Methods for determining roof fall risk in underground mines

Reducing the number of roof fall injuries is a goal of the NIOSH mine safety research program. Central to this effort is the development of assessment techniques to help identify the nature of the risks associated with working under potentially hazardous roof conditions. This paper discusses a method to determine the roof fall risk using a qualitative risk-analysis technique. The ability to determine roof fall risk has been a long-standing goal of safety professionals and could provide the kind of information needed by on-site personnel responsible for worker safety to mitigate roof fall injuries. What is the state-of-practice for minerals industry risk assessment? The International Organization for Standardization (ISO) and the American National Standards Institute (ANSI) produce standards and guidelines that define the use of risk-assessment and risk-management methods. When applied to a particular industry, the issues unique to that industry require special approaches. For example, the environmental and health sciences have long used risk-assessment and risk-management methods to identify the highest environmental and occupational health and safety risks and to develop controls specific to their operational and regulatory environments (National Research Council, 1983, 1994, 2006). Risk-assessment and risk-management methods for the mining industry are more prevalent in countries with safety standards that emphasize duty-of-care, i.e., Australia, Canada, United Kingdom and South Africa, rather than the prescriptive health and safety regulations, i.e., the United States. Duty-of-care in these countries is defined in legislation that requires employers, suppliers and employees to provide, design for and adhere to reasonable activities that ensure workers are cared for. In Australia, an ISO has been specifically developed (Anon, 2004) to enable organizations to implement environmental management systems (EMS) for continuous improvement in their operations. In the mid-1990s, Australia’s mineral industry became heavily involved in risk-management methods that typically consisting of structured, teambased exercises to review potential problems carefully with new or existing mining methods, new equipment or other operational problems (Joy, 2001). Joy estimates that at least 80 percent of all Australian coal mines have performed some form of structured, team-based risk assessment/risk management. Tools used in these exercises include HAZOP (Hazard and Operability Analyses), FMECA (Failure Modes, Effects and Criticality Analysis), WRAC (Workplace Risk Assessment and Control) and the BTA (Bow Tie Analysis). All of these tools and techniques are defined in a framework by Joy (2006) to explain the management of risk in the minerals industry. Lastly, the Minerals Industry Safety and Health Center (MISHC) Web site is an excellent source for information on Australia’s diverse risk-assessment/risk-management approaches (www.mishc.up.edu.au). Examples of risk assessment applied to ground control issues In the early 1990s, the United Kingdom (UK) developed a code of practice (now referred to as Industry Guidance) for rock bolt use as roadway supports that included geotechnical assessment, initial design, design verification and routine monitoring (Arthur et al., 1998). Cartwright and Bowler (1999) provided a UK example of a procedure to assess the risk associated with potential failure or overloading of rock-bolt support systems. In the mid-1990s, South African mines developed codes of practice to combat rock fall and rock burst accidents, as required by its 1996 Mine Health and Safety Act (Gudmanz, 1998). Swart and Joughin (1998) discussed the importance of rock engineering in developing this code of practice. Van Wijk et al. (2002) developed a riskassessment method for use in South African coal mines. This risk-assessment method aims to optimize resources and focuses attention on the areas where it is most required. Lind (2005) demonstrated an integrated riskmanagement method that required a basic assessment of physical parameters such as coal seam characteristics, depth below surface and mining conditions. The Minerals Council of Australia (MCA) helped produce a national guideline for the management of roof fall risks in underground metalliferous