1 

Assessing Roof Fall Hazards for Underground Stone Mines: A Proposed Methodology 

A. Iannacchione, NIOSH, Pittsburgh, PA 

L. Prosser, NIOSH, Pittsburgh, PA 

G. Esterhuizen, NIOSH, Pittsburgh, PA 

T. S. Bajpayee, NIOSH, Pittsburgh, PA 

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% 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 of 7 m 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 or part of the mine. 

 A methodology to assess the risk for a roof fall is proposed in a 

preliminary fashion based on engineering judgment acquired from 

extensive underground stone mine experience and examination of 

related literature. The proposed method utilizes an observational 

technique to identify the risk of roof falls in three categories. Case 

study scenarios provide a realistic picture of model implementation. 

Providing the mine level decision maker with an accurate assessment 

tool to ascertain the level of risk related to ground conditions is 

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

presences of danger can be overcome with a clear picture of quantified 

ground conditions. 

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 [US] (Anon, 2004). Fatal occupational injury rates in 2002 were 

highest in mining (23.5 per 100,000 workers), 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% of the fatal and 16% of the lost-workday 

injuries in the years from 2000 to 2004 (Anon, 2005). The National 

Institute for Occupational Safety and Health (NIOSH) has a focused 

research program to enhance the recognition of hazardous conditions 

and practices, and then 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 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 preliminary qualitative method to determine a roof fall risk 

index, or 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 proposed. In the early 1990s, the 

United Kingdom (UK) developed a code of practice (now Industry 

Guidance) for rockbolt use as roadway supports that included 

geotechnical assessment, initial design, design verification, and routine 

monitoring (Arthur, Waite and Altounyan, 1998). Cartwright and 

Bowler (1999) provided a UK example of a procedure to assess the 

risk associated with potential failure or overloading of rockbolt 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 

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 U.S., Duzgun and 

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 using 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 (widths >10 m 

and <17 m wide) and high roofs (>7 m), 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. 

A 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 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 wide openings (>10m and <17m), high roofs or back 

(>7m), relatively flat lying strata, and uses 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 determine roof fall risk for this 

mining sector. 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 U.S. The criteria used to rate strata defects are 

based on past experience and engineering assessments during 

examination of more than 50 different underground stone mines. Ten 

measurable and observable categories are 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 10 defect categories fall into four broad groups: 

geologic factors, mining induced failures, roof profile, and moisture 

factors (Table 1, see Appendix A). 
2 

Geologic Factors 

 The following are the geologic conditions that most often result in 

increased instabilities in underground stone mines: 1) large angular 

discontinuities, 2) joint frequency, and 3) roof layer thickness and 

bedding contact strength. Parameters used in assigning an 

assessment value are identified in table 1. 

Large Angular Discontinuities - Large angular discon-tinuities include 

faults, slips, and any other significant geologic structures (figure 1, see 

Appendix B). They can act to weaken competent roof rock and are 

often zones where deformations are initiated (mobilized). The 

influence of angular discontinuities on roof strata stability is well 

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

non-existent, then a value of 1 is assigned. A value of 5 is assigned to 

roof strata with multiple angular discontinuities and associated weak 

(low strength) 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 stability (Krausse, et al., 1980). Joints refer to 

the steeply inclined (nearly vertical) fractures that often naturally occur 

in rock formations (figure 1). 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, 

1980, 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 (figure 1). 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 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 in thickness, the associated beam 

deformation or sag can increase, raising the probability of failure. 

Layers less than 0.25 m 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 reflection of strata defects 

produced as a result of mining. There are four important categories of 

mining induced failures in underground stone mines: 1) shear rupture 

surfaces, 2) joint separation, 3) lateral strata shifting, and 4) vertical 

strata separation. 

Shear Rupture Surfaces - Shear rupture surfaces are typically found in 

association with buckling of roof layers less than 1m 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 (figure 1). If the 

occurrence of angular discontinuities is unknown, the assessment 

value is 5. 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 (figure 1). 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 are 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 

(figure 1). 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 largescale 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 2 cm of offset is 

observed where the surface intercepts the mine roof or rib, then the 

assessment value is 3. If the offset is > 2cm, 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 (figure 1). 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 (>0.5 cm), 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 

upon its shape. This damage can be inherent to the rock or it can 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 (figure 1). 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 man-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 (figure 1). 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. 
3 

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, large-opening 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 flow 

conditions. Standing or flowing water in prominent fracture systems 

can exert considerable destabilizing forces within the roof. 

