RI 9698 

REPORT OF INVESTIGATIONS/2015 

Facilitating the Use of 

Built-in-place Refuge Alternatives 

in Mines 
 

 

Report of Investigations 9698 

Facilitating the Use of Built-in-place Refuge Alternatives in Mines 

Jack D. Trackemas, Edward D. Thimons, Eric R. Bauer, Michael J. Sapko, R. Karl 

Zipf, Jr., Joseph Schall, Elaine Rubinstein, Gerald L. Finfinger, Larry D. Patts, and 

Niccolle LaBranche 

DEPARTMENT OF HEALTH AND HUMAN SERVICES
 Centers for Disease Control and Prevention
 

National Institute for Occupational Safety and Health
 

Office of Mine Safety and Health Research
 

Pittsburgh, PA • Spokane, WA
 

April 2015 
This document is in the public domain and may be freely 

copied or reprinted. 

 

 

 

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Suggested Citation

NIOSH [2015]. Facilitating the use of built-in-place refuge alternatives in mines. By Trackemas 

JD, Thimons ED, Bauer ER, Sapko MJ, Zipf RK, Schall J, Rubinstein E, Finfinger GL, Patts LD, 

LaBranche N. Pittsburgh, PA: U.S. Department of Health and Human Services, Centers for 

Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS 

(NIOSH) Publication No. 2015-114, RI 9698. 

DHHS (NIOSH) Publication No. 2015–114 

April 2015 

Cover photo by NIOSH
Contents

Executive Summary ........................................................................................................ 1

Background ............................................................................................................... 1

Summary of Approach ................................................................................................ 2

Summary of Findings .................................................................................................. 3

Locating BIP RAs at Greater Distances from the Working Face ..................................... 3

Designing RA Stoppings for MSHA Approval.................................................................. 6

Delivering Clean, Breathable Air to a BIP RA ................................................................. 6

Considering Economic Drivers when Designing New BIP RAs ....................................... 6

Introduction ..................................................................................................................... 8

Potential Advantages and Disadvantages of BIP RAs .................................................. 10

Locating BIP RAs Further from the Face....................................................................... 12

Approach 1: Based on SCSR-Mandated Storage Cache Locations ............................... 13

Approach 2: Based on Worst-Case SCSR Usage Times .............................................. 14

Approach 3: Based on NIOSH and U.S. Bureau of Mines Established Travel Times 

and Escape Probabilities........................................................................................... 17

Discussion of Three Approaches to Locating BIP RAs Further from the Face................. 18

Discussion of How Locating BIP RAs Further from the Face Impacts RA Moves ............ 21

Design and Approval of RA Stoppings .......................................................................... 21

Structural Design Considerations for RA Stoppings ..................................................... 21

Recommended Design Pressure-Time Curve for an RA Stopping ................................. 24

Criteria for Designing an RA Stopping and Approval of RA Stopping Designs ................ 26

Structural Design Criteria...............................................................................................27

Foundation Design ........................................................................................................27

Location.........................................................................................................................27

Leakage ........................................................................................................................27

Quality Control...............................................................................................................28

Guidelines for Effective RA Stopping Design............................................................... 28

Example Design for an Effective RA Stopping ............................................................. 30

Delivering a Reliable Supply of Clean, Breathable Air to a BIP RA............................... 32

Considerations for Facilitating the Use of BIP RAs ....................................................... 34

Locating BIP RAs at Greater Distances from the Working Face .................................... 34

Designing RA Stoppings for MSHA Approval............................................................... 36
Delivering Clean, Breathable Air to a BIP RA .............................................................. 36

Considering Economic Drivers when Designing New BIP RAs ...................................... 36

Acknowledgments ......................................................................................................... 38

References.................................................................................................................... 39

Appendix A—An Overview of Built-in-place Refuge Alternatives in U.S. 

Underground Coal Mines .............................................................................................. 43

Introduction.............................................................................................................. 43

Review of Refuge Alternatives ................................................................................... 43

Mobile RAs....................................................................................................................44

BIP RAs.........................................................................................................................45

Current Role of BIP RAs ........................................................................................... 45

Built-in-place Shelters as Part of a Comprehensive Escape and Rescue Plan ..............45

Regulatory Requirements for BIP RAs...........................................................................46

Summary of Mine Visits............................................................................................. 47

General Information on BIP RAs Observed ...................................................................47

BIP RA Stopping Construction.......................................................................................49

Supplemental Support ...................................................................................................51

Location (Distance Apart) for BIP RAs...........................................................................52

Location (Physical/Actual) of BIP RAs ...........................................................................52

Breathable Air Supply Systems for BIP RAs..................................................................52

Occupancy/Capacity......................................................................................................56

RA Supplies...................................................................................................................56

Waste (Toilet) Facilities in BIP RAs ...............................................................................57

Internal Facilities ...........................................................................................................59

Appendix B—Statistical Analysis of Times to Complete Needed Actions in a Mine 

Emergency .................................................................................................................... 63

Approach................................................................................................................. 63

Results .................................................................................................................... 65

Appendix C—Design Example for a 15-psi RA Stopping Using Rebar-Reinforced 

Concrete........................................................................................................................ 71

Design Inputs........................................................................................................... 71

Specify design load .......................................................................................................71

Specify allowable failure ................................................................................................71

Specify safety factor ......................................................................................................71

Determine dynamic load factor (DLF)............................................................................71
Specify coal mine entry geometry..................................................................................71

Determine support conditions ........................................................................................72

Specify material properties ............................................................................................72

Determine dynamic increase factor ...............................................................................72

Foundation Design (RA Stopping Anchorage) ............................................................. 72

Determine approximate yield line location .....................................................................72

Determine approximate shear forces.............................................................................73

Determine number of rock bolt anchors.........................................................................73

RA Stopping Structure Design ................................................................................... 74

Estimate reinforcing steel requirements.........................................................................74

Determine diagonal shear reinforcement .......................................................................75

Determine actual vertical moment capacity ...................................................................75

Determine actual horizontal moment capacity ...............................................................76

Determine static properties of the design.......................................................................77

Determine actual yield line location and ultimate resistance ..........................................78

Determine direct shear capacity of concrete..................................................................78

Determine dynamic response of the RA stopping structure ...........................................79
Figures

Figure 1. Number of active underground coal mines by seam height, 2012.................. 20

Figure 2. Number of coal mine employees working at underground locations by 

seam height, 2012, excluding office employees............................................................ 20

Figure 3. Recommended design pressure-time curve for refuge 

alternative stoppings. .................................................................................................... 25

Figure 4. Pressure-time curve showing pressure decay with distance and shock 

wave development from LLEM test #485 with 60-ft gas zone. ...................................... 26

Figure 5. Design example for a 15-psi RA stopping using conventional 

rebar-reinforced concrete.............................................................................................. 31

Figure A1. Example of a MICON built-in-place refuge alternative stopping and 

door system................................................................................................................... 49

Figure A2. Mine-designed and MSHA District Manager-approved built-in-place 

refuge alternative stopping and door system................................................................. 50

Figure A3. Posts and chain link fence placed to prevent rib rolls in a higher seam 

built-in-place refuge alternative. .................................................................................... 51

Figure A4. Permanent blower air supply system. .......................................................... 53

Figure A5. Connection to borehole for a permanent air supply system......................... 54

Figure A6. Portable, diesel-powered blowers for borehole air supply systems. ............ 55

Figure A7. Breathable air supply borehole opening into a built-in-place refuge 

alternative with no collar................................................................................................ 56

Figure A8: A marine-type flushable toilet....................................................................... 58

Figure A9: A mine-constructed isolation room for a toilet.............................................. 59

Figure A10. Metal benches in a built-in-place refuge alternative................................... 60

Figure A11. Storage shelves in a built-in-place refuge alternative. ............................... 61

Figure A12. Typical storage boxes in a built-in-place refuge alternative. ...................... 62

Figure B1. Histogram of simulated values for total time to travel to and access an 

RA (rounded to the nearest minute). ............................................................................. 68

Figure B2. Comparison of frequency distributions for total time to travel to and 

access an RA in first and second simulation runs. ........................................................ 69
Tables

Table 1. Travel distances to SCSR storage locations ................................................... 14

Table 2. Summary of non-escapeway travel activities based on worst-case 

SCSR usage times........................................................................................................ 16

Table 3. Calculated 90-minute travel distances in smoke-filled escapeways based 

on entry height .............................................................................................................. 17

Table 4. Maximum distances of refuge alternatives from the face ................................ 18

Table 5. Effect of various long duration blast overpressures and the associated 

maximum wind speeds on various structures and the human body .............................. 23

Table 6. Maximum distances of BIP RAs from the face ................................................ 35

Table A1. Historical review of mine barricading efforts as a result of fires and 

explosions in coal mines ............................................................................................... 43

Table A2. Comparison of characteristics for mobile RAs and BIP RAs......................... 44

Table A3. Summary of in-mine observations of BIP RAs .............................................. 48

Table B1. Specifications for generation of simulated data for four activities required 

to travel to and access a refuge alternative................................................................... 63

Table B2. First 12 cases in master data file that correspond to data from Table B1 ..... 64

Table B3. Descriptive statistics on simulated values for four phases required to 

travel to and access an RA ........................................................................................... 65

Table B4. Descriptive statistics on simulated values for total time to travel to 

and access an RA......................................................................................................... 66

Table B5. Frequency distribution for total time to travel to and access an RA .............. 67

 
ACRONYMS AND ABBREVIATIONS

ASTM American Society for Testing and Materials

BIP built-in-place

BOM Bureau of Mines

CFR Code of Federal Regulations

chgovr changeover

CH4 methane

CMU concrete masonry unit

CO carbon monoxide

CO2 carbon dioxide

Dec decision

DLF dynamic load factor

HBA Hubble Breathable Air

NA not applicable

LLEM Lake Lynn Experimental Mine

NIOSH National Institute for Occupational Safety and Health

OMSHR Office of Mine Safety and Health Research

PVC polyvinyl chloride

RA refuge alternative

SCSR self-contained self-rescuer

Seq sequence

Trvl travel

UFC Unified Facilities Criteria
UNIT OF MEASURE ABBREVIATIONS

CI confidence interval

diam diameter

fpm feet per minute

ft feet

ft3 cubic feet

hr hour

in inch

ksi kilopounds per square inch

lb-in pounds inch

m meter

min minute

ppm parts per million

psi pounds per square inch

SD standard deviation

sq square

1 

Facilitating the Use of Built-in-place 

Refuge Alternatives in Mines

Jack D. Trackemas,

1

1 Mining Engineer, NIOSH OMSHR, Pittsburgh, PA.

Edward D. Thimons,2

2 Senior Physical Scientist (retired), NIOSH OMSHR; currently Senior Technical Advisor, URS

Corporation, Pittsburgh, PA.

