Production Blasts Research Articles

Blast monitoring instrumentation and analysis techniques, with an emphasis on field applications

Abstract Detailed monitoring on hundreds of single hole and multi-row blasts in full scale varying blast environments were completed for specific analyses using high-speed photography and/or videography, laser surveying, refraction seismographs, digital seismographs to measure ground vibration/airblast, fragmentation and jointing analysis systems, borehole inspection cameras, multi-channel recorders and continuous VOD instrumentation. The majority of field tests were instrumented with multi-instrumentation systems in order to generate the most meaningful interpretation of the results for use in blast design optimization. Very rarely was only one type of instrumentation system used on full scale tests because it would severely restrict the scope of information required to generate site-specific custom blast designs, (Chiappetta 1991). For example, those that promote a single piece of equipment such as a seismograph to measure a single disturbance point in all of space, and then attempt to use the result to redesign a blast for improving on fragmentation, select optimum delay intervals, advising on explosive selections and recommending general blast design parameters, etc. can be misleading. Likewise, redesigning a blast based strictly on high-speed photography results, and assuming that the explosives, detonators and primers performed perfectly in the production blast can also have grave consequences. Stand alone velocity of detonation data, if not coupled to information obtained from the other instrumentation systems can also be meaningless from a design standpoint, unless obvious explosive malfunctioning or performance is evident. The point here is that unless the data are reviewed collectively from all of the instrumentation systems, true blast design optimization might never be realized, and the resulting design could be flawed along with a false economic assessment. This paper outlines the field instrumentation systems used and the information obtained for use in blast optimization.

Controlled blasting in an open-pit mine for improved slope stability

A case study is presented where a controlled blasting programme (presplitting) was adopted in an open-pit copper mine in order to improve the stability of the resulting cut slopes. The aim was to reduce over break and minimize damage to the final pit walls from production blasts. The use of presplitting allowed considerably higher benches to be adopted with much steeper overall slope angles. The extra cost of presplitting was fully justified by the improved face stability and the resulting reduction in overburden quantity.

Fragmentation monitoring of production blasts at MRICA : Proc 3rd International Symposium on Rock Fragmentation by Blasting, Brisbane, 26–31 Aug 1990 P283–289. Publ Parkville: AusIMM, 1990

Fragmentation monitoring of production blasts at MRICA : Proc 3rd International Symposium on Rock Fragmentation by Blasting, Brisbane, 26–31 Aug 1990 P283–289. Publ Parkville: AusIMM, 1990

Frequency control of blast-induced ground vibrations.

A site-specific approach is presented for controlling the amplitude, frequency, and duration of ground vibrations induced by blasting operations. Using measured single-charge response functions, and proper frequency control of the pulse train excitation applied to the overburden, significant reductions in vibration amplitude and duration may be achieved for multiple-charge production blasts. A case study is used to illustrate these effects, even though the charge weight per delay was increased multifold.

Application of refraction tomography to map the extent of blast-induced fracturing : Cumerlato, C L; Stachura, V J; Tweeton, D R Key Questions in Rock Mechanics: Proc 29th US Symposium, Minneapolis, 13–15 June 1988P691–698. Publ Rotterdam: A A Balkema, 1988

The Bureau of Mines has 8oonducted high-resolution refraction surveys at a limestone quarry both before and after standard production blasts where variations in the blast design were made. The data collected were processed using a tomographic inversion algorithm to produce maps (tomograms) of the seismic velocity field of a shallow refracting layer in the rock left standing. Overlapping of surveys, velocity measurements from shotholes (preblast) and measurements of predominant joint sets were used to verify results. Resulting tomograms show sublinear low velcrc1ty zones whose orientations correspond to those of measured joint sets. Low velocity zones near pitwalls are interpreted as the extent of intense fracturing caused by blasting. These results demonstrate that refraction tomography can be a powerful and economical tool for locating predominant jointing and areas of preexisting or blast-induced fracturing; an important aid to operators during premine planning and blast designing.

Modelling of blast vibrations

A hybrid method for modelling the time history of blast vibrations from production blasts is developed. The seismic signal produced by the detonation of a single hole charge is recorded in situ. This record contains information about the complex mechanism of radiating seismic energy from an explosive source as well as the filtering effect of the travel path of the signal. Assuming a linear superposition of the seismic effects of the individual holes in a row shot, the vibrations produced by a production blast are simulated. The single shot signal is convolved with an impulse series that represents the firing sequence. By a systematic variation of firing times in the simulation procedure, their effects on the vibrations are studied. The peak amplitudes of vibrations can be reduced. The application of an electronic initiation system allows the realization of optimized firing times in production blasts and a successful validation of the synthetic seismograms.

Initiation timing influence on ground vibration and airblast

ABSTRACT A major concern with blasting at surface mines is generation of ground vibrations and airb1ast and their effects on nearby residences. This Bureau of Mines report looks at the use of millisecond delays in blast design and their effect on the resulting ground vibrations and airb1ast. A total of 52 production blasts were instrumented and monitored at a surface coal mine in southern Indiana. Arrays of seismographs were used to gather time histories of vibrations and airblast. The data were analyzed for peak values of vibration and airblast and for frequency content. Various delay intervals were used within and between rows of b1astho1es.De1ay interva1s within rows were 17 and 42 ms, and those between rows ranged from 30 to 100 ms; these interva1s are equivaIent to 0.5 and 1.3 ms/ft within rows and 1.2 to 4.3 ms/ft between rows. Subsonic delay interva1s with in rows reduced airb1ast by 6 dB. Large delay interva1s between rows reduced the amp1itude of ground vibrations; their frequency depended primar...

