Rock Excavation Handbook Rock Excavation

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10 Rock Excavation Handbook Rock Excavation


<p>62</p> <p>3. Principles of Rock Excavation TechnologiesR O C K E X C AVAT I O N H A N D B O O K</p> <p>63</p> <p>3.1 MECHANICS OF ROCK BREAKINGWhen a tool is loaded onto a rock surface, stress is built up under the contact area. The way the rock responds to this stress depends on the rock type and the type of loading, for example, the drilling method. Rock breakage by percussive drilling can be divided into four phases:</p> <p>CRACK PROPAGATIONAfter the energy barrier has been overcome, spontaneous and rapid propagation follows, zone II in FIGURE 3.1.-1. At a lower depth than the contact dimension, the tensile driving force falls below that necessary to maintain growth, thus the crack again becomes stable. The crack is then said to be well developed.</p> <p>CHIPPING CRUSHED ZONEAs the tool tip begins to dent the rock surface, stress grows with the increasing load and the material is elastically deformed, zone III in FIGURE 3.1.-1. At the contact surface, irregularities are immediately formed and a zone of crushed rock develops beneath the indenter (the button or insert of a drill bit) FIGURE 3.1.-1. The crushed zone comprises numerous microcracks that pulverize the rock into powder or extremely small particles. 70-85% of the indenters work is consumed by the formation of the crushed zone. The crushed zone transmits the main force component into the rock. When the load reaches a sufficient level, the rock breaks and one or more large chips is formed by lateral cracks propagating from beneath the tip of the indenter to the surface. This process is called surface chipping. Each time a chip is formed, the force temporarily drops and must be built up to a new, higher level to achieve chipping. Crushing and chipping creates a crater.</p> <p>3.2 TOP-HAMMER DRILLINGThere are four main components in a drilling system, FIGURE 3.2.-1. These components are related to the utilization of energy by the system for attacking rock in the following way: 1. The piston inside the rock drill is the prime mover, converting energy from its original form (fluid, electrical, pneumatic or combustion engine drive) into mechanical energy to actuate the system. 2. The shank adapter transmits impact energy from the piston to the rod(s) or tube(s). Additionally, rotation torque is delivered via the shank adapter. 3. The rod or tube transmits impact energy and rotation torque. 4. The bit applies the energy in the system, mechanically attacking rock to achieve penetration.</p> <p>FIGURE 3.1.-1. Rock breakage in percussive drilling</p> <p>CRACK FORMATIONAs the process continues, dominant cracks begin to form in the rock. This initial stage of restricted growth is described as an energy barrier to full propagation. The placement of major cracks depends on the indenter shape. Generally, the dominant placement of major cracks with blunt indenters, such as a sphere, is located just outside the contact area, pointing down and away from the surface.</p> <p>FIGURE 3.2.-1. Percussion dynamics</p> <p>64</p> <p>3. Principles of Rock Excavation TechnologiesR O C K E X C AVAT I O N H A N D B O O K</p> <p>65</p> <p>In tophammer drilling, the piston inside the rock drill accelerates to the desired velocity before striking a shank adapter or drill rod. Upon impact, the drill rod particles achieve velocity, and therefore, a displacement in a direction away from the piston. The particles transmit this motion to adjacent particles which then repeat the process, creating a stress wave pulse that travels down the rod. The shape of the stress wave is determined by piston and drill string geometry. A rectangular stress wave is formed when a piston with a uniform cross-section area hits a rod made of identical material and the same cross-section area. Hydraulic rock drills produce rectangular stress wave, FIGURE 3.2.-1.</p> <p>PERCUSSIVE DRILLING PARAMETERSPercussive drilling consists of four drilling parameters which affect performance: percussion power (percussion energy and frequency), feed force, bit rotation speed and flushing.FIGURE 3.2.-2.</p> <p>Percussion power Percussion output power in percussive drilling is produced by the rock drills impact energy and frequency. Pneumatic drilling has a typical impact frequency of between 1,600 3,400 hits per minute; hydraulic drilling, 2,000 - 4,500 hits per minute. Percussion output power is a function of hydraulic or pneumatic pressure and flow rates. Compared to pneumatic drills, hydraulic drills are capable of higher percussion power and faster penetration rates. The net penetration rate achieved with TAMROCK hydraulic rock drills as a function of drill hole diameter and rock drillability shown in FIGURE 3.2.-3.FIGURE 3.2.-2. Top-hammer drilling. FIGURE 3.2.-3. Net penetration rates achieved with TAMROCK rock drills.