 Eric R. Bauer,3

3 Senior Mining Engineer (retired), Human Factors Branch, NIOSH OMSHR, Pittsburgh, PA.

Michael J. Sapko,

4

4 Senior Research Physical Scientist (retired), NIOSH OMSHR; currently Senior Principal Advisor, URS 

Corporation, Pittsburgh, PA.

 R. Karl Zipf, Jr.,5

5 Senior Mining Engineer (retired), Fires and Explosions Branch, NIOSH OMSHR, Pittsburgh, PA.

Joseph Schall,

6

6 Health Communications Specialist, Health Communications, Surveillance and Research Support 

Branch, NIOSH OMSHR, Pittsburgh, PA.

Elaine Rubinstein,

7

7 Senior Service Fellow, Health Communications, Surveillance and Research Support Branch, NIOSH 

OMSHR, Pittsburgh, PA.

Gerald L. Finfinger,8

8 Senior Scientist (retired), NIOSH OMSHR; currently Senior Technical Advisor, URS Corporation,

Pittsburgh, PA.

Larry D. Patts,9

9 Physical Scientist, Dust, Ventilation and Toxic Substances Branch, NIOSH OMSHR, Pittsburgh, PA.

 and Niccolle LaBranche10

10 Formerly Mining Engineer, NIOSH OMSHR, Pittsburgh, PA.

Executive Summary

Background

Three major coal mining disasters occurred in 2006, involving either fires or explosions, 

killing 19 miners. In all of these tragedies, miners survived the initial disaster but were unable 

either to escape or successfully isolate themselves from the poisonous gases present in the mine 

environment resulting from the disaster. In response, Congress passed the Mine Improvement 

and New Emergency Response Act (MINER Act) of 2006 (Public Law 109-236). Among its 

mandates, the MINER Act called for the National Institute for Occupational Safety and Health 

(NIOSH) Office of Mine Safety and Health Research (OMSHR) to “provide for the conduct of 

research, including field tests, concerning the utility, practicality, survivability, and cost of 

various refuge alternatives in an underground coal mine environment, including commerciallyavailable portable refuge chambers.” The primary function of a refuge alternative (RA) is to 

provide safe refuge for miners unable to escape their work area immediately after a disaster due 

to toxic gases or a blocked escapeway. To be effective, the RA must survive the initiation of the

disaster, whether it is an explosion or fire. Furthermore, it would additionally be beneficial if it 

would protect the miners inside the RA from the blast impacts of a secondary explosion. 

In response to the MINER Act mandate, NIOSH OMSHR conducted research on refuge

alternatives and provided its findings in the form of a report to Congress [NIOSH 2007]. In sum,

this report found that refuge alternatives were practical for use in most underground coal mines 

to facilitate escape and to serve as a refuge of last resort, provided that mine operators develop

comprehensive escape and rescue plans incorporating refuge alternatives and training on their

 
2 

use. These findings applied to both portable RAs and built-in-place (BIP) RAs, with the BIP RAs

emerging as being able to provide a superior environment. They offer the potential to provide 

miners with an improved psychological and physiological environment, which can be greatly 

advantageous to their health and safety in the stress of an emergency, and they can be provided 

with fresh air via a borehole to the surface or a protected compressed air line. 

Despite these advantages, current BIP RA designs cannot be moved frequently from a 

practical standpoint—with movement of the RA location required to keep up with dynamic 

mining production—and as such it would generally be impractical to keep them within 1,000 feet 

of the nearest working face, as prescribed in 30 CFR11 75.1506(c). Notwithstanding, OMSHR’s 

2007 report to Congress on refuge alternatives concluded that, as compared to portable RAs, the 

strengths of built-in-place refuge alternatives “are so significant that consideration should be 

given to allowing extended distances, if in-place shelters are used to provide refuge for face 

workers” [NIOSH 2007]. 

Currently, there are approximately 30 BIP RAs in U.S. underground coal mines; however, all 

of these are located outby the face area and none are designed to be advanced with the working 

face. The usage of BIP RAs that can be advanced with the working face will only be practical if

three issues can be addressed: (1) locating BIP RAs further from the face; (2) providing a 

consistent process for the design and approval of RA stoppings; (3) delivering a reliable supply 

of clean, breathable air to a BIP RA. In this Report of Investigations, these three issues are

explored in detail.

Summary of Approach

To investigate the first issue—the possibility of locating BIP RAs further from the face—

OMSHR researchers developed three different approaches to determine how far from the face 

area miners could travel given the 120 minutes of breathing time afforded them by currently 

available self-contained self-rescuers (SCSRs). This available 120 minutes of breathing time is 

based on 30 CFR 75.1714-4(a)(1), which calls for the mine operator to provide “at least one 

additional SCSR, which provides protection for a period of one hour or longer, for each person at 

a fixed underground work location.” It should be noted that the Refuge Alternatives for 

Underground Coal Mines Final Rule [73 Fed. Reg.

12 80656(2008)] makes it clear that the times 

included are based on a worst case of only one SCSR being available to each miner.

The three different approaches to determine how far from the face area miners could travel

are as follows:

• Approach 1 was based on SCSR-mandated storage cache locations, and examined the 

MSHA-established criteria for distances between SCSR storage caches as a method 

of determining acceptable distances from the face area for RA locations.

• Approach 2 was based on worst-case SCSR usage times, with OMSHR performing a 

timeline study of an assumed worst-case scenario for miners involved in a disaster, 

 

11 Code of Federal Regulations. See CFR in references. 12 Federal Register. See Fed. Reg. in references.
3 

beginning with the time they first don their initial SCSR and including the times it 

would take to travel to the face area, assemble as a group, make decisions, and 

perform switchover to a new SCSR if needed. 

• Approach 3 was based on established travel times and escape probabilities

determined from NIOSH and U.S. Bureau of Mines research, including research from 

2011 involving actual miners traveling in airways filled with dense smoke in high 

coal without lifelines, and research from 1990 that considered the probability of 

making a successful mine escape in high coal with a single SCSR. 

To address the second issue—design and approval of RA stoppings—OMSHR analyzed the 

criteria that engineers must consider when submitting RA stopping designs for approval under 

the requirements of MSHA’s Refuge Alternatives for Underground Coal Mines [Federal Register 

2008] and guidelines for coal mine seal design applications [MSHA 2008]. Using the MSHA 

application guidelines as a model, OMSHR developed extensive guidelines for RA stopping 

design applications as well as specifications for an RA stopping design to serve as an example. 

The example RA stopping design is presented to illustrate the application of the proposed design 

guidelines in preparing a design submittal to MSHA District Managers for approval. 

Significantly different RA stopping/door systems for BIP RAs have already been approved by 

MSHA for use, and NIOSH believes that if BIP RAs become more commonly used in U.S. coal 

mines then many more acceptable and economically viable stopping/door designs that meet the 

15 psi static pressure criteria will be developed. 

To explore the third and critical issue—delivering a reliable supply of clean, breathable air to

a BIP RA—OMSHR considered the available technologies approved by MSHA for providing

breathable air to an RA via a protected compressed air line. OMSHR also analyzed the practical

and technical considerations for the surface compressor station and the protected compressed air

line.

To obtain information relevant to the above three issues, OMSHR researchers visited a 

number of underground coal mines to view BIP RAs and discuss with mine officials why they 

employ them. Researchers gathered extensive information on the construction, location, capacity, 

air supply, and provisioning. Mines in Colorado, Kentucky, Montana, and New Mexico were 

visited, with the specific mines selected to obtain a cross section of the industry including high 

and low coal and eastern and western coal mines.

Summary of Findings

Locating BIP RAs at Greater Distances from the Working Face

Based on Approaches 1 and 2 as described above—using SCSR-mandated storage cache

location criteria and worst-case SCSR usage times—OMSHR determined that the worst-case

time required for non-escapeway travel activities to the face would be 30 minutes. A detailed

statistical analysis along with MSHA-established values of realistic travel distances to SCSRs in

emergency situations were used to establish this 30-minute figure. Mine workers will have two

SCSRs available to them, providing 120 minutes of breathing time. In OMSHR’s assumed

worst-case scenario, if miners use 30 of their 120 minutes of SCSR time to travel to the face, this

leaves 90 minutes for travel time from the face to the RA.
4 

Based on Approach 3 of using established travel times and escape probabilities determined 

from published research, and choosing the most conservative estimates of travel times needed, 

OMSHR calculated 90-minute travel distances in smoke-filled escapeways based on entry 

height. These worst-case scenarios combined three pieces of information: the regulations in 30 

CFR 75.1714-4(c)(2)(ii) for 30-minute travel distances in various entry heights; the fact that 

smoke-filled airways increase travel times, and the above-outlined discussion that escaping 

miners should have 90 minutes of SCSR breathing time available for travel from the face to an 

RA. The resulting evidence clearly justifies the argument that RAs can be located further from 

the face, and that greater maximum distances of the RA from the face can be used in parallel 

with greater entry heights. 

Combining the results from these three approaches, conservative maximum distances from 

the face to the RA for various entry heights were established, grounded in the assumption that 

miners have 90 minutes of available travel time in the escapeway to reach the RA. Based on 

these findings, OMSHR believes that mines could locate built-in-place refuge alternatives at 

distances from the working face based on the guidance provided by the table below. 

The following table presents a summary of these findings, rounding down the most 

conservative distances arrived at in this report as the maximum BIP RA distances from the face, 

with these distances varying based on entry height. 

