Recent Blog's


09 Aug 2015

Engineering blasting operations

POSTED BY - Gulshan Kumar


1.1 INTRODUCTION

Rock is blasted either to break it into smaller pieces such as in most mining and quarrying operations or large blocks for dimensional stone mining and some civil engineering applications, or to create space. The conditions under which blasting is carried out also affect the operations and the results. Precise engineering of operations are needed to achieve the desired objectives. The engineering of blasting operations needs clearly defined objectives, materials, skilled techniques, the necessary theoretical background of the process of rock fragmentation and effect of rock conditions and experience in combining them.

 

1.2 BLASTING OBJECTIVES

In mining and quarrying, the main objective is to extract the largest possible quantity at minimum cost. The material may include ore, coal, aggregates for construction and also the waste rock required to remove the above useful material. The blasting operations must be carried out to provide quantity and quality requirements of production in such a way that overall profits of mining or quarrying operation are maximised. In-situ rock is reduced in size by blasting and crushing into the required size or with additional grinding, into a finer powder suitable for mineral processing. Large blocks needing secondary breakage or an excess of fines, can result from poorly designed blasts or due to adverse geological conditions. A well designed should produce shapes and sizes that can be accommodated by the available loading and hauling equipment crushing plant with little or no need for secondary breakage. While optimising the fragmentation, it is also important, for safety and ease of loading, to control the throw and scatter of fragments. However, other times controlled displacement is provided, as in the case of casting of overburden or explosive mining, where part of the overburden is thrown to such a distance that it need not be handled again.

In civil engineering, rock is removed to create tunnels or caverns, or deep excavations at the ground surface for road cuts, foundations, or basements. The emphasis is not on high rates of production, although the job must be done as quickly and as cheaply as possible, but on creating space and leaving behind stable rock walls that are either self-supporting or require little reinforcement and lining. Requirements smooth walls and long-term stability exist also in mine shafts, crusher stations and to a lesser extent, in mine development drifts that must remain open for moderate periods. Problems in the blasting of civil engineering works are most often associated with overbreak and underbreak. Special blasting techniques are applied in carrying out controlled blasting to produce smooth walled excavations. Often adjustments to conventional procedures needed.

In some civil engineering operations large blocks are needed for many operations such as dam construction and water construction. To meet these requirements techniques and explosive materials used are modified as practiced for conventional blasting. Underwater blasting is carried out to deepen harbours or river ways which also calls for specialised blasting operations. Similarly there are many other specialised operations like sting in fire zones, and blasting of ice, both of which need materials and techniques different from the conventional blasting operations.

The size of blasting operations varies greatly from those needing a small charge to those requiring several hundreds of kilograms of explosives and also from those involving a few cubic meter rock to millions of cubic meter of rock. The equipment to charge holes is available for various sizes and types of explosives and initiating systems. For surface blasting operations such as quarries, foundations, trenches, the environment needs to be protected. Blast damage due to vibrations and noise needs to be avoided by careful blasting. Controlled blasting is often used to limit the projection of flyrock. For underground blasting operations care needs to be taken so that fumes and dust produced is minimal to protect persons working underground, Care needs to be taken of extraneous hazards while handling and during storing and also while carrying out blasting operations to prevent accidental explosions, Mining and quarrying operations have production as a primary goal, but precautions must be also taken to avoid damaging the rock left behind, at least to an extent sufficient to preserve safe working conditions while the mine remains in production. In terms of mining economics, the optimum excavating method is one that maximises production and safety and minimises dilution, excavation costs and environmental costs. Long-term stability needs to be preserved, to avoid unnecessary subsidence or if the space is to be converted to other uses

 

1.3 THE PROCESS OF FRAGMENTATION

The processes involved in rock fragmentation by blasting have been the subject of large amount of experimentation and studies. However, much of this is qualitative or semi-qualitative in nature. The difficulty is that at the time of fracture, fragmentation and displacement of rock, persons or sensors may not be close to the point of action.

As a consequence, the results of various studies are useful but they do not represent definitive scientific results and provide only for adopting techniques and materials for engineering blasts. Nevertheless, without basic theoretical considerations and understanding of the phenomena involved precise blasting results are difficult to achieve.

