25th International Conference on Ground Control in Mining 

128 

ABSTRACT 

 The Roof Fall Risk Index (RFRI) is a new method introduced 

by the National Institute for Occupational Safety and Health 

(NIOSH) to assist the underground stone mine operator in 1) 

assessing defects the mine strata and 2) rating the relative roof fall 

risk these defects pose. The RFRI utilizes observational techniques 

to identify defects in the roof strata caused by local geologic, stress, 

and mining conditions. Assessment values for ten defined defect 

categories provides the foundation for estimating a range of risk 

conditions between 0 and 100 that define the potential for a roof 

fall. This paper examines how the defect information is collected 

using two field verification sites and proposes methods to analyze 

the RFRI data. These examples provide information on how well 

the method replicates observed conditions and how it might be 

applied in practice. 

INTRODUCTION 

 Many of the hazardous conditions present in the underground 

mining environment are caused by a combination of geologic and 

in-situ stress factors which can be further affected by local mining 

activity. Recognizing and assessing roof stability conditions is a 

fundamental part of any proactive effort to address falls-of-ground 

injuries. The implementation of this process allows decision 

makers at all levels to estimate the potential for a roof fall. The 

RFRI was previously introduced at the 2006 SME Annual Meeting 

in St. Louis (Iannacchione et al., 2006). Methods to assess roof fall 

risk in underground mines were examined and the role of the RFRI 

was outlined; however, site-specific examples were not provided. 

In its current revised form, the RFRI varies from 0 to 100, higher 

numbers indicating a greater potential for a roof fall. This paper 

provides field evidence to demonstrate how this method works and 

how it may be used to assess the roof conditions. 

 The RFRI is for use in underground stone mines where the 

defects that result in unstable strata conditions are difficult to see 

and current assessment techniques are typically limited and 

subjective in nature. The assumptions used in this analysis are that 

the underground stone mine has wide openings (> 10m), 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 current use of the RFRI is relevant only to this experience base. 

 This method is applicable to 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 twenty years of ground control experience and 

engineering assessments from examination of more than 50 

underground stone mines. 

OBSERVING AND ASSESSING STRATA DEFECTS 

 Ten measurable and observable categories are proposed that 

represent a significant range of defects found at underground stone 

mines (table 1). An assessment value (AV) from 1 to 5 is assigned 

within each defect category. Increasing values represent more 

severe defects. The assessment value of 3 is also used when 

information on a parameter is unknown. The ten categories and the 

parameters used for assigning individual AVs are: 

• Category 1 (Large Angular Discontinuities) notes the 

occurrence of significant geologic structures such as faults and 

slips that cut the roof rock at angles from 10 to 70 degrees 

from horizontal (Figure 1, No.1). These discontinuities act to 

weaken competent roof rock and can be zones where 

displacements are initiated (mobilized). The influence of 

angular discontinuities on roof strata stability is documented 

by Lagather (1977) and Moebs (1977). If these parameters are 

non-existent, then an AV of 1 is assigned. An AV of 5 is 

assigned to roof strata with a significant angular discontinuity, 

containing weak (low strength) contact. 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. 

• Category 2 (Joint Frequency) focuses on the importance of joint 

frequency on roof stability (Krausse et al., 1980). Joints refer 

to the steeply inclined (nearly vertical) fractures that often 

naturally occur in rock formations (Figure 1, No.2). Joint 

frequency is comprised of several parameters that help 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). Widely spaced joints yield an AV of 2, 

while closely spaced joints have an AV of 5. 

FIELD VERIFICATION OF THE ROOF FALL RISK INDEX: 

A METHOD TO ASSESS STRATA CONDITIONS 

Anthony T. Iannacchione, Acting Geotechnical Engineering Section Chief

Gabriel Esterhuizen, Senior Service Fellow 

NIOSH-Pittsburgh Research Laboratory 

Pittsburgh, PA, USA 

Scott Schilling, Engineering Geologist 

Graymont (PA), Inc. 