Entry height

Approach 1: 

Based on SCSRmandated 

storage cache 

locations 

Approach 2:

Based on 

worst-case 

SCSR usage 

times

Approach 3: Based 

on NIOSH and 

BOM established 

travel times and 

escape probabilities

Maximum BIP RA 

distance from the 

face 

<40 inches 2,200 feet 2,640 feet NA 2,000 feet

>40–<50 inches 3,300 feet 3,960 feet NA 3,000 feet

>50–<65 inches 4,400 feet 5,280 feet 6,000 feet 4,000 feet

>65 inches 5,700 feet 6,480 feet 6,500–7,000 feet 5,000 feet

Allowing mines to locate BIP RAs at greater distances from the working face as outlined in 

this table introduces a number of advantages: (1) a higher likelihood of the BIP RA avoiding 

damage from both primary and secondary explosions that often occur at the face area, which also

increases the likelihood that the communication system to the RA survives a disaster; (2) a 

reduction in the number of BIP RAs required to be constructed; and (3) the introduction of a 

wider variety of BIP RA designs, which could potentially improve the safety as well as the 

psychological and physiological comfort and mental well-being of confined miners.

Locating BIP RAs further from the face requires a trade-off of the advantages of locating an RA

close to the face—such as the ability to easily assist injured miners in reaching the RA location 

and a shorter travel distance for miners to the RA—against the significant potential advantages

gained from the use of a BIP RA. This argument is consistent with OMSHR’s position in its

2007 Report to Congress on refuge alternatives [NIOSH 2007]. Therefore, consideration should 

be given to allowing mines to locate RAs further from the face only if they employ new RA

technologies that meet the following three criteria:
 

 

 

	 

	 

	 

5
 

1.	 Provide a constant supply of air to the RA either via a protected compressed air line 

into the RA or a borehole from the surface into the RA. 

2.	 Provide additional RA space per occupant. The original minimal space requirement of 

85 ft3 per occupant as noted in the NIOSH report to Congress on refuge alternatives 

[NIOSH 2007] is recommended. 

3.	 Maintain the RA interior atmosphere under positive pressure when not in use to 

ensure that the RA contains breathable air immediately upon entry and to keep 

contaminated air from entering the RA with miner entry. It would be advantageous to 

have a differential pressure gauge on the RA which could be inspected regularly to 

ensure that the positive differential pressure is maintained with the RA door closed. In 

many cases it will not be feasible to maintain the RA under positive pressure when it 

is not occupied. This would be the case when the air supply system for the BIP RA is 

not permanently located at the borehole but is only transported to the borehole in the 

event of a mine disaster. While the air supply to the BIP RA might be established 

before escaping miners arrive at the RA, this cannot be assured. Further, miners 

entering the RA prior to the ventilation system being established could bring 

contaminants into the RA. In these situations, additional SCSRs must be available in 

the RA to sustain miners until the RA can be adequately purged of contaminated air 

by the air supply to the RA. Importantly, to investigate these issues, NIOSH OMSHR 

is currently constructing a BIP RA in its Experimental Coal Mine in Pittsburgh to 

research issues related to BIP RA contamination with miner entry, the advantages of 

airlocks, airlock purging systems, and other means of keeping contaminants out of the 

RA—such as a membrane entry system or a series of overlapping plastic strips 

through which miners would enter. Research will also be completed on pressure relief 

valves and ways to ventilate the BIP RA during everyday operation to ensure against 

the buildup of methane or other harmful gases in the RA. 

It should be noted that while BIP RAs may not be practical for all underground coal mine 

applications, OMSHR believes that they should play an important role in the available suite of 

RA options. As an example, one consideration would be to locate a BIP RA with a borehole to 

the surface or a protected compressed air line and communication to the surface at the mouth of 

each section. The BIP RA would be built in a crosscut or blind cutout with stopping/door 

systems at the ends set back to better survive possible explosion forces. Depending on the 

distance to the face, the RA could be the first one that miners travel to from the face or may be 

an additional outby RA. Also, the BIP RA being at the mouth of the section could provide 

miners a good location at which to decide on a further course of action with respect to continued 

escape or continued refuge with continuous fresh air supply and communications with the 

surface and rescue teams. Additionally, a combination of BIP RAs located at distances presented 

in the table above along with mobile RAs much nearer to the face would give miners more 

options for refuge. 

Lastly, it is important to emphasize that in the event of any mine disaster, if at all possible, 

the primary goal of miners should always be to evacuate the mine before considering taking 

refuge. 
6 

Designing RA Stoppings for MSHA Approval

Based on OMSHR’s analysis of design and approval criteria for RA stopping design, and 

using the MSHA guidelines for coal mine seal design applications as a model, OMSHR believes 

that the guidelines set forth in this report could be used by industry when submitting stopping 

design applications for approval by MSHA. 

OMSHR’s recommended design pressure-time curve for an RA stopping has a magnitude of

15 psi, a rise time of 0.1 seconds, a fall time of 0.1 seconds, and a duration of 0.2 seconds.13

13 These criteria are specified in 30 CFR 7.505 and further clarified in the April 29, 2009, section of the 

document, “Questions and Answers: MSHA’s Refuge Alternatives Requirements.”

Because of the slow rise time, this pressure-time curve always has a dynamic load factor of 1.0

and an equivalent static design pressure of 15 psi. This design pressure-time curve is illustrative

of the pressure-time history close to an explosion originating in a face area of a development 

entry. It is similar to reported explosion pressures in recent underground coal mine disasters, and

is commensurate with the blast pressure tolerance of the human body as analyzed in this Report

of Investigations. This curve is identical to that developed in OMSHR’s 2007 report to Congress

on refuge alternatives [NIOSH 2007]. The justification of the 15 psi design criterion for an RA

stopping is based on human survivability to explosion pressures. Finally, following the extensive

guidelines recommended here for RA stopping design, OMSHR provides an example RA

stopping design consisting of a conventional rebar-reinforced concrete wall that can withstand 

the expected15-psi explosion pressures.

Delivering Clean, Breathable Air to a BIP RA

As noted in this report, most of the advantages of a BIP RA disappear if the RA is not 

guaranteed a constant and highly reliable supply of clean, breathable air. A borehole from the 

surface directly into the BIP RA is the most advantageous and reliable approach. However, a 

borehole from the surface to the BIP RA is often impractical due to such factors as drilling costs 

and surface rights issues. Instead, using a protected compressed air line carrying clean, 

breathable air to a BIP RA is emerging as a practical and achievable goal. One such system 

already has MSHA approval14

14 The Hubble Breathable Air Units (Models HBA75, HBA 100, and HBA 250) have already been 

approved by MSHA (Approval No. 07-LCA110001) for use in providing breathable air to an RA via a

protected compressed air line.

 and it is anticipated that other systems will be developed if there 

is an opportunity for BIP RA designs to be advanced with the working face of coal mines. 

OMSHR believes that a constant supply of clean, breathable air can be provided to BIP RAs 

either from a borehole to the surface or through a protected compressed air line system. Several 

efforts to develop and test such systems are already being undertaken by private companies. 

Considering Economic Drivers when Designing New BIP RAs

Based on the findings gathered by OMSHR researchers during mine visits, all of the 

approximately 30 BIP RAs with a constant air supply (all via borehole to the surface) in the U.S. 

have MSHA-approved stopping and door systems, and all are located outby the face areas. Each 

of these BIP RAs provides more than the 85 ft3

 of space per occupant recommended in the 

NIOSH report to Congress on refuge alternatives [NIOSH 2007]. The costs of these existing BIP 

RAs range from $50,000 to $150,000, depending mainly on stopping and door system costs and 

 
7 

borehole costs. Given these costs, it does not appear to be economically viable to provide these 

types of RAs within 1,000 feet of the working face of mines. However, if RAs can be located at 

greater distances from the face, as suggested here, OMSHR believes it may become more 

economically viable to introduce new BIP RA designs. Some work in this area is already being 

undertaken by private companies. 

In the context of economic drivers, design needs for new BIP RAs that can be advanced with 

the face are (1) designs that employ stopping and door systems inexpensive enough to build and 

leave in place, or systems designed to be quickly and easily disassembled, moved, and 

reassembled; (2) novel BIP RA concepts that provide more space and breathable air per occupant

and can be easily moved with the face; and (3) designs that provide a protected compressed air

line that supplies breathable air to the RA occupants and can be advanced with the face.
 

 

 

 

 

 

 

Introduction 

Three major coal mining disasters occurred in 2006. At the Sago Mine in Upshur County, 

West Virginia, 12 miners died after succumbing to the smoke and toxic gas while trapped 

underground for two days following an explosion. One miner was rescued in critical condition 

and recovered. The Sago miners had hung curtains recovered from the face area to build a crude 

barricade for themselves, but it was not enough to keep them in fresh air. One by one they lost 

consciousness while they were awaiting rescue [MSHA 2007a]. At the Aracoma Alma No. 1 

Mine in Logan County, West Virginia, two miners died when a conveyor belt caught fire and 

they became lost and disoriented while trying to escape. Due to poorly constructed stoppings, the 

fire spread to the intake airway, impairing the visibility in the primary escapeway. The two 

miners were separated from the 10 other crew members in the dense smoke and died of carbon 

monoxide (CO) poisoning [MSHA 2007b]. Finally, at the Darby Mine No. 1 in Harlan County, 

Kentucky, a methane explosion occurred, resulting in five fatalities. The forces from the 

explosion resulted in fatal injuries to two miners, and while attempting to escape, three miners 

died due to carbon monoxide poisoning along with smoke and soot inhalation [MSHA 2007c]. In 

all of these tragedies, miners survived the initial disaster but were unable either to escape or 

successfully isolate themselves from the poisonous gases present in the mine environment 

resulting from the disaster. 

In response to the above mine disasters, Congress passed the Mine Improvement and New 

Emergency Response Act (MINER Act) of 2006 (Public Law 109-236). The MINER Act calls 

for each underground coal mine operator to develop and adopt a written accident response plan 

to “provide for the maintenance of individuals trapped underground in the event that miners are 

not able to evacuate the mine.” The MINER Act also charges the National Institute for 

Occupational Safety and Health (NIOSH) Office of Mine Safety and Health Research (OMSHR) 

to “provide for the conduct of research, including field tests, concerning the utility, practicality, 

survivability, and cost of various refuge alternatives in an underground coal mine environment, 

including commercially-available portable refuge chambers.” Significant research has been 

conducted for decades on refuge alternatives (RAs) as potential safe havens for miners while 

waiting for rescue crews to reach them in the event of an emergency; however, RAs have not 

always been fully embraced by either labor or industry, both of which believe that the primary 

focus understandably and correctly should be on methods and resources to help ensure successful 

escape rather than refuge. Refuge should always be a “last resort” decision. 