When an explosive detonates in a hole the pressures can exceed 10 GPa (100.000 atmosphere), sufficient to shatter the rock near the hole, and also generate a stress wave that travels outward at a velocity 3000-5000 m/s. The leading front of the stress wave is compressive, but it closely followed by the tensile stresses that are mainly responsible for rock fragmentation. A compressive wave reflects when it reaches a nearby exposed rock surface, and on reflection, becomes a tensile strain pulse. Rock breaks much more easily in tension than in compression, and backward from the free surface. The gas pressures generated during the process also act to widen and extend stress-generated cracks or process which takes place is the combined effect of the above two, the role is dependent on the rock conditions, blasting geometry, explosive mate and initiation systems. The relative role of stress waves and gas pressure is not fully understood and hence an accepted process of rock fragmentation by blasting lacking.

 

Much of the effort in recent years has been on understanding the vibration effect of blasting, determining damage criteria, and developing techniques to reduce the dam to protect the structures near the blasting site. Some efforts have also been made to understand the damage to the remaining rock and flyrock accidents with the aim of reducing these impacts.

The theoretical understanding has also developed from practising blasting personnel which allows one to adopt modifications in blasting parameters depending on rock and geological conditions as well as the desired results. Based on these, many empirical design calculations have been developed culminating in the use of computerised blasting design calculations for open pit blasting, tunnelling and stope blasting operations. Use of high speed photography and vibration monitoring equipment has been extensive as well as the monitoring of explosive and initiating system behavior has been developed

 

1.4 BLASTING MATERIALS

For carrying out blasting operations, explosives (in cartridged shape or free flowing form) initiating devices for these explosives, loading systems and techniques are needed. The use of black powder known to be used in the 13th Century has continued till now. NG (Nitroglycerine based explosives developed a century ago dominated the usage till about 30 years ago. Recent years have seen developments with the discovery of new generation of explosives which offer greater flexibility in their range and application, and are safer in usage. With the increased popularity of new generation of explosives (like ANFO, slurries, emulsions) the demand for NG based explosives has declined. ANFO, consists an oxidiser and a fuel. Ammonium Nitrate in prilled form (coated with an inert absorbent material, or treated with a surfactant to promote thorough mixing), sensitised with fuel oil s the least expensive, and when suitably employed performs as well as NG based explosives and is safer to handle and use. ANFO is supplied either in bulk or in waterproof polyethylene bags.

The separate components, delivered for bulk mixing on site, are not classified as explosives and can be shipped without incurring the extra costs of transportation and storage precautions. However, it is susceptible to absorbing water, and therefore can be employed in wet conditions. In addition, the density of material is low, therefore, may not provide enough energy per unit rock as compared to similar other blasting agents.

To overcome these disadvantages and still retain the cheapness, performance and safety of ANFO specially designed based slurry and explosives have become widely used A combustible fuel is mixed with granular and a thickener and with just enough aqueous AN dispersed with a sensitizer mixture containing up to 20% water. The solution of AN to give a semifluid such as or even TNT Thick combustible fuel component can be material emulsifier Sensitiseners include starch, water soluble vegetable or oil with an ers include pigment bubbles, soluble organic nitrates. The mixture can be sold in final thickened form, or a gelling agent is added before or during loading so that it forms a thick gel after charging the slurries can be used in watery conditions. Recently the use of emulsified ANFO has increased. Sold as Heavy ANFO, this retains advantages of ANFO and increases their energy density.

Specially formulated explosives are used for blasting operations such as smooth blasting, dimensional stone blasting or for hot material blasting the necessary impetus. Initiation of blasts needs devices which provide place. There are basically two such as flame or spark to the explosive at the desired time and basically two systems -electrical and non-electrical and safety fuses. The safety fuse

About 100 years ago it was the use of detonators slowly contains of black powder. Hit with hot the powder bums or standard rate (120 sm). At the end of safety fuse is a blasting detonator blasting cap inserted in a cartridge of high explosive. This detonator provides a powerful to initiate detonation in a commercial explosive. Later on electric detonators initiating of primary explosive and a charge of secondary explosive, but they also contain wire and an ignition charge that ignites when electric current is passed through the wire. Port or power is utilised to provide an electric impulse. Later on to achieve the same objective, detonating cord of explosives a plastic sheath and protective wrapping started to be used and is most often attached to a primer or booster, which in turn initiates the charge. Unlike the safety fuse it has a high velocity of detonation (6 km/s), and is initiated by a plain or electric detonators. It is used for mass initiation of large blasts.

The detonating cord is much safer to handle than a detonator, is extremely water resistant and is comparatively safer. There are some problems, like discontinuity in the cord and others. To overcome these problems and that of electric blasting, use is made of other non-electric blasting systems which utilise about 1-2 ghm of initiating explosive or use hollow tube coated with initiating agents to provide initiation. These overcome many problems of electric detonation and the detonating cord, and also some of advantages of both the systems like delay and bottom hole initiation.

