Methods for determining roof fall risk

in underground mines
Background bolts are in use” or as “an unplanned
A realistic goal for underground
A. IANNACCHIONE, L. PROSSER, roof or rib fall in inactive workings
mines trying to reduce the incidence G. ESTERHUIZEN AND T. BAJPAYEE that impairs ventilation or impedes
of miner injuries associated with passage” (Anon, 2005). In general,
A. Iannacchione, member SME, L. Prosser, G. Esterhuizen,
roof falls is to assess the conditions roof fall reporting requirements
member SME, and T. Bajpayee, member SME, are principal
that pose a roof fall risk. If mine consist of time and date, location in-
research engineer, research scientist, senior research fellow and
operators properly assess roof fall formation, mining method involved,
lead reasearch engineer, respectively, with the National Institute for
hazards, they can better reduce roof equipment involved and a narrative
Occupational Safety and Health (NIOSH), Pittsburgh Research Labo-
fall risks with appropriate engineer- that fully describes the conditions
ratory, Pittsburgh, PA. Paper number TP-06-047. Original manuscript
ing and administrative controls. Any contributing to the roof fall and that
submitted online Dec. 2006 and accepted for publication June 2007.
methodology that helps attain this also quantifies the damage. The stan-
Discussion of this peer-reviewed and approved paper is invited and
goal can be thought of as a roof fall dard also requires the operator to
must be submitted to SME Publications Dept. prior to Feb. 29, 2008.
risk-assessment method. An effec- take steps to prevent a recurrence.
tive roof fall risk-assessment method The mining law, however, does not
includes the ability to observe vari- specifically require that information
able roof conditions and assess how much these condi- about hazards that could cause roof falls be displayed on
tions represent the potential for a roof fall capable of mine maps or communicated to mine workers.
injuring miners. This methodology should rank the risks
associated with varying conditions, should be reasonably Why is a roof fall risk-assessment method
reproducible and should clearly indicate roof fall risk to important for improving miner safety?
all mine personnel responsible for the design, approval The potential for roof falls in underground mines is
or installation of controls that either stabilize the roof a significant danger for mine workers. In 2006, 10 fatal
or lessen the exposure to roof falls. This paper focuses ground fall injuries occurred (Table 1). Also, during the
on the risk-assessment issues, leaving the roof fall risk- 10-year period from 1996 through 2005, 7,738 miners
management process, where controls are designed and were injured from roof falls in underground coal, metal,
used to reduce risk, to another discussion. nonmetal and stone mines (MSHA, 2005). Coal mines
One of the most important safety issues at any mining had the highest rate, 1.75 roof fall injuries per 200,000
site is the need to identify the location and nature of roof hours worked underground (Table 2). While this rate
fall hazards. The mining law requires that roof falls be dropped over this period, there were still 581 recorded
reported to enforcement agencies by Form 7000-1. Roof roof fall injuries in 2005, with many classified as severe.
fall locations are to be displayed on mine maps and made Fatal injury trends from 1996 to 2005 were equally trou-
available to miners or their representatives. The Code of bling, with 100 roof fall fatalities. While coal mining had
Federal Regulations, Title 30 Part 50 Section 2, defines a the highest number with 82, metal mining had the high-
reportable roof fall as “an unplanned roof fall at or above est rate with 0.03 fatalities per 100,000 miners (Table 1).
the anchorage horizon in active workings where roof These statistics attest to the seriousness of this safety
issue, although roof fall injuries decreased from 1.71 in
1996 to 1.19 in 2005 per 200,000 hours worked (Table
Abstract 2). Clearly, progress in miner safety has been made, but
further improvement is possible. It is imperative that new
Reducing the number of roof fall injuries is a goal of the safety techniques and methodologies continue to be de-
NIOSH mine safety research program. Central to this ef- veloped, so this downward trend in roof fall injures can
fort is the development of assessment techniques to help be maintained.
identify the nature of the risks associated with working Most safety decisions in the U.S. mining industry are
under potentially hazardous roof conditions. This paper guided by company policy and the requirements of state
discusses a method to determine the roof fall risk using and federal regulations. These decisions have been suc-
a qualitative risk-analysis technique. The ability to deter- cessful in reducing roof fall injuries (Table 2). For this
mine roof fall risk has been a long-standing goal of safety study, the author’s underlying assumption is that incorpo-
professionals and could provide the kind of information rating risk-assessment and risk-management methods to
needed by on-site personnel responsible for worker safety the existing decision-making process will help to further
to mitigate roof fall injuries. reduce miner injury rates.
