WALL CONTROL BLASTING TECHNIQUES TO MINIMIZE DAMAGE TO THE ROCK AT THE LIMITS OF SURFACE AND UNDERGROUND EXCAVATION, IN ORDER TO ENHANCE SAFETY STANDARD AND ECONOMY *** Author: Partha Das Sharma, B.Tech(Hons.) in Mining Engineering, E.mail:
[email protected], Blog/Website: http://miningandblasting.wordpress.com/ ABSTRACT Wall failures are costly and often life threatening. The goal of efficient wall control blasting is to make transition from a well fragmented rock mass to an undamaged slope in as short a distance as possible. This can be quite challenging due to the many factors that influence wall damage. To develop efficient designs one must have a basic understanding of wall failure mechanisms as well as limitations of wall control procedures. In addition, it is imperative, design be precisely implemented, evaluated and refined on a continuous basis. The release of energy during blasting produces reactive forces, which cause the deterioration of the remaining rock face. Pre-splitting and trim blasting are the key techniques adopted to protect final rock faces. However, even these well known techniques are applied; slope failures and back damage may persist. The key parameters within the control of the blasting engineers are type and energy in the hole, drilling pattern, hole depth, hole diameter, hole angle, bench geometry and blast timing. An understanding of mechanisms of all the aspects is needed for good designing for blast for wall control and slope stability.
1. INTRODUCTION: Wall control blasting is the technique used to obtain a pit wall, free of backbreak and loose rock that will stand safely at the required wall angle for extended periods of time. Direct damage to the excavation limit due to blasting is usually found in the form of backbreak or overbreak, crest fracture and loose rock on the face. The mine operator has a number of tools available for minimizing or eliminating these problems. Techniques include changing the explosive type, or changing the blasthole diameter, by decoupling the explosive, by decking, and by changing the burden and spacing. Changing the depth of subgrade drilling or the stemming height can reduce crest fracture and any resultant narrowing of the width of safety benches. Changing the millisecond delay timing and the rotation of the round may also be helpful in eliminating these problems. The rock characteristics and geology must be considered when designing controlled blasts as these have an important influence on the final results. The compressive strength, crushing strength and tensile strength of the rock should be known. The frequency and orientation of joints and fractures in the rock are also important parameters. These variables cannot be controlled but must be determined by suitable field and laboratory techniques.
Author: Partha Das Sharma, (B.Tech-Hons., Mining Engg.), E.mail:
[email protected], Website: http://miningandblasting.wordpress.com/
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WALL CONTROL BLASTING TECHNIQUES ----------------------------------------------------In other words, wall control blasting techniques have been used in surface and underground blasting in the mining, quarrying and construction industries for many years. The specific reasons for the use of controlled blasting techniques may vary according to the industry and project; however, two generally applicable reasons can be identified: a. To insure that the rock is broken to the excavation limit but not beyond to keep the host rock intact. b. To insure the subsequent safety of personnel and equipment, working under / side of the wall, by avoiding back-break and loose rock on the face. In open pit operations breakage beyond the pit limit is costly. Excessive back-break at the perimeter generally results in an overall pit wall angle less than designed, and may result in the need for costly artificial support techniques. In fact, failure to properly control blasting at the final pit wall can cost a large open pit mine many millions of dollars expenses in additional waste removal for the same ore mined. There are four principal controlled blasting techniques used, which are: • Pre-splitting • Cushion blasting • Buffer blasting • Line drilling Pre-splitting is the most commonly used technique especially in surface work. This is followed by cushion blasting, also known as trim blasting in open pits. Smooth blasting, used underground, is similar to cushion blasting. Pre-splitting provides a preferential fracture plane behind the blast to terminate cracks growing from blast holes, while trim blasting reduces rate of energy release against the final wall. Buffer blasting may be used alone in cases where the rock is quite competent, but this is not a common approach. However, a properly designed buffer row at the back of the final production shot is essential to the success of most pre-splitting and cushion blasting applications. Line drilling involves the drilling of closely spaced small diameter holes at the perimeter of the excavation. These holes are not loaded with explosive, but form a discontinuity at the excavation limit. This method is costly because of the many boreholes drilled and is therefore only seen in blasting for civil works projects, where back-break can be a very expensive result. Modified forms of line drilling may be used in mining and quarrying in special circumstances. Geology can have pronounced effects on the results of controlled blasting / wall control blasts. For example, it is known that trim blasting does not work well in the presence of relatively shallow dipping joint planes dipping into the excavation. It may not always be possible to obtain the classic result when adverse geology is encountered. However, if back-break, crest fracture and face loose rock have been minimized, then the result will be far more acceptable than a wall in the same rock where no controlled blasting has been performed.