Pleasant Gap, PA USA 

Tom Goodwin, Mine Foremen

Hanson Aggregates North America, 

Connellsville, PA USA 
25th International Conference on Ground Control in Mining 

129 

• Category 3 (Roof Layer Thickness and Bedding Contact(s) 

Strength) evaluates roof layer thickness and bedding contact 

strength. Past experience has demonstrated how these 

geologic conditions interact to affect individual roof beam 

conditions and the potential for separations between these 

same layers (Moebs, 1977, Hylbert, 1980, Iannacchione and 

Prosser, 1998). Massive strata, void of distinct geologic layers, 

tend to have few continuous, horizontal bedding plane 

contacts, resulting in few defects (Figure 1, No.3). These 

strata have an AV of 1. By necessity, 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. Layers less than 0.25 m thick have often 

been observed as sagging and present a high probability for 

eventual roof beam failure, especially when they are bounded 

by weak contacts. In this case, an AV of 5 is assigned. 

• Category 4 (Shear Rupture Surfaces) considers the influence of 

shear rupture surfaces, typically caused by buckling of roof 

layers less than 1 m thick (Figure 1, No.4). When excessive 

levels of horizontal stress are present, a roof layer can buckle, 

producing a low angle shear rupture surface with a powderlike residue (Gale et al., 2001; Iannacchione el al. 2003). The 

shear failure or roof cutters are typically comprised of a 

concentrated zone of defects. They are typically assigned an 

AV of 5 but if present in very small lengths (< 1 m) are given 

an AV of 3. 

• Category 5 (Joint Separations) takes into account the separation 

of vertical fractures or joints. This type of defect occurs only 

when the strata loses compression in the direction 

perpendicular to the joint orientation (Figure 1, No.5). Joint 

separation signals that strata extension has occurred. Because 

most underground stone mine roofs have some level of 

vertical jointing and horizontal bedding plane contacts, most 

roofs are comprised of block segments of varying size that are 

supported by the confining stresses in the immediate roof 

beam. When strata extension occurs, the roof blocks are no 

longer confined and can fall to the ground under the forces of 

gravity. If no joint separation is observed, then the AV is 1. 

Because separation is a severe defect, any noticeable or 

measurable separation of a vertical joint is assigned an AV of 

5 

• Category 6 (Lateral Strata Shifting) examines defects associated 

with lateral strata shifting where roof layers move in opposing 

directions along bedding contacts (Figure 1, No.6). While it is 

difficult to directly link this category with roof falls, it is 

commonly recognized as an unstable condition (Zhang and 

Peng, 2001). In some mines, lateral strata shifting are 

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

large angular discontinuity (Iannacchione et al., 1981). 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 occur, then the AV is 1. If less than 2cm of 

offset is observed where the surface intercepts the mine roof 

or rib, then the AV is 3. If the offset is greater than 2cm, the 

AV 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 into the roof 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). 

• Category 7 (Strata Separation) encompasses the vertical strata 

separation caused when roof layers separate from one another 

and sag into the mine entry (Figure 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 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 bore scope, or a roof 

deflection monitor. If no separations exist in the immediate 

roof, then the AV is 1. If the separation is barely detectable or 

open, then the AV is 3. If the separation is easily detectable 

(>0.5cm), then the AV is 5. 

• Category 8 (Roof Rock Debris on the Floor) assesses the 

amount of defects in the roof strata by estimating the amount 

of roof rock deposited on the mine floor (Figure 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 not recorded, then this 

information needs to be inferred. Debris from blasting and 

scaling the roof and ribs must be differentiated from roof 

rocks that have fallen. If no roof rock debris is observed, then 

the AV is 1. Increasing amounts of debris produce a higher 

AV. An AV of 5 is typically associated with a pile of broken 

rocks that obstructs or hinders walking in that portion of that 

mine entry. 

