A Method to Characterize Risk Associated With Mine Roof Conditions

Objective 

 To characterize the roof fall risk associated with 

underground limestone mining. 

Background 

During the first 5 years of this decade, more than 

one-fourth of the underground mining fatalities 

occurred as the result of fall of ground. The large 

openings in underground stone mines present limitations in observing ground conditions because of 

floor-to-roof heights ranging from 20 to 60 ft or more. 

Generally, the status of ground conditions in underground stone mines is based on observation and experience obtained during mining development. Some 

mines use monitoring instruments to gain information 

on roof conditions, but this practice is typically 

focused and localized to address issues in a particular 

area or section of the mine. 

 The Roof Fall Risk Index (RFRI) described here 

provides a method of systematically characterizing 

the risk of roof falls to enhance the safety of mine 

workers. 

Approach 

As in many industries, a defective product signals 

the potential for failure. Similarly, defective rock or 

strata in an underground stone mine presents a 

potential risk. Assessing ground conditions relative 

to the concentration of defects observed provides a 

way to index or rate these conditions and the 

subsequent risk. Through observations at approximately 50 underground limestone mines in the United 

States, strata defects were categorized to develop 

a quantitative method for determining the RFRI. This 

method is specifically geared to underground limestone mines because strata defects are difficult to see 

and limited by the experience and knowledge of the 

observer. Overall, 10 defect categories were determined: 3 that relate to geology, 4 that result from 

mine development, 2 that are indicative of the roof 

profile, and 1 that addresses moisture or water flow 

(see RFRI Chart 1). The defect categories are also 

weighted to reflect the conditions indicative of roof or 

strata movement or separation (categories 4 through 

8). These weights are multiplied by the assessment 

value to determine the total category value. The 

benefit of using monitoring techniques to gain 

information on roof conditions is also factored into 

the RFRI for overall assessment of an area in the 

mine. Each defect category is assessed with a value 

of 1 to 5 based on parameters as defined in each 

defect category, with 5 indicating the highest degree 

or level of defect. The assessment value of 3 is also 

used when information on a parameter is unknown. 

The parameters for the defect categories are 

illustrated in RFRI Chart 2 for use in determining the 

assessed value. 

How It Works 

The RFRI provides a means to categorize the risk 

for roof falls in underground stone mines. The concept 

of this calculation is aimed at providing an indication 

of the level or degree of defect relative to the area of 

the mine examined. The RFRI for the calculation 

produces a distribution where RFRI values approaching 0 represent a stable condition and those approaching 100 represent an unstable condition. 

No. 520 October 2006 
To calculate the RFRI in your mine, simply assess 

the defect categories by parameter, multiply by the 

weight, add the category values (as shown at the 

bottom of RFRI Chart 1), and multiply by 1.11. 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 gained, it may 

be necessary to amend the list of defect categories, 

adjust category weights, or change category assessment values. For example, if microseismic or roof 

deformation monitoring data are available, these data 

can be used to adjust the RFRI, as shown at the bottom 

of RFRI Chart 1. Other factors such as roof support 

would also impact the RFRI. 

Summary 

 NIOSH research in underground stone mines has 

provided the framework for developing an RFRI. 

This method is centered on examining 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 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. In addition, 

it can be used as a training method to help less 

experienced miners identify defective rock conditions. Lastly, decision-makers can use the RFRI to 

examine changes in mining conditions and to help 

develop plans for proactive actions during the course 

of mine development. 

To increase the use of the RFRI, NIOSH will 

provide a full-sized laminated card containing RFRI 

Charts 1 and 2 so it can readily be used in an underground mine. See “For More Information” below on 

how to obtain an RFRI card for your mine. 

Accomplishments 

The RFRI was introduced at the annual meeting of 

the Society for Mining, Metallurgy, and Exploration 

in St. Louis, MO, on March 26–29, 2006. An update 

on the RFRI was presented at the 25th International 

Conference on Ground Control in Mining in 

Morgantown, WV, on August 1–3, 2006. The 

published papers for these presentations are listed 

below and are available upon request: 

Iannacchione AT, Prosser LJ Jr., Esterhuizen GS, 

Bajpayee TS [2006]. Assessing roof fall hazards for 

underground stone mines: a proposed methodology. 

SME preprint 06–059. Littleton, CO: Society for 

Mining, Metallurgy, and Exploration, Inc. 

Iannacchione AT, Esterhuizen GS, Schilling S, 

Goodwin T [2006]. Field verification of the roof fall 

risk index: a method to assess strata conditions. In: 

Peng SS, Mark C, Finfinger GL, Tadolini SC, Khair 

AW, Heasley KA, Luo Y, eds. Proceedings of the 

25th International Conference on Ground Control in 

Mining. Morgantown, WV: West Virginia University, 

pp. 128–137. 

For More Information 

For more information on the Roof Fall Risk Index 

(RFRI) or to obtain a full-sized laminated RFRI card, 

contact Lou Prosser (412–386–4423, LProsser@cdc.gov), 

or Anthony T. Iannacchione, Ph.D., P.E. (412–386–

6581, AIannacchione@cdc.gov), NIOSH Pittsburgh 

Research Laboratory, P.O. Box 18070, Pittsburgh, PA 

15236–0070. 

To receive other information about occupational 

safety and health topics, call 1–800–35–NIOSH 

(1–800–356–4674), or visit the NIOSH Website at 

www.cdc.gov/niosh 

Mention of any company name or product does not 

constitute endorsement by the National Institute 

for Occupational Safety and Health. 
RFRI Chart 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 (>3 ft) 2 

 Moderately spaced (1–3 ft) 4 

 Closely spaced (<1 ft) 5 

2 – Joint 

frequency 

 Unknown 3 

1 

 Massive (>3-ft layers) 1 

 Strong bedding contacts in immediate roof (0–3 ft) 2 

 Weak bedding contact(s) in immediate roof (0–3 ft) 3 

 Rock layers 1–3 ft with weak bedding contact(s) 4 

 Thin layers (<1 ft) with strong bedding contact(s) 4 

 Thin layers (<1 ft) with weak bedding contact(s) 5 

Geologic factors 

3 – Roof layer 

thickness and 

bedding contact 

strength 

 Unknown 3 

1 

 None 1 

 Small shear (cutter <3 ft) 3 

4 – 

 Large shear (cutter >3 ft) 5 

Shear 

rupture 

surface(s) Unknown 3 

2 

 None 1 

 Noticeable or measurable 5 

5 – 

Joint 

separation

 Unknown 3 

2 

 None 1 

 <1 inch of offset or partial vertical drill hole offset 3 

6 – 

 >1 inch of offset or complete vertical drill hole offset 5 

Lateral 

strata 

shifting 

 Unknown 3 

2 

 None 1 

 Slight (barely detectable) 3 

 Significant (>0.2 inch) 5 

Mining-induced failures 

7 – 

Vertical 

strata 

separation 

 Unknown 3 

2 

 None 1 

 Slight (widely spaced) 2 

 Moderate 4 

 Significant (continuous) 5 

8 – Roof 

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 

factors 

 Steady flow 5 

10 –

Moisture/ 

ground 

water 

inflow 

 Unknown 3 

1 

Sum of all category values .............................................................................................................

Multiply 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 ............................................................................................................................................