Author: Partha Das Sharma, (B.Tech-Hons., Mining Engg.), E.mail:
[email protected], Website: http://miningandblasting.wordpress.com/
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WALL CONTROL BLASTING TECHNIQUES ----------------------------------------------------Underground, over-break in the stope results in costly ore dilution. Poor breakage control at the perimeter of drifts and shafts means more scaling of the walls and roof and more difficulty installing support and facilities. In construction blasting breakage beyond the designed limits may lead to the removal of many tons of rock not specified in the contract. Added scaling and support may be needed for the long term stability of the wall. The consumption of concrete and other construction items may well increase. All of this is expensive. Equally important as cost, in every industry, is the need to provide a safe working environment. Pit and quarry walls that have sustained substantial back-break are prone to hazardous rock falls. Safety benches, intended to arrest the fall of loose material will typically be narrow and ineffective. Drifts and stopes experiencing excessive over-break will be more prone to hazardous rock falls. Similar hazards will also exist in construction work as well. Therefore, any organization that emphasizes safety will want to control blasting at the limits of an excavation. Thus, wall control blasting techniques are the system of controlled blasting which refers to various techniques used to minimize damage to the rock at the limits of an excavation due to the action of the ground shock wave and the high pressure explosion gases, generated during the blast. 2. GENERAL PRINCIPLES OF WALL CONTROL BLASTING TECHNIQUES: a. Controlling energy input given by explosive and the borehole pressures exerted - A fundamental goal of all wall control blasting is to reduce the energy input and the borehole pressures at the perimeter of the excavation. The borehole pressures generated by commercial explosives, which are fully coupled to the hole, are much greater than the rock strength and will cause extensive damage around the blasthole. Therefore, these pressures must be reduced. The borehole pressure for a fully coupled hole can often be obtained from the manufacturer of the product being considered for use. CALCULATION OF BOREHOLE PRESSURE: Borehole pressure can also be calculated using the following formula given. Generally, borehole pressure is function of VOD of explosives used. (Pb)c = 2.5 x 10-6 x ρ x V2 ; where, ‘(Pb)c’ is borehole pressure in kilobar, when fully coupled explosive used, ‘ρ’ is density of explosives and ‘V’ is Velocity of Detonation (VOD) of explosives in m/s. While the above equation may not yield exact results it has proven quite adequate for practical design requirements. However, the equation has some limitation in the case of aluminized explosives. The velocity of detonation is reduced because the initial reactions of the oxidizer with aluminium are endothermic. However, beyond the detonation zone the equilibrium shifts to the very rapid formation of exothermic reaction products. Therefore, the actual borehole pressure will be considerably higher than that calculated from the detonation velocity. Low density explosives produce low borehole pressures because the detonation velocity is reduced.
Author: Partha Das Sharma, (B.Tech-Hons., Mining Engg.), E.mail:
[email protected], Website: http://miningandblasting.wordpress.com/
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WALL CONTROL BLASTING TECHNIQUES ----------------------------------------------------Decoupling and decking are the techniques basically used for reduction of borehole pressure. A primary means of reducing the borehole pressure is to decouple the charge from the hole. This means that the diameter of the explosive charge is less than the diameter of the hole. Pressure may be further reduced by decking, whereby gasbags, wooden or cardboard spacers etc., are used between charges or the charges are taped to detonating cord with a gap left between individual cartridges. For a given hole diameter and explosive the usual approach is to decouple radially first. If this is insufficient to reduce the borehole pressure enough, additionally decks can be employed. When a charge is decoupled from the blasthole the explosion gases expand to fill the hole volume before exerting borehole pressure. Therefore the decoupled borehole pressure will be much less than the coupled value.
CALCULATION OF COUPLING RATIO AND DECOUPLED PRESSURE: Coupling Ratio (CR) can be expressed by: CR = (C)1/2 x ( dc/dh), where ‘C’ = the percent of explosive column actually loaded, ‘dc’ = charge diameter and ‘dh’ = hole diameter. Decoupled pressure may be calculated from the following formula: (Pb)dc = (Pb)c x (CR)2.4 where ‘(Pb)dc’ = The borehole pressure for a decoupled and/or decked charge, ‘(Pb )c’ = The borehole pressure for a fully coupled charge and ‘CR’ = Coupling ratio. In using these equations it is necessary to have an idea of what an acceptable decoupled borehole pressure will be. In pre-splitting it has been found that the pressure should be in the range of 2 to 5 times the uniaxial compressive strength. In larger hole diameters it is often better to set the decoupled borehole pressure near to the uniaxial compressive strength of the rock because of the greater radius of rupture that may result around larger diameter boreholes, when the borehole pressure exceeds the compressive strength of the rock. This potential for large rupture radius around the borehole can lead to a wall more prone to unravel over time.