FIGURE 1 problems carefully with new or existing mining meth-
ods, new equipment or other operational problems (Joy,
Flow diagram depicting the generalized structure of roof
2001). Joy estimates that at least 80 percent of all Aus-
fall risk assessment activities and its relation to risk
tralian coal mines have performed some form of struc-
management activities.
tured, 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 manage-
ment of risk in the minerals industry. Lastly, the Miner-
als 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) de-
veloped 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 poten-
tial failure or overloading of rock-bolt support systems.
In the mid-1990s, South African mines developed codes
What is the state-of-practice for minerals of practice to combat rock fall and rock burst accidents,
industry risk assessment? as required by its 1996 Mine Health and Safety Act
The International Organization for Standardization (Gudmanz, 1998). Swart and Joughin (1998) discussed
(ISO) and the American National Standards Institute the importance of rock engineering in developing this
(ANSI) produce standards and guidelines that define code of practice. Van Wijk et al. (2002) developed a risk-
the use of risk-assessment and risk-management meth- assessment method for use in South African coal mines.
ods. When applied to a particular industry, the issues This risk-assessment method aims to optimize resources
unique to that industry require special approaches. For and focuses attention on the areas where it is most re-
example, the environmental and health sciences have quired. Lind (2005) demonstrated an integrated risk-
long used risk-assessment and risk-management meth- management method that required a basic assessment
ods to identify the highest environmental and occupa- of physical parameters such as coal seam characteristics,
tional health and safety risks and to develop controls depth below surface and mining conditions.
specific to their operational and regulatory environ- The Minerals Council of Australia (MCA) helped
ments (National Research Council, 1983, 1994, 2006). produce a national guideline for the management of
Risk-assessment and risk-management methods for roof fall risks in underground metalliferous mines
the mining industry are more prevalent in countries (MOSHAB, 1997). Potvin and Nedin (2003) published
with safety standards that emphasize duty-of-care, i.e., a “Reference Manual” in support of the MCA guide-
Australia, Canada, United Kingdom and South Africa, lines meant as a collection of techniques and examples
rather than the prescriptive health
and safety regulations, i.e., the United
States. Duty-of-care in these countries Table 1
is defined in legislation that requires Fatal roof fall injuries in underground coal mines during 2006.
employers, suppliers and employees
to provide, design for and adhere Date Mine Company State
to reasonable activities that ensure
1/10/06 #1 Maverick KY
workers are cared for. In Australia, an
1/29/06 Aberdeen Andalex UT
ISO has been specifically developed
2/1/06 #18 Tunnel Long Branch WV
(Anon, 2004) to enable organizations
2/16/06 HZ4-1 Perry County KY
to implement environmental manage-
3/29/06 #4 Jim Walter AL
ment systems (EMS) for continuous
4/20/06 #1 Tri Star KY
improvement in their operations.
10/6/06 #2 D&R KY
In the mid-1990s, Australia’s min-
10/12/06 #7 Jim Walter AL
eral industry became heavily involved
10/20/06 Whitetail
in risk-management methods that typ-
Kittanning Alpha Natural Resources WV
ically consisting of structured, team-
12/17/06 Prime #1 Dana Mining WV
based exercises to review potential
of good roof control practices. FIGURE 2
(a) RFRI values for the 226 measurement area that comprised the study area
Roof fall hazard-assessment and (b) histogram of RFRI frequency.
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 inter-
act to create varying roof conditions,
commodity-specific or activity-based
hazard-assessment techniques and as-
sociated risk-analysis techniques are
needed to locate potential risk with-
in 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 as-
sessment, they have had only limited
application when applied to roof fall
risk assessment. the defects. To calculate the RFRI, one must determine
To help improve the link between hazard assessment the assessment value for each defect category, multiply
and risk assessment, NIOSH developed a tool called the by an assigned weight (either 1 or 2), add all category
roof fall risk index (RFRI) to systematically identify values together and multiply by 1.11. Ideally, values
roof fall hazards. The RFRI is specifically developed for approaching zero represent safer roof conditions, while
underground stone mine and is mentioned here as an an RFRI approaching 100 represents a serious roof fall
example that could be adapted to mining conditions. The hazard.
