Technique to assess hazards 

in underground stone mines: 

the roof-fall-risk index (RFRI) 

A.T. IANNACCHIONE, L.J. PROSSER, 

G. ESTERHUIZEN AND T.S. BAJPAYEE 

Introduction 

Mining has been identified as one of 

four sectors with injury rates that are 

consistently higher than all other 

industries within the United States 

(NIOSH. 2004). Fatal occupational 

injury rates in 2002 were highest in 

mining (23.5 per 100,000 workers). 

Mining was followed by agriculture, 

forestry and fishing (22.7); construction 

(12.2); and transportation and public 

utilities (11.3). Within the underground 

mining sector, falls of ground comprised 

about 28 percent of the fatal and 16 

percent of the lost-workday injuries from

2000 to 2004 (NIOSH, 2005). The National Institute for Occupational 

Safety and Health (NIOSH) has a focused research program to t

enhance the recognition of hazardous conditions and practices and to 

develop engineering interventions that mitigate conditions most often 

associated with fall-of-ground injuries. 

Many of the hazardous conditions present in the underground 

mining environment are caused by a combination of geologic and i

mining-induced factors. Recognizing and assessing the different 

stability conditions of mine roof strata is a fundamental part of a 

proactive effort to address falls-of-ground injuries. The implementation 

of this process allows decision-makers at all levels to determine the 

potential for a roof fall, a fundamental component of methods to assess 

risk. This paper proposes a qualitative method determine a roof-fallrisk index (RFRI) as one possible 

method to assess the ground-fall hazards 

associated with underground mining. 

Background. Methods aimed at 

improving the quantitative nature of 

roof-stability assessment have been 

developed and used in mining. In the 

early 1990s, the United Kingdom (UK) 

developed a code of practice (now 

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 

o assess the risk associated with potential failure or overloading of 

rock bolt support systems. In the mid 1900s, 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 

mportance of rock engineering in developing this code of practice. 

Van Wijk et al. (2002) developed a risk-rating system for use in South 

African coal mines. This risk-rating system aimed to optimize 

resources and ensure that focus is placed on the areas where it is most 

required. Lind (2005) demonstrated an integrated risk-management 

approach that required a basic assessment of physical parameters such 

as coal seam characteristics, depth below surface and mining 

conditions. In the United States, Duzgun and 

Abstract 

The potential for roof falls in underground mines remains a clear and 

present danger for mine workers. An investigation of ground 

conditions in nearly 50 percent of the nation’s underground stone 

mines found that the state of roof stability is primarily determined in a 

limited and subjective manner. These large-opening mines, with roof 

heights typically 7 m (23 ft) or more, make physical observation 

difficult. Although some mines use monitoring techniques to gain 

additional information on roof stability, this practice is usually short 

term and localized to address ground conditions in a particular section 

of the mine. A roof-fall hazard-assessment technique was developed 

based on engineering judgment acquired from extensive underground 

stone mine experience and on examination of the related literature. 

This technique utilizes observational processes to determine roof fall 

likelihood. Case-study scenarios offer a realistic picture of model 

implementation. Providing the mine level decision-maker with an 

accurate assessment tool to ascertain the level of ground fall hazards is 

expected to reduce mine worker injuries and fatalities. Moreover, the 

presences of danger can be overcomewith a clear picture of qualified 

ground conditions. 
Table 1 

Defect categories for determining the RFRI in underground stone mines.

