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 Bl ID: 912758
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Web-based Class Projecton Rock Mechanics
Report prepared as part of course CEE 544: Rock MechanicsWinter 2015 SemesterInstructor: Professor Dimitrios ZekkosDepartment 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:
Slide2Overview
1.0 Introduction2.0 Mechanics of Rock Blasting3.0 Types of Blasting4.0 Typical Components and Terms of Blasting5.0 Damages and Mitigation6.0 Case Studies
Slide3Introduction
Rock blasting as a practice dates back many centuries and general rules were developed from experienceRock blasting as a science, however, is fairly newOne of the goals to understanding rock blasting better is to limit the extent of damage in the remaining rock massThe two techniques investigated are presplitting and smooth blastingFor this, two case studies will be looked at – The Ekati Mine and the Excavation of High Rock Slopes in China
Slide4Mechanics of a Blast
Release of energy from explosives in the form of waves and expanding hot gasesCrushing – Compression failureRadial Cracking – Tension failure Spalling – Tension failureExpelling and escape of gases
Slide5Mechanics of a Blast
From USACE EM-1110-2-3800 (1972)
Slide6Types of Blasting
Two main categories: Underground blasting and surface blastingSurface blasting is used for excavation and mines or quarriesWhy does it matter? Scale of the blast, fragmentation of material to be removed, and finally, the extent of damage to the remaining rock mass
Slide7Typical Terms and Components
Parameters of Blasting
1. Explosives2. Blast Hole Diameter and Drilling
3. Burden
4. Bench Height
5. Spacing
6. Detonation7. Stemming
8. Properties
of the Rock Mass
Slide8Typical Terms and Components - Explosives
(Langefors 1978)
Where
:V= burdenK= bench
heightE=spacing between blast holesh= height of the charged
= hole diameters= weight strengthp= density of explosiveu=detonation velocityci
= rock characterization
Slide9Typical Terms and Components - Explosives
USACE EM-1110-2-3800 (1972)
Slide10Typical Terms and Components
ParameterPossible Damage Effects from Improper DesignDiameter and Drilling
Poor fragmentation, excessive burden, too little burden, uneven floor, etc.>> BurdenVibrations, poor fragmentation
<< BurdenFlyrock, airblast>> Bench HeightU
neven bore holes, explosive cutoff<< Bench Height
Airblast, flyrock>> SpacingUneven final face
<< SpacingCratering, crushingDetonation timeVibrations, fragmentation, flyrock
>> StemmingVibrations, unbroken rock at surface<< Stemming
Airblast
,
flyrock
Slide11Typical Terms and Components – Properties of the Rock Mass
Blasting coefficient, powder factor, hardness of rock massPre-existing weaknesses can create paths for explosive energyBaker (1982)
Slide12Damages
Slide13Damages
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 failureRelease 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
Slide14Damages and Relation to Vibrations
The most widely used way of monitoring and gauging the effects of blasting is measuring the PPVTwo main factors that affect vibrations are weight of the charge and distance from detonation, among others, as mentioned previouslyThere 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
Slide15Damages and Relation to Vibrations
Rock QualityThreshold Limits
Excavation in poor quality rock200-600 mm/s (0.66 – 1.97 ft/s)Excavation in good quality rock
600-2000 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)
Slide16Mitigation – Smooth Blasting
Main Objective – Leave a berm between main blasts and final faceThen, use lower charge weights and smaller spacing to form a continual crack between holesThis takes advantage of several mechanical aspects of blasting e.g. stress concentration, crack length to fracture density, simultaneous blasting
Slide17Mitigation – Presplit Blasting
Main Objective – Form a free face before main production blastDrill closely spaced holes and lightly pack with explosives and detonate firstTakes advantage of same mechanisms in smooth blasting except more contained
Slide18Mitigation
Hu et al (2013)
Slide19Mitigation
Hoek (2007)
Slide20Case 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)
Slide21Ekati Mine
Diamond mine located in Northwest Territory, Canada that used presplit blasting for final pit walls3 blasts were monitored by measuring PPV and gas pressureOne production blast, one blast after presplitting (wall control blast), and presplitting blast itselfConcluded that heave and gas penetration were main causes of damage, not vibrationsThus not overbreak but excavation instability
Slide22Ekati 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 rockThis limits thrust and movement of the remaining wallPeterson also stressed importance of overall blast design in limiting damage because it is production blasts that are responsible for most of the damage
Slide23High Rock Slopes in China
Used numerical modeling to estimate damages to the final rock faceCompared smooth blasting with presplit blastingFor each case, there were production blasts, buffer blasts, and then either a smooth or presplit blast
Slide24High Rock Slopes in China – Smooth Blast
Damage is predominately a result of the first two production blasts, penetrating into the rock massLittle columnar damage was noted around the final smooth blast hole
Slide25High Rock Slopes in China – Smooth Blast
Slide26High Rock Slopes in China – Presplit Blast
Most damage was from the presplit blast itself, specifically around the blast hole in a columnar shapeHowever, extent and depth of damage was minimized to the main rock mass in comparison to the smooth blast
Slide27High Rock Slopes in China – Presplit Blast
Slide28High Rock Slopes in China - Recommendations
Use a combination of the two techniquesThis will take advantage of the smaller confinement of smooth blasting and the limit of damage from production blasts as a result of presplit
Slide29References
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.
Slide30Questions?
Slide31More Information
More detailed technical information on this project can be found at:http://www.geoengineer.org/education/web-based-class-projects/rock-mechanics