Moisture/Ground Water Inflow - The assessment values for 

moisture/ground water inflow characteristics are the following: 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 are all drill-and-blast 

operations, every mine in the U.S. has the basic ability to access and 

drill the roof. However, if this were the only information that was 

available to decision makers, then our 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. 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 roof deflection 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, 1973; Maleki 

and McVey, 1988; Kaiser, 1981). 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 simply tools, 

such as a scratch tool, that allow the operator to remotely feel or detect 

the crack or separation within a roof borehole. 

Vertical Strata Separation Adjustment - Because roof-to-floor 

convergence and roof beam sag monitoring instruments provide an 

opportunity to collect measurable values of roof deflection, these data 

can be used to either reduce or increase assessment values for 

Category 7 – Vertical Strata Separation (table 1). Three general 

conditions are characterized when measuring roof deflection. The first 

condition is no measurable roof deflection, indicating the strata is or 

has become stable. If this occurs, the assessment value for this 

category is 1. 

 The second condition is when a measurable level of roof 

deflection persists for longer than a few days. 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 may not be on a path to a roof 

fall. In many ways, this condition represents a semi-stable state 

(Iannacchione et al., 2004). There are many examples where roofs 

with this kind of deflection have subsequently stabilized, and, in some 

cases, for long periods of time. If this condition occurs, the 

assessment value is multiplied by 2. It should be noted that when roofdeflection is measured, it is 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 month to the next. An increasing rate of roof 

deflection is a well documented precursor of roof failure. This 

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

assessment value is multiplied by 4. 

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 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 

Spottiswoode, 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., 2001 and 2004). 

Shear Rupture Surfaces Adjustment – Microseismic emissions have 

been shown to be particularly good at characterizing the parameters in 

Category 4 – Shear Rupture Surfaces. As a result, Category 4’s 

assessment value can either be reduced or increased based on the 

level and clustering characteristics of these emissions (table 1). 

Emissions that are equivalent to background levels have been found to 

represent stable conditions, effectively reducing the assessment value 

to 1. Elevated emissions that cluster in well-defined locations have 

been associated with roof falls, resulting in an increased assessment 

by a multiple of 3. 

Lateral Strata Shifting Adjustment - Monitoring microseismic emissions 

has the added benefit of identifying large scale bedding plane shears 

(Gale et al., 2001), albeit to less of a degree than shear rupture 

surfaces. When emissions increase and cluster, they represent a 

reasonably good opportunity to further characterize the parameters in 

Category 6 - Lateral Strata Shifting (table 1). Emissions that are 

equivalent to background levels have been found to represent stable 

conditions, while elevated emissions that cluster in well-defined 

locations have been associated with roof falls. 

Determining the Relative Probability of Roof Falls 

 The risk of roof falls can be characterized as a risk index. A 

mathematical expression can be used to calculate the roof fall risk 

index (RFRI) and is defined as: 

(1) 

Where: 

 AV = the assessment value for each defect category 

 MAV = the maximum of assessment value of each 

category or 6 

 W = the weighting of each category 

 Because the defect categories affect 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 4, 5, 6, 7, and 8 

and were assigned a weight of 2. The other categories, i.e., roof 

shape (9), moisture/water inflow (10), and all of the geologic related 

factors (1, 2, and 3), were each weighted at 1. 

 The RFRI for the mathematical expression shown in equation 1 

produces a distribution where RFRI values approaching 0 would 

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

condition. The minimum and maximum RFRI values without 

adjustment factors range between 0.17 and 0.83 (figure 2). If the 

maximum adjustment factors are applied to Categories 4, 6, and 7, an 

RFRI value of 1.46 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 0.5, or equally between the stable and unstable 

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

RFRI = ' (AV * W) / ' (MAV * W), 
4 

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

three risk categories: low, medium, and high (Figure 2). It is important 

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

ranking hazardous conditions, and it is, therefore, inappropriate at this 

time to equate the proposed risk categories with a prescribed action. 

