Flyrock Phenomena And Area Security In Blasting-Related Accidents

Abstract
In both the mining and construction industries, blasting is the predominant method for fragmentation of consolidated mineral deposits. The blasting process, however, remains a potential source of numerous hazards to people and surrounding objects. This paper presents the results of the research study on flyrock phenomena and blast area security related accidents in surface mining. The study revealed that a total of 45 fatal and 367 non-fatal accidents in coal, metal and non-metal surface mines had occurred between 1978 and 1998 where the primary causes were the lack of blast area security, flyrock, premature blast, and misfires. The lack of blast area security and flyrock accounted for 281 (68.2%) accidents. Investigations of flyrock accidents have revealed one or more of the following contributing factors: discontinuity in the geology and rock structure, improper blasthole layout and loading, insufficient burden, very high explosive concentration, and inadequate stemming.

The study also shows that accidents due to lack of blast area security are caused by failure to use appropriate blasting shelter, failure to evacuate humans from the blast area, and inadequate guarding of the access roads leading to the blast area. The research results should have a positive impact on hazard awareness, prevention, and safe blasting practices in mining and construction industries. 

1. Introduction
The main purpose of blasting operations in surface mining is the rock fragmentation, and is considered to be essential to the success of mining operations. This process provides appropriate material granulation that will be suitable for excavation and transportation. According to the US Geological Survey (2000), the US coal, metal and non-metal surface mining industry uses almost 1.8 billion kilograms of explosives annually. Between 1989 and 1999, surface coal mines have used 16.2 billion kilograms and 3.3 billion kilograms have been used in non-metal mines and quarries (Kramer, 2000).

The blasting process, however, remains a potential source of numerous hazards. Even though the mining industry has improved its blasting safety, there are still reports indicating blasting-related accidents involving both people and various structures. Investigations carried out by the Mine Safety and Health Administration (MSHA, 1994; MSHA, 1999a,b) provide clear evidence regarding the severity of these accidents. Figs. 1 and 2 show the fatal and non-fatal blasting accidents from 1978 to 1998 for coal, metal and non-metal surface mining (Verakis and Lobb, 2001). A total of 45 fatalities occurred during the entire period, an average of 2.14 fatalities per year. Coal mining accounted for 44.68% of the fatalities, while metal and non-metal operations for 55.32%. For the same period, a total of 367 non-fatal injuries have occurred, an average of 17.47 per year.

Further historical data summarized by Verakis and Lobb (2003) shows that for the period of 1978–2001, a total of 195 blasting accidents occurred in US surface coal mine operations. Of the 195 accidents, 89 accidents (45.64%) were directly attributed to lack of blast area security, 54 accidents (27.69%) to flyrock, 33 (16.92%) to premature blast, and 11 (5.64%) to misfires (Fig. 3). Since flyrock and a lack of blast area security constitute the majority of all blastingrelated accidents, the cause and control of these hazards and activities are discussed.

Fig. 1. Number of fatal and non-fatal blasting accidents in coal surface mining.


Fig. 2. Number of fatal and non-fatal blasting accidents in metal and non-metal surface mining.


Fig. 3. Blasting accident causes in coal surface mining (1978–2001)


2. Flyrock phenomena
Flyrock is defined as the rock propelled beyond the blast area by the force of an explosion (IME, 1997). The uncontrolled material fragments generated by the effects of a blast are one of the prime causes in blasting-related accidents. When these rock fragments are thrown beyond the allowable limits they result in human injuries, fatalities and structure damages. Fig. 4 shows flyrock occurrences during the blasting process.

Previous experimental and theoretical work about flyrock phenomena has been performed by Langefors and Kishlstrom (1963), Ladegaard-Pedersen and Persson (1973), Lundborg (1974, 1981), Holmeberg and Persson (1976), and Roth (1979). Ladegaard-Pederson and Persson (1973) have performed experiments in Plexiglas (polymethylmethacrylate—PMMA). Their drilling experiment involved a single hole in a block of PMMA, and variation of the explosive charges by a factor approaching 2. They concluded that as the charge increases, the fragmentation and the velocity of the broken material increases as well. They also found that the gaseous venting from the blast penetrated the fracture planes perpendicular to the hole axis and broke the material up and propelled them. They also conducted a series of bench blast tests with a single hole in rock boulders. The drillhole diameter was 25 mm. After each single hole blast, the distance from the hole and the angles of the flyrock were determined.

Holmeberg and Persson (1976) studied flyrock in field experiments with high-speed cameras. They concluded that most of the collar flyrock are thrown in a direction following the drillhole axis. Their experiments also confirmed that the scatter of the angle of throw increases as the unloaded hole length decreases.