• Category 9 (Roof Shape) uses the shape of the roof profile to 

estimate the concentration of defects in the roof strata 

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

desirable in underground stone mining and typically represents 

a roof that has not been damaged by blasting or scaling 

(Figure 1, No.9). Smooth roof is given an AV of 1. 

Conversely, if the roof is highly irregular with pronounced 

swales and troughs, the potential for increased amounts of 

defects increases the AV to 5. Sometimes this condition is 

caused by inherent weakness within the roof rocks. Other 

times the rough looking roof is a result of roof rocks damaged 

by blasting, scaling or excessive stress conditions. 

• Category 10 (Moisture/Ground Water Inflow) appraises the 

amount of defect damage by moisture/ground water inflow 

characteristics (Figure 1, No.10). Different AV’s are assigned 

as the conditions change from dry (AV=1), to damp (AV=2), 

to dripping (AV=4), and finally to the steady flow of water 

from the roof (AV=5). 
25th International Conference on Ground Control in Mining 

130 

 

Figure 1. Sketch of parameters associated with the ten defect categories. 
25th International Conference on Ground Control in Mining 

131 

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

Grouping Category Parameter Assessment 

Value Weight Category 

Value 

None 1 

One, strong contact 2 

One, weak contact 3 

More than one, strong contact 4 

1 - Large 

More than one, weak contact 5 

angular 

discontinuities 

Unknown 3 

1 

None 1 

Widely spaced (>1m) 2 

Moderately spaced (0.25 to 1m) 4 

2 - Joint 

Closely spaced (<0.25m) 5 

frequency 

Unknown 3 

1 

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 

Geologic factors 

3 - Roof layer 

thickness and 

bedding contact 

strength 

Unknown 3 

1 

None 1 

Small shear (cutter < 1m) 3 

Large shear (cutter > 1m) 5 

4 - 

Shear 

rupture 

surfaces 

Unknown 3 

2 

None 1 

5 - 

Noticeable or measurable 5 

Joint 

separation 

Unknown 3 

2 

None 1 

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

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

6 - 

Lateral 

strata 

shifting 

Unknown 3 

2 

None 1 

Slight (barely detectable) 3 

Significant (>0.5cm) 5 

Mining induced failures 

7 - 

Strata 

separation 

Unknown 3 

2 

None 1 

Slight (widely spaced) 2 

Moderate 4 

8 - Roof 

Significant (continuous) 5 

rock debris 

on floor 

Unknown 3 

2 

Smooth 1 

Intermediate 3 

Rough 5 

Roof profile 

9 - Roof 

shape 

Unknown 3 

1 

None 1 

Damp roof 2 

Drippers 4 

Moisture 

Steady flow 5 

factors 

10 - 

Moisture/ 

ground 

water 

inflow

Unknown 3 

1 

Sum all category values

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
25th International Conference on Ground Control in Mining 

132 

CALCULATING THE RFRI 

 A fairly simple and straight forward mathematical expression is 

used to calculate the RFRI and is defined as: 

 (1) 

Where: AV = the assessment value for each defect category 

 SV = the scaling value (SV = 6.0) 

 W = the weighting of each category 

 Because the individual defect categories reflect the ground 

stability of underground mine entries to different degrees, it is 

necessary to weight (W) each of the ten categories (Table 1). The 

defect categories viewed as more detrimental to roof conditions are 

4, 5, 6, 7, and 8 and were assigned a W 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 assigned a W of 1. 

 A scaling value (SV) of 6.0 is assigned for the current defect 

categories and associated W. The SV = 6.0 was selected so that 

AV of 3 for all ten defect categories will result in an RFRI of 50. 

The SV can be modified if the W or defect categories are modified 

in future versions of the RFRI, to maintain this relationship. When 

these SV’s and W’s are used, the RFRI can be calculated with the 

following equation: 

(2) 

 

 For the SV and W used above, the RFRI distribution ranges 

between 17 and 83. Adjustments can alter this range (see below), 

but the RFRI should never be greater than 100. When RFRI values 

approach 0, very low concentrations of defects are measured and 

roof conditions are potentially stable. When RFRI values approach 

100, very high concentrations of defects are measured and roof 

conditions are potentially unstable. 