Assessment Category Parameter value Weight Category 

value

Geologic factors:

1 Large angular discontinuities None 1 1 ____

One, strong contact 2

One, weak contact 3

More than one, strong contact 4

More than one, weak contact 5

Unknown 3

2 Joint frequency None 1 1 ____

Widely spaced, > 1 m (>3.3 ft) 2

Moderately spaced, 0.25 to 1 m (0.82 to 3.3 ft) 4

Closely spaced, <0.25 m (<0.82 ft) 5

Unknown 3

3 Roof layer thickness and Massive. >1 m (>3.3 ft) layers 1 bedding contact strength: Strong bedding contacts in immediate roof, 0 to 3 m (10 ft)

Weak bedding contact(s) in immediate roof, 0 to 3 m (10 ft) 2

3 Rock layers 0.25 to 1 m (0.82 to 10 ft) with weak bedding 4 contact{s) 4 Thin layers, <0.25 m (<0.82 ft) with strong bedding contact{s) 5 Thin layers, <0.25 m (<0.82 ft) with weak. bedding contact{s) 3 Unknown

1 ____

Mining induced failures:

4 Shear rupture surfaces None 1 2 ____

Small shear, cutter <1 m (3.3 ft) 3

Large shear, cutter>1 m (>3.3 ft) 5

Unknown 3

Microseismic emission at background level 1

Microseismic emission elevated and clustered 3

5 Joint separation None 1 2 ____

Noticeable or measurable 5

Unknown 3

6 Lateral strata shifting None 1 2 ____

<20 mm of offset or partial vertical drill hole offset 3

>20 mm of offset or complete vertical drill hole offset 5

Unknown 3

7 Strata separation None 1 2 ____

Slight (barely detectable) 3

Significant, >5 mm 5

Unknown 3

Roof profile:

8 Roof rock debris on floor None 1 2 ____

Slight (widely spaced) 2

Moderate 4

Significant (continuous) 5

Unknown 3

9 Roof shape Smooth 1 2 ____

Intermediate 3

Rough 5

Unknown 3

Moisture factors: 

10 Moisture/groundwater None 1 1 ____

Damp roof 2

Drippers 4

Steady flow 5

Unknown 3

Sum all Category value = ____

Multiplied by 1.11 = ____

Microseismic activity adjustment: no microseismic<: clustering subtract 5; clustering add 25: 0 If unknown

____

Roof deformation rate adjustment: no roof deflection movement subtract 5; constant deflection add 15; 

____

accelerating deflection add 30; 0 If unknown

____

RFRI=

Einstein (2004) used a statistical analysis of available roof-fall data 

from mines in the Appalachian Basin to assess the roof-fall risks 

associated with underground coal mining. In India, Rahaman et al. 

(2004) discussed the use of microseismic monitoring systems to assess 

the risk of roof falls. All of these reports either demonstrate or postulate 

the use of geotechnical parameters to determine the mining system's 

potential for failure, a fundamental step towards managing the risk 

associated with fall-of-ground hazards. In many underground U.S. 

stone mines, especially those with large openings, i.e., > 10 and <17 m 

(>33 and <56 ft) wide with roofs >7 m (>23 ft) high, the state of roof 

stability is primarily determined in a limited and subjective manner. 

Therefore, the development of accepted procedures to help determine 

potential areas of unstable roof will inevitably lead to lower miner 

exposure to hazardous environments and a measurable reduction in 

falls-of-ground injuries. 

Technique to determine a roof-fall-risk index (RFRI) 

The purpose of this paper is to present a qualitative method for 

determining the RFRI. This method is specifically aimed at 

underground stone mines where the strata defects that comprise 

hazardous conditions are difficult to see and where the on-site 

assessment techniques are typically limited and subjective in nature. 

The assumptions made in this analysis are that the typical underground 

stone mine has the following characteristics: wide openings, i.e., >10 

and <17 m (>33 and <56 ft); high roofs or back, i.e., >7 m (>23 ft); and 

relatively flat lying strata. Typical underground stone mines also use 

blasting techniques to break the rock, scaling to remove loose rocks 

and. on occasion, some form of rock reinforcement and roof 

monitoring. The use of this RFRI is relevant only to this experience 

base and is solely intended to assist in developing a quantitative 

method to recognize hazardous ground conditions. The target 

population is the 70 to 90 underground relatively flat lying limestone 

room-and-pillar mines in the central and eastern portion of the United 

States. The criteria used to rate strata defects arc based on past 

experience and engineering assessments during examination of more 

than 50 different underground stone mines. Ten measurable and 

observable categories arc proposed, representing a significant range of 

defects found at these mines. 