The term refuge alternative can be applied to either mobile chambers that are advanced as the 

mining face advances or to built-in-place (BIP) RAs that are generally permanently located in the 

outby areas of a mine. Although the commercial availability of mobile RAs15

15 Also referred to as portable RAs. 

8 

was almost 

nonexistent at the time of passage of the MINER Act, a task force from the State of West 

Virginia developed the initial RA requirements [WV 2007] from which a number of 

manufacturers began developing, marketing, and selling mobile RAs. These RAs were initially 
 

 

 

 

 

 

 

approved by the State of West Virginia and subsequently “grandfathered in” by the Mine Safety 

and Health Administration (MSHA) when federal regulations were developed, adopted, and took 

effect on March 2, 2009 [30 CFR 7 and 30 CFR 75]. 

NIOSH OMSHR completed its mandated research into RAs in 2007 and provided its 

findings in the form of a report to Congress [NIOSH 2007]. In sum, this report found that refuge 

alternatives were practical for use in most underground coal mines to facilitate escape and to 

serve as a refuge of last resort, provided that mine operators develop comprehensive escape and 

rescue plans incorporating refuge alternatives and training on their use. These findings applied to 

both portable RAs and BIP RAs, with the built-in-place refuge alternatives emerging as being 

able to provide a superior environment, in particular because they could supply a constant supply 

of breathable air via a borehole to surface or through a protected compressed air line. BIP RAs 

also offer the potential to provide miners with an improved psychological and physiological 

environment, both because the available air makes the space more comfortable and due to the 

larger amount of space provided per occupant. Boreholes or protected compressed air line air 

supply systems also provide a much higher probability of there being communications to the RA. 

Despite these advantages, current BIP RA designs cannot be moved frequently—with 

movement of the RA location required to keep up with dynamic mining production—and as such 

it would be impractical to keep them within 1,000 feet of the nearest working face, as prescribed 

in 30 CFR 75.1506(c). Notwithstanding, OMSHR’s 2007 report to Congress on refuge 

alternatives concluded that, as compared to portable RAs,16 the strengths of built-in-place refuge 

alternatives “are so significant that consideration should be given to allowing extended distances, 

if in-place shelters are used to provide refuge for face workers” [NIOSH 2007]. OMSHR also 

envisions that future BIP RA designs will be less expensive and easier to advance with the face. 

Currently, there are approximately 30 BIP RAs in U.S. underground coal mines;17 however, 

all of these are located outby the face area and none are designed to be advanced with the 

working face. The usage of built-in-place refuge alternatives that can be advanced with the 

working face will only be practical if three issues can be addressed: 

16 As a follow-up to OMSHR’s 2007 report to Congress, OMSHR also undertook an extensive research 

program to investigate improvements that could be made to portable RAs. This research on portable RAs
 

and specific recommendations about their use is reported in two NIOSH Report of Investigations,
 

published in 2014: “Investigation of Purging and Airlock Contamination of Mobile Refuge Alternatives,”
 

NIOSH Publication No. 2014-116, and “Investigation of Temperature Rise in Mobile Refuge Alternatives,”
 

NIOSH Publication No. 2014-117.
 17 This number is based on discussions between OMSHR researchers and members of industry.
 

1. Locating BIP RAs further from the face; 

2. Design and approval of RA stoppings; 

3. Delivering a reliable supply of clean, breathable air to a BIP RA. 

In this report, these three issues will be discussed in detail after a background summary of the 

potential advantages and disadvantages of BIP RAs. 

9 
 

 

 

 

 

 

 

 

In developing considered responses to these issues, OMSHR personnel visited a number of 

mines currently employing BIP RAs to obtain site-specific information on characteristics 

including their construction, location, air supply, and provisions and inventories. Additionally, 

OMSHR conducted an analysis of the number of BIP RAs currently in U.S. underground coal 

mines, the number of coal companies and mines employing them, how they are currently used, 

and their role as part of a mine’s comprehensive escape and rescue plan. To give full context to 

the OMSHR findings that follow, this detailed summary of the current status of BIP RAs is 

presented in Appendix A. 

Potential Advantages and Disadvantages of BIP RAs 

Based on numerous discussions with industry and labor and observations by OMSHR 

personnel, properly constructed BIP RAs provided with a constant supply of breathable air hold 

a number of substantial advantages over mobile RAs. These potential advantages result from 

how the BIP RAs are constructed, the larger space available to miners, a constant clean air 

supply, and greater quantities of supplies and personal comfort features. The primary potential 

advantages of BIP RAs over mobile RAs are as follows: 

1.	 A BIP RA with a continuous supply of fresh breathable air will likely minimize or

eliminate the need for RA purging.18 

18 Current regulations in 30 CFR 7.508 require mobile refuge alternatives to purge the internal 

atmosphere from 400 parts per million (ppm) of carbon monoxide to 25 ppm. 

It may be possible to create a positive pressure

with clean breathable air in the shelter prior to entry, which should eliminate the need

for purging. However, in many cases it will not be feasible to maintain the RA under

positive pressure when not occupied. This would be the case when the air supply

system for the BIP RA is not permanently located at the borehole but is only

transported to the borehole in the event of a mine disaster. While the air supply to the

BIP RA might be established before escaping miners arrive at the RA, this cannot be

assured. Further, miners entering the RA prior to the ventilation system being

established could bring contaminants into the RA. In these situations, additional

SCSRs must be available in the RA to sustain miners until the RA can be adequately

purged of contaminated air by the air supply to the RA. Importantly, to investigate

these issues, NIOSH OMSHR is currently constructing a BIP RA in its Experimental

Coal Mine in Pittsburgh to research issues related to BIP RA contamination with

miner entry, the advantages of airlocks, airlock purging systems, and other means of

keeping contaminants out of the RA—such as a membrane entry system or a series of

overlapping plastic strips through which miners would enter. Research will also be

completed on pressure relief valves and ways to ventilate the BIP RA during

everyday operation to ensure against the buildup of methane or other harmful gases in

the RA.

2.	 A BIP RA with a continuous supply of fresh breathable air does not require carbon

dioxide (CO2) scrubbing, which is necessary in an occupied mobile RA because there

is minimal or no air exchange capability. When continuous breathable air is supplied

10 
 

 

 

 

 

 

11
 

at or above the rate specified in 30 CFR 7.506(c) (12.5 ft3/minute per occupant), the 

need for CO2 scrubbing is generally eliminated. Additionally, eliminating the need 

for CO2 scrubbing makes the operation of the RA less complex and provides more 

room in the RA. It will be necessary to ensure that the flow of air from the air inlet is 

distributed in a fairly uniform fashion throughout the RA volume. 

3.	 In BIP RAs, the thermal issues that accompany the occupation of a mobile RA can be 

minimized. Mobile chambers rely on compressed O2 to be provided at the rate of 1.32 

ft3

/hour per occupant to maintain adequate O2 for miners (30 CFR 7.506(d)). Due to 

size restraints, mobile chambers cannot provide the alternative of 12.5 ft3/min per 

occupant of compressed air. The much greater flow rate of 750 ft3/hr of air per 

occupant rather than1.32 ft3/hr of O2 per occupant creates a much higher rate of air 

changeover in the RA, resulting in a more efficient dissipation of heat and humidity. 

4.	 The communication system in a BIP RA can be designed to have a greater chance of 

surviving an explosion or fire. For example, a reliable communication system could 

be provided from the surface through a borehole or via a protected compressed air 

line to the BIP RA. 

5.	 BIP RAs can be made larger and provide more available room for refuged miners 

than can be made available with mobile RAs. The fact that only the BIP stopping has 

to be advanced with the face rather than the entire BIP RA allows for this larger size 

benefit. The larger size also affords the opportunity to include in the shelter more 

food, water, and personal comforts. Finally, this larger size could minimize many of 

the psychological and ergonomic issues that could be associated with taking refuge in 

a smaller mobile RA. 

6.	 There is a higher likelihood of the BIP RA surviving a secondary explosion by 

comparison to a portable RA employing a tent-type design. This assumes that the BIP 

RA is farther from the face and possible explosion source, and that the stoppings are 

built to withstand 15 pounds per square inch (psi) overpressure for 0.2 seconds prior 

to deployment, as specified in 30 CFR 7.505. Portable tent-type mobile RAs are 

especially problematic in relation to surviving secondary explosions as have occurred 

in many mine disasters. 

7.	 BIP RAs with constant breathable air supply have fewer operating requirements than 

portable refuge alternatives; thus miners should be able to easily learn how to operate 

these shelters. There would likely be no purging and CO2 scrubbing systems to learn 

to operate because they are not needed as outlined in points 1 and 2 above. Further, 

preparation and entry time into a BIP RA should be much shorter than with a mobile 

RA. 

8.	 The 96-hr survival time as mandated in 30 CFR 7.506 for breathable air sustainability 

is no longer a serious limitation because there is likely to be an unending supply of 

fresh, breathable air and access to additional food and water. Thus, the “ticking 

clock” for refuged miners is eliminated. The race against time for rescuers is also 

diminished, which should allow rescue teams more time for decision making. 
 

 

 

 

 

 

 

 

 

9.	 The number and order in which miners arrive at a BIP RA are much less important by 

comparison to a mobile RA, because purging is generally not needed and thus the 

availability of purge air is not as critical as it would be for mobile RA. Also, if a need 

arises for some miners to leave a BIP RA and others to stay, there is less of a problem 

of RA contamination because there is adequate breathable air available to keep the 

RA livable, even if some outside contaminated air enters the RA during the process of 

miner departure. 

In addition to the above advantages, some potential disadvantages associated with the use of 

BIP RAs are also important to note, as follows: 

1.	 The cost of current BIP RA designs, if they are required to be kept within 1,000 ft of 

the active mining face, would be prohibitive as compared to using mobile RAs. 

However, if the use of BIP RAs becomes more universally accepted, it is likely that 

much more economical designs will evolve. 

2.	 Economically viable BIP RA stopping/door systems will need to be developed which 

are either inexpensive enough to be abandoned in place or which can be 

disassembled, moved, and reassembled at a new BIP RA location closer to the face. 

Several such designs are currently being investigated by manufacturers and 

universities. 