Fig. 4. Flyrock generation in blasting process (Cameron et al., 2003).

The theory for predicting flyrock from blasting operations in hard rock such as granite has been developed by Lundborg (1974). The charge hole diameter has been established as d = k/qv, where k is a constant. To determine the constant k, measurements of the /qv were made for different d values. By doing so, the relation 10d ¼ /qv 2600 was obtained where q is the density of the rock in kg/m3 (2600 kg/m3 is the average density of granite), / is the fragment size diameter in meters, and v is the fragment velocity in m/s. To investigate the validity of this equation, and to determine the factor of proportionality, several blasts were photographed and the flyrock velocities measured. In a number of blasts, the maximum distance of throw and the diameter of each flyrock fragments were also measured. By using the previous equation, the maximum throw was calculated as Rmax = 260d2/3, where R is in meters.

Additional studies on flyrock phenomena can be found in Persson et al. (1984), Bajpayee et al. (2000), Fletcher and D Andrea (1986), Rehak et al. (2001), Shea and Clark (1998), and Siskind and Kopp (1995). Generally, flyrock is caused by a mismatch of the explosive energy with the strength of the rock mass surrounding the explosive charge. Investigations of flyrock accidents have revealed one or more of the following contributing factors: (i) discontinuity in the geology and rock structure, (ii) improper blasthole layout and loading, (iii) insufficient burden, (iv) very high explosive concentration, and (v) inadequate stemming.

2.1. Geology and rock structure
The rock structure and rock properties may vary considerably from a location to location even within the same blast area. Discontinuity in the geology and rock structure causes a mismatch between the explosive energy and the resistance of the rock. Existence of fissures, joints, weaknesses, and voids are likely to assist in the creation of flyrock. The compressive strength, abrasiveness and the rock density also play a very important role in the blasting process, as does the spatial distribution of rock properties. Base information (e.g. consolidation, voids, etc.) regarding the rock structure and properties of the material to be blasted can be routinely obtained from drill hole logs, and must be considered prior to hole loading. A much more in-depth analysis of geologic characteristics can be achieved through modeling. Realistic representation of geological domain requires a form of a spatially referenced database that provides means for modeling a 3-D body from all geological and geophysical data. 

Depending upon the rock type, data can be analyzed and modeled using:
(a) Stratigraphic modeling, where a set of grid surfaces and subsequent intervals represent a sedimentary deposit; or
(b) Discrete fracture network (DFN) approach and incorporates deterministic, conditioned, and stochastic features; or
(c) Block modeling where each block consists of unique rock strata property information.

As a result, a 3-D model can be generated providing information and spatial visualization of the rock structure as shown in Figs. 5–7. Spatial analysis offers a number of advantages. Rock properties data can be obtained or predicted at each location, and can be quickly investigated and visualized in 3-D. This would allow quick response to changes in rock conditions by providing an opportunity for early identification of possible problems.

Secondly, it is very important that the surface rock is inspected for faults and planes before blasthole charging. Previous excavations can give significant information about the rock structure. Best-in-class safety performance can be achieved when regular geologic hazard or exception mapping occurs by trained foremen and/or mine geologists. Incorporating geologic variability can be routine by including exception mapping into the periodic stripping plan. Most surface mine operations plan pit sequencing and stripping on a weekly to a monthly basis.

Fig. 5. Stratigraphic model of sedimentary deposit (Mincom, 2004)


Fig. 6. Block model of rock properties (Surpac, 2004)


Fig. 7. Discrete fracture network model (Golder Associates, 2004)



2.2. Blast hole pattern
Inaccuracies in the design of blasting patterns, including incorrect blasthole angle can cause large deviations from the planned pattern resulting with flyrock occurrence. Commonly, the graphical design of drilling and blasting patterns is performed by using 2-D computer aided design (CAD) tools, or is generally determined by drill operator experience.

However, 2-D design techniques do not consider spatial characteristics of rock properties and usually use the average value of a parameter that is of interest. An engineer's ability to analyze interactions among rock properties, geology, and pattern design could be enhanced considerably using 3-D graphics. Fig. 8 shows an example of pattern design using state-of-the-art technology such as MineScape (Mincom, 2004). The entire drilling and blasting domain can be visualized from different angles, thus, forewarning about possible trouble spots before drilling. More detailed description on 3-D design of drilling and blasting patterns can be found in Kecojevic et al. (2003), Kecojevic and Wilkinson (2003), and Wilkinson and Kecojevic (2004).