An assessment value from “1” to “5” is assigned within each 

category. Increasing values represent higher potential for failure. The 

assessment value of “3” is also used when information on a parameter 

is unknown. The ten defect categories (identified as Nos. 1 through 10 

in Table 1) fall into four broad groups: geologic factors, mining 

induced failures, roof profile and moisture factors. 

Geologic factors. The following are the geologic conditions that 

most often result in increased instabilities in underground stone mines: 

large angular discontinuities, joint frequency and roof layer thickness 

and bedding contact strength. Parameters used in assigning an 

assessment value are identified in Table 1. 

Large angular discontinuities: Large angular discontinuities 

include faults, slips and any other significant geologic structures (Fig. 1,

No. 1). They can act to weaken competent roof rock and arc often 

zones where deformations are initiated (mobilized). The influence of 

angular discontinuities on roof strata stability is well documented 

(Moebs, 1977; Lagather, 1979). If these parameters are nonexistent, 

then a value of “1” is assigned. A value of “5” is assigned to roof strata 

with multiple angular discontinuities and associated weak (lowstrength) contacts, implying a high potential for instability from this 

category. Typically, strong contacts are comprised of sharp surfaces 

with relatively rough profiles, while weak contacts are comprised of 

smooth surfaces that are either polished or filled with fine-grained 

material. If the occurrence of angular discontinuities is unknown, the 

assessment value is "3." 

Joint frequency: Joint frequency has been identified as an 

important factor influencing roof salability (Krausse et al., 1980). Joints 

refer to the steeply inclined (nearly vertical) fractures that often 

naturally occur in rock formations (Fig. 1, No. 2). Joint frequency is 

comprised of several parameters that help to define the frequency or 

spacing of joints. Typically, the joints will occur in preferential 

orientations that can cluster in one or more groupings. It is 

recommended that the cluster with the lowest average distance 

between joints be used to evaluate this parameter (Table 1). 

Roof layer thickness and bedding contact(s) strength: Roof layer 

thickness and bedding contact strength have long been recognized as 

important factors in determining strata stability (Moebs, 1977, Hylbert, 

1978, Iannacchione and Prosser, 1998). It is the interaction of these two 

characteristics that controls the development of separate roof beams 

and partially controls how they deform (Fig. 1, No. 3). Massive strata, 

void of distinct geologic layers, tend to have few continuous, horizontal 

bedding plane structures, making for stable strata conditions. These 

strata have an assessment value of “1.” Almost without exception, 

mine roofs with wide spans are comprised of relatively strong layers. 

Layers greater than 1 m (3.3 ft) in thickness are often observed as 

stable. If these layers are bonded by weak bedding contacts, then the 

strata are typically less stable. As the roof layers incrementally, thin 

below 1 m (3.3 ft) in thickness, the associated beam deformation or sag 

can increase, raising the probability of failure. Layers less than 0.25 m 

(0.82 ft) thick have often been observed as unstable and present a high 

probability for excessive roof beam sag, especially when they are 

bounded by weak contacts. In this case, an assessment value of “5” is 

assigned. The parameters in this category could easily be modified to 

match local mining experiences. 

Mining-induced failures. Mining-induced failures are a direct 

reaction of strata defects produced as a result of mining. There are four 

important categories of mining-induced failures in underground stone 

mines: shear rupture surfaces, joint separation, lateral strata shifting and 

vertical strata separation. 

Shear rupture surfaces: Shear rupture surfaces arc typically found 

in association with buckling of roof layers less than 1 m (3.3 ft) thick. 