3.	 Depending on the circumstances at the mine, providing a constant supply of air either 

via a borehole to the surface or via a protected compressed air line will require 

significant planning and may be costly. It would be desirable to maintain a constant 

low flow of breathable air to the BIP RA to prevent a buildup of methane and to keep 

the BIP RA under constant positive pressure fresh air. Research is needed in this area 

to develop guidelines for protected compressed air lines that will survive the 15-psi 

over 0.2-sec pressure requirement. 

4.	 Locating BIP RAs further from the face than the currently required 1,000 ft or less 

distance could make it more difficult for injured miners, either on their own or with 

assistance, to travel to the RA. 

Locating BIP RAs Further from the Face 

MSHA’s 30 CFR 75.1506(c) states that refuge alternatives shall be provided “within 1,000 

feet from the nearest working face and from locations where mechanized mining equipment is 

being installed or removed.”19 

19 The regulation also includes an exception for anthracite coal mines with no electrical face equipment, in 

which case RAs are provided if the nearest working face is greater than 2,000 feet from the surface. 

However, this exception does not relate to the specific findings reported here. 

To meet this 1,000-feet requirement, all underground coal mines 

are currently employing mobile RAs (tent-type and rigid steel-type structures). Approximately 

1,700 mobile units are currently located in underground coal mines in the United States. 

12 
13 

In order to facilitate the introduction of new BIP RA concepts into U. S. underground coal 

mines, OMSHR has investigated practical distances from the working face for the placement of 

RAs.

20

20 This investigation was designed solely for determining the allowable distances that new and innovative

RA concepts, such as BIP RAs, or other novel RA concepts that offer the advantages of BIP RAs, can be

located in the active working sections of underground coal mines. This analysis was not intended for and

should not be used to locate currently mobile RAs currently employed in underground coal mines.

OMSHR’s goal is to provide maximum safety for miners while allowing for a wider 

variety of RA designs. Applying more habitable RA designs could increase the likelihood of 

miners using an RA when escape is impossible as well as improve their psychological and 

physiological comfort.

To address the issue of locating BIP RAs further from the face, OMSHR researchers

developed three different approaches to determine how far from the face area miners could travel

given the 120 minutes of breathing time afforded them by currently available SCSRs.

21

21 Every miner working at the face has at least two SCSRs available—an initial SCSR and an additional 

one. The requirement for an additional SCSR that provides protection for at least one hour for each 

person at a fixed underground work location can be found in 30 CFR 75.1714-4(a). With at least two 

available SCSRs, each miner generally has a total of 120 minutes of reliable SCSR life. Additionally, on 

some faces, miners actually have access to additional SCSRs. Some longwall mines locate SCSRs at the 

longwall tailgate and midway on the longwall face. At mines where 20-minute SCSRs are employed, 

miners have two additional 60-minute units available to them prior to beginning travel down the 

escapeway. 

 Each of

these approaches assumed that new RA technologies with BIP designs will be employed under

the following conditions: to provide a constant supply of air to the RA either via a protected 

compressed air line or a borehole to the surface; to provide additional RA space per occupant;22

22 Based on the 2007 report to Congress on refuge alternatives [NIOSH 2007], it is recommended that the 

original NIOSH minimal space recommendation parameter of 85 ft3 per person be used.

to maintain the RA’s internal atmosphere under positive pressure when not in use to ensure that

the RA contains breathable air immediately upon entry and to keep contaminated air from

entering when miners enter.

Employing the 120 minutes of breathing time and the application of new technologies that 

meet the conditions outlined above, OMSHR considered three approaches to locating BIP RAs 

further from the face, as described in the following subsections. 

Approach 1: Based on SCSR-mandated Storage Cache Locations

This approach examined the MSHA-established criteria for distances between SCSR storage 

caches as a method of establishing acceptable distances from the face area for RA locations.

MSHA’s 30 CFR 75.1714-4(c)(2) defines these distances as follows: “Storage locations shall be 

spaced along each escapeway at 30-minute travel distances no greater than the distances 

determined by: (i) calculating the distance an average miner walks in 30 minutes by using the 

time necessary for each miner in a sample of typical miners to walk a typical length of each 

escapeway; or (ii) using the SCSR storage location spacing specified in [Table 1], except for 

escapeways with grades over 5 percent.” 

 
14 

Table 1. Travel distances to SCSR storage locations (adapted from 30 CFR 75.1714-4)

*Travel rates in fpm have been added to the original table.

Average entry height Maximum distance between 

SCSR storage locations Travel rate* 

<40 in (crawl) 2,200 ft 73 fpm 

>40–<50 in (duck walk) 3,300 ft 110 fpm 

>50–<65 in (walk head bent) 4,400 ft 146 fpm 

>65 in (walk erect) 5,700 ft 190 fpm 

MSHA has established the values in Table 1 as realistic travel distances to SCSRs in 

emergency situations, and OMSHR concurs that these travel distances are realistic.

23

23 It is important to note that in most situations, travel in escapeways to both SCSR caches and RAs 

would be done with escapeway lifelines, which have been shown to significantly facilitate travel speeds.

Because 

these distances are acceptable for SCSR locations, OMSHR posits that they should also be 

acceptable travel distances to BIP RAs if those RAs are spaced based on entry heights,

24

24 Put simply, greater entry heights allow for faster travel.

 and if 

new BIP RA designs that are under positive fresh air pressure and require minimal deployment 

time prior to entry are used. Very little SCSR breathing time is required for RA deployment and 

entry with these new BIP RA designs. 

Approach 2: Based on Worst-case SCSR Usage Times

To develop this approach, OMSHR performed a timeline study of an assumed worst-case

scenario for miners involved in a disaster, beginning with the time they first donned their initial

SCSR. For this analysis, researchers assumed the maximum realistic times for each step; that 

miners would be walking in a smoke-filled travelway without lifelines; and that they would be

traveling in entries with different heights. The timeline included times for donning their initial

SCSR, traveling to the assembly location on the face and deciding on a course of action,

switching over to their second SCSR, traveling to the BIP RA, and preparing and entering the

RA where there is a safe, breathable atmosphere. Maximum times were assigned to each of these

actions, with the exception of the travel time to the RA. The travel time to the BIP RA was

determined by calculating the breathable air time the miners had remaining from their SCSRs

after times for all other actions were subtracted out. This time was used to estimate the distance

that BIP RAs could be placed from the face as a function of entry height.

The following is a summary of the assumed worst-case timeline scenario for miners located

at the face area deciding to seek refuge in a new BIP RA design during a mine disaster. 

 
15 

1. SCSR Donning. When miners at the face become aware of a disaster situation either 

via self-recognition or through a warning notification, in the worst-case scenario, they 

would immediately don their SCSRs, and the time clock for their 120 minutes of 

available breathable air would begin. In this worst-case scenario, it is assumed that it 

takes miners 5 minutes from the time they start breathing into their SCSRs to 

complete the donning of the device. It can also be assumed that at this point they 

would, either alone or with their co-workers, start to escape (towards a BIP RA) or 

travel to an assembly point near the start of the escapeway.

2. Face Area Travel, Assembly, and Decisionmaking. Once miners leave their working 

location, they head directly to the escapeway or meet at an assembly location near the 

start of the escapeway to decide on a course of action or to communicate with the 

surface to learn about the circumstances. There are many considerations in calculating 

how much of a miner’s available SCSR breathable air time is used in face area travel, 

assembly, and decisionmaking. Depending upon the situation, miners may don their 

SCSRs before they begin travel to the start of the escapeway or they may travel along 

the face to the escapeway without starting the use of their SCSRs. Based on which 

occurs, significant differences can result in travel distances and these distances could 

result in face travel times that exceed 15 minutes. However, in many mines with long 

face travel times such as longwall mines, there are generally additional SCSRs at 

locations along the face which would provide extra breathable air time.25

25 Some longwall mines provide additional SCSRs at the tailgate and mid-face. In some mines, miners 

have 20-minute SCSRs, which they wear on their person, but have additional 60-minute units available to 

them at face locations, which results in the miners adding additional breathing time to their available 120 

minutes. Additional SCSRs could be mandated on faces where travel times on the face to the escapeway 

exceed 15 minutes.

If miners do meet at an assembly location, they would generally make a rapid 

decision as to their course of action and immediately proceed to travel the escapeway 

either to an RA or out of the mine. Taking into account that miners working on faces 

with long travel distances have additional SCSRs available to them and that miners in 

a disaster situation would assemble and make a decision about their course of action, 

it is reasonable to assume that miners will generally not use more than 15 minutes of 

their available 120 minutes of SCSR breathable air time in the course of face area 

traveling, assembly, and decisionmaking. 

3. SCSR Switchover. Working with a breathable air time of 120 minutes assumes that 

miners will switch from their original SCSR to a new one. In a worst-case scenario, 

the maximum switchover time would be 5 minutes. In cases where a miner starting 

out with a 20-minute SCSR has to accomplish a switchover to a new 60-minute SCSR 

before traveling to a BIP RA, the extra 5 minutes required for the additional 

switchover would be more than made up for by the initial 20 minutes of breathable air 

from the 20-minute SCSR.

4. RA Preparation and Entry. When using current mobile RA designs, preparation and 

entry time can be considerable. According to 30 CFR 7.505, which defines the 

structural components of an RA, the process of deploying an RA once it is reached by 

 
16 

trained persons, without the use of tools, must be achieved within 10 minutes of the 

RA being reached. The removal of harmful gases for RAs is covered in 30 CFR 

7.508, which states that the time between deployment of and entry into the RA cannot 

exceed 20 minutes. Summing these two amounts, currently available mobile RA units 

could require up to 30 minutes for preparation and entry.26

26 By contrast to this 30-minute time frame required for RA preparation and entry, the proposed new BIP

RA designs can maintain the RA’s inside atmosphere under positive pressure when not in use to ensure 

that the RA contains breathable air immediately upon entry. Further, these designs would likely only 

require that miners turn a compressed air valve to prepare the RA for use. Since no purging is needed, all 

miners can enter the RA immediately. For these BIP RAs, the maximum preparation and entry time 

should not exceed 5 minutes. This should also result in simpler and more effective emergency training.