ASSESSMENT VALUES ADJUSTMENTS 

 Two factors make it necessary to develop ways to adjust RFRI 

values: 1) all underground mines have conditions that are unique, 

and 2) additional information about local strata conditions is 

sometimes available. As additional experience is added, it maybe 

necessary to append the list of defect categories, adjust category W 

or change category AV. This first factor is beyond the scope of this 

paper, but should be addressed in the future. Today, many mines 

collect additional observational and monitoring information on the 

character and behavior of their strata, often through drill holes in 

the roof. Monitoring instruments are typically deformation sensors, 

either of the roof sag or roof-to-floor convergence variety, but can 

also include a range of geophysical techniques including 

microseismic monitoring. Two RFRI adjustments are presented 

below, but others may be added to help address the unique 

characteristics at every operating mine. 

Microseismic Activity Adjustment 

 Microseismic emissions are known to be particularly good at 

characterizing shear rupture surfaces (Iannacchione et al., 2004). It 

follows that 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). 

Roof Deformation Rate Adjustment 

 Roof deflection measurements are known to produce nonambiguous assessments of strata separation characteristics. 

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

an opportunity to collect values of roof deflection measurements 

that can be used to adjust the RFRI values. Three general 

conditions are characterized when measuring roof deflection. If no 

roof deflection is measured, 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 month to the next. An increasing rate of roof 

deflection is a well documented precursor of roof failure (Maleki 

and McVey, 1988). This condition suggests the roof is in an 

unstable state. If this condition occurs, the RFRI is increased by 30. 

FIELD VERIFICATION CASE STUDIES 

 The RFRI was field tested at two underground stone mines. 

These field trials consist of three components: 1) collection of 

defect information from the mine, 2) analysis of RFRI values, and 3) 

assessment of RFRI performance. 

Field Verification Site No. 1: Comparing Roof Stability 

Conditions 

 At the first field verification site, the mine was driving a 4-entry 

system to a nearby outcrop to enhance local mine ventilation. Roof 

conditions were generally good in this part of the mine, but were 

expected to become more difficult as mining approached the 

outcrop. The mine was interested in knowing the general condition 

of the roof in this area and wanted to examine how conditions 

changed with time. The principal verification test was to evaluate 

how well the RFRI characterized actual roof conditions. 

Collection of defect information from the mine: During one mine 

visit, a total of 3 hours was spent mapping the new 4-entry 

development. The geologic, stress and mining related defects were 

observed and placed on a mine map (Figure 2a). The mapped 

region was then divided into 46 measurement areas (Figure 2b). A 

general summary of the AV for the most important Defect 

Categories used to determine the RFRI for typical measurement 

areas at the first site are as follows: 

RFRI = Σ (AV * W) * 1.11 

RFRI = [Σ (AV * W) / Σ (SV * W)] * 100 
25th International Conference on Ground Control in Mining 

133 

o Category 1 – Large angular discontinuities: Several faults 

intersected the roof strata at dips ranging from 20 to 50 degrees. 

These faults generally contained clay material and were sources 

of water infiltration. These large angular discontinuities 

produced significant defects. The AV was 5 for these fault 

conditions. 

o Category 3 – Roof layer thickness and bedding contact strength: 

An AV was generally 2 indicating strong bedding contacts in 

the immediate roof. Whenever weak red beds were 

encountered, an AV of 5 was assigned. 