</p> <p>steels life time simultaneously decreases. Possible increased hole deviation and its impact on burden and spacing must also be taken into account. Feed Feed force is required to keep the shank in contact with the drill and the drill bit in contact with the rock. This ensures maximum impact energy transfer from the piston to the rock. When percussion pressure is increased, feed pressure must also be increased. Optimum feed force depends on the percussion pressure levels, rock condition, hole depth, drilling angle, and the size and type of drill steels. Broken rock should be drilled at low percussion and feed pressures. In top-hammer drilling systems, the drill normally travels on a feed. The required feed force is transferred to the rock drill cradle by chains or cylinders. Optimal feed pressure can easily be observed by monitoring penetration, bit wear and steel thread wear. Quite often, visual monitoring of feed and rotational smoothness during drilling is sufficient to determine the optimum feed pressure. Low feed force results in: - Poor transmission of percussion energy, shank damage and increased thread wear since couplings tend to loosen - Reduced penetration rates due to poor percussion energy transfer through the drill string - Almost no resistance to rotation and low torque</p> <p>One limitation in percussion drilling is the capacity of the drill steel to transmit energy. Only maximum kinetic energy is transmitted through a particular steel before excessive drill string deterioration occurs. For field drilling, the optimum percussion pressure setting depends on financial aspects. Higher penetration rates are achieved through increased percussion power, however, the drill</p> <p>66</p> <p>3. Principles of Rock Excavation TechnologiesR O C K E X C AVAT I O N H A N D B O O K</p> <p>67</p> <p>- Increased inner bit button wear - Overheated and rattling coupling Very high feed force leads to: - Unnecessary bending and drill steel and shank wear - Flushing problems - Rapid button wear on the bits due to increased drag against the hole bottom and because bit is forced to work in inclined position when drill string bends - Increased hole deviation - Uncoupling becomes difficult due to excessively tight threads - Lower penetration rates Bit Rotation The main purpose of bit rotation is to index the drill bit between consecutive blows. After each blow, the drill bit must be turned to ensure there is always fresh rock under the inserts or buttons. Bit rotation speed is adjusted to the point where the penetration rate is at its maximum. The following factors affect optimum bit rotation speed: - Rock type - Rock drill frequency - Drill bit diameter - Gauge button diameter (in case of button bit) At optimum rotation speed, the size of the disintegrated chips is greatest and thus the penetration rate is maximum. FIGURE 3.2.-4.</p> <p>According to a rocks drilling properties, the optimum travel distance of gauge buttons between consecutive blows is the diameter of gauge button times 0,5 - 1,5. The smaller optimum travel distances applies to conditions in which the formation of chips during bit denting is poor, resulting in slower net penetration rates. Bit rotation speed can be calculated through the following equation: n= Where S 60 d</p> <p>S = Gauge button travel distance between consecutive blows [mm] n = Bit rotation speed [RPM] = Impact frequency of the hammer [1/s] d = Bit diameter [mm]</p> <p>However, for smaller rod diameters (1 1/2 and less), the optimum penetration rate disappears after the second rod due to poor indexing, which is caused by reduced drilling string stiffness. Therefore it is necessary to: - Decrease RPM values by 2030% for a combination of small rod diameters and long drill holes to extend the bit regrind limit For cross bits, the following RPM values relative to button bits are suggested: - Diameters 35 - 51 mm cross bits, RPM 5 - 10% lower values - Diameters 76 mm or larger cross bits, RPM 5 - 10% higher Insufficient bit rotation speeds result in energy loss due to recutting and result in low penetration rates. However, sometimes bit rotation speed is intentionally set under its optimum value since lower RPMs</p> <p>FIGURE 3.2.-4. Penetration</p> <p>rate in granodiorite through bit rotation speed.</p> <p>- Reduce in-hole bit deviation - May be required in very abrasive rocks to maintain gauge button velocity under critical wear speed Excess rotational speeds result in excessive bit wear as rock is forced to break by rotation instead of percussion. High rotational speeds also lead to excessively tight couplings, which result in uncoupling problems. Flushing Flushing is used to remove rock cuttings from the drillhole and to cool the drill bit. The flushing medium - air, water, mist or foam - is forced to the bottom of the drillhole through</p> <p>68</p> <p>3. Principles of Rock Excavation TechnologiesR O C K E X C AVAT I O N H A N D B O O K</p> <p>69</p> <p>the steels flushing hole and the holes in the drill bit. Loose rock cuttings mix with the flushing agent and are forced out of the drillhole through a space between the drill steel and the hole wall. Insufficient flushing leads to low penetration (increased recutting), decreased drill steel life (bit wear and jamming of the steels) and