This buckling failure is caused by excessive levels of horizontal stress, 

producing a low-angle shear rupture surface with a sharp contact and 

covered with a powder-like rock dust residue (Fig. 1, No. 4). If the 

occurrence of angular discontinuities is unknown, the assessment 

value is “3.” When the immediate roof layer buckles, the relatively 

straight shear rupture surface is observable. 
Joint separations: Joint separations occur when nearly vertical 

fractures begin to expand or open up (Fig. 1, No. 5). This can signal a 

potentially unstable condition, confirming that strata extension is 

occurring and the strata have lost considerable strength. Because most 

underground stone mine roofs have some level of vertical jointing and 

horizontal bedding plane contacts, most roofs are comprised of blocks 

of varying sizes that arc supported by the confining stresses in the 

immediate roof beam. When strata extension occurs, the roof blocks 

are no longer confined and are prone to fall to the ground under the 

forces of gravity. If no joint separation is observed, then the assessment 

value is “1.” Because the parameters used to define separation are 

limited, any noticeable or measurable separation of a vertical joint is 

assigned a value of' “5.” 

Lateral strata shifting: Lateral strata shifting is a condition caused 

when roof layers move in different directions along bedding contacts 

(Fig. 1, No. 6). While it is difficult to directly link this category with 

roof falls, it is commonly recognized as a hazardous condition (Zhang 

and Peng, 2001). In some mines, lateral strata shifting is associated 

with large-scale movement along a fault plane or a large angular 

discontinuity. The level of strata offset on either side of the shifting 

surface can be an indication of the magnitude of movement. If no 

lateral strata shifting occurs, then the assessment value is “1.” If less 

than 20 mm (0.8 in.) of offset is observed where the surface intercepts 

the mine roof or rib, then the assessment value is “3.” If the offset is 

>20 mm (0.8 mm), the assessment value is “5.” Many of these lateral 

offsets do not intercept the mine roof or rib and can be hidden from 

view within the immediate roof. A proven technique to detect these 

surfaces is to drill vertical boreholes on a regularly spaced pattern. This 

technique has been used in coal mining to successfully determine the 

magnitude and direction of strata shifting (Mucho and Mark, 1994). 

Vertical strata separation: Vertical strata separation is a condition 

caused when roof layers separate from one another and sag into the 

mine entry (Fig. 1, No. 7). The association of roof layer deflection with 

roof falls is well established and has been a subject of many 

investigations (Parker, 1973; Maleki and McVey, 1988; Iannacchione 

and Prosser, 1998). While vertical strata separation can be determined 

by many methods, a basic requirement is a vertical borehole drilled 

into the roof and some means to observe and locate separations and 

determine their magnitude. Often, this is accomplished with devices 

such as a simple scratch tool, a borescope or a roof deflection monitor. 

If no separations exist in the immediate roof, then the assessment value 

is “1.” If the separation is barely detectable or open, then the value is 

“3.” If the separation is easily detectable (>5 mm or 0.2 in.), then the 

value is “5.”

Roof profile. The profile of the roof provides a good indication of 

what damage has occurred to the roof and potentially what damage 

will occur based on its shape. This damage could be inherent to the 

rock or it could be induced by blasting or scaling. The two categories 

that help to define the roof profile are the roof rock debris on the floor 

and roof shape. 

Roof rock debris on the floor: If an entry is being or has been 

damaged by existing defects or by blasting or scaling, evidence of this 

damage is typically found deposited on the mine floor (Fig. 1, No. 8). It 

is vitally important that this information be retained by the mining 

operation in some manner. If the floor is cleaned after debris has fallen 

from the roof and no record is made of it, then this valuable piece of 

information will be lost. One has to make sure that debris from blasting 

and scaling the roof and ribs is not confused with roof rocks that have 

fallen without this operational-induced assistance. If no roof rock 

debris is observed, then the assessment value is “1.” Increasing 

amounts of debris produce higher assessment values. A value of “5” is 

typically associated with a significant pile of broken rocks that covers a 

portion of the mine's entry. 

