Analysis of Safety Aspects And Mining Practices For Effective Ground Control in Surface Mining

Fatalities caused by highwall/spoilbank failure in the surface mines, coal and non-metal, increased to an alarming rate of seven during 1999. To determine the causes of slope failure and successful mining practices, the National Institute for Occupational Health and Safety undertook a study. The study included: 1. A review of accident statistics; 2. A review of Federal and state mining laws pertaining to surface mining; 3. A literature review, and; 4. Mine visits. The study emphasized surface mines in the states of West Virginia, Ohio, and Pennsylvania. The review of a decade’s accident statistics using the Mine Safety and Health Administration’s (MSHA) database showed that approximately 40% of all ground control related incidents reported to MSHA occurred in just four eastern states: Kentucky, West Virginia, Pennsylvania, and Ohio. The comprehensive literature search provided a historical perspective of highwall stability issues. Eleven mines were visited to obtain data on their mining practice or design. Commodities included coal, sandstone, and limestone. Based on the visits, five case studies were developed to represent typical mining methods and effective ground control practices used in eastern surface mines. Benching was found to be a common technique to reduce the overall highwall slope angle. Decking in the softer zones, such as shale, proved useful in controlling damage due to blasting. INTRODUCTION In recent years, highwall/spoilbank failures in the surface mines have resulted in significant loss to human lives, property, and production. A highwall is always changing as the process of extracting coal or ore continues. The challenge, therefore, is to maintain a stable highwall throughout the mine's operating life. A stable highwall requires an optimum slope under given conditions. Effective slope design includes determining safe and workable bench height, bench face angle , and bench width. To determine the relationship of mine design parameters/practices and effective ground control in surface mining, this study was undertaken. It included coal and nonmetal mines, and consisted of four parts: • A review of accident statistics for the 1988-1997 period and Mine Safety and Health Administration’s (MSHA) accident investigation reports for the 1996-1999 period; • A review of pertinent Federal mining laws and the State laws of Pennsylvania, West Virginia, and Ohio; • A comprehensive review of relevant published literature over the past decade; and • Visits to eleven operating surface mines representative of the tri-state area. The mining laws, both Federal and state, guide the design of surface mines; significant regulations are summarized in Appendix A. The data obtained from the mine visits was utilized to prepare five case studies to show the area’s different geological settings, operating parameters, and effective ground control practices under existing conditions. The knowledge gained from this research will enable better mine planning so that the health and safety of the surface mine worker can be improved. ACKNOWLEDGMENTS The authors thank the managements of mining companies for their participation in this study, and Deno M. Pappas of the National Institute for Occupational Safety and Health, Pittsburgh Research Laboratory for assisting with the MSHA data base on accident statistics. ACCIDENT ANALYSIS Period 1988 1997 Highwall accident statistics from the MSHA data base were analyzed for the ten-year period for incident frequency, degree of injury, nature of injury, equipment involved, coal and non-metal breakdown, worker activity at the time of accident, and other relevant parameters. Salient results of the analysis are presented below. In all, there were 428 accidents caused by the highwall instability in active coal and non-metal mines. Four mines had four incidents each, 13 mines had 3, 35 mines had 2, and the remaining 303 mines had one highwall incident. Thus, a total of 355 mines reported 428 incidents during the period. Mines with multiple incidences, three and four, are listed in Appendix B. For each mine, number of incidents, year of occurrence, failure type (highwall/spoilbank), degree of injury, and the commodity mined are provided. The table 1 shows the breakdown of all accidents by degree of injury: Table 1. Accidents by degree of injury Degree category (definition) Occurrence, % 0 (No injury) . . . . . . . . . . . . . . . . . . . . . . . . . 5.0 1 (Fatal) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.0 2 (Permanent disability) . . . . . . . . . . . . . . . . 0.0 3 (Days away from work) . . . . . . . . . . . . . . . 57.0 4 (Days away and restricted activity) . . . . . . 5.0 5 (Days of restricted activity) . . . . . . . . . . . . 4.0 6 (No days lost) . . . . . . . . . . . . . . . . . . . . . . . 22.0 Total . . . . . . . . . . . . . . . . . . . . . . . . . 100.0 Of the 28 fatal accidents, the majority (70%) occurred in non-metal mines. This represents approximately three fatal accidents per year over the 10-year period. Considering all accident incidents (428), the distribution between coal and nonmetal mines is about 50-50. On worker exposure basis, the incidence rate at surface coal mines is about twice that at surface nonmetal mines. Refer to the section on surface mine worker population. The 428 accidents were categorized by the nature of injury caused and are as follows (table 2): Table 2. Accidents by nature of injury Nature of injury Occurrence, % Contusions, bruises . . . . . . . . . . . . . . 23.0 Multiple injuries . . . . . . . . . . . . . . . . 18.0 Cut, laceration, puncture . . . . . . . . . . 17.0 Sprain, strain . . . . . . . . . . . . . . . . . . . 8.0 Others* . . . . . . . . . . . . . . . . . . . . . . . . . 34.0 Total . . . . . . . . . . . . . . . . . . . 