Table 2. Weightings of defect categories. 

Category 

number Category description Weighting 

1 Large angular discontinuities 1 

2 Joint frequency 1 

3 Roof layer thickness and bedding 

contact strength 

1 

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 

Hypothetical Case Studies 

 Two case studies of the use of the proposed methodology to 

assess roof fall risk 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 

investigations and related studies found within the literature, was used 

to identify: 1) the number and kind of defect categories, 2) the 

parameters used to determine an assessment value for each category, 

3) the weightings of categories, and 4) the adjustments for monitoring 

activities. 

Case 1: Shear Rupture Surfaces with Rock Debris on the Floor 

 One ground condition that adversely affects approximately 20% 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 (first 2m of strata), propagating in 

a direction perpendicular to the principal stress direction (Emery, 1964; 

Parker, 1966). The shear rupture surface is typically comprised of 

multiple surfaces that fracture the roof, forming a cutter 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. 1 assumes that the decision makers at the mine have 

no knowledge of the defect categories discussed above with the 

following exceptions: 1) the entry has a large shear rupture surface (>1 

m in length), and 2) 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 

assessment value of 3 for all other categories with a RFRI of 0.58 

(No. 1, table 3, see Appendix A). This is within the moderate risk zone 

for a roof fall (No. 1, figure 3). 

 Adding information about site conditions provides additional 

examples, to help explain the proposed methodology and test the 

method against the author’s experience. As more favorable 

characteristics about the site are obtained, such as favorable geologic 

conditions, smooth roof profile, dry roof conditions (No. 2, table 3 and 

figure 3), and when drill holes show no lateral or vertical movement 

(No. 3, table 3 and figure 3), the RFRI falls to a low of 0.35. This is 

close to the low risk zone. Indeed if all the additional information 

indicates the site has stabilized for the current time interval, then a 

lower risk is apparent. 

 Conversely, when additional information about the site conditions 

provides less favorable characteristics, such as drill holes showing 

lateral strata separation and measurable vertical strata movement (No. 

4, table 3 and figure 3), and elevated and clustered microseismic 

emissions (No. 5, table 3 and figure 3), the RFRI rises to a high of 

0.83. This is within the high risk zone for a roof fall. 

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) 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 (Iannacchione et al., 1998). 

 Case No. 2 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 0.52 (No. 6, table 3 

and figure 4). This is within the moderate risk zone. 

 A more favorable characterization of the site, where categories 1, 

4, 5, 6, 7, 8, and 9 are all 1, and categories 2 and 10 are 3 (No. 7, table 

3 and figure 4), produces a RFRI of 0.25. This is within the low risk 

zone. However, by simply observing some lateral and vertical strata 

separation (No. 8, table 3 and figure 4) or by actually measuring an 

accelerating rate of roof deflection within the site (No. 9, table 3 and 

Example 1

Example 2

Example 3

Example 4

Example 5

0.00

0.33

0.67

1.00

Examples

Roof Fall Risk Index (RFRI)

Moderate 

 risk 

High risk 

Low risk

Figure 3. Five potential examples of the RFRI for roof strata that 

contain shear rupture surfaces with rock debris on the floor and 

their impact on risk assessment (Case No.1). 

Figure 2. General RFRI distributions for high, moderate, and 

low risk categories. 

Low

Median or unknown 

case

High

0.00

0.33

0.67

1.00

Examples

Roof Fall Risk Index (RFRI)

Moderate 

 risk 

High risk 

Low risk 
5 

figure 4), the RFRI can rise to 0.42 and 0.73, respectively. This is well 

within the high risk zone. 

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 stability 

assessment techniques are limited by difficulties with assessing 

conditions in high roofs (>7 m). 

 The proposed roof fall hazard assessment methodology is 

comprised of ten 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 are 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, roof stability is directly impacted by a range of mining 

induced failures, which include shear rupture surfaces, joint 

separations, lateral strata shifting, and vertical strata separation. 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 influence of moisture on roof stability is 

determined by observing wetness and ground water inflow 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 their 

site ground conditions are better able to make a more accurate hazard 

assessment. 