Roof shape: It has been established that the shape of the roof can 

provide some indication of the future performance of the roof 

(Iannacchione and Prosser, 1998). In general, a smooth roof is 

desirable in underground stone mining and typically represents a stable 

state (Fig. 1, No. 9). In this case, the assessment value is “1.”

Conversely, if the roof is highly irregular with pronounced swales and 

troughs, the potential for unstable conditions increases and the 

assessment value is “5.” Sometimes this condition is caused by 

inherent weakness within the roof rocks. Other times the rougher 

looking roof is a result of roof rocks damaged by blasting or scaling. 

Moisture factors. In mining, the physicochemical effects of water 

can act to reduce the strength of a mine roof (Unrug, 1997). Also, 

water pressure in fractures may be strong enough to cause roof 

instabilities. This condition is particularly acute in shallow, largeopening stone mines where extreme humidity conditions, especially in 

the summer months, reduce roof rock strength. Additionally, the 

closeness of the mine to the surface places the mines above drainage. 

This condition promotes the development of weathered joints with 

variable water How conditions. Standing or flowing water in 

prominent fracture systems can exert considerable destabilizing forces 

within the roof. 

Moisture\groundwater inflow: The assessment values for 

moisture/groundwater inflow characteristics are the following: if the 

roof is dry and no water is observed, the assessment value is “1”; if the 

roof is damp, the value is “2”; if dripping occurs, the value is "'4": and if 

the flow of water from the roof is steady, the value is “5.” 

Monitoring data and its impact 

on assessment values 

To this point, parameter characteristics of the proposed method to 

assess roof-fall hazards have been determined with information readily 

available at any mine site with a means of accessing and drilling the 

roof. Because underground stone mines arc all drill-and-blast 

operations, every mine in the United States has the basic ability to 

access and drill the roof. However, if this were the only information 

that was available to decision makers, then ones ability to more 

accurately assess stability conditions would be limited. In fact, some 

mines use advanced monitoring techniques to gain additional valuable 

information about roof stability. This practice has developed, in part, 

because of difficulties in accurately observing roof conditions when 

room heights exceed 7 m (23 ft). Another reason is the need to assess 

roof-rock behavior above the immediate roof, which is entirely out of 
the decision-maker's view. As a result, a diverse range of roofdeflection monitoring devices and some geophysical techniques have 

been or are being used to detect roof-rock defects. 

Roof-deflection measurements. Roof-deflection monitoring 

techniques have long been employed in underground mining to 

monitor roof behavior (Parker, 1973b; Kaiser, 1981; Maleki and 

McVey, 1988; Iannacchione et al., 2004a). Typically, these are 

mechanical or electro-mechanical devices that allow for the 

measurement of displacement between two or more known points 

within a roof borehole or between the mine's roof and floor. 

Sometimes they are simple tools, such as a scratch tool that allow the 

operator to remotely feel or detect the crack or separation within a roof 

borehole. Roof-deflection measurements are known to produce 

unambiguous assessments of strata separation characteristics. 

Monitoring roof beam sag and roof-to-floor convergence provides an 

opportunity 10 collect values of roof deflection measurements that can 

be used to adjust the RFRI values. 

Roof-deformation-rate adjustment: Three general conditions are 

characterized when measuring roof deflection. If measurements 

indicate that no roof deflection is taking place, the strata can be 

temporarily considered to be stable. In this first condition, the RFRI is 

reduced by 5 (Table 1). The second condition is when a measurable 

level of roof deflection persists for a period. The magnitude of this 

value is site specific in nature and has been found to range between a 

few tenths of a millimeter to several millimeters per day. This condition 

suggests the roof is no longer stable but still may not be on a path that 

will lead to a failure. There are many examples where roofs with this 

amount of deflection have temporarily stabilized, in some cases for 

long periods of time. If this condition occurs, the RFRI is increased by 

15. It should be noted that when roof deformations occur, it might be 

advisable to construct some form of notification and/or barrier to limit 

entry into the area. The third condition is when the rate of deflection 

increases on some type of regular basis. such as from one day to the 

next or perhaps one week or one month to the next. This condition 

suggests the roof is in an unstable state. If this condition occurs, the 

RFRI is increased by 30. 