Delay blasting by use of delay electric detonators or the non-electric delay system, commonly used tends to give lower muckpile and longer throw, increased fragmentation, reduced vibration and reduced overbreak and back break. Electric delay detonators give a controlled time gap between pressing the plunger and initiating the charge. A large array of charges can be fired and in a controlled sequence. Electric delay detonators are manufactured with nominal half-second time intervals and also 8 to 100ms.

Delay firing can also be achieved using non electric millisecond delay into a detonating cord trunk line. These are copper tubes about 75 mm long with an explosive charge at by delay needed checking electric circuits, locating extraneous hazards and for current leakages, etc. Another major Requirement cable is the modem of high speed of explosives. High speed cartridge loaders are used for long holes and for achieving higher loading density per meter of holes.

 

 1.5 BLASTING TECHNIQUES

Rock is in general blasted towards a free face. Bench blasting is often carried out in surface operations and even in large underground tunnels, caverns or stopes, when a bench face does not exist, a release cut (simply called cut) is made by drilling, cratering, or cutting. In bench blasting several holes are drilled in a pattern, loaded with explosives and initiating devices and then fired in a particular initiation sequence (see Chapter 12). Several variables are involved in bench blasting with respect to holes diameter, depth & location, explosives and loading, initiation sequence. By making adjustments, the blasting engineer can obtain desired results in terms of fragmentation, reduced damage to the remaining rock walls, reduced costs, reduced vibration and airblast and throw, Small diameter holes (32-35 mm) are used for very small operations whereas for larger operations hole diameters range from 100 mm to 400 mm. Bench blasting operations are usually accomplished by parallel rows of drill holes, detonating first the row nearest to the exposed face to give better release for successive rounds (see Chapter 9). Small diameter holes are generally loaded with explosive cartridges and larger holes are loaded with ANFO or other blasting agents. Many parameters which characterise surface blasting are related to burden. The burden is the distance between the exposed rock face and the nearest line of blastholes. The burden is kept about 30 times the hole diameter with averages between 20 and 40. The sub drilling is kept about 0.3 of the burden, except that no subgrade drilling is needed when joints run parallel to the floor or when blasting on coal benches. The length of stemming averages 0.7 times the burden and ranges from about 0.5 to 10 of the burden. Spacing is kept between 1 and 2 times the burden. These parameters need to be oriented and adjusted between the exposed rock face and the nearest line of blast holes.

Another important parameter is the sequence of firing, which includes the number of blast holes detonated in any one round and the time delay between successive rounds, The sequence can be varied using delay charges, to minimise vibration levels and unwanted rock damage and to give a more efficient pattem of rock removal.

In engineering projects, which are often in densely populated areas, special attention has to be given to prevent damage to nearby buildings and services, and to safeguard the public (see Chapter 18). Thus blast coverings are placed over the area being blasted to limit the throw of the flyrock.

In engineering projects road cuts, trenches and foundation blasting are carried out. In these, special attention given to designs to the rock forming cuts, blasting is made difficult by height of of more than blasted in more and are two methods, with overburden still in place or with it removed. The trench is advanced by wards a free face. For narrow trenches say up 600 mm wide, a single row of may suffice, but better results are usually attained by staggered or paired holes 300 600 mm of and deeper trenches the holes are drilled in rows and tend to be larger in diameter, of referred as blasting or explosive mining) not only breaks rock but also dam and earth either handling of overburden (Chapter 16) to build a rock a

Underground blasting needs release of rock within the confines of underground rock excavations. The quantity of explosive required unit excavated because of the confinement greater than in the case of open cut excavations. When blasting a rock the holes to be detonated (those with shortest create a cut, an opening forward successfully blasted. Subsequent delays blast the rock into which the rest of rock is the cut in a pattern of rings of increasing diameter until they reach the perimeter, which is outside the line of holes, often with reduced charge density. There are two main types of cut firstly an angle cut, in which the blast holes meet each other; and secondly a parallel cut, in which the holes are drilled parallel to each other in the direction of advance. Nowadays, for larger openings parallel h drilling is often used in which large empty holes are drilled by jumbo. Typically these have diameters of 57, 76 or 125 mm. The smaller charged holes have diameters in the range of 30-50 mm and are loaded with a charge concentration ranging between 0.25 and 0.55 kg per meter. Tunnels and mine drifts of a diameter smaller than about 8 m and larger headings in good quality rock are excavated full face by drilling and blasting the entire face in a single round. As rock conditions deteriorate the heading becomes larger, an upper section (top heading) is often removed first, followed by supporting the roof and then removal of the remaining bench. The length of advance per round is limited by the quality of rock and by the diameter of the excavation. Pull varies between 0.5m in very weak ground to as much as 3 m in massive and self-supporting rock.