Figure 2. The first field verification site measurement region: a) defects map, b) 46 measurement areas, and c) roof fall risk

map displaying the individual RFRI values over the measurement region. 

b)

a)

c)
25th International Conference on Ground Control in Mining 

134 

o Category 4 – Shear rupture surfaces: The AV ranged between 1 

and 5; 1 if no shears were present, 5 when extensive shears 

were present. 

o Category 7 – Strata separation: The AV was typically 1; 

however, higher values were assigned to areas where 

separations were detected from roof drill holes. 

o Category 8 – Roof rock debris on floor: AV was 1 if no roof 

rock spalling occurred and higher if guttering was observed, i.e. 

common in association with shear ruptures. 

o Category 9 – Roof shape: Whenever smooth roof was observed 

an AV of 1 was assigned. Rough roofs were assigned an AV of 

5. Typically these roofs contained shear ruptures or multiple 

brows outlining individual roof layers. 

o Category 10 – Moisture/ground water inflow: The AV was 1 

for dry conditions, 2 for damp areas and 3 to 5 for areas where 

significant water flow was observed. 

 Category 1 and 4 had the most pronounced affect on the RFRI 

followed by 3, 7, 8, 9, and 10. 

Analysis of RFRI values: The RFRI values in the measurement 

region ranged from a low of 19 to a high of 58 (Figure 3). A roof 

fall risk map was constructed from the 46 RFRI measurements 

(Figure 2c). The highest percentage of these values occurred at the 

lowest RFRI indicating that most of the mine’s roof in this section 

had low concentrations of defects. In fact, 591 m or 69 % of the 

measured mine entries had an RFRI < 25. Approximately 25 % or 

220 m had an RFRI between 25 and 50. The main factor for the 

elevated RFRI values in these areas was faults. Small increases in 

the percentage of RFRI values from 30 to 35 and 45 to 50 were 

caused by faults (Figure 3), the latter were faults with some vertical 

strata separation and rough roof conditions. 

Comments on the assessment of the RFRI field test: The maximum 

RFRI value of 58 was calculated for a section of entry 52 m long 

that contained shear ruptures, rough roof, vertical strata separations 

and flowing water conditions. The next highest RFRI value was 46. 

The mine was concerned about the stability of this area and 

constructed berms to restrict entry into the area. The fact that the 

mine operator and the RFRI both identified this entry as having an 

elevated risk for roof failure validates the performance of the RFRI. 

Field Verification of Site No. 2: Comparing Mine Design 

Changes 

 At the second field verification site, two mine layout designs 

were implemented to improve roof conditions. The first, Design A, 

used headings (entry paralleling the main direction of mine 

development) developed in the S30W direction and crosscuts (entry 

between headings) developed in the S80W direction (Figure 4). 

Figure 3. Percentage of RFRI values collected from the two 

field verification sites. 

0%

10%

20%

30%

40%

50%

60%

70%

20

25

30

35

4045

50

5560

More

RFRI Values

First Site Second Site

Open joints

Faults Faults with 

roof damage

Shear ruptures Roof falls=100

Restricted 

entry

0%

10%

20%

30%

40%

50%

60%

70%

20

25

30

35

4045

50

5560

More

RFRI Values

First Site Second Site

Open joints

Faults Faults with 

roof damage

Shear ruptures Roof falls=100

Restricted 

entry

Figure 4. The second field verification site measurement region showing the two mine layout designs. Major defects 

are also shown where shear ruptures are drawn as tracked features and open joints are drawn as solid lines. 

Design A

Design B Crosscut

Heading

Crosscut

Heading

S

W

Design A

Design B Crosscut

Heading

Crosscut

Heading

S

W

Design A

Design B Crosscut

Heading

Crosscut

Heading

S

W
25th International Conference on Ground Control in Mining 

135 

The pillars are rectangular with the long axis oriented parallel to the 

headings. Because the operator had wanted to eliminate the 4-way 

intersections, crosscuts were offset, resulting in only 3-way 

intersections. 

 Next, Design B was implemented which optimizes the mine 

layout when developing in excessive horizontal stress conditions 

(Emery, 1964; Parker, 1966; Shaffer and Petersen, 2000; 

Iannacchione et al., 2003). This design orients the headings and the 

associated rectangular pillars parallel to the principal horizontal 

stress direction. The crosscuts are offset and oriented roughly 

perpendicular to the headings. The principal verification test was 

to evaluate how well the RFRI characterized entry performance 

under these different design conditions. 

