12 Jul 2016
POSTED BY - MineExcellence
1. Introduction
Mining industry is heading towards a technology driven optimization process and cost effective operations. Traditional working practices and techniques are unable to improve efficiency and mitigate environmental hazards. One of the reasons for hindering improved mining efficiency is poor blasting efficiency. Improving blasting efficiency is an important issue in all-mining processes. It might appear, on the face of it that the regular mining activity of producing suitably sized blasted rock is a relatively uncomplicated exercise. However, there are many examples, throughout the mining industry where, through an apparent lack of understanding of
the basic principles governing efficient drilling and effective blasting, mines are not operating at their optimum – in terms of overall production costs, energy utilization, resource management and rock stability whilst, at times, jeopardizing safety or creating an adverse environmental impact, as a result of blasting nuisance such as ground and air vibration hazards and dangerous fly rock incidents.
In order to improve performance, the drilling and blasting industry the world over is rapidly adopting new technology in all forms. In some countries (e.g. Great Britain), the law now stipulates that before any blast can be loaded in surface mines, the rock face must be completely profiled and results used to design and load the blast, thus fly rock and vibration problems are minimized. In Australia for quarrying
within metropolitan area, blasts as large as using 75 tonnes of explosive breaking 286,000 tonnes of rock in one shot has been carried out with housing settlements as close as 250m from the blast. This has been possible because the basic steps in blast engineering are followed--- design the blast according to rock face conditions and results required with environmental considerations, execute the blast while controlling the actual drilled pattern and then appropriately load explosive products, monitor during the blast event and evaluate the outcome of the blast. Results are stored online for retrieval and analysis. Recently developed technologies can enhance and streamline the process for the optimization of blast results with safety and environmental controls. The objective of this paper is to enumerate innovative blasting practices which could be adopted for safety and controlling adverse environmental controls in blasting operations.
In India neither in organized sector nor in unorganized sector, blasting performance is satisfactory. In surface coal mines, limestone mines or iron ore mines there is often inadequate control on fragmentation, ground vibrations, flyrock or airblast. Much litigation is pending in courts and many adverse judgments have been delivered leading to closure of mines and even mining areas.
Blasting operations often result in large boulders on one hand and generation of fines on the other hand. Inadequate attention has been paid to reduce the generation of fines. Many fines get airborne causing environmental dust problems. Inadequate control of ground and air vibrations and flyrock leads to complaints and agitations and so far it has been practice to vacate the nearby villages and pay huge compensations (Adhikari, 1999). Where it is not
possible to move people then the regulatory bodies do not give permission to blast within 100m of boundary. As a result several hundred million tonnes of valuable minerals cannot be mined. Mines are resorting to smaller blasts to overcome the hazards like ground and air vibrations, flyrock and dust. This leads to poorer fragmentation, more manpower requirement and complaints from neighbors. In underground blasting advance per blast has remained static over the last several decades whereas the world over this has increased substantially. In most of Indian mines it is well below 2m. Blasting operations when improperly carried out lead to damage to remaining rock. Often poor rock is blamed for high fatalities due to fall of roof whereas blasting is often a cause of damage to roof, though it can be controlled by appropriate techniques and products.
2. Tools and Techniques
Several tools can be used in assessing blast face conditions, assist in designing blast, executing blasts, during and after blast monitoring, recording all results and analysing records. Many tools and techniques have taken the guesswork out of explosives loading and blasting operations (Rodgers, 1999). Some of them are:
Face profiler systems using laser technology profilesthe rock face by pointing profiler to the floor, toe and crest – then take extremely accurate measurements and calculates bench heights, minimum and optimum burdens, computes drill hole angles and offsets, and hole depths. Drills may have a tendency to follow the faults, weakness planes, weak rock, cavity or similar other geological weakness creating borehole deviation. When operators start using electronic Hole Deviation measurement tools, operators discover
light burden areas and are able to thwart safety issues. The cable deviation measurement tools uses sensors that measure borehole deviation at fixed intervals from the collar position and transmit the findings instantly to a field computer.
The second tool is the 3-D laser profiling system, which includes equipment and software that allows the user to make precise burden measurements without venturing near the crest of the bench. This system, too, transmits its findings instantly to a field computer. From these field computers, the data from both tools are merged in blast design software to adjust the blasting plan by compensating for any initial \"guesstimation\" of the burden and for drilling inaccuracy.