Discussion on Borehole pressure on type of controlled blasting - In some presplitting applications a concentrated charge is used in or near the bottom of the hole with the remainder of the borehole left void. Upon detonation the explosion gases are free to expand up the hole and exert a suitable decoupled pressure on the surrounding rock. This method has been used extensively in active highwall pre-splitting when blast casting in dragline mines. It has also been used in other types of Author: Partha Das Sharma, (B.Tech-Hons., Mining Engg.), E.mail:
[email protected], Website: http://miningandblasting.wordpress.com/
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WALL CONTROL BLASTING TECHNIQUES ----------------------------------------------------mining, generally being most successful if the ground is reasonably competent thereby avoiding damage at the bottom of the hole and excessive leakage of gases as these expand up the borehole. b. The buffer row - Occasionally buffer blasting alone may be sufficient to protect a final excavation limit from damage. However, when pre-splitting or cushion blasting the last row of the final production blast must be a buffer row. The exceptions to this rule would be when active highwall pre-splitting for a dragline operation or in small diameter work underground where a buffer row is not always used. The buffer row must be designed with a sufficient charge to break the rock between the buffer hole and the final wall. However, the explosive consumption in the buffer row should not be so great as to cause breakage beyond the plane of the final wall or the controlled blasting effort would have been wasted. Often, when damage is observed beyond the final wall limit the problem is the buffer row design rather than the presplit or trim row. As buffer row is designed with less explosive in the hole than is in production blasting boreholes; because the explosive is kept low, in the hole, with a greater length of stemming above, there is less potential for crest fracture and face loose rock. The toe between the buffer hole and the excavation limit can still be adequately broken. The low centre of gravity of the charge in the buffer hole causes it to behave like a spherical charge, for which cube root scaling applies. In a buffer row a scaled depth of burial (SDOB) of about 1.5 times the optimum scaled depth of burial for the given explosive in the given rock type should be used. The scaled depth of burial is simply the depth from the surface to the centre of the charge column divided by the cube root of the total explosive weight in the column. Ideally the charge should have a length not exceeding 8 times the diameter of the borehole. If, because of the hole depth or diameter, the charge length exceeds 8 times the diameter the calculation should be performed using the depth to the centre of a charge column equal in length to 8 times the diameter and located at the top of the charge. c. Effect of water on a decoupled explosive charge - When a decoupled charge is surrounded by water the pressure generated by the detonating explosive, at the borehole wall, will be considerably higher than would be the case if the explosion gases were free to expand across an air filled gap. The degree of decoupling achieved will be much less than that calculated assuming the charge is surrounded by air. In fact because water is quite incompressible the pressure transferred to the borehole wall may be quite similar to that of a fully coupled hole. The explosive charge will need to be decoupled to a greater extent than normal. If the area can be dewatered prior to final wall blasting this will be the best solution. When height of water column is above the concentrated presplit charge at the bottom of a large diameter hole, another problem can develop. The water column tends to behave as stemming and the explosion gases are inhibited from freely expanding up the hole. There will be more damage around the bottom of the hole. The presplit crack may not extend the full length of the borehole. These holes will work best if pumped before explosive loading. They should be loaded and fired promptly to minimize the water column that forms above the explosive charge.
Author: Partha Das Sharma, (B.Tech-Hons., Mining Engg.), E.mail:
[email protected], Website: http://miningandblasting.wordpress.com/
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WALL CONTROL BLASTING TECHNIQUES ----------------------------------------------------3. INFLUENCE OF SITE AND STRATA CONDITIONS: The properties of the rock and the site geology are of significant importance when designing a controlled blast. If these factors are ignored the results will be, at best, a hit and miss affair. Serious backbreak, crest fracture, face loose rock or sliding of weak portions of the wall are all possible outcomes. It is also important to recognize that in complex geological settings it may not be possible to achieve the classic result. However, even though the half-casts of all the holes are not visible on the face the controlled blast will still have been successful if a safe, stable wall has been achieved at an economical cost. a. Important rock properties to be considered - The most important rock properties are the tensile strength, compressive strength and crushing strength. Also very important are the nature, frequency and orientation of joints and fractures, the rock density, longitudinal wave velocity and Young's Modulus. Ideally these properties should be measured in-situ. In-situ values reflect the effects of weathering and structural features in the rock. A rock which tests as quite strong in the laboratory may be considerably weaker when weathering, groundwater alteration, presence of structures such as open joints, bedding or foliation planes and fractures due to previous blasting are accounted for. However, at this time methods for measuring rock properties in-situ are not very satisfactory and are usually costly. Therefore, laboratory tests are generally relied on. Laboratory data can be adjusted by a site factor to account for in-situ conditions. Deciding what the site factor should be is not a simple task and will be an approximation. Most practical is to design the controlled blast based on the laboratory results and observe the results in the field. Then the design can be adjusted to account for any problems until an optimum result is obtained. It may then be possible to back calculate the in-situ uniaxial compressive strength and tensile strength. Backbreak and radial crushing around the borehole result when the stress produced in the rock by the explosion exceeds the crushing strength of the rock. The crushing strength is typically two to five times the uniaxial compressive strength. Major backbreak problems are likely if an explosive loading that was successful in competent ground is subsequently used in highly jointed or fractured ground, even though the rock type is the same. Therefore, powder factors and decoupled borehole pressures must be adjusted to account for structural conditions and the actual crushing strength of the rock surrounding the hole. Another point one has to observe is the joints orientation. The orientation of the joints has a major influence on the controlled blast results. When joints or fractures strike parallel to the excavation face a smooth clear wall may be obtained. When the joints are steeply dipping (>70°) the wall can be made to conform to the joint planes. When the joints are shallow dipping it is undesirable to cause the wall angle to conform to these planes. There is greater chance that planes will undercut the face. When this occurs it is more difficult to obtain a classic result because there is a greater likelihood that portions of the wall will Author: Partha Das Sharma, (B.Tech-Hons., Mining Engg.), E.mail:
[email protected], Website: http://miningandblasting.wordpress.com/
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WALL CONTROL BLASTING TECHNIQUES ----------------------------------------------------slide off along these structured planes. Large diameter cushion blasting has been found unsuited to these conditions. Presplitting may be more successful if great care is taken to design the presplit and buffer rows to minimize the disruption experienced on the joint planes. It takes relatively little movement along the plane to destroy cohesion resistance and cause the material resting on the joint to be more prone to slide. When steeply dipping joints dip back into the wall while striking parallel to the face, sliding on undercut planes is not possible. However, toppling failures may occur. In the presence of these features the final wall should not be vertical. An angle of 70 to 80 degrees is more suitable. A toe buttressing effect is provided and the wall is far more likely to remain safe and in good condition for the long term. When structural features strike at angles other than parallel to the face the amount of backbreak depends on the nature of the joints and fractures and their strike. Open joints are likely to break back more than tight, infilled joints. Planes striking at 45 degrees to the face are likely to break back further than near vertical joints striking at 90°. Again, the frequency of jointing is important. Jointing begins to interfere with wall control results when the joint spacing is less than the hole spacing. In pre-splitting the hole spacing should not exceed twice the major joint spacing. Frequent jointing can lead to greater crest fracture. The explosive collar height must be increased or the upper column load reduced. When the stress due to the reflected ground shock wave at the free face, near to a blast, exceeds the rock tensile strength slabbing can occur. If joints, bedding planes or foliations exist, striking parallel to the face, the potential for slabbing is greatly increased. Slabbing is especially a hazard when blasting near to tunnels or when blasting in a pit that is in close proximity to the walls of another pit. Reduced explosive loading may be necessary for better result. Where rock breakage is not desired, as in the case at the final excavation limit, rock properties that relate to the in-situ rock strength are important. The Young's Modulus of Elasticity is a measure of the brittleness of a rock and its susceptibility to backbreak. Rock with a high Young's Modulus has a higher crushing strength and is harder to break. Higher borehole pressures may be permissible at the perimeter. Rocks with a higher longitudinal wave velocity are also usually found to be stronger. Weaker rock or strata that have been weakened by weathering, alteration or fracturing due to dense jointing or previous blasting exhibits a lower longitudinal wave velocity. This fact leads to the seismic techniques for determining overburden depth, depth of broken rock, radius of rupture, jointing and density. As an in-situ method these techniques may be particularly valuable for determining the nature of the in-place rock. 4. WALL CONTROL PRACTICES IN SURFACE OPERATIONS: a. Explanation of various methods – (i) Buffer Blasting - This is perhaps, the simplest form of wall control shooting. The last row of the production blasting pattern is altered to limit the energy input at the final wall. The explosive loading Author: Partha Das Sharma, (B.Tech-Hons., Mining Engg.), E.mail:
[email protected], Website: http://miningandblasting.wordpress.com/
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WALL CONTROL BLASTING TECHNIQUES ----------------------------------------------------is reduced and as a consequence the burden and spacing are also decreased. As described above, explosive loading is often reduced by selecting a scaled depth of burial greater than would normally be used. Another approach is to use decoupled bagged powder above a toe load of fully coupled explosive.
Buffer blasting can only be used as the sole controlled blasting technique when the ground is quite competent. Some minor backbreak or crest fracture may develop but this will be much less than would be caused by the production blast holes. Where buffer blasting can be used alone the cost of wall control will be quite economical. In most cases buffer blasting is used in conjunction with another wall control blasting technique. A properly designed buffer row is very important to most successful presplit or trim blasts. Design of the buffer row is the same as when the technique is used alone. It becomes important to insure that the buffer row is at the correct location relative to the presplit or trim row. Typical design for the buffer row includes using a scaled depth of burial at the top of the charge of 1.5 times the production hole value and reducing the powder factor to 0.5 - 0.8 times the production row powder factor. Burdens range from 0.5 to 0.75 times the production burden. The spacing should not be less than the burden and will usually be 1.0 to 1.25 times the buffer row burden. Author: Partha Das Sharma, (B.Tech-Hons., Mining Engg.), E.mail:
[email protected], Website: http://miningandblasting.wordpress.com/
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WALL CONTROL BLASTING TECHNIQUES ----------------------------------------------------To avoid backbreak and crest fracture the buffer row holes must be properly located in front of the intended plane of the final wall or the presplit line. This distance must be sufficiently large to insure that the stress at the final wall is adequately attenuated to avoid crushing beyond the plane of the wall. Following figure shows as an example, how the stress generated by detonating buffer row holes attenuates with distance from the blasthole.