RFRI focuses on the character and intensity of defects The RFRI is a hazard-assessment technique that
associated with specific roof conditions and attempts can be used as both a training tool and a communica-
to incorporate some of the characteristics discussed tion tool. This technique requires that roof fall hazards
in the other hazard assessment techniques (Iannac- be mapped and the spatial distribution within the un-
chione et al., 2006;
Iannacchione et al., Table 2
2007). The defects
Roof fall injury and fatality rates over then 10-year period from 1996 to 2005 for
measured within the
underground mines.
RFRI can be caused
by a wide range of Coal Metal Nonmetal Stone Total
local geologic, min- Injury Fatal Injury Fatal Injury Fatal Injury Fatal Injury Fata
ing and stress fac- Year rate rate rate rate rate rate rate rate rate rate
tors and are equated
directly to changing 1996 1.8 0.029 2.08 0.016 0.36 0.0 0.58 0.116 1.71 0.028
roof conditions caus- 1997 1.9 0.02 2.12 0.032 0.43 0.0 0.5 0.055 1.8 0.022
ing roof fall hazards. 1998 2.03 0.033 2.07 0.052 0.44 0.0 0.52 0.0 1.89 0.032
A significant range 1999 1.89 0.031 1.82 0.061 0.59 0.0 0.92 0.051 1.77 0.033
of defects found at 2000 1.98 0.011 1.63 0.023 0.4 0.0 0.45 0.0 1.79 0.011
underground stone 2001 1.79 0.03 1.01 0.09 0.31 0.0 0.52 0.0 1.58 0.032
mines are classified 2002 1.75 0.011 0.94 0.0 0.31 0.0 0.59 0.0 1.55 0.009
into 10 categories 2003 1.51 0.009 0.86 0.0 0.3 0.0 0.43 0.0 1.34 0.007
(known as defect 2004 1.5 0.008 0.68 0.0 0.25 0.0 0.31 0.0 1.31 0.007
categories), each of 2005 1.34 0.023 0.81 0.0 0.33 0.0 0.24 0.0 1.19 0.019
which is assigned
an assessment value Total 1.75 0.021 1.51 0.03 0.38 0.0 0.5 0.021 1.6 0.021
ranging from 1 to
5, with the numeri- Injury rate = Roof fall injuries (Degree of Incident, Class 1-6) per 200,000 hours worked under-
cal value increasing ground.
with the severity of Fatal rate = Roof fall fatalities per 100,000 miners.
FIGURE 3 estimated with the RFRI, while injury
potential is estimated by the miner’s
Miner activity (fictional example) and related miner exposure for the 226 mea- exposure to hazardous roof conditions.
surement areas. Miner exposure is dependent on the
frequency of an activity within an area
versus the percentage of the work-
force involved in that activity (Table
4). The activity frequency can range
from many times per shift to once per
month, while the percentage of the
workforce involved in that activity can
range from many (>50 percent) to few
(<5 percent). Roof fall probability and
miner exposure can be determined for
all areas of the mine accessible by the
miner.
The consequence term in the risk
equation typically refers to the sever-
ity of the event. When a miner is in-
jured from a roof fall, some medical
attention is required. For example, of
the 7,738 miners injured from roof falls
between 1996 and 2005, 1.3 percent re-
derground workplace determined. The RFRI strives sulted in a fatality, the rest required medical attention
to assess roof conditions over large, continuous areas, (Table 2). For most of the nonfatal injuries, the rock that
producing a comprehensive assessment of changing struck the miner was probably relatively small. Because
roof conditions than was previously possible. it is beyond the author’s abilities to forecast the size of
a roof fall, it was assumed that any roof fall could seri-
Moving from hazard assessment ously injure a miner. Therefore, the consequence should
to risk assessment always be considered severe and assigned a unit value of
Hazard assessment in conjunction with the mine’s 1. This effectively takes away the consequence term from
individual roof control plans can be thought of as an im- this analysis. Therefore, a more appropriate definition for
plicit form of risk assessment. Ideally, a hazard-assess- roof fall risk is
ment technique should be capable of ranking the various
hazards and communicating these hazards to the persons Roof fall risk = Roof fall probability x Miner exposure
or groups in need of this information. The outcome of this to roof falls (2)
process can aid in establishing minimum roof support
standards for a mining operation where a general class of Qualitative approach to measure roof fall risk
hazards is being addressed. The focus of this roof fall risk- A qualitative approach allows for estimations of roof
assessment approach is on identifying areas of highest fall probability and miner exposure. Roof fall probabil-
risk so that additional controls can be applied.