Under this assumed worst-case scenario as outlined above, the sum of the non-escapeway 

travel activities is 30 minutes as shown in Table 2. In this assumed worst-case scenario, if miners 

use 30 of their 120 minutes of SCSR time to carry out the activities described above, this leaves 

90 minutes for travel time from the face to the RA. Note that this worst-case scenario assumes 

the longest time needed to accomplish each of the tasks. Appendix B presents a statistical 

analysis to establish a realistic range of times for miners to accomplish these four tasks.27

27 Appendix B establishes that it is highly unlikely that a miner would require more than 26 minutes to 

accomplish the four tasks of SCSR donning; face travel time, assembly, and decisionmaking; SCSR

changeover; and RA deployment and entry—with a mean time for miners of about 20 minutes. This

supports the argument that miners with two 60-minute SCSRs would have at least 90 minutes of travel

time in the escapeway to the RA. Another consideration is that this worst-case scenario assumes that 

miners don their SCSRs before starting to leave their work location. Depending on the circumstances,

they may not actually don their SCSRs until after they have traveled the face, assembled, and chosen a 

course of action, which would increase the SCSR available breathing time for traveling the escapeway to 

the RA.

Table 2. Summary of non-escapeway travel activities based

on worst-case SCSR usage times

Nontravel activity Worse-case SCSR usage times

SCSR donning 5 min

Face area travel, assembly, decisionmaking 15 min

SCSR switchover 5 min

RA preparation and entry 5 min

Total 30 min

In addition to this 30-minute requirement for non-escapeway travel activities, knowing the 

time available for escapeway travel is critical in determining how far from the working face a 

BIP RA can be placed. In the published literature, there is a great deal of discussion about how 

smoke-filled airways reduce travel speeds, but few concrete numbers are published. Some 

available information comes from Harteis et al. [2011], who conducted studies of travel times for 

miners in high coal (greater than 65-in entry height) through dense smoke and without lifelines. 

 
17 

The study of 53 miners found that the average miner could travel 77.9 feet per minute (fpm) in 

heavy smoke. In MSHA’s 30 CFR 75.1714-4(c)(2)(ii), it is assumed that an average miner can 

travel 5,700 feet in 30 minutes in high coal in clean air. This is a travel rate of 190 fpm. 

Taking these two pieces of information into account, one can calculate that travel in smokefilled airways without lifelines results in miners traveling only about 40% as fast as in clean 

airways (77.9 fpm/190 fpm = 41%). This would indicate that travel rates in heavy smoke are 

reduced by about 60%. This 60% reduction is further confirmed by the work done by Kriel et al. 

[1995], which stated that without lifelines travel rates in smoke-filled airways are reduced to 

about 40% of normal travel rates. 

Emerging from the above discussion is Table 3, which calculates 90-minute travel distances 

in smoke-filled escapeways based on entry height. This table combines three pieces of 

information: the regulations in 30 CFR 75.1714-4(c)(2)(ii) for 30-minute travel distances in 

various entry heights; the fact that smoke-filled airways reduce travel times by about 60%; and 

the above-outlined worst-case scenario evidence that escaping miners should have 90 minutes of 

SCSR breathing time available to them for travel from the face to an RA.

Table 3. Calculated 90-minute travel distances in smoke-filled

escapeways based on entry height

Average entry height 90-minute travel distance 

with 60% reduction

90-minute travel distance 

with lifeline (assuming a 40% 

reduction)* 

<40 in 2,640 ft 3,960 ft

>40–<50 in 3,960 ft 5,940 ft

>50–<65 in 5,280 ft 7,920 ft

>65 in 6,840 ft 10,260 ft

*This column assumes that with a lifeline in the escapeway, miners would be able to travel at 60% of their travel 

speed in clean air. This 60% travel rate with lifelines is conservative with respect to the findings of Kriel et al. 

[1995], who reported that miners wearing SCSRs and traveling in very low visibility move at approximately 75% of 

normal travel speeds when aided by a lifeline.

Approach 3: Based on NIOSH and U.S. Bureau of Mines Established Travel Times and 

Escape Probabilities

Approach 3 uses travel times established by recent OMSHR research from actual miners 

traveling in airways filled with dense smoke in high coal (greater than 65-in entry height) 

without lifelines [Harteis et al. 2011], and U.S. Bureau of Mines (BOM) research from 1990 that 

considers the probability of making a successful mine escape in high coal with a single SCSR 

[BOM 1990]. 

Looking solely at the OMSHR research from the 2011 study, which measured travel times 

for 53 miners, the findings were that the average miner could travel 77.9 fpm, while the slowest 

miner traveled at 73.2 fpm. In the 90 minutes of available SCSR breathing time arrived at in 

Approach 2, the average travel distance would be over 7,000 feet, while the slowest travel 

distance would be over 6,500 feet. These two travel distances fall just above and below the 6,840 
18 

travel distance established in Approach 2, Table 3, for high entry heights in smoke-filled 

airways. This evidence further justifies the argument that RAs can be located further from the 

face based on entry height.28

28 As noted in an earlier discussion, it is also important to point out that a person traveling with a lifeline to 

the RA would normally travel at a faster rate than without a lifeline.

The 1990 U.S. Bureau of Mines research showed that miners who were proficient in donning 

and using an SCSR had a high probability of escaping up to 2,000 meters (approximately 6,000 

feet) employing a single SCSR, walking bent over (assuming a 50- to 65-in entry height), and 

traveling without lifelines. This 6,000-feet travel distance is significantly higher than the 4,400-

feet travel distance suggested in Approach 1, and is somewhat higher than the 5,280-feet travel 

distance arrived at in Approach 2. These findings from the U.S. Bureau of Mines study clearly 

support the argument for locating RAs at greater distances from the working face.

Discussion of Three Approaches to Locating BIP RAs Further from the Face

Taking into account the findings from the three approaches discussed above, conservative 

maximum distances from the face to the RA for the various entry heights can be established 

(Table 4). Table 4 assumes that all other factors from approach 2 remain the same—i.e., SCSR 

donning and switchover times; time to travel the face, assemble, and make a decision; and time 

to prepare and enter the RA. Therefore, Table 4 is grounded in the assumption that miners have 

90 minutes of available travel time in the escapeway (based on available SCSR breathing time) 

to reach the RA.

Table 4. Maximum distances of refuge alternatives from the face

Average entry height Maximum distance of RA 

from the face

Maximum distance between

SCSR storage locations 

<40 in 2,000 ft 2,200 ft

>40–<50 in 3,000 ft 3,300 ft

>50–<65 in 4,000 ft 4,400 ft

>65 in 5,000 ft 5,700 ft

The travel distances to RA locations based on entry height in Table 4 closely correspond to 

the distances between SCSR cache locations in 30 CFR 75.1714-4(c)(2)(ii)—therefore, it would 

be practical to use the same travel distances for both. Using the same travel distances to both 

SCSR caches and RAs, as well as locating SCSR caches in the same place where possible, would 

be logical given the following facts: 

1. Additional SCSRs would be located at the RA.

2. Miners would have unused SCSRs at the RA should they have issues with RA 

deployment. 

 
19 

3. Miners would have a specific and clearly defined decision point—“Grab another 

fresh SCSR and go on” or “Deploy RA.”29

29 Having a clear decision point can be in important factor in an emergency circumstance. In this case, 

because miners will have already traveled some distance and can better assess their abilities and 

limitations, they may be in a better position to make an informed decision about the next steps to either 

self-escape or take refuge.

4. The compliance distance requirement would be a single number for both SCSRs and 

RAs.

5. Firebosses would be able to examine both the RA and SCSRs at the same location. 

6. Storage of the SCSRs could potentially be incorporated inside the BIP RA, providing 

them with additional protection from explosive forces. 

Expanding on the argument of defining travel distances to RA locations based on entry 

height and using the same travel distances to both SCSR caches and RAs, Figures 1 and 2 offer 

perspective on the number of underground coal mines and underground coal miners that would 

be positively affected by this approach. Figure 2 shows that of the 45,099 coal mine employees 

in underground mines, almost 65% (29,191) work in mines with seam heights greater than 65 in, 

and almost 84% (37,721) work in mines with seam heights greater than 50 in.

30

30 Figures 1 and 2 show MSHA data based on coal mine seam heights. The entry heights in coal mines,

especially lower coal mines, are always equal to or greater than the actual coal seam height. Therefore,

the numbers reporting that almost 65% of underground coal miners work in mines with seam heights

greater than 65 in and almost 84% work in mines with seam heights greater than 50 in are conservative.

Therefore—

comparing the data from Table 4 and Figure 2—based on seam height, the vast majority of 

underground coal miners would be able to travel at least 4,000 feet to RAs given the available 

SCSR breathing time.

Considering the most conservative maximum distance of the RA from the face as defined in 

Table 4, OMSHR believes that mines could locate built-in-place refuge alternatives at distances

from the working face according to the guidance provided in Table 4. This is further discussed in 

the report section, “Considerations for Facilitating the Use of BIP RAs.” 

Finally, it should be noted that while BIP RAs may not be practical for all underground coal

mine applications, OMSHR believes that they should play an important role in the available suite

of RA options. For example, a consideration would be to locate a BIP RA with a borehole to the

surface or a protected compressed air line at the mouth of each section. The BIP RA would be

built in a crosscut or blind cutout with seals at the ends set back to better survive possible

explosion forces. Depending on the distance to the face, the RA could be the first one that miners

travel to from the face or could be an additional outby RA. Also, the BIP RA being at the mouth

of the section could provide miners a good location at which to decide on a further course of 

action with respect to escape or refuge.

 
20 

Figure 1. Number of active underground coal mines by seam

height, 2012. (data source: MSHA 2013) 

73

169

108

218

0

50

100

150

200

250

<40 in >40–<50 in >50–<65 in >65 in

Number of mines

Seam height

Figure 2. Number of coal mine employees working at underground locations

by seam height, 2012, excluding office employees. (data source: MSHA 2013) 

1,851

5,527

8,530

29,191

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

<40 in >40–<50 in >50–<65 in >65 in

Number of employees

Seam height
21 

Discussion of How Locating BIP RAs Further from the Face Impacts RA Moves

Using the maximum distances for locating BIP RAs from the face presented in Table 4, it is 

possible to estimate the frequency of BIP RA moves as compared to currently employed mobile 

RAs. The most rapid face advance would generally occur with a three-entry continuous miner 

development entry, where advance rates would range from about 1,000 to 1,300 feet per month. 