It has been observed that, in quite soft rock, such as coal mine overburden, spacing the buffer row 10 feet (about 3.5 m) or more in front of the presplit line may indeed be prudent. In hard rock the spacing at the toe needs to be much less to break the rock between the buffer row and the presplit line. However, breakage beyond the presplit can be avoided. Moreover, to avoid crest fracture in competent rock, drilling the presplit holes on an angle is advantageous. One can space the presplit and buffer hole closely at the toe for breakage while obtaining a greater standoff at the crest. In hard rock it has been found that the toe of the buffer row should be 3 to 5 feet (1 to 1.5 m) from an intended face angled at 80 degrees. In soft rock, such as coal overburden, it has been necessary to move the toe of the buffer row out as much as 15 feet (4.6 m) to keep the zone of crushed material from extending beyond the planned wall location.
Author: Partha Das Sharma, (B.Tech-Hons., Mining Engg.), E.mail:
[email protected], Website: http://miningandblasting.wordpress.com/
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WALL CONTROL BLASTING TECHNIQUES ----------------------------------------------------(ii) Presplitting - Presplitting is the most common controlled blasting technique and has proven successful in applications from large open pit mines to civil construction. This method involves the drilling of closely spaced holes at the planned excavation perimeter which are lightly loaded with explosives in order to generate an appropriate borehole pressure. Presplitting is being done using hole diameters ranging from 2 inches to 12¼ inches. Often, small diameter presplitting is preferred for technical reasons and because the cost per square foot of wall may be lowers. Other mines use large diameter holes in order to employ the same drills for presplitting as for production drilling. This approach has worked especially well in active highwall pre-splitting designs associated with blast casting operations. It has not always been as successful in other types of mining applications. In small diameters (6 inches, 152 mm) hole spacing of 5 to 18 feet (1.5 to 5.5 m) have been employed. As spacing become larger geological structure becomes an increasingly important control on this dimension. •
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Spacing Between Holes - The spacing between the holes is a function of the hole diameter, decoupled borehole pressure and the tensile strength of the rock. The spacing between presplit holes may have to be varied in different areas of the pit if differing rock types exist with different uniaxial compressive strengths and tensile strengths. Therefore characterization of the geology is important. Not only do the rock properties affect the spacing, but the geological structure is also an important control. As a rule of thumb the hole spacing should not exceed twice the spacing between major, open joints. Pre-splitting on an Angle - Observations in open pit mines and quarries has shown that presplitting at an angle less than vertical contributes to a wall that remains in better condition for extended periods of time than one that is presplit vertically. This has been observed in iron mines, coal mines and quarries. Vertical presplit may be appropriate where the rock is particularly competent, or special circumstances preclude an angled wall. Presplit angles typically range between 70 and 80 degrees, with 80 degrees being perhaps the most common. In construction blasting a vertical presplit is likely to be more common. An angled wall may lead to greater construction cost. However, in deep road cuts for example an angled presplit should still be considered. A principal advantage to angle hole pre-splitting results from the toe of the presplit face being moved out from the crest. Therefore, if isolated blocks of rock fall from the face near the toe the entire face is not undercut, as would typically be the case for a vertically presplit wall. Another primary advantage occurs when steeply dipping joints or bedding planes dip back into the wall and strike near parallel to the face. Under these conditions the wall may be subject to toppling failures. The stability of a wall prone to these failures can be enhanced by the toe buttressing effect of an angled presplit wall. The third important advantage to angled presplit holes relates to the relative position of the presplit and buffer rows. When the presplit holes are angled and the buffer row is vertical it is possible to locate the toe of the buffer hole close to the presplit line for good breakage, while maintaining a greater standoff at the crest to avoid excessive crest fracture.