Collection of defect information from the mine: During two mine 

visits, 10 hours were spent mapping a portion of the mine that 

contains parts of the two distinct mine layouts (Figure 4). The 

geologic, stress and mining related defects were observed and 

placed on the mine map. The mapped region was then divided into 

226 measurement areas. Each measurement area generally 

encompassed one of three mine entry characteristics: headings, 

crosscuts and 3-way intersections. The length of these different 

measurement areas ranged from 12 to 66m. In this study, the 

measurement areas are equivalent to distinct segments of mine 

entries. This facilitated analyzing the influence of mine layouts on 

RFRI. A general summary of the AV for the most important 

Defect Categories used to determine the RFRI for typical 

measurement areas at the second site are as follows: 

o Category 2 – Joint frequency: The AV ranged between 2 and 4 

where joint spacing ranged from 0.3 to 1m. Most of the higher 

AV’s (3 and 4) were associated with the major water bearing 

fracture system that cuts across the measurement region. 

o Category 3 – Roof layer thickness and bedding contact strength: 

The AV was generally 2, indicating pronounced bedding 

contacts in the immediate roof. 

o Category 4 – Shear rupture surfaces: The AV ranged between 1 

and 5: 1 if no shears were present, 5 when extensive shears 

were present. 

o Category 5 – Joint separation: The AV ranged between 1 and 

5: 1 if the joints were closed, 5 if they were open. 

o Category 7 – Strata separation: The AV was typically 1, 

however higher values were used if separation was observed 

along brows or within drill holes (typically locations were 

marked on the roof by driller). 

o Category 8 – Roof rock debris on floor: The AV ranged 

between 1 and 5: 1 if the floor was clear of debris, 5 if the 

debris pile was difficult to negotiate. 

o Category 9 – Roof shape: The AV was 1 for smooth roof and 3 

for uneven roof profiles, i.e. associated with guttering, shearing, 

blasting or fracturing. 

o Category 10 – Moisture/ground water inflow: The AV was 1 

for dry conditions, 2 for damp areas and 3 to 5 for areas where 

significant water flow was observed. 

Category 4 and 5 had the most pronounced affect on the RFRI at 

this site followed by 7, 8, 9, and 10. 

Figure 5. (a) Roof fall risk map displaying the 226 RFRI values over the measurement region. (b) RFRI for 

different entry characteristics in the Design A and Design B. 

0

10

20

30

40

Headings Crosscuts 3-way

intersections

Avg. RFRI 

Design A

b) Design B

S

W

a)

0

10

20

30

40

Headings Crosscuts 3-way

intersections

Avg. RFRI 

Design A

b) Design B

S

W

a)
25th International Conference on Ground Control in Mining 

136 

Analysis of RFRI values: The distribution of the 226 RFRI values 

collected from measurement sites is shown in Figure 5. RFRI 

values ranged from a low of 19 to a high of 53, with the exception 

of the pre-survey roof fall areas that are assigned an RFRI of 100 

(Figure 5a). As would be expected, most of the locations had low 

RFRI values. For example, 61 % of the measured entry areas had 

an RFRI less than 30. Twenty-seven percent of the entries had 

RFRI values between 30 and 40, and were often adversely 

influenced by open joints and to a leaser extent by shear ruptures 

and flowing water. Twelve percent of the entries were dominated 

by shear ruptures. 

 A roof fall risk map was constructed from the 226 measured 

RFRI values (Figure 5a). The average RFRI for the different mine 

designs were calculated and shown in Figure 5b. In general, 

Designs A and B have similar overall average RFRI (A = 28 and B 

= 29); however, small, but important, differences are observed 

when individual entry characteristics are considered. For example, 

the headings of the Design B had an RFRI of 25 as compared to the 

30 for the headings of the Design A. The opposite trend was 

observed for the crosscuts where Design B had a higher RFRI (26 

versus 34). The RFRI for the 3-way intersections was very similar 

for both designs. This data indicates that Design A had similar 

concentrations of defects in both the headings and crosscuts, 

whereas Design B had relative low concentration of defects in the 

headings and higher concentrations in the crosscuts. 