Fig. 8. 3-D representation of blasting pattern

2.3. Burden
Insufficient burden is one of the primary causes of flyrock (Fig. 9). Too short a distance to the bench slope wastes energy, while too great a burden distance causes improper fracturing of the rock, creating oversize boulders. Due to irregularity of bench slopes, energy generated during blasting pose the hazard at the weakest point of the bench. Furthermore, any deviation during the drilling process can increase or reduce the burden. A common problem in small mining operations is the lack of knowledge and accurate technology to identify and recognize the specific anomaly or weakness in the rock structure that leads to the subsequent flyrock problem. The blaster is aware that flyrock can occur if the hole deviates from the intended direction and goes to close to the free face.


Fig. 9. Typical blasting hole in surface mining (Fernberg, 2003).


Fig. 10. A Global Positioning System (GPS) installed on the drill system (Modular Mining System, 2002)

Until recently, a convenient means of gathering drilling records were not available. Wireless technology applied at the drilling rig may help to resolve this problem. Drilling machines can be instrumented with the variety of sensors, from which data can be digitized and transmitted to any location for analyses. A global positioning system (GPS) installed on the drill system can provide the precise locations of boreholes drilled (Modular Mining System, 2002). Each borehole can be surveyed to provide an as-built record of the drilling accuracy accomplished at each location (Fig. 10). The operator also can provide the onthe-spot assessment of situations that result in drill downtime, or unusual performance of the system at the given location. In such an arrangement, the machine location, changes in geology, unusual rock strata features and machine defects could all be documented at the same setting.

The Aquila Mining Systems (2004) has developed a production monitoring system, a material recognition system, and a guidance system for vertical and inclined drilling. The production monitoring system provides the operator with immediate information on drilling productivity and performance, while the material recognition system is equipped with vibration sensors and pattern recognition software to determine hole geology while drilling. Guidance systems for vertical and inclined drilling enable the operator to position the blast hole with centimeter accuracy.

2.4. Blasthole loading
Blasthole overloading is one of the frequent causes of flyrock occurrence. Such overloading generates excessive release of energy. It appears due to the loss of powder in fissures, joints, voids, and cracks. In order to prevent hole overloading, it is necessary to load holes as designed using the correct charge weight. Additionally, a blast ratio should be ensured sufficiently high to eliminate the possibility of excessive charging, and holes have to be monitored to check the rise of the powder.

2.5. Stemming
Stemming material provides confinement and prevents the escape of high-pressure gases from the blasting holes. This material must be free from rocks and properly tamped. Inadequate stemming results in stemming ejections from the holes resulting with flyrock. In general, the stemming length should be not less than 25 times the blast hole diameter (Sheridan, 2002). Konya and Walter (1990) recommend a steaming length of 0.7 times the burden.

3. Blast area security
The US Code of Federal Regulations—CFR, Title 30 defines 'Blast Area' as the area in which concussion (shock wave), flying material, or gases from an explosion may cause injury to persons. Furthermore, the CFR states that the blast area shall be determined by considering geology or material to be blasted, blasting patterns, blasting experience of the mine personnel, delay systems, type and amount of explosive material, and type and amount of stemming.

During the last two decades, lack of securing blast areas caused 45.64% of the fatal and non-fatal accidents in coal surface mining due to failure to use appropriate blasting shelter, failure to evacuate blast area from humans, and inadequate guarding of the access roads leading to the blast area. Failure to evacuate humans from blast areas is complicated by the increase in accessibility to rough terrain brought on by the substantial increase in use of all terrain vehicles (ATV's). Areas inspected to be all-clear can be infiltrated by nonmining personnel on ATV's within seconds. The issue of blast area security can be successfully addressed by providing appropriate training and education of personnel involved in blasting operations to apply the best safety practices, as well as state and government regulations.

Furthermore, the blast area must be inspected to determine distances to nearby structures, roads, public places, and due consideration must be taken in determining the degree of protection necessary, including the use of line-of-sight inspection methods to guard against ATV's. It is of primary importance to clear all employees from the blast area, guards should be posted at the entrance to all access roads leading to the blast area, and the blaster should communicate to the foreman about the impending blast. The blaster must go outside the blast area or stay inside a blasting shelter, and after receiving the
feedback from the foreman and guards, blast signal needs to be sounded. A detailed study on safeguarding blast-affected areas is given by D'Andrea and Bennett (1984).