Author: Partha Das Sharma, (B.Tech-Hons., Mining Engg.), E.mail:
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WALL CONTROL BLASTING TECHNIQUES -----------------------------------------------------
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Choosing the Hole Diameter - Current open pit and quarry designs call for multiple benches to be brought back to the final limit between safety benches. In general it is not possible to drill an angled hole flush to the wall using large hole equipment. Small diameter percussive drills, however, can perform this task quite readily by drilling back under the machine. Therefore, these machines are commonly used where the above criteria are to be met. In some cases a larger diameter drill may be used to produce the angle presplit, as in blast casting operations for example, if there is sufficient clearance room for the drill to set up on the holes. Again, the use of small diameter holes is not appropriate if the boreholes are quite deep. The limit is about 50 feet (15.2 m) on hole depth, although 60 feet (18.3 m) is possible in highly competent rock. In heavily fractured ground 40 feet (12.2 m) is likely to be the maximum depth to which small diameter holes can be accurately drilled. Also, in very wet ground small diameter holes are more difficult to drill with the desired degree of accuracy, if the holes are more than 40 feet (12.2 m) deep. Increasing the decoupled borehole pressures beyond the compressive strength of the rock has been more successful in small diameter holes than in large. The radius of rupture around a smaller hole is less. Therefore, any cracking that occurs around the borehole is less likely to cause long term unraveling of the wall of the excavation. From the spacing equation one can see that an increased decoupled borehole pressure results in a wider spacing between presplit holes,
Author: Partha Das Sharma, (B.Tech-Hons., Mining Engg.), E.mail:
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WALL CONTROL BLASTING TECHNIQUES -----------------------------------------------------
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thereby reducing the cost. Thus the cost of small diameter presplitting will not always exceed the cost incurred using large diameters as is sometimes believed. Each situation should be assessed according to the factors discussed above and the best option selected. Shooting the Presplit Line - The presplit line may be shot with the final production blast or before the final shot is laid out in the field. Both approaches are workable. When the presplit line is detonated with the final blast it should be initiated approximately 100 milliseconds before the final wall blast. In delayed blasts care should be taken that the presplit line does not precede the detonation of the adjacent buffer row holes by too great a time. A delay may need to be introduced into the presplit line periodically in order to avoid the possible disruption of nearby buffer holes from the detonating presplit holes. However, as many holes as possible should be shot instantaneously taking into account the lead time and any vibration control requirements, because this yields a better defined presplit. If the final wall shot is quite narrow the presplit row should be detonated with the final blast. Detonating the presplit holes in advance may lead to the mass of rock sliding off the wall, leaving very poorly fragmented material to be cleaned up. Following figure is an example of a final wall blast incorporating two production rows, a buffer row and the presplit holes angled at 80 degrees. This example is for an iron ore mine in competent rock.
Active Highwall Pre-splitting in Dragline Operations - The pre-splitting technique has also been used to control the successive highwalls in a blast casting operation. The standard method involves drilling large diameter holes on the designed highwall location and loading these with a concentrated charge of explosive in or near the bottom of the borehole. Active highwall pre-splitting has two advantages. First, it allows a very regular highwall to be produced. Therefore, front row burdens on the next casting shot can be well controlled for
Author: Partha Das Sharma, (B.Tech-Hons., Mining Engg.), E.mail:
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WALL CONTROL BLASTING TECHNIQUES ----------------------------------------------------maximum casting efficiency. Second, in wet ground the presplit, fired in advance of drilling off the production blast, can be used to dewater the block to be shot thereby reducing explosives cost. When dewatering is a goal the presplit row will be drilled along the back and both sides of the block to be shot to isolate the area from recharge by groundwater. Active wall pre-splitting has often been accomplished using vertical drill holes. However in some mines this has lead to shallow slope failures on the newly formed highwalls, largely due to the presence of steeply dipping joint planes dipping into the highwall. When this occurs vertical pre-splitting cannot be used. A much improved result has been achieved by using an angle presplit. An angle of 70 degrees has often been employed in this application. Best results are obtained if the subsequent production blast is drilled on the same angle, so that a constant burden from crest to toe can be maintained on the front row. The weight of charge can be obtained by calculating the diameter required of a distributed decoupled charge of the explosive.
(iii) Cushion blasting - Cushion blasting is a common controlled blasting technique in surface operations, second to pre-splitting as the most common method. Cushion blasting is used to slash or trim excess material from the bench face to leave a smooth, clean wall with little backbreak, which will remain stable for extended periods. Blastholes are drilled in a line along the planned excavation limit and are loaded with a reduced charge capable of slashing material from the wall without damaging the rock behind the holes. Charges are usually decoupled for this purpose. Common diameters used in cushion blasting have been 4-7 inches (102-178 mm), but large holes have often been used in open pit mines. In the common range of diameters hole spacing of 5-8 feet (1.5-2.4 m) have been typical. In large diameters hole spacing are greater. As a general rule the spacing in feet should be 1.25 to 2.0 times the hole diameter in inches. The lower value is to be used in hard, competent rock while the higher value applies to soft, highly fractured rock. Following figure illustrates a single row cushion blast using 12¼-inch (311 mm) boreholes. A coupled toe charge is followed by a decoupled column charge. Note the projected break line which will leave an angled face at the excavation limit. The coupling ratio in this case is about 0.37 for the column charge. As an alternative to a decoupled charge low density explosives could be used in a cushion row. Gassed slurries or emulsions are an Author: Partha Das Sharma, (B.Tech-Hons., Mining Engg.), E.mail:
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WALL CONTROL BLASTING TECHNIQUES ----------------------------------------------------example. The density of ANFO can be reduced by adding micro-balloons or perlite. As the density is decreased the velocity of detonation and the borehole pressures also decrease.