high bit wear. The amount of flushing required to remove cuttings from the drillhole depends on the hole size, cuttings produced and hole length. Air flushing is typically used in surface drilling; water in underground drilling operations. If water is not allowed underground, air, airwater mist or a variety of foam flushing agents can be used. Air flushing in a closed space requires excellent dust collection systems.Experience shows that the minimum required flushing velocity for successful cuttings removal is 15 m/s for air and 1.0 m/s for water flushing. Over flushing is also a risk. With air flushing, excess flushing pressures are used. The bit-body is eroded by the sand-blasting effect through a mixture of cuttings and air. High bit body wear is a problem in especially abrasive rock types. In water flushing, overly high flushing pressures decrease the penetration rate because water cushions the bit against the rock. Required air flow for sufficient flushing can be estimated from the minimum velocity stated in Table 3.2.-1. The amount of water used in tunneling is approx. 50 liters per minute for each rock drill. In longhole drilling, drilling direction has a large impact on the required water consumption. Downholes need more flushing water compared to upholes. Sometimes foams are used together with air flushing to bind the dust and to stabilize drill hole walls. Airwater mists are also used for these purposes. Normally the amount of water added to the flushing air varies between 2-5 l/min. Air and air-water mist flushing increases penetration rates up to 1020% compared to water flushing.</p> <p>Table 3.2.-1. Required air volume for sufficient hole flushing. Steel diameter, mm 25 Hole size, mm 32 38 45 51 64 76 89 102 115 127 32 38 45 51 64 76 87</p> <p>Required air volume m3/min (air velocity = 15 m/s) 0.28 Drifting 0.58 0.30 0.71 0.41 1.11 0.82 Long hole drilling 2.17 1.87 1.46 3.06 2.65 2.24 1.19 Tubes 4.17 3.76 2.70 1.52 5.52 4.46 3.27 2.00 7.51 5.27 4.00 6.05</p> <p>HYDRAULIC DRILLINGHydraulic percussive drilling rigs were introduced to the market in the early 1970s. These new, high-power rock drills not only doubled drilling capacities but also improved the drilling environment. The introduction of hydraulics to rock drilling also led to improvements in drilling accuracy, mechanization and automation. Working principle of the hydraulic rock drill The general working principle of a hydraulic percussion rock drill is presented inFIGURE 3.2.-5.</p> <p>a. Piston at front end b. Piston moves backwards c. Piston in rear position d. Piston moves forwards</p> <p>FIGURE 3.2.-5 a, b, c and d. The working principle of the</p> <p>hydraulic rock drill.</p> <p>70</p> <p>3. Principles of Rock Excavation TechnologiesR O C K E X C AVAT I O N H A N D B O O K</p> <p>71</p> <p>TAMROCK rock drills are designed with a minimum of modules and parts. The hydraulic rock drill has a reliable percussion cartridge structure, comprising a piston and a distributor. This design allows large and short flow channels, which ensures maximum flow efficiency. The compact modules for percussion, rotation and flushing minimize the number of joints (FIGURE 3.2.-6).</p> <p>3.3 PRINCIPLE OF DTH DRILLINGDTH (Down-The-Hole) drilling, also known as ITH (InThe-Hole) drilling, is a method in which the percussive hammer works in the hole during drilling, as opposed to above the hole in top-hammer drilling. DTH hammers are used in underground benching operations. In DTH hammers, the rock drilling bit is a continuation of the shank, which the rock drill piston strikes directly. DTH machines are driven by compressed air and require a fairly large compressor to operate effectively. Since the piston is in almost direct contact with the drill bit, little energy is lost. This gives a nearly constant penetration rate regardless of hole length. Hole accuracy is also good. DTH machines are limited by their relatively low penetration rates and poor mobility, because they require a large separate compressor. Rotation is usually hydraulic. Energy consumption is also large compared to top-hammer drills.The hole sizes most commonly used for underground DTH drilling are 89 - 165 mm in diameter, and can extend up to 1,100 mm. Hole lengths in underground benching operations vary up to 60 meters.(FIGURE 3.3.-1.)</p> <p>FIGURE 3.2.-6. Modular design of modern rock drill.</p> <p>PNEUMATIC DRILLINGPneumatic rock drills and mechanized pneumatic jumbos were most popular during the 1960s and early 1970s. The power source for the pneumatic rock drill is existing compressed air lines or a portable compressor.</p> <p>COMPARISON BETWEEN HYDRAULIC AND PNEUMATIC DRILLINGTable 3.2.-2. Comparison between hydraulic and pneumatic drilling. Hydraulic drilling Efficient 50% higher drilling capacity Reliable, constantly high efficiency level Easily adjustable to changing rock and drilling conditions, smoother drilling Ergonomic Less noise, moisture, mist, no surrounding temperature fluctuation Economical High capacity, independent, minimal labo...</p>


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