These controls can range from additional Table 3
monitoring to supplemental roof support.
Identifying areas of highest roof fall risk is A generalized risk matrix used in many qualitative risk-analysis
accomplished through standard risk-analysis techniques.
methods, where the probability of occurrence Probability of occurrence
and its consequence are determined using
Consquences High value Medium value Low value
Risk = Probability of occurrence x Conse-
quence (1) High value High risk
Medium value Moderate risk
Of the many different risk-assessment Low value Low risk
methods discussed in the literature, only a few
risk analysis techniques apply to the roof fall Table 4
problem. For example, when determining the
probability of occurrence, two very different Exposure of miners within a particular work area.
approaches are available: qualitative assess- Frequency
ment and quantitative assessment. This paper Percent of Many times/shift 1/day 1/week 1/month
focuses on a qualitative risk-analysis technique workforce
using a risk matrix as shown in Table 3.
For a roof fall event, the Probability of oc- Most >50% A A B C
currence term in Eq. (1) consists of two factors: Many – 30% A B C D
the probability of a roof fall occurring and the Several – 10% B C D E
potential for a miner being injured by this roof Few <5% C D E E
fall. Roof fall probability in this analysis can be
ity can be qualified by calculating the FIGURE 4
RFRI over regions of an underground
(a) Ranked risk for roof fall injuries over the 226 measurement areas comprising
mine and by grouping RFRI values to
the study area and (b) histogram of roof fall risk categories. The study area is a
appropriate roof fall probability cat-
fictional case presented here as an instructional example.
egories that range from very unlikely
to very likely. RFRI values approach-
ing 0 represent low defect conditions
typically associated with stable roof
conditions and imply a very unlikely
roof fall probability. Conversely, RFRI
values approaching 100 represent ex-
cessive defect conditions typically as-
sociated with unstable roof conditions,
implying a highly likely roof fall prob-
ability. Intermediate RFRI values fall
into the unlikely, possible and likely
roof fall probability category.
The other input for calculating roof
fall risk, miner exposure, requires an
estimation of miner activity through
these same measured areas used in the
RFRI analysis. These estimated pa-
rameters are used within a risk matrix
(Table 5) to assign the relative roof fall
risk for any accessible area within a
mine. As roof conditions and patterns
of miner activity change within a mine,
roof fall risk changes accordingly. The ultimate utility of series of roof fall risk levels tied to changing roof condi-
the risk rankings, shown in Table 5, lies in ones ability to tions. Because risk can be ranked throughout the mine,
identify areas with the highest risk and to design controls risk-management methods can be used to determine how
that mitigate risk in a logical and thoughtful fashion. to mitigate the risk.

Characteristics of a roof fall Demonstration of a roof fall

risk-assessment method risk-assessment method
The process to assess risk and implement controls to The intention of the following example is to detail
manage risks can be thought of as a series of steps (Fig. a comprehensive risk-analysis method and to apply it
1). The first step is to recognize and rank defective roof to an experience at a mine setting. In a previous paper
conditions within active portions of the mine. By doing (Iannacchione et al., 2006), the RFRI values at an active
this, hazards are identified and some attempt can be made underground stone mine were calculated and placed on
to rank these hazards from low to high. The next step uses a mine map (Fig. 2 (a)). The study area was divided into
a wide variety of risk-analysis techniques to determine 226 measurement areas that ranged in size from that of a
roof fall probability associated with specific conditions. 15 x 15 m (50 x 50 ft) intersection to the 15- to 30-m- (50
Miner exposure, a key element in assessing risk, is next to 100 ft-) long entries between intersections. The RFRI
determined by estimating the amount of time miners are frequency distribution is shown in Fig. 2 (b). Roof fall
expected to occupy the different locations within the ac- probability is implied directly from the RFRI values and
tive underground workings. Combining the probability of divided into five categories: very unlikely, unlikely, pos-
roof falls with the estimations of miner exposure yields a sible, likely and very likely.