Cushion blasting is also performed using multiple row blasts. These usually incorporate larger diameter holes in the 9 7/8—12 ¼ -inch range (251—311 mm). However, in surface gold mining diameters are more typically 6 3/4—7 7/8-inch (171—200 mm). These blasts typically consist of three to four rows including the cushion row. This type of final wall blast is typically called a trim shot and the cushion line is then termed the trim row. These blasts consist of three components similar to a presplit blast. (a) The trim row, (b) The buffer row and (c) One or more production rows. The trim row should be suitably decoupled. The coupling ratio typically does not exceed 0.45. Decoupling is often achieved using undersized cardboard tubes. An alternative is to use undersized plastic liners manufactured for use in pre-splitting and trim blasting. A third approach is to place a suitable charge in the bottom of the hole and allow the gases to expand into the void above. The trim row must do sufficient work on the surrounding rock to slash excess material off the wall. Therefore, borehole pressures greater than that required for pre-splitting are necessary and these need to be sustained for longer periods. Thus when a concentrated charge is loaded in the bottom of the hole the use of an airbag and stemming may be a good way to contain the explosion gases for a longer time while still allowing the borehole pressures to be attenuated. As with pre-splitting the last row of the production blast is a buffer row. The design is essentially the same as is used in pre-splitting. A greater scaled depth of burial is achieved by increasing the stemming thereby avoiding cratering back through the trim row at the crest. Blast timing for both the shots is standard for the production and buffer rows and could be a V-1 tie-in for a square pattern or a V-2 arrangement for a staggered square or equilateral pattern. The trim row should detonate one delay period after the adjacent production holes.
Author: Partha Das Sharma, (B.Tech-Hons., Mining Engg.), E.mail:
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WALL CONTROL BLASTING TECHNIQUES ----------------------------------------------------Two or more trim holes can be shot per delay provided these do not outrun the production holes and any vibration considerations are accounted for. Good relief for the trim blasts to move away from the final wall is essential. Firing across two free faces will be very useful. Adequate delay time should be provided to allow for good relief. For cushion blasting in general accurate drilling is required. Coupling ratio of 0.45 or less should normally be used. (iv) Line Drilling - This method is seldom used in open pit mines because the closely spaced holes are costly. However, it has been used in some cases where the rock was very weak and difficult to presplit or cushion blast. It is more commonly used in civil construction projects where overbreak can be very costly. The typical hole sizes for line drilling are 21½ to 3-inches (64-76 mm). However, large diameter rotary drill holes can also be used. When the spacing between the holes remains constant regardless of hole diameter the cost is comparable in small and large diameter work. If the spacing can be increased as larger holes are used, then the larger diameters will be more economical. In small diameter work hole depths should be restricted to 30 to 40 feet (9.1-12.2 m) to minimize hole wander. Greater depths are possible when larger diameters are used. No sub-grade drilling is needed. Drilling must be very accurate for line drilling to be successful. The holes must be drilled so that they all lie in one plane which corresponds to the angle of the final pit wall. Unequal spacing between holes will lead to variable results. A buffer row is once again essential to good results. The design of the buffer row would be as discussed earlier. (v) Air deck-Air shock techniques - Air-decking is a method which involves the use of a concentrated charge in the blasthole with a void above the explosive. The idea was originally expounded by Melnikov in 1940 but widespread use of the technique only developed during the 1980's. It has been used in pre-splitting where a charge is placed in the bottom of the hole and an air-bag is placed near the top of the hole with stemming above. The gases from detonation freely expand into the void and the pressure is attenuated as would be the case with a distributed charge or a concentrated charge when no stemming is used. However, the explosion gases are contained in the blasthole for a longer period of time, due to the stemming, and exert pressure on the borehole wall for a longer time. Thus the stress generated in the ground between holes is sustained for more time and there is greater potential for wedging action to further open the presplit crack. Experience in the industry has been that the explosive consumption should be 8 to 11 percent of the total blasthole volume and 14 to 18 percent with respect to the air-deck volume above the charge. However, one should also check the decoupled borehole pressures using the methods above to insure that these pressures will suit the rock being presplit. When an air-bag and stemming are used it may be possible to increase the hole spacing. However, this needs to be assessed on a site-by-site basis. Geology will play an important role in determining whether spacing can be expanded beyond those used in conventional techniques.
