Rock Engineering Practice & Design - ISRM

1 Rock EngineeringRock EngineeringPractice & DesignPractice & DesignLecture 11: Lecture 11: Excavation MethodsExcavation Methods1 of 45 Erik Eberhardt UBC Geological Engineering ISRM EditionAuthor s Note:Author s Note:The lecture slides provided here are taken from the course Geotechnical Engineering Practice , which is part of the 4th year Geological Engineering program at the University of British Columbia (V C d ) Th k ii d (Vancouver, Canada). The course covers rock Engineering and geotechnical Design methodologies, building on those already taken by the students covering Introductory Rock Mechanics and Advanced Rock Mechanics Rock Mechanics. Although the slides have been modified in part to add context, they of course are missing the detailed narrative that accompanies any l l d h h l lecture. It is also recognized that these lectures summarize, reproduce and build on the work of others for which gratitude is extended. Where possible, efforts have been made to acknowledge th v ri us s urc s ith list f r f r nc s b in pr vid d t th the various sources, with a list of references being provided at the end of each lecture.

2 Errors, omissions, comments, etc., can be forwarded to the 2 of 45 Erik Eberhardt UBC Geological Engineering ISRM EditionErrors, omissions, comments, etc., can be forwarded to the author at: Excavation ProcessThe Excavation ProcessIt is instructive to consider the fundamental objective of the excavation process which is to remove rock material (either to create an opening or to obtain material for its inherent value). In order to remove part of a rock mass it is necessary to induce additional fracturing and rock mass, it is necessary to induce additional fracturing and fragmentation of the rock. This introduces three critical aspects of excavation: The peak strength of the rock must be in situ block size distribution must be changed to the required fragment size what means should the required energy be introduced into the rock?3 of 45 Erik Eberhardt UBC Geological Engineering ISRM Editioninto the rock?In SituIn SituBlock and Fragmentation DistributionBlock and Fragmentation DistributionRock is naturally fractured and consists of rock blocks of certain sizes.

3 The fracturing of rock during excavation changes this natural block size changes this natural block size distribution to the fragmentsize distribution. The engineer can consider how best to move from one curve to the other in the excavation process. 4 of 45 Erik Eberhardt UBC Geological Engineering ISRM EditionHudson & Harrison (1997)Energy and Excavation ProcessEnergy and Excavation ProcessAlthough the objective during excavation is large-scale fragmentation, at the same time we wish to minimize any damageto the wall rock as this would work towards weakening the rock mass which may result in ground control problems Fragmentation and rock mass damage are both related control problems. Fragmentation and rock mass damage are both related to the amount of energy used and whether its applied instantaneously or continuously. The tensile strength of rock is about 1/10th the compressive strength and the energy beneath h hl the stress-strain curve is roughly its square.

4 Therefore, breaking the rock in tension requires only 1/100th of the energy as that in Hudson & Harrison (1997)5 of 45 Erik Eberhardt UBC Geological Engineering ISRM Editioncompression. Energy and Excavation ProcessEnergy and Excavation ProcessOne objective in the excavation process may be to optimize the use of energy, the amount of energy required to remove a unit volume of rock (specific energy= J/m3). There are two fundamental ways of inputting energy into the rock for excavation:inputting energy into the rock for excavation:Blasting:Energy is input in large h quantities over very short durations (cyclical drill then blast, drill then blast, etc.).Machine Excavation:Energy is input in smaller quantities of 45 Erik Eberhardt UBC Geological Engineering ISRM EditionConventional Drill & BlastConventional Drill & BlastThe technique of rock breakage using explosives involves drilling blastholesby percussion or rotary-percussive means, loading the boreholes with explosives and then detonatingthe explosive in each explosives and then detonatingthe explosive in each hole in sequenceaccording to the blast explosion generates a stress wave and significant gas pressure.

5 Following the local fracturing at the local fracturing at the blasthole wall and the spalling of the free face, the subsequent gas pressure then provides the necessary energy to disaggregate the broken rock7 of 45 Erik Eberhardt UBC Geological Engineering ISRM & Harrison (1997)Conventional Drill & BlastConventional Drill & BlastDrillLoadBlastSurveyBlastVilSurveyV entilateBoltScoopScale8 of 45 Erik Eberhardt UBC Geological Engineering ISRM EditionDrill & Blast Drill & Blast -- DrillingDrillingpercussionhammerthrustfo rcerotationwaterflush(chip 9 of 45 Erik Eberhardt UBC Geological Engineering ISRM Edition(premoval)Thuro (1997)Drill & Blast Drill & Blast -- DrillingDrillingTh bit' bilit t t t th k ffi i tl dd th t t The bit's ability to penetrate the rock efficiently depends on the contact surface of the buttons, their shape and number, the bits' flushing characteristics and the brittleness, or drillability, of the rock.)