Another technology that has great importance for drilling accuracy, and the integration of drilling and blasting operations is GPS as applied to drill positioning on individual blastholes. GPS based systems allow the blast plan with hole locations to be downloaded to the drill. Some form of moving display is used to guide the drill onto the designed hole location. The drill can be positioned to within about one third meter of the designed location. Therefore, the drill can be accurately spotted without the need for extensive field surveying. Equally important, the systems records exactly where the hole is drilled. The final design is downloaded to the drill equipped with GPS capability. The drill machine can then drill the blastholes accurately on the designed locations.
If the mine is new, selection will have to be made between cartridge and bulk explosives depending on the proposed volume of excavation per month. Once this is made, the choice can be exercised amongst ANFO, HANFO, slurry and emulsion depending on the characteristics of rock vis-à-vis its response under dynamic loading during blasting. Selection of initiation system and timing need to be made keeping in view the environmental constraints.
The most significant changes in blast technology have taken place in product-delivery systems. One factor is the continuing trend away from the use of cartridged products in favor of bulk products for both surface and underground operations: new surface and underground delivery-vehicle technologies that boost blast accuracy and safety: high-precision pumps and blending and measurement devices, robotic arms that place the product in the hole, and remote controls. When considering blasting technologies, operating companies tend to be highly cost conscious, which mitigates opportunities to develop value-added or innovative products.
Use of Shock Tube/Signal Tube initiation has indeed made down hole initiation possible allowing much safer blasting operations. Electronic detonators are now commercially available(Cunningham, 2004). Advantages include more-precise delay timing (resulting in increased blast efficiency and control) and greater compatibility with remote-controlled loading of explosives and wireless detonation. However, these initiating systems have higher costs. Whilst most of the effort by the manufactures is being directed towards enhanced fragmentation, there are undoubtedly areas relating to environmental control that, can bring benefits to operators, regulators and local residents. The trend towards use of electronic detonators has not been to replace other systems such as signal tube or shock tube which has its own advantages.
Another trend is outsourcing of blasting-related services, ranging from consulting on safety to providing comprehensive packages priced according to the volume of “shot rock” on the ground or ore processed. As a result blast-optimization effort based on the volume of explosives/blast agents per unit of shot rock is slowly decreasing. Explosives are a mature technology and have become a commodity product, motivating suppliers to look at the entire blasting-related value chain for opportunities to capitalize on their expertise. As a result, there is need for explosive providers to focus from products to service.
3. Blast Execution, Monitoring and Control
Front row burden variation is the important cause of poor fragmentation and release of explosive energy. The charge in the holes of first row should be based on profile of the face. Face profile can be drawn with the help of face profiler survey instrument. Designed patterns be laid out by surveyor taking into consideration elevation and bench height. Larger blasts need to be carried out so that equipment movement is reduced thus saving several hours of idle time.
The stemming length should be according to requirement of blasting. Avoid stemming shorter than the burden. Too short stemming may create crater effects. Presently drill cuttings are used as stemming material. It is suggested coarse aggregate material be used as stemming material rather than drill cuttings. This improves blast results and reduces dust being raised in atmosphere.
In different zones and benches variation in terms of type of rock, joint structure in strata normally exists. Patterns need to be changed according to strata.
Correct delay sequence and timing of delays between the rows and between the holes are required. Timing of delays between the rows and between the holes often can result in successful or failed blast. Timing of delays can be changed according to actual drilled holes before blast can be charged.
In mines blast records are maintained but are generally inadequate for analysis and study. In general only location and average parameters are maintained. Post blast performance details, site information such as geology, geometrical information of blast site and detailed parameters are not recorded. It is important to record blast performance zone wise, explosive and accessories used and also unusual happenings. Use of information technology in recording and retrieving information would help in future blast design. Ideally, the blast need to be designed with computer assistance using information from laser surveys, geotechnical and other data and hole locations are placed on blast plan maps. Several software are available to keeps records of blasts.