Comments on the assessment of the RFRI field test: When 

excessive levels of horizontal stress exist, shear ruptures are 

typically oriented perpendicular to the principal stress direction. 

This orientation has high compressive stresses and represents the 

least favorable mining direction. Conversely, orientations parallel 

to the principal stress direction are typically free of shear ruptures 

but can often have tensile fractures. This orientation represents the 

most favorable mining direction. As the horizontal stress field 

becomes more bi-axial, or less equal, these tension fractures can 

extend and open, hence open joints can sometimes occur 

perpendicular to the shear ruptures. 

 At the second verification field site, these very conditions were 

observed. Shear ruptures occurred in orientations that cluster 

around S20E as shown by the tracked pattern on Figure 4. 

Additionally, a significant number of open fractures exist in the 

S70W orientation suggesting a highly bi-axial horizontal stress 

field (see the solid lines on Figure 4). For this reason, the main 

headings in Design B were laid out along that direction to take 

advantage of favorable conditions. Conversely, crosscuts were 

oriented 53 degrees off the S70W bearing (S17W) and offset by 

rectangular pillars to inhibit the extension of roof cutters in the least 

favorable mining direction (S20E). The crosscuts at this mine 

could not be developed in the S20E due to steep dips in that 

direction. Because the RFRI has produced trends that support the 

collective knowledge of roof conditions in the mapped region of 

the study site, this field trial was viewed as a success. 

SUMMARY AND CONCLUSIONS 

 NIOSH’s RFRI was developed to help assess the risk of roof 

falls associated with underground stone mining. This method is 

centered on an examination of the defects contained in the roof 

caused by a wide range of local geologic, mining and stress factors. 

The RFRI is based on observations and should be considered as a 

part of an overall strategy deployed by mining operations to assess 

the risk of roof falls. Clearly more information leads to less 

uncertainty and potentially reduces the risk associated with ever 

changing mining conditions. NIOSH’s aim is to develop a method 

to help mine safety personnel identify and track changing roof 

conditions. 

 The RFRI is an assessment technique that can be used to rate 

roof fall risk in important parts of a mine or potentially the entire 

mine property. It should also be viewed as a powerful 

communication tool that helps to track changes in roof conditions. 

It can also be used as a training method to help less-experienced 

miners identify defective rock conditions. Lastly, decision makers 

can use it to examine changes in mining conditions and to help 

develop plans for proactive actions during the course of mine 

development. 

 The purpose of this paper was to provide field examples that 

demonstrate how rock defect information is collected and analyzed 

and to verify that output from the RFRI are meaningful and, at least, 

partially repeatable. The first field verification test was conducted 

to examine how well the RFRI assessed roof conditions as a new 

set of entries was developed. In this case, the RFRI showed that 

elevated risks were associated with mining under major geologic 

discontinuities (faults) and that areas identified by the mine 

operator as the most hazardous also contained the highest RFRI 

values. 

 In the second field verification test, a new stress control mine 

layout was compared with a previous layout to determine what 

affects these design changes had on roof conditions. In this case, 

headings developed in a favorable mining direction had lower 

RFRI values then crosscuts developed in less favorable directions. 

In both field verification tests, the RFRI was found to perform as 

designed. 

REFERENCES 

Emery, C.L. (1964). In Situ Measurements Applied to Mine 

Design. Proceedings, 6th Symposium on Rock Mechanics, Rolla, 

MO, pp. 218-230. 

Gale, W.J., Heasley, K.A., Iannacchione, A.T., Swanson, P.L., 

Hatherly, P. and King, A. (2001). A Rock Damage 

Characterization from Microseismic Monitoring. Proceedings, 38th

U.S. Rock Mechanics Symposium, July 7-10, Washington, DC, 

pp. 1313-1320. 