Author: Partha Das Sharma, (B.Tech-Hons., Mining Engg.), E.mail:
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WALL CONTROL BLASTING TECHNIQUES ----------------------------------------------------The best approach will be to initially design the presplit shot, using the air-deck approach, on the normal presplit spacing. If the results are of high quality increase the spacing by 20 percent. If good results are still obtained increase the spacing in 10 percent increments until the optimum is achieved. In the final analysis geology is likely to be the determining factor for the success of airdecking on the presplit row. The method has been used to good effect in strata with horizontal bedding that is relatively widely spaced. Moreover, highly fractured rock tends to lead to a poorer result. Containing the decoupled borehole pressure for a longer time can loosen existing joints and fractures as well as further defining a presplit crack. The hole spacing is more likely to be controlled by the distance between major joints than by the application of air-deck technology. Often in mining and construction, blasting takes place in proximity to housing and other unowned structures. Under these circumstances presplit holes cannot be left un-stemmed because the resulting airblast becomes excessive. In larger diameters the use of an airbag allows the hole to be sealed such that it can be stemmed and airblast reduced to acceptable levels. In small diameters the hole may be plugged by simply pushing a wad of plastic hole liner down to the desired depth. Airdecking technology may have good application on the buffer row as well. A bulk loaded charge could be placed in the blasthole with an air-deck above and then stemming above the inflated air-bag. In this manner the borehole pressure could be reduced while being distributed evenly throughout the hole. Crushing around the hole and crest fracture can be avoided provided the plug is placed at the correct depth. When active highwall pre-splitting is employed in deep holes the weight of explosive needed to provide a suitable decoupled borehole pressure can become large. This can lead to excessive fracturing around the toe of the hole. Thus there could be an advantage to splitting the charge into two and placing these at different locations in the hole to reduce the potential damage. An air-bag could be placed at the appropriate location and the upper charge placed above it thereby reducing the potential for damage. Air-decking technology may have good application on the buffer row. A bulk loaded charge could be placed in the blasthole with an air-deck above and then stemming above the inflated air-bag. In this manner the borehole pressure could be reduced while being distributed evenly throughout the hole. Crushing around the hole and crest fracture can be avoided provided the plug is placed at the correct depth. When active highwall pre-splitting is employed in deep holes the weight of explosive needed to provide a suitable decoupled borehole pressure can become large. This can lead to excessive fracturing around the toe of the hole. Thus there could be an advantage to splitting the charge into two and placing these at different locations in the hole to reduce the potential damage. An air-bag could be placed at the appropriate location and the upper charge placed above it thereby reducing the potential for damage. b. Blast Design for Final Wall Shots - Successful wall control blasting involves not only the wall control row and the buffer row but also the design of the associated production blasts. If the overall design is improper results will be poor, even though the wall control row has been well designed. A key to successful wall control blasting is to allow excellent relief for the blast to pull away from the
Author: Partha Das Sharma, (B.Tech-Hons., Mining Engg.), E.mail:
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WALL CONTROL BLASTING TECHNIQUES ----------------------------------------------------excavation limit. Achieving this result is a function of the orientation and millisecond delay timing of the shot. When two free faces are available the blast is better able to pull away from the final wall. The shot can be delayed to systematically pull the buffer row holes away from the presplit or trim line one hole at a time. The potential for freezing material to the face or wall damage is greatly reduced. When tieing-in the blast the orientation can be V-1 at 45° to the free face if the pattern is square. If it is a staggered square or staggered equilateral pattern the shot may be tied-in on the V-2 orientation along the long axis at a 34 degree or 30 degree angle to the principal free face respectively. These latter patterns have often given good results, based on the substantial burden reduction across the tie-in lines and the consequent ability to displace the material away from the wall. If only one free face is available then a full echelon tie-in can be used, oriented to the single free face. Blast Damage Mechanisms: There are a number of basic principles that can be applied for limiting blast damage, but first, we need to understand the mechanism of blast damage along final walls. There are a few possible mechanisms that need to be considered, these being damage due to gas energy penetration into preexisting crack systems in the rock behind the blast, vibration related damage and geometrical effects. These are briefly summarised below. • Damage by gas penetration - Originally, it was believed that blast damage was caused mostly by explosion gases entering planar weaknesses in the rock and forcing them open. However, research work carried out by Brent and Smith (1999) illustrated that the pressures in the rock behind a blast, even as close as one burden, are negative and not positive. In other words, damage in the final wall is unlikely to be the result of gas pressure penetration into a pre-existing network of joints, bedding planes and faults. • Damage caused by high vibration amplitudes - At the same time, Rorke and Milev presented information on rock damage as a function of vibration amplitude. Their measurements indicated that fresh cracks (damage) in quartzite occurred at amplitudes above 650 mm/s. Therefore, in this case, we can consider vibration amplitude as being a primary driver of rock damage. The variables that affect vibration amplitude are: i. Hole diameter ii. Charge mass per delay iii. Firing delays and sequence of firing iv. Firing time accuracy v. Level of confinement (burden) vi. The presence or absence of air decks These factors can be applied to alter the predicted vibration generated by the back row of holes in a blast. Predicting near field vibration requires a different attenuation model than the standard scaled distance equation. In normal vibration prediction, the source (such as a blasthole or a blast) can be regarded as a single point source because it is far from the point of concern. However, close to blastholes, predicting vibration is a little more comple, as each individual element of charge contributes to the vibration as a point charge. • Damage related to pit-wall and blast geometry - Very often, pit-wall and blast geometry are ignored. However, they can be a major source of unwanted wall damage. The width, length and height of the trim blast have an impact. The angle of the faces along the final wall (presplit plane angle if presplitting is being done) and whether double or single bench pre-splitting is applied will also influence the damage results. Therefore, during the design stages of the final pit geometry, decisions about the presplit angles and presplit heights should be made by considering the nature of blast related damage that can occur as a result of poor choices.
Author: Partha Das Sharma, (B.Tech-Hons., Mining Engg.), E.mail:
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WALL CONTROL BLASTING TECHNIQUES ----------------------------------------------------The millisecond delay timing must be sufficient to allow the rock mass to displace freely. Delay times of 2 to 3 times the effective burden on the tie-in should be considered minimum. In some quarry applications delay times of 5 to 7 times the effective burden have proven most effective. In weak overburden (
