Presplitting and Smooth Blasting Web-based Class Projecton Rock Mechanics Prepared by: Report prepared as part of course CEE 544: Rock Mechanics Winter 2015 Semester Instructor: Professor Dimitrios Zekkos Department of Civil and Environmental Engineering University of Michigan Charles Krolikowski With the Support of:
Overview • 1.0 Introduction • 2.0 Mechanics of Rock Blasting • 3.0 Types of Blasting • 4.0 Typical Components and Terms of Blasting • 5.0 Damages and Mitigation • 6.0 Case Studies
Introduction • Rock blasting as a practice dates back many centuries and general rules were developed from experience • Rock blasting as a science, however, is fairly new • One of the goals to understanding rock blasting better is to limit the extent of damage in the remaining rock mass • The two techniques investigated are presplitting and smooth blasting • For this, two case studies will be looked at – The Ekati Mine and the Excavation of High Rock Slopes in China
Mechanics of a Blast • Release of energy from explosives in the form of waves and expanding hot gases • Crushing – Compression failure • Radial Cracking – Tension failure • Spalling – Tension failure • Expelling and escape of gases
Mechanics of a Blast • From USACE EM-1110-2-3800 (1972)
Types of Blasting • Two main categories: Underground blasting and surface blasting • Surface blasting is used for excavation and mines or quarries • Why does it matter? Scale of the blast, fragmentation of material to be removed, and finally, the extent of damage to the remaining rock mass
Typical Terms and Components - Explosives • (Langefors 1978) • Where: • V= burden • K= bench height • E=spacing between blast holes • h= height of the charge • d= hole diameter • s= weight strength • p= density of explosive • u=detonation velocity • ci= rock characterization
Typical Terms and Components - Explosives • USACE EM-1110-2-3800 (1972)
Typical Terms and Components – Properties of the Rock Mass • Blasting coefficient, powder factor, hardness of rock mass • Pre-existing weaknesses can create paths for explosive energy • Baker (1982)
Damages • Overbreak – Creation of new cracks from the blast. About 80% reduction of strength with an extent approximately the burden length. • Excavation Stability – Movement of rock blocks from vibrations and gases can disrupt interlocking of joints/fractures. This lowers shear strength and additional excavation leaves the slopes susceptible to failure • Release of Load – Rubber mat analogy. This can create vertical fractures up to 55m behind a new face. • Hoek-Brown parameter D in rock strength equation is example of importance between good and bad blasting
Damages and Relation to Vibrations • The most widely used way of monitoring and gauging the effects of blasting is measuring the PPV • Two main factors that affect vibrations are weight of the charge and distance from detonation, among others, as mentioned previously • There have been studies that correlate PPV to strains experienced by the rock, and therefore, the likelihood new fractures will form or slippage along existing discontinuities
Mitigation – Smooth Blasting • Main Objective – Leave a berm between main blasts and final face • Then, use lower charge weights and smaller spacing to form a continual crack between holes • This takes advantage of several mechanical aspects of blasting e.g. stress concentration, crack length to fracture density, simultaneous blasting
Mitigation – Presplit Blasting • Main Objective – Form a free face before main production blast • Drill closely spaced holes and lightly pack with explosives and detonate first • Takes advantage of same mechanisms in smooth blasting except more contained
Mitigation • Hu et al (2013)
Mitigation • Hoek (2007)
Case Studies • Blast Damage Mechanisms at EkatiTM Mine by Peterson (2001) • Comparison of Blast-Induced Damage between Presplit and Smooth Blasting of High Rock Slope by Hu et al. (2013)
Ekati Mine • Diamond mine located in Northwest Territory, Canada that used presplit blasting for final pit walls • 3 blasts were monitored by measuring PPV and gas pressure • One production blast, one blast after presplitting (wall control blast), and presplitting blast itself • Concluded that heave and gas penetration were main causes of damage, not vibrations • Thus not overbreak but excavation instability
Ekati Mine • Why was presplitting beneficial to the final wall face? • It was suggested by Peterson that the blast pattern can be setup in such a way that the blasted rock moves along the presplit face instead of into the remaining rock • This limits thrust and movement of the remaining wall • Peterson also stressed importance of overall blast design in limiting damage because it is production blasts that are responsible for most of the damage
High Rock Slopes in China • Used numerical modeling to estimate damages to the final rock face • Compared smooth blasting with presplit blasting • For each case, there were production blasts, buffer blasts, and then either a smooth or presplit blast
High Rock Slopes in China – Smooth Blast • Damage is predominately a result of the first two production blasts, penetrating into the rock mass • Little columnar damage was noted around the final smooth blast hole
High Rock Slopes in China – Presplit Blast • Most damage was from the presplit blast itself, specifically around the blast hole in a columnar shape • However, extent and depth of damage was minimized to the main rock mass in comparison to the smooth blast
High Rock Slopes in China - Recommendations • Use a combination of the two techniques • This will take advantage of the smaller confinement of smooth blasting and the limit of damage from production blasts as a result of presplit
References • Dick, R. A., Fletcher, L. R., and D’Andrea, D. V. (1982). Explosives and Blasting Procedures Manual, Bureau of Mines, Washington, D.C. • Hoek, E. (2007). “Blasting Damage in Rock.” Practical Rock Engineering. • Hu, Y., Lu, W., Chen, M., Yan, P., and Yang, J. (2013). “Comparison of Blast- Induced Damage Between Presplit and Smooth Blasting of High Rock Slope.” Rock Mechanics and Rock Engineering, 47 (4), 1307-1320. • Langefors, U., and Kihlstrӧm, B. (1978). The Modern Technique of Rock Blasting, 3rd Ed., Wiley and Sons Inc., NY. • Peterson, J. A. (2001). Blast Damage Mechanisms at Ekati(TM) Mine (Order No. MQ69811). Available from ProQuest Dissertations & Theses A&I; ProQuest Dissertations & Theses Full Text; ProQuest Dissertations & Theses Global. (304744467). • U.S. Army Corps of Engineers (USACE). (1972). Systematic Drilling and Blasting for Surface Excavations Engineering Manual. Engineer Manual 1110-2-3800, U.S. Army Corps of Engineers, Washington, D.C.
More Information More detailed technical information on this project can be found at: http://www.geoengineer.org/education/web-based-class-projects/rock-mechanics