mines (MOSHAB, 1997). Potvin and Nedin (2003) published a “Reference Manual” in support of the MCA guidelines meant as a collection of techniques and examples FIGURE 1 Flow diagram depicting the generalized structure of roof fall risk assessment activities and its relation to risk management activities. Date Mine Company State 1/10/06 #1 Maverick KY 1/29/06 Aberdeen Andalex UT 2/1/06 #18 Tunnel Long Branch WV 2/16/06 HZ4-1 Perry County KY 3/29/06 #4 Jim Walter AL 4/20/06 #1 Tri Star KY 10/6/06 #2 D & R KY 10/12/06 #7 Jim Walter AL 10/20/06 Whitetail Kittanning Alpha Natural Resources WV 12/17/06 Prime #1 Dana Mining WV Table 1 Fatal roof fall injuries in underground coal mines during 2006. of good roof control practices. Roof fall hazard-assessment techniques Risk-assessment methods provide a systematic approach to identifying and characterizing risks, especially those associated with low-probability, high-consequence events such as roof falls. The first step in utilizing a roof fall risk-assessment method requires identification of the potential roof fall hazards. Because local geologic, stress and mining conditions interact to create varying roof conditions, commodity-specific or activity-based hazard-assessment techniques and associated risk-analysis techniques are needed to locate potential risk within workplaces throughout the mine. Many hazard-assessment techniques generally can be classified into one of the following three groups: hazard maps, rock-mass classification systems and monitoring data. While all three techniques are useful in hazard assessment, they have had only limited application when applied to roof fall risk assessment. To help improve the link between hazard assessment and risk assessment, NIOSH developed a tool called the roof fall risk index (RFRI) to systematically identify roof fall hazards. The RFRI is specifically developed for underground stone mine and is mentioned here as an example that could be adapted to mining conditions. The RFRI focuses on the character and intensity of defects associated with specific roof conditions and attempts to incorporate some of the characteristics discussed in the other hazard assessment techniques (Iannacchione et al., 2006; Iannacchione et al., 2007). The defects measured within the RFRI can be caused by a wide range of local geologic, mining and stress factors and are equated directly to changing roof conditions causing roof fall hazards. A significant range of defects found at underground stone mines are classified into 10 categories (known as defect categories), each of which is assigned an assessment value ranging from 1 to 5, with the numerical value increasing with the severity of the defects. To calculate the RFRI, one must determine the assessment value for each defect category, multiply by an assigned weight (either 1 or 2), add all category values together and multiply by 1.11. Ideally, values approaching zero represent safer roof conditions, while an RFRI approaching 100 represents a serious roof fall hazard. The RFRI is a hazard-assessment technique that can be used as both a training tool and a communication tool. This technique requires that roof fall hazards be mapped and the spatial distribution within the unFIGURE 2 (a) RFRI values for the 226 measurement area that comprised the study area and (b) histogram of RFRI frequency. Coal Metal Nonmetal Stone Total Injury Fatal Injury Fatal Injury Fatal Injury Fatal Injury Fata Year rate rate rate rate rate rate rate rate rate rate 1996 1.8 0.029 2.08 0.016 0.36 0.0 0.58 0.116 1.71 0.028 1997 1.9 0.02 2.12 0.032 0.43 0.0 0.5 0.055 1.8 0.022 1998 2.03 0.033 2.07 0.052 0.44 0.0 0.52 0.0 1.89 0.032 1999 1.89 0.031 1.82 0.061 0.59 0.0 0.92 0.051 1.77 0.033 2000 1.98 0.011 1.63 0.023 0.4 0.0 0.45 0.0 1.79 0.011 2001 1.79 0.03 1.01 0.09 0.31 0.0 0.52 0.0 1.58 0.032 2002 1.75 0.011 0.94 0.0 0.31 0.0 0.59 0.0 1.55 0.009 2003 1.51 0.009 0.86 0.0 0.3 0.0 0.43 0.0 1.34 0.007 2004 1.5 0.008 0.68 0.0 0.25 0.0 0.31 0.0 1.31 0.007 2005 1.34 0.023 0.81 0.0 0.33 0.0 0.24 0.0 1.19 0.019 Total 1.75 0.021 1.51 0.03 0.38 0.0 0.5 0.021 1.6 0.021 Injury rate = Roof fall injuries (Degree of Incident, Class 1-6) per 200,000 hours worked underground. Fatal rate = Roof fall fatalities per 100,000 miners. Table 2 Roof fall injury and fatality rates over then 10-year period from 1996 to 2005 for