100.0 *Has over 30 injury classifications. Approximately 26% of all accidents caused by highwall failure involved heavy earth moving equipment, such as the drill, front-end loader, dozer, shovel/dragline, and trucks. Approximately half of the injuries were caused when the failed material hit the victim directly. The remaining 24% involved miscellaneous equipment items (over 40). Figure 1 shows this breakdown. Handling supplies or material was found to be the single most frequent (11%) worker activity when injured, followed by idle category (7%). Handling explosives, drilling face, and surface equipment (not elsewhere classified) each represent about 6% of all accidents (18% total). The remaining 64% are distributed over 80 mine worker activities. Figure 2 shows this breakdown. Figure 1. Accidents by worker involvement Surface Equipm ent 6% Drilling Face 6% Handling Explosive 6% Handling Supplies 11% Idle 7% M isc. Activities 64% Figure 2. Accidents by worker activity Four eastern states, Kentucky, West Virginia, Pennsylvania, and Ohio, experienced some 40% of all highwall incidents during the study period. Accident narratives do not provide accurate information on the size of rock falls. It can generally be estimated that about 50% of highwall instability incidents are caused by falling of small rocks or fragments. A review of months of incident occurrence does not provide a significant correlation. In other words, winter months were not significantly accident prone due to freeze-thaw cycles. Current Data MSHA's Fatalgrams and accident investigation reports were reviewed for fatalities in 1998 and 1999. In 1998, there was one highwall failure accident in a coal operation. In 1999, there were seven fatalities, four in coal and three in non-metal mines. This is over two-fold increase compared to the average of approximately three fatalities per year during the ten-year study period. Salient findings/data for the eight accidents are presented below. Two accidents were caused when spoilbank collapsed, the remaining six resulted due to falling rock from the highwall of active mining operations. In three cases, failed highwall material directly hit the victim; whereas, workers were operating equipment in the remaining situations. The equipment involved in the accidents include highwall drill, excavator (shovel), and truck. The occupation of workers included all phases of operations, i.e., truck driver, drill operator, excavator/shovel operator, and a company owner. The size of rock falls varied between unspecified amount to massive or extremely large. Some reports mentioned the size as one kilogram, one tonne, and several tonnes. Best practices, recommended by MSHA, to avoid the accidents were given as: • Examine and monitor highwall often. • Follow ground control plan. • Train miners to recognize hazardous highwall conditions. • Scale-down or support the hazardous highwall areas. • Keep drill and other mobile equipment operators away from highwall face or highwall hazards by positioning them in safe locations. • Employ mining methods that will maintain wall, bank, and slope stability in places where persons work or travel. • Provide adequate berms to prevent over-travel at dump locations. MSHA’s accident investigation reports for the 1996-1999 period were thoroughly analyzed. Twelve fatality reports are summarized in Appendix C. The table includes data such as the commodity mined, types of failure, accident description, worker activity at the time of accident, weather conditions, citations, victim’s occupation, equipment involved, and the height of highwall. SURFACE MINE WORKER POPULATION Mine Injury and Worktime, Quarterly from MSHA provided approximate surface mine employment data for 1998: Commodity No. of workers Coal (includes anthracite and contractors) . 36,350 Non-metal (includes stone and contractors) 68,300 Total . . . . . . . . . . . . . . . . . . . . . . 104,650 As can be seen from the above, over 104,000 workers are exposed to the hazards of highwall instability in the United States mining industry. LITERATURE REVIEW Literature review pertaining to slope stability is provided below in three categories. Mine Planning and Design Mining parameters in a surface mine are influenced by strata conditions and their subsequent design for actual mining process. The structural aspects of overburden and floor material play a significant role in the predictive behavior of rock masses in response to the mining operations, especially of highwall stability and the formation of spoil dumps (1), and therefore, the choice of mining parameters. These parameters include bench width, height, and slope angle. Planning of open-pit mines on a business-risk basis (2) emphasizes that confidence for slope design should be categorized using the same fundamental approach as that adopted for resource definitions–such as proven and possible reserves. For example, a proven slope angle requires that the continuity of the stratigraphic and lithological units within the affected rock mass is confirmed in space from adequate intersections. A possible slope angle, whereas, corresponds to typical slope angles based on experience in similar rocks and along discontinuities. A risk balance between mineral resources and slope angle is needed for developing efficient mine plans. Study of South Wales open-pit site (3) provides significant relationships between the slope design and structural geological factors. The geological parameters that are most relevant to rock slope stability are: • The persistence, attitude, and nature of discontinuities within the rock mass. • The shear strength characteristics both within the rock mass and along discontinuities. • The rock density. • The potential for build-up of water pressure in the rock mass and in tension cracks in rock slopes. Bedding planes are the most common through-going discontinuities, followed by joints, in coal measure sequences. Transcurrent (strike-dip) faults, unless major, do not lead to slope instability. Open-pit designs should attempt to avoid the presence of adversely oriented structurally controlled discontinuities in the highwall. Gregg River Mine (4) in Alberta, Canada, had two highwall slopes 160 and 219.5 m (525 and 720 ft) high excavated in a dipping strata in approximately the same lithologic and structural environment. In the earlier phase of mining, the highwall experienced instability. In the later phase, using flatter bench face angles, the slope did not experience any significant movements. It was found that 50° bench face angle prevented toppling failure. Also, horizontal drain holes in critical areas of slope were successful in keeping water away. A slope stability analysis performed at the Beulah lignite mine (5) indicated that most failures were in spoil piles more than 27.4 m (90 ft) high. Circular arc failures mostly occurred when the spoil slope were too steep for the height. The study recommended decreasing the pit width and lowering the overall slope angle by use of a spoil bench.