Microseismic emissions. Numerous geophysical techniques exist 

for detecting zones of potential roof instability, including cross-hole 

seismic tomography, ground-penetrating radar and the monitoring of 

microseismic emissions. Maleki et al. (1992) detected the 

development of mine roof fractures up to 15 m (50 ft) into the mine 

roof. Also, Molinda et al. (1996) used ground-penetrating radar to 

image a known geologic discontinuity at NIOSH's underground Lake 

Lynn Laboratory. The use of microseismic emissions information has 

been discussed to assess risk for South African deep hard rock mine 

stability (Stewart and Spolliswoode, 1996) and Indian coal mine roof 

falls (Rahaman et al., 2004). Recently, microseismic emissions have 

been used to identify zones of roof rock instability at an operating stone 

mine in Pennsylvania (Iannacchione et al., 2004b). 

Microseismic activity adjustment: An adjustment to the RFRI 

value can be made if adequate microseismic monitoring information 

exists. Clustering of microseismic events in time, and within a 

relatively well-defined area of the mine, can signal that rock fracturing 

is occurring and that the strata may be unstable. Clustering in time is 

defined by microseismic activity far in excess of the normal 

background rate. Clustering in space is defined by the microseismic 

activity occurring within the same general area. The location accuracy 

of microseismic events can greatly influence spatial clustering. If 

microseismic activity does not cluster, the strata are most likely not 

producing new fracture surfaces. In this case, the RFRI is reduced by 5 

(Table 1). If microseismic emissions cluster, then the RFRI is increased 

by 25 (Table 1). 

Determining the relative probability of roof falls 

Roof-fall-hazard variations can be expressed as a risk index. A 

mathematical expression can be used to calculate the roof-fall-risk 

index (RFRI) and is defined as 

RFRI = ∑(AV*W)/ ∑ (MAV*W) 

where 

AV is the assessment value for each defect category: 

MAV is the maximum of assessment value of each category, or 6: 

and 

W is the weighting of each category. 

Because the defect categories defect the performance of 

underground stone mine entries to different degrees. it is necessary to 

independently weight each of the ten categories (Table 2). The defect 

categories more detrimental to entry performance are Categories 4, 5, 

6, 7 and 8 were assigned a weight of “2.” The other categories, i.e., roof 

shape (Category 9). Moisture/water inflow (Category 10) and all of the 

geologic related factors (Categories 1, 2 and 3), were each weighted at 

“1.” 

The RFRI for the mathematical expression shown in Eq. (1) 

produces a distribution where RFRI values approaching 0 would 

represent a very stable condition and those near 1 represent a very 

unstable condition. The minimum and maximum RFRI values 

without adjustment factors range between 17 and 83 (Fig. 2). If the 

maximum adjustment factors are applied to Categories 4.6 and 7, an 

RFRI value of 146 is possible. It is also possible to calculate the RFRI 

if nothing is known about any of the defect categories. This produces a 

RFRI equal to 50, or equally between the stable and unstable 

conditions. This is a desired outcome of the mathematical expression. 

A logical outcome of these three conditions is to divide the RFRI into 

three risk categories: low, moderate and high (Fig. 2). It is important to 

note that the objective of this paper is to develop a method of ranking 

hazardous conditions. Therefore, it is in appropriate at this time to 

equate the proposed risk categories with a prescribed action. 

Hypothetical case studies 

Two case studies of the use of the proposed methodology to assess 

roof-fall hazards are given below. These cases are meant to 

demonstrate the use of the method through realistic scenarios. 