Currently employed mobile RAs are moved about every 500 feet of advance in order to comply 

with the regulation to keep them within 1,000 feet of the working face. Therefore, they would 

need to be advanced two to three times per month. In high coal, if a BIP RA could be located up 

to 5,000 feet from the working face as proposed in Table 4, its construction could be started 

about 500 feet from the working face. Assuming a very conservative construction time of two 

weeks and an advance rate of 500 feet during that time, the BIP RA would be located about 

1,000 feet from the working face when it is ready for use. The working face could then be 

advanced another 4,000 feet before a new BIP RA nearer to the face would be needed. 

Therefore, a new BIP RA—with construction completed within 1,000 feet of the original 

working face—would not be needed for another 3 to 4 months depending on the face advance 

rate.

Design and Approval of RA Stoppings

Structural Design Considerations for RA Stoppings

Refuge alternative stoppings31

31 An RA stopping is a hand-built barrier designed to create and isolate a BIP RA in either a crosscut or a 

cutout of an underground coal mine. It must include a built-in door at the front end of the RA constructed 

in a crosscut or a cutout to allow entry into the RA. It can also be a uniform stopping with no door as could 

be used at the back end of a BIP RA constructed in a crosscut. Some BIP RAs constructed in crosscuts 

have doors in the RA stoppings at both ends of the RA. Either the uniform stopping or the stopping/door 

combination must meet the MSHA criteria for RA stoppings of surviving a 15-psi explosion with a 0.1-

second rise time and a 0.1-second fall time. These criteria are specified in 30 CFR 7.505 and further 

clarified in the April 29, 2009, section of the document, “Questions and Answers: MSHA’s Refuge 

Alternatives Requirements.”

are the essential life-critical component of a BIP RA—put 

simply, if the stoppings fail, then lives are at risk. To understand the significant structural design 

considerations of RA stoppings, a literature review outlining explosion pressure considerations 

provides a useful beginning. Regarding the magnitude of explosion pressure that an RA stopping 

that is constructed in a crosscut and out of line with the direction of explosion propagation could 

be subjected to, there are two important factors to consider: (1) explosion pressure estimates 

from past mine disasters, and (2) the durability of the human body, as measured from 

experimental studies.

Explosion pressures from past mine disasters are documented in the literature in a recent 

report by Foster-Miller [2007], which summarized the characteristics of 12 underground coal 

mine disasters that occurred in active mining (non-sealed) areas between 1970 and 2006. These 

explosion pressures are estimates based on observed damage by accident investigators and are 

not actual measurements of explosion pressures. Post-explosion overpressure estimates are 

 
22 

primarily from the study of damage to stoppings that were located in crosscuts perpendicular to 

direction of the explosion propagation. Of those 12 disasters, 9 were explosions and 3 were fires. 

Two of the explosions occurred within sealed areas that affected the active area of the mine. The 

estimated explosion overpressure within 1,000 feet of the source was 15 to 20 psi in one case, 2 

to 5 psi in two cases, and low (< 2 psi) in the remaining six cases. The estimated explosion 

pressure within 2,000 feet of the source was 2 to 5 psi in three cases, and low (< 2 psi) in six 

cases.

Three recent coal mine disasters not considered in the above report are the Willow Creek 

Mine explosions [MSHA 2000], the Jim Walter Resources No. 5 Mine explosions [MSHA

2001], and the Upper Big Branch Mine explosion [MSHA 2010]. Four explosions occurred at the 

Willow Creek Mine with pressures of about 5 psi near the origin at the face, decreasing to about 

2 psi some 500 feet away where ventilation control devices were destroyed. Four injured 

survivors remained in the mine after the first explosion, and they were rescued within four hours 

before the subsequent explosions occurred [MSHA 2000]. 

The first explosion at Jim Walter Resources No. 5 Mine originated about 400 ft from a 

development face, and its pressure was estimated at about 4 psi near the source. The second 

explosion originated about 1,000 feet from the face and had an estimated peak pressure of about 

12 psi near the source [MSHA 2001]. 

The explosion at the Upper Big Branch Mine developed overpressures32

32 The pressures discussed in this report are static overpressures above atmospheric pressure and acting 

in all directions. A dynamic overpressure in the direction of the explosion wave propagation is associated 

with the static overpressure, and it can also have significant magnitude. Reflected blast waves at normal 

incidence can be several times the incident explosion wave overpressure, which is the combined static 

and dynamic overpressure.

 ranging from 6 to 25 

psi depending on location and local conditions in the mine. Near the source of the explosion in 

the tailgate near the longwall panel, the explosion pressure is believed to have been about 14 psi. 

Along the longwall face, the explosion pressure is believed to have ranged from 7 to 14 psi. 

Elsewhere in the mine, explosion pressures may have reached 20 to 25 psi. In the Headgate 25 

area, calculations indicated pressures on the order of 52 to 65 psi, and these pressures could have 

resulted in reflected overpressures up to 105 psi [MSHA 2010]. 

To detail the effects of blast pressures, Table 5—based on Department of Defense data from 

Glasstone and Dolan [1977] and Sartori [1983]—summarizes the effects of progressively 

increased blast pressures on various structures and the human body. These data originate from 

weapons tests and blast studies and provide some guidance on the possible effects of mine 

explosions on miners. 

The human body can survive relatively high blast overpressure without experiencing 

barotrauma, which is injury caused by changing air pressure. A 5-psi blast overpressure will 

rupture eardrums in about 1% of subjects, and a 45-psi overpressure will cause eardrum rupture 

in about 99% of all subjects. The threshold for lung damage occurs at about 15 psi blast 

overpressure. A 35-psi to 45-psi overpressure may cause 1% fatalities, and a 55-psi to 65-psi 

overpressure may cause 99% fatalities [Glasstone and Dolan 1977; TM 5-1300 1990; UFC 3-

340-02 2008]. 

 
23 

Table 5 also shows the maximum wind speed associated with a given overpressure. In mine 

explosions, as in war-related explosions, it is the blast wind resulting from the blast overpressure 

that leads to injuries and fatalities. The human body may be thrown violently into objects and 

receive blunt force trauma; conversely, large objects may be thrown into persons resulting in 

crush injuries, or projectiles launched by the blast wind may penetrate the body. The 

susceptibility of personnel to blast effects depends on their proximity to nearby objects and 

possible projectiles. Miners standing in the open and away from projectiles may survive higher 

blast overpressures than those standing near a solid wall or object. Personnel sitting within the 

confines of mining machines may receive some protection from both blunt force trauma and 

projectiles. While it is impossible to determine the exact correlation between blast wave 

overpressure and fatality rate for personnel in an active underground mine, the data in Table 5

provide useful guidance. 

Table 5. Effect of various long duration blast overpressures and the associated maximum 

wind speeds on various structures and the human body (based on data from Glasstone 

and Dolan [1977] and Sartori [1983])

Peak 

overpressure

Maximum 

wind speed Effect on structures Effect on the human body

1 psi 38 mph Window glass shatters Light injuries from 

fragments occur

2 psi 70 mph 

Moderate damage to houses 

occurs (windows and doors 

blown out and severe damage 

to roofs) 

People are injured by 

flying glass and debris

3 psi 102 mph Residential structures collapse

Serious injuries are 

common, fatalities may 

occur 

5 psi 163 mph Most buildings collapse Injuries are universal, 

fatalities are widespread

10 psi 294 mph 

Reinforced concrete buildings 

are severely damaged or 

demolished 

Most people are killed

20 psi 502 mph 

Heavily built concrete 

buildings are severely 

damaged or demolished 

Fatalities approach 100%
24 

The above review of recent explosion disasters shows that maximum explosion pressures in 

the active parts of coal mines generally do not appear to exceed 15 to 20 psi.33

33 It should be stressed that, in relation to worst-case explosion pressures, if an initial small mine 

explosion disrupts or destroys much of the ventilation system, it is conceivable that large volumes of

flammable methane-air mixtures could accumulate. If these mixtures were ignited, it is possible that 

explosion pressures greater than 120 psi could develop.

This same

estimated pressure range of 15 to 20 psi was reported within 1,000 feet of the explosion origin at 

the Darby Mine explosion [MSHA 2006]. The recent explosion at Upper Big Branch produced 

estimated explosion pressures up to 25 psi, and possibly higher in some locations. According to 

Table 1, miners are unlikely to survive an explosion with pressure greater than 10 psi and 

therefore would be unable to reach a nearby RA unassisted. Consideration of recent explosions 

such as Willow Creek Mine in 2000, the Jim Walter Resources No. 5 Mine in 2001, and others 

mentioned in the Foster-Miller report [1983], suggests that secondary explosions are a very real 

concern. Therefore, if miners are inside an RA, it should protect them from a secondary 

explosion that could develop. The pressures developed by secondary explosions are not well 

understood, but they too have not been observed to exceed 15 to 20 psi. 

Based on the above literature review and known structural design considerations for RA 

stoppings, the following needs emerge for designing stoppings that can withstand the expected 

explosion pressures: 

1. A recommended design pressure-time curve for an RA stopping. 

2. Criteria for designing an RA stopping and approval of RA stopping designs. 

3. Suggested guidelines for an RA stopping design application. 

4. An example design for RA stoppings meeting the proposed design criteria. 

Recommended Design Pressure-Time Curve for an RA Stopping

The essential structural design criteria for an RA stopping are the pressure-time curve and

elasticity of design applied to (1) the RA stopping structure itself; (2) the RA stopping 

foundation or anchorage; and (3) the entrance door. All components must meet the design 

pressure-time curve requirements and remain within the linear elastic range of their materials.

The RA stopping foundation must have adequate preparation to develop favorable support 

conditions. 

Based on the considerations detailed in the previous section, a 15-psi pressure-time curve 

with a 0.1-second rise time, 0.1-second fall time, and 0.2-second duration is suggested for the 

design of RA stoppings.34

34 It should be noted that this recommended pressure-time curve is less conservative than the curve 

developed by Foster-Miller [1975; 1983] in its reports. These prior studies recommended a 20-psi 

instantaneous rise time curve with an equivalent static pressure of 40 psi.