4. Blast Design Software
Optimum blasting just does not happen. It requires suitable planning, good blast design, accurate drilling, the correct choice of explosives and initiation system and methods, adequate supervision and considerable attention to detail. An approach is needed that considers factors that interact with each other during blasting. The rock type and structure; size, length and inclination of blast holes, drilling pattern and accuracy, type, quantity and distribution of explosives; charging and initiating techniques all play a significant role in the overall efficiency of a mining operation. During the design stage environmental constraints such as vibration limits or
flyrock restriction with respect to any structure can be prescribed. Blast design software can be used which considers all the above aspects.
5. Blast Information Management System (BIMS)
BIMS is software which helps to store, access and manage the information needed to take critical decisions for their mine/quarry operations. The system stores blast details, blast parameters, blast pattern, face profile, explosive consumption, charging details, costs, weather information, pre-blast survey, post-blast evaluation data, fragmentation information, photograph(s), videos, accidents, misfires, vibration record and information for vibration analysis (Fig. 1). The stored blast information data can be retrieved quickly for analysis.
The system generates reports for individually identified blast, monthly explosive consumption report, cost report, vibration monitoring report, and monthly blast performance report. The storage of this information in database format allows querying to retrieve scenarios, which meet certain criteria, and to use this information to further optimize the outcome from a blast. Use of software provides the mine manger’s to take quick decision based on accurate and reliable information.
6. Environmental Considerations
Increasing numbers of mining operations are coming under pressure to monitor and reduce blasting related safety and environmental hazards. Ground vibrations, air over-pressure, fly-rock, dust, blasting fumes and in some cases leaching of chemicals in the blast holes and polluting ground water are some of the undesired events associated with blasting which collectively affect the surrounding environment adversely.
Much work has been carried out on the environmental aspects such as ground vibration and airblast control (Richards and Moore, 2002).Operators are now aware about the steps which need to be taken. Norms and standards regarding ground vibration and air blast as specified by regulating agencies must be complied with. In India adequate regulations do not exist with respect to ground vibrations and airblast vibrations. Self regulatory limits need to be set by the organizations while taking neighbors into confidence. There is a need for severe penalty and stoppage of work if the limits are exceeded. Current norms were initially intended to prevent structural damage to adjacent properties, however nowadays they are being employed in an attempt to minimize human nuisance. Thus these values are now set at much lower levels than those based on damage criteria but still above human perception level and as a consequence complaints
still arise. A statutory vibration limit be included on a sites operational license which must be adhered to at a specified confidence level at the nearest occupied property. It is therefore, vital for the industry to do all that it can to reduce the vibration levels experienced at these adjacent properties without imperiling the financial viability of the enterprise.
Figure 2 shows ground vibration contours for blasts for a particular maximum instantaneous charge. When these contours are superimposed on the site plan or photo, the maximum extent of vibration levels for blasting anywhere in the mine on the surrounding area can be seen. Scaling methods have been used for many years to determine relations between charge mass distance and blast vibration levels. The vibration is predicted by either square root or cube root scaling formulae relating vibration to charge mass and distance for a particular site. Excessive air and ground vibration are then controlled by a reduction in the explosives charge mass being fired at the one instant of time, or within a small time period of up to 8ms. These scaling methods do not allow for the time taken for vibration wave fronts to travel from each blasthole, and cause reinforcement of vibration wave front.
Wave front reinforcement has been found to cause substantial increases in both air and ground vibrations (Richards and Moore, 1995). Simple alterations to firing pattern can prevent wave front reinforcement.
Pattern Analyser is a graphical software programfor the design which can be used to alter design and initiation pattern to avoid reinforcement and thus lowering the vibration levels.
Blasting operations can generate large quantities of dust.However, dust is as yet not assumed to becausing problems. Whereas this dust when released in an uncontrolled manner, can cause widespread nuisance and potential health concerns for on-site
personnel and surrounding communities. Though the blasting dust plume is raised for few minutes but most of the dust settles in and around mining area and some of it is dispersed before settling down. Depending on meteorological conditions the dust dispersal can travel to substantial distances endangering health of communities. Generation of fines and dust is influenced by several blasting and rock parameters.
Meteorological conditions such as wind speed and direction, temperature, cloud cover and humidity will affect the dispersion of airborne dust. Atmospheric stability affects dispersion of the emitted plume, determining the extent of the vertical and horizontal, transverse and axial spread of the emitted particulates. Thus, dispersal of dust plume resulting from blasting is an important area which needs attention. A computer model has been developed to simulate the dispersal of dust (Kumar and Bhandari, 2002).