This analysis uses logically assumed miner exposure
data that rep-
Table 5 licated a main
A risk matrix comparing roof fall probability with miner exposure. The exposure period used for haulage route
this example could range from one to six months. running north-
south in the
Roof fall probability
center of the sec-
Risk
tion, a secondary
exposure Highly likely Likely Possible Unlikely Very unlikely
haulage route
(From Table 5) (RFRI > 50) (RFRI 41 to 50) (RFRI 31 to 40) (RFRI 21 to 30) (RFRI < 21)
running along
the western
A 1 2 4 7 11
portion of the
B 3 5 8 12 16
section, active
C 6 9 13 17 20
development
D 10 14 18 21 23
faces along the
E 15 19 22 24 25
southern perim-
eter of the sec-
tion, an idle Table 6
section be-
hind the faces A potential correlation between a risk ranking and the levels of acceptability of existing and new
and between control measures to mitigate roof fall injuries.
the haulage
r o u t e s, a n d Risk ranking Risk category Percentage Risk management response
restricted ar-
eas to the east 1–5 High 12 Unacceptable, additional controls needed
(Fig. 3). The 6–10 Elevated 13 Undesirable, additional controls should be examined
m a i n h a u l - 11–15 Moderate 20 Acceptable with management review and approval
age route in 16–20 Minimal 34 Acceptable with monitoring and auditing
a stone mine 21–25 Low 21 Acceptable
is the area
of highest
miner activity, where most of the workforce is in that • investigate strategic or tactical controls;
location many times per shift. Therefore, using Table 4, • monitor the performance of the controls; and
an exposure value of A is assigned. The secondary haul- • modify them as needed, in an iterative process, thus
age route and the development faces are assigned an continually addressing the highest roof fall risk ar-
exposure value of B, because approximately 30 percent eas.
of the shift workforce is in these areas at least once per These methods help to rule out the option of doing
day. Idle sections away from the haulage routes and the nothing by introducing required actions in certain situa-
development areas are assigned an exposure value of C tions through structured decision-making.
where approximately 10 percent of the shift workforce is There are four basic steps to the roof fall risk-assess-
in these areas once per day. The restricted area, and the ment method used in this paper:
escapeway that services this section, is assigned the low-
est exposure value of E. • Recognize and rate defective roof conditions that
It is now possible to use the 5 x 5 risk matrix shown represent roof fall hazards: This is accomplished with
in Table 5 to estimate the risk associated with each of the the RFRI hazard assessment technique.
226 measurement areas within the study area. Twenty-five • Determine the roof fall probability for specific roof
risk rankings are identified ranging from 1, the highest, conditions: This is accomplished using qualitative
to 25, the lowest (Table 5). Within these rankings, five analysis techniques where RFRI values were grouped
risk categories are subjectively assigned, ranging from into logical probability categories.
high to low (Table 6). Seventy-five percent of the study • Evaluate the exposure of miners to roof falls in the
area measurement areas are within the moderate-to-low study area.
risk categories. A potential correlation between the risk • Rank the roof fall risk for all active workplaces within
ranking and the action taken to manage risk are given the mine using a risk matrix: Rating or ranking roof
in Table 6. Clearly, a risk ranking method such as this fall risks helps to identify what areas should be moni-
allows the mine operator to focus attention on high-risk tored most closely by the mine operators and miners
areas in the main haulage and development entries where alike. It is also critical for prioritizing the areas where
proactive tactical and strategic controls to mitigate these administrative and/or engineering controls are needed
hazardous conditions can be applied. To ensure effective most to reduce these risks.
implementation of these controls, it is necessary that the
mine operator strive to: This paper demonstrates how roof fall risk can be as-
sessed by appropriately designed hazard assessment and
• understand how roof falls occur, qualitative risk-analysis techniques. These techniques
• decide how to deal with crucial roof fall warning help to rate hazards, rank roof fall risk over a mine prop-
signs, erty, provide a means to communicate information with
• develop triggers to action, all levels of the mining operation, track changing condi-
• specify what kind of actions are mandatory and who tions as the mine develops, train less-experienced miners
is responsible for taking action and to recognize hazardous conditions and develop controls/
• put decisions in writing along with reasons. plans that are the hallmark of a proactive approach to
mitigate risk to miners. ■

Summary and conclusions References

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