Furthermore, blasting regulations (30 CFR Part 77.1303) require that ample warning shall be given before blasts are fired, and all persons shall be cleared and removed from the blast area unless suitable blasting shelters are provided to protect persons endangered by concussion or flyrock from blasting.

4. Safety evaluations
The administration of industrial safety rests on the foundation that accident investigation results in the identification of cause followed by the appropriate response or correction in procedure. This approach can be referred as reactive safety, since the safety response mechanism occurs after an accident. The proactive safety response mechanism occurs when corrective action is taken after a non-event called a near-miss. A benchmark safety study done in 1969 involving over 3 billion man-hours revealed for every serious or disabling or serious injury, 10 minor injuries occur, 30 property damage events occur, and 600 incidents occur with no visible injury or damage (Bird, 1974). The 1-10-30-600 relationships indicate the essential value of proactive safety and the prevention of accidents depends on addressing the near-misses.

Dyno Nobel Corporation, like other large explosive manufacturers, provides product delivery to the minesite, and in fact into the very drill hole in mining operations. Exposure of an explosive's manufacturer's employees is therefore equal to miners. Dyno Nobel North America implemented a proactive safety program of which one the main elements was a near-miss reporting and evaluation procedure for their employees. The results have been excellent. For example, for the period from 1995 to 1998, lost time injury frequency rate decreased from 4.44 to 1.11, and lost time injury severity rate decreased from 95 to 29. The approach of reporting near-misses affected safety performance in a large magnitude.

5. Education and training
Effectively training the workforce in blasting hazard recognition and avoidance, and the safe use of explosives is an essential activity in reducing blasting incidents. In the United States, the use of explosives in mining is regulated at both the Federal and states- level. The federal government and individual state governments maintain and enforce health and safety, and training standards to help minimize blasting mishaps that endanger life and property. While each government entity works toward the same goal, the federal government and state governments assume somewhat different approaches to achieving the goal. Under federal training regulations contained in Title 30, CFR, Parts 48 and 46, (US Department of Labor, 2002) mining companies are required to train miners in the hazards related to explosives and safe blasting requirements through training curriculum content presented in either what is known as comprehensive training courses, i.e., new miner, annual refresher, newly-hired experienced, new task training, or hazard training (typically provided for contactors working on mine sites, or occasional visitors and service workers).

It would be accurate to say that if the mine uses explosives, the miner or contractor will be instructed at a minimum in blasting hazards and avoidance, and if the miner is assigned to a blasting crew, in the safe use of explosives. This instruction on the safe use of explosives would be provided in a task training course or a task training session within a new miner training course. The federal training regulations, if fully complied with, ensure that all miners and visitors are, at a minimum, trained in basic blast hazard awareness.

State level involvement in achieving the goal of safe blasting activities typically includes the establishment and implementation of a program of blasters' training, examination, and certification. The general purposes of these programs are to ensure that ‘‘blasts’’ are designed, supervised, and executed by trained and competent personnel (Alabama Surface Mining Commission Administrative Code, 2004). As an example of the training curriculum content, Pennsylvania's programs includes, but is not limited to, discussion and instruction on regulations, scaled distances, blast design, blasting materials, initiation systems, and record-keeping (Bureau of Mining and Reclamation, 2003). While the particulars of the programs differ by state, applicants to the program must pass a competency examination before being awarded a blaster's license. The licensed blaster becomes the ‘‘blaster-in-charge’’ for each blast. Such licensing programs attempt to maximize blasting accident prevention by ensuring that the blast be designed and executed in strict accordance with the statutory rules, and that adequate supervision, monitoring, and control of all blasting activities be administered by a certified person.

6. Conclusion
The historical trend over the 23-year period is a general decrease in the number of injuries and fatalities from blasting accidents for coal, metal and non-metal operations. Even though blasting accidents for all types of mining operations have declined, they continue to occur and cause fatalities and injuries. Mining personnel continue to suffer fatal and disabling injuries from blasting accidents. An analysis performed shows that the lack of blast area security and flyrock accounted for 281 (68.2%) accidents during the analyzed period.

A major challenge facing users of blasting techniques is how to apply the state-of-theart technology to assist them in evaluation of the potential to cause harm to workers, and to develop effective strategies for control and to minimize occupational health hazards associated with blasting. Training and education of personnel involved in blasting operations play a critical role in preventing fatalities and injuries, and should be focused on: codes and standards, workplace responsibility, assessing and developing accident prevention strategies, developing workplace safety procedures, implementing work practices that meet specified legislation and standards, identifying strategies for monitoring and updating safety and health information, effective occupational health and safety communications, and improving occupational health and safety performance.