However, production of fines and dust need to be reduced and controlled for better environmental conditions (Hagan, 1979). Dust generation and dispersion from blasting operations depends on factors such as meteorology, bench height, blast design information, and rock. (Bhandari, et al., 2004). Concern is expressed about nitrogen-oxide (NOx) releases from blast sites and their potential health impacts on workers, as well as their aesthetic and environmental impacts on nearby communities.
7. Flyrock Hazard
Explosives handling and blasting operations are high consequence risk activities. There are many safety hazards associated with use of explosive, transport
and storage. One of the safety hazards during blasting is flyrock. Damage due to flyrock from blasting is one of the main causes of strained relations between mining operation and neighbors. Flyrock distances can range from zero for a well controlled mine blast to nearly 1.5 km for a poorly confined large, hard rock mine blast and many fatalities have occurred.
In a circular, The Director General of Mines Safety India in 1982 had recommended that personnel be removed up to 500 m, though previous limit was 300 m only. Thus, where large diameter blasting is carried in hard rock mining, extra precautions are required to control the flyrock damages in the surroundings. There is a ‘safe’ blasting area in blasting is dependent on the knowledge of distance to which flyrock will propel.
A software for predicting distance to which a flyrock will travel has been developed (Richards and Moore, 2004). Inputs to the software are charge mass, burden or stemming height, and a site constant that lies within a general range that can be tightened by site calibration. The output is the distance that rock will be thrown, and this quantification can be used to establish both safe clearance distances, and the critical range of burdens and stemming heights where the situation changes rapidly from safe to hazardous. Using safety factors danger zones for machinery and persons respectively. If it is not possible remove any structure or person then one can change charging of holes.
For a blast which uses 152 mm diameter hole maximum throw for would be 141.1 m and taking safety factors into consideration people be removed up to 423.5 m and equipment be removed up to 282.2 m (Fig. 4).
8. Conclusions
In Indian mining and construction industry there is very urgent need for improving explosive and blasting performance. The paper has identified those key factors which impact upon optimum blasting performance, whilst highlighting, in particular, some opportunities for utilizing modern surveying technology, bulk explosives and initiating systems development and others to better appreciate the interdependence of the various factors which impact upon environmentally effective blasting. Whilst there is an urgent need for Indian mining management to control and improve its current blasting performance if the image of mining, and blasting in particular, is to be improved.
By use of several predictive tools for blasts can be fired with the minimum amount of noise and airblast overpressure by effective distances associated with surface mine operations, is very unlikely to create ground vibrations which could cause structural damage and flyrock damage.
References
Adhikari, G.K., Studies on Flyrock at Limestone quarries, Rock Mechanics and Rock Engineering 32(4): 291-301; 1999
Bhandari, S., 1997, Engineering Rock Blasting Operations, A.A. Balkema Publishers, Rotterdam, Netherlands / Brookfield, U.S.A., 370p
Bhandari, S., Bhandari, A. and Arya, S. 2004 Dust Resulting From Blasting in Surface Mines and its Control, EXPLO 2004 Conference, Perth, August, Bhandari, S. & Bhandari, A. 2006.; Blast Operations Information Management System, Journal of Mines, Metals and Fuels, Vol. 54 no.12
Bhandari, S. 2011, Information Management for Improved Blasting Operations and Environmental Control, 3rd Asia- Pacific Symposium on Blasting Techniques, August 10~13, Xiamen, China Cunningham, C. V. B. (2004) Electronic Detonators: growing success in transforming rock breakage, Ann. Conf. on Explosives and Blasting Technique, International Society of Explosive Engineers, New Orleans
Hagan, T.N. 1979. The control of fines through improved blast design, Proc. Aust. Inst. Min. & Metal, pp 9.
Kumar, P. and Bhandari, S. 2001, Modelling dust dispersal near source after opencast mine blast in weak wind conditions over flat terrain in tropical conditions, Explo 2001 Conference, Hunters Valley, October 28-31,
Kumar, P. and Bhandari, S. 2002, Modelling dust dispersal near source after surface mine blast over undulated terrain in weak wind conditions, APCOM
–2002, Phoenix, February 25-27, Proceedings of the 29th International Symposium on Computer Applications in the Minerals Industries.