Engineering judgment, based on extensive underground stone mine 
Table 2

Defect category weights

Category 

number Category description Weight

1 Large angular discontinuities 1

2 Joint frequency 1

3 Roof layer thickness and bedding contact 1

strength

4 Shear rupture surfaces 2

5 Joint separation 2

6 Lateral strata shifting 2

7 Vertical strata separation 2

8 Roof rock debris on floor 2

9 Roof shape 1

10 Moisture/ground water inflow 1

investigations and related studies found 

within the literature, was used to identify: 

• the number and kind of defect 

categories. 

• the parameters used to determine 

• an assessment value for each category. 

• the weightings of categories and 

• the adjustments for monitoring activities. 

Case 1: Shear rupture surfaces with 

rock debris on the floor. One ground 

condition that adversely affects 

approximately 20 percent of U.S. 

underground stone mines is the occurrence of 

roof falls in conjunction with excessive levels 

of high horizontal stresses (Iannacchione, 

2003). Mines with this problem often have a 

shear rupture surface in the immediate roof, 

i.e., first 2 m (6.6 ft) of strata, propagating in a 

direction perpendicular to the principal stress 

direct ion (Emery, 1964: Parker, 1966). The 

shear rupture surface is typically comprised 

of multiple surfaces that fracture the roof, 

forming a cutler or gutter type structure in the 

roof. As the rock fails, it falls to the ground 

below the shear rupture surface and begins 

to form a debris pile. The size of the pile 

depends on the size and shape of the shear 

rupture surface. 

Case No. 1a (Table 3) assumes that the 

decision-makers lit the mine have no 

knowledge of the defect categories discussed 

above with the following exceptions: the 

entry has a large shear rupture surface. i.e.,

>1 m (>3.3 ft) in length, and a continuous 

pile of rock debris has accumulated on the 

floor beneath the shear rupture surface. This 

first example produces an assessment value 

of "5" for defect Categories 4 and 8 and an 

assessment value of “3” for all other 

categories with a RFRI 58.8 (Table 3). This is within the moderate 

RFRI level (Fig. 2). 

Adding information about site conditions provides additional 

examples to help explain the proposed methodology and to test the 

method against the authors’ experience. As more characteristics about 

this same site are obtained, such as favorable geologic conditions, 

smooth roof profile and dry roof conditions, and when drill holes show 

no lateral or vertical movement (Case No. 1b, Table 3), the RFRI falls 

to 316. This is just within the low RFRI level (Fig. 2). 

Conversely, when additional information about the site conditions 

provides less favorable characteristics, such as drill holes showing 

lateral strata separation and elevated and clustered microseismic 

emissions (Case No. 1c, Table 3), the RFRI rises to 70.5. This is within 

the high RFRI level. 

Case 2: Thinly bedded strata with weak bedding contacts.

The impact of thinly bedded strata on roof rock stability is well 

documented (Hebblewhite and Lu, 2004). Add to this the wide room 

spans. >15 m (50 ft) and non-uniform use or rock reinforcement, and it 

is easy to see why this condition has been linked to many underground 

stone mine roof falls. Euler’s formula provides general performance 

parameters for bedded stone roof beams where the critical stress 

defining the onset of beam buckling is highly dependent on beam 

thickness. 

Case No. 2d (Table 3) assumes that the decision-maker has no 

knowledge of local defect categories with only one exception: the site 

is known to have thinly bedded strata with weak bedding contacts. 

This condition gives an assessment value of “5” for defect Category 3 

and assessment values of “3” for all other categories with a RFRI of 

52.2 (Table 3). This is within the moderate RFRI level (Fig. 2). 
Table 3

RFRI values for two cases.