6 Button ShapeCharacteristicsApplication "non aggressive" shape minimum drilling ratesRock with high UCS and high abrasivity Sphericalminimum drilling rates low bit wear excavation mainly by impact "aggressive" shapeand high abrasivity ( quartzite, granite, gneiss, amphibolite)Rock with mid UCS Semi-Ballisticggp moderate drilling rates moderate bit wear excavation mainly by shearing/cuttingand less abrasivity ( slate, sandstone, limestone, weathered rock)Conical(Ballistic) "very aggressive" shape maximum drilling rates high bit wear excavation mainly by shearing/cuttingRock with low UCS and low abrasivity ( shale, weak sandstone, phyllite)10 of 45 Erik Eberhardt UBC Geological Engineering ISRM EditionThuro (1997)Drill & Blast Drill & Blast Drill Bit ButtonsDrill Bit Buttonsl.(2002)nninger et of 45 Erik Eberhardt UBC Geological Engineering ISRM EditionPlinDrill & Blast Drill & Blast Blast Pattern DesignBlast Pattern DesignOne of the basic principles of designing the configuration and sequential detonation of blastholes in a one blast, is the presence of a free face parallel or sub-parallel to the blast holes, as detonation occurs.

7 In some cases these free faces may already be present (benches in an open pit cases, these free faces may already be present (benches in an open pit mine), but in other cases may need to be created by the blast itself (a tunnel face). Practical application of the free-face concept using one 12 of 45 Erik Eberhardt UBC Geological Engineering ISRM Editionfreeface concept using one form of the burn cut. Hudson & Harrison (1997)Drill & Blast Drill & Blast Burn CutBurn CutTh t d i f bl t t t ith th fi t h lt b dt td The correct Design of a blast starts with the first holeto be detonated. In the case of a tunnel blast, the first requirement is to create a voidinto which rock broken by the blast can expand. This is generally achieved by a wedge or burn cutwhich is designed to create a clean void and to ejectthe ggjrock originally contained in this void clear of the tunnel cut designs using millisecond delays. 13 of 45 Erik Eberhardt UBC Geological Engineering ISRM EditionyDrill & Blast Drill & Blast Blast Pattern DesignBlast Pattern Design14 of 45 Erik Eberhardt UBC Geological Engineering ISRM EditionSpecialized Blasting TechniquesSpecialized Blasting TechniquesD i bl ti th li d t l di During blasting, the explosive damagemay not only occur according to the blasting round Design , but there may also be extra rock damage behindthe excavation boundary.)

8 To minimize damage to the rock a presplit blast (surface excavation) or smoothwall blast rock, a pre-split blast (surface excavation) or smooth-wall blast (underground) may be used to create the final excavation blast: First a series of small-diameter, parallel boreholes are ison (1997)drilled along the plane of the required final excavation boundary ( rock cut slope) dson & Harr15 of 45 Erik Eberhardt UBC Geological Engineering ISRM Editionrock cut slope). HudPrePre--Split & SmoothSplit & Smooth--Wall BlastingWall BlastingThe principle is then to tailor the explosive parameters such that detonation of the l i i h iiil explosives in these initial holes will primarily create a plane of intersecting holes through the coalescence of through the coalescence of several induced fractures. The smooth-wall blast follows a similar process to the pre-split blast, except in the reverse order. First a rough opening is formed using a large bulk opening is formed using a large bulk blast, and then the smooth-wall blast follows along a series of closely spaced and lightly charged parallel holes.

9 16 of 45 Erik Eberhardt UBC Geological Engineering ISRM EditionHudson & Harrison (1997)PrePre--Split BlastingSplit BlastingWhen, subsequently, the main body of rock is blasted to form the cutting, the pre-split reflects the stress waves back into the rock being excavated and dissipates excess gas pressure, such that the bulk blast has little effect of the rock behind the pre-split plane blast has little effect of the rock behind the pre-split plane. pre-split bl t dnormal bulk bld17 of 45 Erik Eberhardt UBC Geological Engineering ISRM EditionblastedblastedDrill & Blast Drill & Blast ExplosivesExplosivesCommercial explosives are mixtures of chemical compounds in solid or liquid form. Detonation transforms the compounds into other products, mostly gaseous. The following are the main criteria applied to select an explosive for a given type of blasting: available energy per unit weight of explosive ( strength) density of the explosive detonation velocity sensitivity (ease of ignition)ti t reaction rate temperature and pressure stability (chemical and storage)18 of 45 Erik Eberhardt UBC Geological Engineering ISRM Editiony(g )Drill & Blast Drill & Blast ExplosivesExplosivesThe following are the more common explosives used in hard rock excavation: dynamites (nitroglycerin made stable by dissolving it in an inert bulking agent moderate bulk strength)bulking agent moderate bulk strength) ANFO (Ammonium Nitrate & diesel Fuel Oil low bulk strength) slurries (water gels high bulk strength for wet conditions) emulsionsANFO is the most prevalent explosive used in mining because it is the least expensive and the safest to transport and handle.

10 ANFO type explosives are susceptible to water and 19 of 45 Erik Eberhardt UBC Geological Engineering ISRM Editionexplosives are susceptible to water and, therefore, are not suitable for wet blastholes. Drill & Blast Drill & Blast StemmingStemming Stemming materials ( pea-sized gravel), are used to top-off the blastholes. The stemming material acts to provide confinement acts to provide confinement, preventing the explosive gases and energy from travelling (venting) up through the drill hole, and instead are id ihih k contained withinthe rock mass. Effects of poor 20 of 45 Erik Eberhardt UBC Geological Engineering ISRM EditionEffects of poor stemming. Drill & Blast Drill & Blast Blasting Caps & DelaysBlasting Caps & DelaysA blasting capis a small explosive device generally used to detonate a larger, more powerful explosive. Most blasting caps contain what is c ll d p im xpl si A p im xpl si is called a primary explosive.