 

In shaft sinking two alternatives for blasting are available, benching and full face. In benching the two halves of the shaft bottom are blasted alternatively. In the full face method angle cuts are most often employed. The average powder factor for a 30 by 4.5 m shaft is 3.25 kg of explosive per cubic meter of rock, varying from about 2 to 6 kg according to shaft size and rock strength.

 

Raises can be driven blind or can follow a pilot hole. Holes are drilled upwards in a regular pattern, most often in pattern. Use of the vertical crater retreat method, when holes are drilled downwards and blasting is in slices, has made raising a much easier operation in recent years.

In stope blasting, depending on the method of mining employed, either short hole blasting or long hole blasting is employed. In traditional short-hole blasting, one the three standard types of rounds is employed overhand stopping or underhand benching. Hand held drills are commonly employed, although jumbo drilling be used to increase production where there is sufficient headroom, particularly in and pillar mining operations. Benching and crater retreat methods of mining make use of long holes of large-diameter (50-200 mm). The production blast is detonated one or two rows at a time, using either full column loading. Decking or bench slicing in which only the bottom of the slice is loaded.

 

1.6 CONTROL OF BLASTING RESULTS

In terms of mine and quarry blasting the optimum result is one that maximises production, fragmentation and safety, minimises dilution and excavation costs and environmental impact. Blasting techniques and designs in mines are optimised over months or years, till blasters gain knowledge of the rock conditions. Even though aided designs (see Chapter 17) are available, their adaptation to actual field conditions is necessary. Control often needed for obtaining fragment distribution, reduction of vibration, airblast and flyrock damage and damage to the remaining rock.

In rock excavation for civil engineering projects and in quarry blast vibrations must be limited to minimise environmental impact damage to nearby structures, and damage to the rock walls of the perimeter. In levels of vibrations can damage the open pit slopes or underground pillar and lead to subsequent problems of safety and subsequent recovery of ore from the blast affected areas. In stopes, damage to the hanging wall can lead to slabbing, high dilution, and need for secondary blasting to relieve ore passes that become blocked by large fragments of ore and waste rock.

Empirical predictions of blast vibration are often in error. This leads to a requirement for vibration monitoring and subsequent blast designs. Seismographs can be used to record the vibrations generated by blasting. Vibration levels can be reduced by limiting the charge weight per delay to an amount sufficient to achieve the required degree of fragmentation. Permissible vibration levels are specified in relation to the levels that can be tolerated by the wall rock, by different types of structures or by people. The most common criterion for prevention of structural damage at the surface is that the peak particle velocity should not exceed 50 mm/s. In another practice structures are categorised and criteria is altered according structure. The frequency spectrum of the transmitted vibration also plays a role to which blasts in determining cause the most complaints. Frequencies in the range of 5-20 Hz are apparently most annoying.

Air transmitted vibrations (called airblast) to be kept within certain limits. Damage can occur, during above-ground demolition, during stemmed blasting of tunnels and shafts, and where large quantities of detonating cord are exposed at the surface, Airblast over pressures greater than 0,7 kPa will almost certainly break all Even in the absence of damage, complaints and legal actions resulting from annoying levels of noise and vibrations close operations down.

 

Damage to the remaining rock occurs when fracturing, including crushing and radial cracking of rock, around the blasthole takes place. It is caused by excessive explosion pressures, excessive burden, inadequate time between rows in multirow or orientation of the blasting row relative to the jointing, or principal stress direction. Also rock is removed specified perimeter. It can result from overblasting, from inaccurate blasthole drilling, or from the action of gravity combined with geological conditions. Remaining the removal of rock beyond the line of blastholes. Underbreak is rock within a specified excavation perimeter that should have been removed by blast. Blasthole in most production blasting applications are in the range times the compressive strength.

Blasthole pressures and consequently wall damage reduced either by reducing the density of the example, by adding an filler to ANFO, which produces fracture radius one half to one-quarter of that of a dynamite), by reducing the charge diameter in the blasthole (decoupling), or by air decking. Special methods for producing smooth surfaces are some of the controlled blast techniques. These include line drilling, pre-splitting, and smooth and cushion blasting.

Blasting operations need to be carried out with the utmost safety of man, machine and environment. Though much safer explosives and accessories have become available, the explosives still need to be treated with the utmost respect while storing, transporting and handling them. Adequate safety precautions are essential to carryout blasting operations without accidents.


Blog Search

Blog Categories