Case 1 Case 2

a b c d e f g 1 2 Category AV WAV AV WAV AV WAV AV WAV AV WAV AV WAV AV WAV

1 3 3 1 1 1 1 3 3 1 1 1 1 1 1

2 3 3 1 1 1 1 3 3 3 3 3 3 3 3

3 3 3 3 3 3 3 5 5 5 5 5 5 5 5

4 5 10 5 10 5 10 3 6 1 2 1 2 1 2

5 3 6 1 2 1 2 3 6 1 2 1 2 1 2

6 3 6 1 2 5 10 3 6 1 2 56 10 5 10

7 3 6 1 2 1 2 3 6 1 2 5 10 5 10

8 5 10 5 10 5 10 3 6 1 2 1 2 1 2

9 3 3 1 1 1 1 3 3 1 1 1 1 1 1

10 3 3 1 1 1 1 3 3 3 3 3 3 3 3

Subtotal 53 33 41 47 23 39 39

Multiply by1.11 58.8 36.6 45.5 52.5 25.5 43.3 43.3

Adjustment 0 -5 25 0 0 15 30

RFRI 58.8 31.6 70.5 52.2 25.5 58.3 73.3

1

AV= Assessment value

2

WAV = Weighted Assessment ValuesIf closer inspection of the sites reveals more favorable 

characteristics (Case No. 2e, Table 3), the RFRI drops to 25.5. This is 

within the low RFRI level (Fig. 2). However, if sensor readings from 

this same area measure a constant downward roof defection (Case No. 

2f, Table 3) or if measurements begin to measure an accelerating rate 

of roof deflection (Case No. 2g. Table 3), the RFRI can rise to 58.3 and 

73.3, respectively. The latter is within the high RFRI level (Fig. 2). 

Summary and conclusions 

This study proposes a roof-fall-hazard-assessment method for 

underground stone mines that can be used to help manage miner 

exposure to unstable roof-rock conditions so that roof-fall-related 

injuries can be reduced. The underground stone-mining industry has 

an acute need for this capability because current roof-stabilityassessment techniques are limited by difficulties with assessing 

conditions in high roofs, i.e., >7 m (>23 ft). 

The proposed roof-fill-hazard-assessment technique is comprised 

of 10 defect categories that cover a range of geologic, mining induced, 

roof profile and moisture factors. Each category has a set of parameters 

that allow for the estimation of an assessment value between “1” and 

“5” These parameters arc based on experience gained from visiting 

more than 50 different operating mines and from an investigation of 

relevant topics in the literature. 

Important geologic factors affecting roof stability include large 

angular discontinuities, joint frequencies and roof-layer thickness and 

bedding-plane contact strength. In addition to these naturally occurring 

strata defects, a range of mining-induced failures, including shear 

rupture surfaces, joint separations, lateral strata shifting and vertical 

strata separation, directly impacts roof stability. A fundamental 

assessment of roof stability is also made by examining the profile of 

the roof, where its shape and the amount of fallen material provide 

evidence of what damage has occurred and. Potentially, what damage 

will occur. Lastly, the innocence of moisture on roof stability is 

determined by observing wetness and groundwater in now conditions. 

These factors are determined with information readily available at any 

mine site with a means of accessing and drilling the roof. 

In practice, much more information about the character and 

performance of a mine's roof can be made with monitoring data. These 

data are generally obtained from roof-deflection monitoring devices 

and some geophysical techniques, all of which help to detect and 

assess hazardous roof rock defects. In this roof-fall-hazard-assessment 

methodology, monitoring data are used to adjust assessment values. If 

monitoring information supports a more stable assessment of roof fall 

potential, then the RFRI is decreased. Conversely, information that 

indicates a less-stable condition yields a higher RFRI. In this way, 

decision-makers who know more about the site ground conditions are 

better able to make a more accurate hazard assessment. 

The technique involves calculating a RFRI. Very stable conditions 

produce RFRI values approaching 0, while unstable conditions 

produce RFRI values approaching 100. In some cases, where 

significant adjustments are made, the RFRI may be in excess of 100. 

Three logical hazard levels are defined as low, moderate and high 

based on the RFRI values. Determining the particular risk for a specific 

underground stone entry will allow decision makers to respond in a 

proactive and measured fashion to hazardous roof rock conditions, 

thereby lowering the potential for fall-of-ground injuries.
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