Web-based Class Project on Rock Mechanics 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 Summary of Surface Blasting with Comparison of Two Mitigation Techniques - Presplitting and Smooth Blasting Prepared by: 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 (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 Parameters of Blasting 1. Explosives 2. Blast Hole Diameter and Drilling 3. Burden 4. Bench Height 5. Spacing 6. Detonation 7. Stemming 8. Properties of the Rock Mass

Typical Terms and Components - Explosives

 USACE EM (1972)

Typical Terms and Components ParameterPossible Damage Effects from Improper Design Diameter and Drilling Poor fragmentation, excessive burden, too little burden, uneven floor, etc. >> BurdenVibrations, poor fragmentation << BurdenFlyrock, airblast >> Bench HeightUneven bore holes, explosive cutoff << Bench HeightAirblast, flyrock >> SpacingUneven final face << SpacingCratering, crushing Detonation timeVibrations, fragmentation, flyrock >> StemmingVibrations, unbroken rock at surface << StemmingAirblast, flyrock

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

Damages and Relation to Vibrations Rock QualityThreshold Limits Excavation in poor quality rock mm/s (0.66 – 1.97 ft/s) Excavation in good quality rock mm/s (1.97 – 6.56 ft/s) Excavation with unfavorable jointing and potential for unstable blocks along walls 100 – 600 mm/s (0.33 – 1.96 ft/s)

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 Ekati TM 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 – Smooth Blast

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 – Presplit 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),  Langefors, U., and Kihlstr ӧ m, 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. ( ).  U.S. Army Corps of Engineers (USACE). (1972). Systematic Drilling and Blasting for Surface Excavations Engineering Manual. Engineer Manual , U.S. Army Corps of Engineers, Washington, D.C.

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