Hylbert, D.K. (1980). Development of Geologic Structural Criteria 

for Predicting Unstable Mine Roof Rocks. U.S. Department of the 

Interior, U.S. Bureau of Mines Open File Report 9-78, 249 p. 

Iannacchione, A.T., Ulery, J.P., Hyman, D.M. and Chase F.E. 

(1981). Geologic Factors in Predicting Coal Mine Roof-Rock 

Stability in the Upper Kittanning Coalbed, Somerset Co., PA, U.S. 

Department of the Interior, Bureau of Mines RI 8575, 40 pp. 

Iannacchione, A.T. and Prosser, L.J. (1998). Roof and Rib Hazards 

Assessment for Underground Stone Mines. Mining Engineering, 

February, pp. 76-80. 

Iannacchione, A.T., Marshall, T.E., Burke, L.M., Melville, R. and 

Litsenberger, J. (2003). Safer Mine Layouts for Underground 

Stone Mines Subjected to Excessive Levels of Horizontal Stress. 

Mining Engineering, April, pp. 25-31. 
25th International Conference on Ground Control in Mining 

137 

Iannacchione, A.T., Coyle, P.R., Prosser, L.J., Marshall, T.E. and 

Litsenberger, J. (2004). Relationship of Roof Movement and Strata 

Induced Microseismic Emissions to Roof Falls. Mining 

Engineering, December, pp. 53-60. 

Iannacchione, A.T., Esterhuizen, G.S., Prosser, L.J. and Bajpayee, 

T.S. (2006). Assessing Roof Fall Hazards for Underground Stone 

Mines: A Proposed Methodology. SME Preprint 06-59 SME 

Annual Meeting, St. Louis, MO, March 27-29. 

Krausse, H.F., Damberger, H.H., Nelson, W.J., Hunt, S.R., Ledvina, 

C.T., Treworgy, C.C. and White, W.A. (1980). Engineering Study 

of Structural Geologic Features of the Herrin (No. 6) Coal and 

Associated Rock in Illinois. U.S. Department of the Interior, U.S. 

Bureau of Mines Open File Report 96-80, Vol. 1, 67 p. 

Lagather, R.B. (1977). Guide to Geologic Features Affecting Coal 

Mine Roof. Mine Safety and Health Administration IR 1101, 18 p. 

Maleki, H.N and McVey, J.R. (1988). Detection of Roof 

Instability by Monitoring the Rate of Movement. U.S. Department 

of the Interior, U.S. Bureau of Mines RI 9170, 12 pp. 

Moebs, N.N. (1977). Roof Rock Structures and Related Roof 

Support Problems in the Pittsburgh Coalbed of Southwestern 

Pennsylvania. U.S. Department of the Interior, U.S. Bureau of 

Mines RI 8230, 30 pp. 

Mucho, T.P. and Mark, C. (1994). Determining Horizontal Stress 

Direction Using the Stress Mapping Technique. Proceedings, 13th

International Conference on Ground Control in Mining, 

Morgantown, WV, Aug. 2-4, pp. 277-289. 

Parker, J. (1966). Mining in a Lateral Stress Field at White Pine. 

Canadian Institute of Mining and Metallurgy Transactions, Vol. 

LXIX, pp. 375-383. 

Parker, J. (1973). How to Design Better Mine Openings: Practical 

Rock Mechanics for Miners. Engineering and Mining Journal, Part 

5, December, pp. 76-80. 

Shaffer, D and Petersen, G. (2000). Practical Rock Mechanics for 

Practical Miners. Aggregate Manager 4(10): 26-31. 

Zhang, Y. and Peng, S.S. (2001). Effects of Bedding Plane Sliding 

and Separation and Tensioned Bolt in Layered Roof. Proceedings, 

20th International Conference on Ground Control in Mining, 

Morgantown, WV, Aug. 7-9, pp. 226-234.