 The proposed method for roof fall hazard assessment involves 

calculating a RFRI. Very stable conditions produce RFRI values 

approaching 0, while unstable conditions produce RFRI values 

approaching 1. In some cases, where significant adjustments are 

made, the RFRI may be in excess of 1. Three logical risk categories 

are defined as low, medium, and high, based upon 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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Example 6

Example 7

Example 8

Example 9

0.00

0.33

0.67

1.00

Examples

Roof Fall Risk Index (RFRI)

High risk 

Moderate risk 

Low risk 

Figure 4. Four potential examples of the RFRI for roof strata that 

contain thin layers with weak bedding contact strength and their 

impact on risk assessment (Case No.2). 
6 

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7 

Appendix A 

Table 1. Defect categories for determining the RFRI in underground stone mines. 

Grouping Category Parameter Assessment Value 

Large angular discontinuities 

None 1 

One, strong contact 2 

One, weak contact 3 

More than one, strong contact 4 

More than one, weak contact 5 

1 

Unknown 3 

Joint frequency 

None 1 

Widely spaced (>1m) 2 

Moderately spaced (0.25 to 1m) 4 

Closely spaced (<0.25m) 5 

Geologic 

factors 

2 

Unknown 3 

Roof layer thickness and bedding contact strength

Massive (>1m layers) 1 

Strong bedding contacts in immediate roof (0 to 3m) 2 

Weak bedding contact(s) in immediate roof (0 to 3m) 3 

Rock layers 0.25 to 1m with weak bedding contact(s) 4 

Thin layers (<0.25m) with strong bedding contact(s) 4 

Thin layers (<0.25m) with weak bedding contact(s) 5 

 3 

Unknown 3 

Shear rupture surfaces 

None 1 

Small shear (cutter < 1m) 3 

Large shear (cutter > 1m) 5 

Unknown 3 

Microseismic emission at background level Reduce to 1 

4 

Microseismic emission elevated and clustered Multiply by 3 

Joint separation 

None 1 

Noticeable or measurable 5 5 

Unknown 3 

Lateral strata shifting 

None 1 

< 2cm of offset or partial vertical drill hole offset 3 

> 2cm of offset or complete vertical drill hole offset 5 

Unknown 3 

Microseismic emissions at background level Reduce to 1 

6 

Microseismic emissions elevated and clustered Multiply by 2 

Vertical strata separation 

None 1 

Slight (barely detectable) 3 

Significant (>0.5cm) 5 

Unknown 3 

No roof deflection Reduce to 1 

Measurable roof deflection Multiply by 2 

Mining 

induced 

failures 

7 

Accelerating roof deflection Multiply by 4 

Roof rock debris on floor 

None 1 

Slight (widely spaced) 2 

Moderate 4 

Significant (continuous) 5 

8 

Unknown 3 

Roof shape 

Smooth 1 

Intermediate 3 

Rough 5 

Roof profile 

9 

Unknown 3 

Moisture/ground water inflow 

None 1 

Damp roof 2 

Drippers 4 

Steady flow 5 

Moisture 

factors 10 

Unknown 3 
8 

Appendix A (cont’d) 

Table 3. Assessment values and RFRI for two Case Studies. 

Case 1 Case 2 Category 1 2 3 4 5 6 7 8 9 

Large angular discontinuities 3 1 1 1 1 3 1 1 1 

Joint frequency 3 1 1 1 1 3 3 3 3 

Roof layer thickness and bedding contact strength 3 3 3 3 3 5 5 5 5 

Shear rupture surface 5 5 5 5 15* 3 1 1 1 

Joint separation 3 3 1 1 1 3 1 1 1 

Strata shifting (lateral movement) 3 3 1 5 10* 3 1 5 5 

Strata separation (vertical movement) 3 3 1 5 5 3 1 5 20* 

Roof rock debris on floor 5 5 5 5 5 3 1 1 1 

Roof profile 3 1 1 1 1 3 1 1 1 

Moisture/ground water inflow 3 1 1 1 1 3 3 3 3 

RFRI 0.58 0.48 0.35 0.52 0.83 0.52 0.25 0.42 0.73 

* - adjustment factors applied 
9 

Appendix B 

Figure 1. Sketch of parameters associated with ten defect categories.