Vibration and air blast noise from surface coal mine blasting : Linehan, P; Wiss, J F Min Engng, V34, N4, April 1982, P391–395

Vibration and air blast noise from blasting were evaluated during a research program sponsored by the Bureau of Mines. Blast variables were studied using small-scale model tests to evaluate specific variables as well as interactive effects, followed by experiments with production blasts at the surface coal mines. Experimental data showed that the primary variables affecting the ground vibration amplitude are the charge weight per delay and the length of the delay interval. Blast variables with a significant affect on the overpressure amplitude include the charge weight per delay, the length of the delay, burden and spacing, stemming amount and type, direction of initiation, charge depth, and amount of detonating cord and cover.

Structure response and damage produced by ground vibration from surface mine blasting : Siskind, D E; Stagg, M S; Kopp, J W US Bureau of Mines report RI 8507, 1980, 74P

Direct measurements were made of ground-vibration-produced structure responses and damage in 76 homes for 219 production blasts. These results were combined with damage data from nine other blasting studies, including the three analyzed previously for Bureau of Mines Bulletin 656. Safe levels of ground vibration from blasting range from 0.5 to 2.0 in/sec peak particle velocity for residential-type structures. The damage threshold values are functions of the frequencies of the vibration transmitted into the residences and the types of construction. Particularly serious are the low-frequency vibrations that exist in soft foundation materials and/or result from long blast-to-residence distances. These vibrations produce not only structure resonances but also excessive levels of displacement and strain. Threshold damage was defined as the occurrence of cosmetic damage; that is, the most superficial interior cracking of the type that develops in all homes independent of blasting. Homes with plastered interior walls are more susceptible to blast-produced cracking than gypsum wallboard. Structure response amplification factors were measured. Typical values were 1.5 for structures as a whole (racking) and 4 for midwalls, at their respective resonance frequencies. For blast vibrations above 40 Hz, all amplification factors for frame residential structures were less than unity. The human responsemore » and annoyance problem from ground vibration is aggravated by wall rattling, secondary noises, and the presence of airblast. Approximately 5 to 10% of the neighbors will judge peak particle velocity levels of 0.5 to 0.75 in/sec as less than acceptable (i.e., unacceptable) based on direct reactions to the vibration. Even lower levels cause psychological response problems, and thus social, economic, and public relations factors become critical for continued blasting.« less

Geology and grade distribution of the Panguna porphyry copper deposit, Bougainville, Papua New Guinea

Information from daily pit mapping of all working benches, since production commenced at Panguna in April 1972, has been used for improved mine production and metallurgical predictions, pit slope control, and the build-up of an orebody model for exploration.The present interpretation of the development of the orebody is as follows. A phase of copper iron sulfide mineralization was associated with the initial intrusion of the Kaverong Quartz Diorite/Biotite Diorite differentiate into the Panguna Andesite country rock. This mineralization was successively upgraded by a remobilization of copper sulfides with the intrusion of the Biotite Granodiorite and by an addition of higher grade copper associated with the Leucocratic Quartz Diorite. Breccias associated with the Biotite Granodiorite are significant sources of higher grade ore. Alteration is generally of weak to moderate intensity and essentially of the potassium and propylitic types. Quartz, sericite, and kaolin may locally become important.The evaluation diamond drill assays conservatively proved overall tonnage and grade, but the 122-m grid is too widely spaced for short-term mine planning. Production blast hole samples average higher Cu values than the evaluation diamond drill assays in low-grade mineralization.

MAGNETITE ASSAYS FROM MAGNETIC SUSCEPTIBILITY MEASUREMENTS IN TACONITE PRODUCTION BLAST HOLES, NORTHERN MINNESOTA

A study was made at U.S. Steel Corp.’s Minntac open‐pit taconite mine in northern Minnesota to assess the applicability of relating magnetic susceptibility measurements made in production blast holes to the magnetite content of the strata penetrated. The measurements were made by continuously recording the changes in self‐inductance of variously constructed air‐core, multiturn coils as they were raised in the holes. These coils were designed so that the magnetic flux which coupled with the wallrock was parallel to the horizontally bedded Biwabik iron‐formation. An analysis of the response for these coils showed that the recorded inductance changes (apparent magnetic susceptibilities) are related linearly to the average weight percent Fe as magnetite within a practical degree of accuracy. In one test hole, 36 assayed intervals, each 18 inches long, fitted a linear line of regression to the corresponding apparent magnetic susceptibilities within a standard error of ±1.2 weight percent Fe. When averaged over three ranges of Fe content, the values fitted a straight line to within ±0.5 weight percent Fe. As susceptibility depends on the mode of occurrence of the magnetite, extrapolation of these results to other areas will require further testing. This method of assay should provide control for the efficient processing of taconite ores. Since the method is rapid and inexpensive, measurements could be made in scores of these accessible holes to delineate cut‐off boundaries between ore and waste. A potential use of these continuous measurements would be found in establishing lithologic contacts from the detailed responses in selected horizons of the iron‐formation.