Because of the slow rise time, this pressure-time curve is expected to

have a dynamic load factor (DLF) of 1.0 and therefore an equivalent static design pressure of 15

 

psi for the anticipated BIP-RA designs. Figure 3 shows the recommended design pressure-time 

curve along with the pressure-time history from an experimental gas explosion conducted at the

 
25 

former NIOSH Lake Lynn Experimental Mine (LLEM). This pressure-time history was 

measured near the explosion source. This recommended pressure-time design curve is identical 

to an earlier one presented by NIOSH OMSHR in its report to Congress on refuge alternatives 

[NIOSH 2007]. 

Figure 3. Recommended design pressure-time curve for refuge alternative stoppings. 

 

The recommended pressure-time curve35

35 The design pressure-time curve is a static overpressure curve above atmospheric pressure. Because 

reflected blast wave overpressure can be several times greater than the static overpressure, BIP-RAs 

must be located out of the direct path of a likely explosion wave such as a crosscut or a similar side 

passageway to minimize the effect from reflected explosion waves. 

with slow rise time is characteristic of an explosion 

pressure-time history near the explosion source. However, as an explosion wave propagates from 

its source, two phenomena occur that affect the character of the wave: first, a shock wave with 

near instantaneous rise time tends to form; second, the magnitude of the pressure wave tends to 

decrease.

Shock wave development occurs because the speed of a pressure wave depends in part on gas

density. At the leading edge of an explosion wave near its source, gas density is less and wave 

speed is slower than at the point of peak pressure or beyond in the trailing part of the explosion 

wave where the gas density is greater. Therefore, the point of peak pressure in the explosion 
26 

wave gains on the leading edge of the wave until a shock wave forms with near instantaneous 

rise time. This process of explosion wave steepening (or “shocking up”) is well understood in the 

gas dynamics and blast wave effects community [Gamezo et al. 2008; Sapko et al. 2000]. 

Figure 4 shows the recommended design pressure-time curve along with the pressure-time 

histories recorded at 10, 526, and 782 ft from the origin for LLEM gas explosion # 485. As the 

pressure-time curve from the gas combustion propagates from its source, shock waves with near 

instantaneous rise times begin to form while the peak pressure tends to decrease. 

Figure 4. Pressure-time curve showing pressure decay with distance and shock wave 

development from LLEM test #485 with 60-ft gas zone.

The recommended 15-psi pressure-time curve with a slow rise time, a dynamic load factor of 

1.0, and an equivalent static pressure of 15 psi is representative of the expected pressure-time 

history close to an explosion source. A 7.5-psi maximum pressure at 526 feet with an assumed 

instantaneous rise time, a dynamic load factor of 2.0, and an equivalent static pressure of 15 psi 

is dynamically identical to the recommended curve, and is representative of the pressure-time 

history further from an explosion. 

Criteria for Designing an RA Stopping and Approval of RA Stopping Designs

Given the analysis provided in the previous subsection, the issues that emerge in relation to 

design and approval of RA stoppings are structural design criteria, foundation design, location, 

leakage, and quality control. 
27 

Structural Design Criteria

1. Pressure-time curve. The required pressure time-curve has a 15-psi constant pressure, 

0.1-second rise time, 0.1-second fall time, and 0.2-second duration. This pressuretime curve is expected to have a DLF of 1.0 for most practical BIP RAs because of its 

slow rise time. The equivalent static design pressure is therefore 15 psi. 

2. Elasticity of design. The RA stopping structure must resist the equivalent static design 

pressure of 15 psi and remain within the linear elastic range of its component 

materials. The RA stopping structure must return to its original shape with no 

permanent deflection when subjected to the 15-psi equivalent static design pressure. 

This elasticity of design requirement is identical to requirements used for the design 

and approval of coal mine seals under the 2008 regulations in 30 CFR 75.335. 

3. Entrance door. The entrance door through the RA stopping must have a least 

dimension of 2 feet and meet the same design pressure-time curve requirements. 

4. Support conditions. The support conditions for the RA stopping structure must match

the mine conditions. The expected support conditions are simple supports with zero 

moment on all four edges in most practical mining conditions. Fixed support 

conditions with moment are possible to develop if the foundation is prepared. The 

design should consider the stopping using simple supports unless the designer 

provides detailed calculations and drawings to demonstrate that the fixed condition is 

a realistic assumption. 

Foundation Design

As to foundation design considerations, the RA stopping must be adequately anchored to the 

surrounding strata. The relevant criteria to consider are:

1. Adequate anchorage capacity. The RA stopping anchorage system must resist the 

shear load around the perimeter of the RA stopping induced by the design pressuretime curve.

2. Elasticity of design. The RA stopping anchorage system must resist the induced shear 

load and remain within the linear elastic range of its component materials.

3. Support preparation. The RA stopping foundation must have sufficient preparation to 

justify the support conditions assumed for the RA stopping structure.

Location

1. The RA stopping must be located in a crosscut or a similar side passageway to 

minimize the magnitude of reflected explosion wave overpressure acting on the 

structure.

Leakage

Given the possibility of leakage, the RA stopping structure and the surrounding strata must 

enable maintenance of a life-supporting atmosphere within the facility. The leakage criteria to 

consider are:

1. Pressure. There must be an ability to maintain a measurable positive pressure inside 

the facility of at least 0.1-in water gauge.
28 

2. Sealant. The RA stopping structure and the nearby roof and rib strata must be coated 

with an approved sealant for fire resistance and leakage control. 

3. Repair. Fresh containers of an approved sealant must be kept inside the facility for 

emergency repair of any leaks. 

Quality Control

In relation to quality control, the design of an RA stopping structure and its foundation must 

be considered a life-critical facility. Therefore, design, construction, and subsequent maintenance 

must follow the highest quality control practices. The practices are similar to those used for coal 

mine seals since adoption of the 2008 regulations prescribed in 30 CFR 75.335. Quality control 

criteria to consider are:

1. Certification by a licensed professional engineer. A design submitted to MSHA for 

approval must be certified by a licensed profession engineer. 

2. Construction of the RA stopping and its foundation. Mine management must certify 

that the RA stopping and its foundation were built according to the design 

specifications of a licensed professional engineer and as approved by MSHA. 

3. Maintenance of the RA stopping and its foundation. Mine management must 

periodically inspect the RA stopping and its foundation to verify that the facility will 

perform as expected.

Guidelines for Effective RA Stopping Design

Design engineers must submit RA stopping designs for approval under the requirements of 

MSHA’s Refuge Alternatives for Underground Coal Mines [Federal Register 2008]. To assist 

them with that task, the list below for an RA stopping design application is adapted from 

guidelines compiled for coal mine seal design applications [MSHA 2008].36

36 These MSHA guidelines call for engineering designs that address the following categories: gas 

sampling pipes, water drainage systems, methods to reduce air leakage, pressure-time curve, fire 

resistance characteristics, flame spread index, entry size, engineering design and analysis, elasticity of 

design, material properties, construction specifications, quality control, design references, and other 

information.

Using the MSHA 

guidelines as a model, an RA stopping design application should include the following elements 

and specifications. 

1. Engineering drawings 

a. Provide front, side, and plan views plus any details as needed at a reasonable 

scale for clarity.

b. Specify the maximum and minimum entry size for which the RA stopping 

design applies.

c. Provide a table giving entry height and width plus relevant structure details of 

the RA stopping design that will change with these dimensions such as 

thickness, amount and size of reinforcement, and anchorage requirements. 

 
29 

2. Material properties descriptions 

a. Provide engineering properties of the materials used in the RA stopping 

design such as compressive strength, yield strength, modulus values, and fire 

endurance properties of the materials. Fire endurance properties should at 

least be consistent with criteria codified in 30 CFR 75.333 Subpart D for the 

design of mine ventilation controls. 

b. Provide laboratory test results for other materials including average and 

standard deviation. 

c. Provide engineering properties of the roof and floor rock of the RA stopping 

site that justify the support conditions assumed in the analysis.

3. Engineering calculations

a. Provide a complete set of calculations for the RA stopping structure and its 

foundation. The calculations must demonstrate that the design is linear elastic 

and satisfies the elasticity of design criterion.

b. Explain all assumptions and justify the support conditions used in the 

structural design. 

c. Cite references for all analysis methods used.

4. Cite applicable design codes followed such as the American Concrete Institute [ACI 

318-08 2008], the American Institute of Steel Construction—Steel Construction 

Manual [AISC 2005], or the Unified Facilities Code [UFC 3-340-02 2008]. 

5. Leakage control measures

a. Demonstrate ability to maintain a measurable positive pressure inside the 

facility of at least 0.1-in water gauge.

b. Describe methods to control leakage out of the facility. 

c. Specify sealants to be used.

6. Door specifications 

a. Provide a complete set of calculations for the entrance door showing that it 

can meet the pressure-time curve requirements and remain linear elastic.

b. Alternatively, provide specifications for a commercially available door that 

meets ASTM Standard F2247-11 [2011] “Standard Test Method for Metal 

Doors Used in Blast Resistant Applications (Equivalent Static Load Method).” 

7. Fire resistance and flame spread index

a. Specify any fire retardants used and their characteristics to meet fire 

endurance criteria of 30 CFR 75.333. 

8. Construction specifications 

a. Provide information on the site location and necessary site preparation for the 

RA stopping. Describe extra ground support to install in the area. Describe 

remediation procedures for any adverse ground conditions that could develop 

in the area.

b. Provide information on the RA stopping foundation such as rock bolt anchors, 

the depth of hitch, or the angle iron size and its anchorage. 
30 

c. Provide information on internal reinforcement such as the diameter and 

spacing of horizontal and vertical reinforcement and stirrups. 

d. Provide information on formwork if used. 

e. Provide information on methods and equipment used to batch, mix, transport, 

convey, and place RA stopping materials.

f. Provide information on the fire resistance properties of the construction 

materials.

9. Quality control plan 

a. Describe how and by whom the site will be investigated prior to installation of 

an approved RA stopping. 

b. Describe how and by whom the RA stopping foundation will be certified 

when it is built.

c. Describe how and by whom the RA stopping structure will be certified when 

it is built.

d. Specify how often the RA stopping should be inspected and maintained 

during its lifetime.

10. Summary of installation procedures 

a. Provide complete, standalone, step-by-step installation procedures for the RA 

stopping and its foundation. 

b. Include drawings and other valuable information to help the installer construct 

the RA stopping according to the engineer’s intent.

Example Design for an Effective RA Stopping

Based on the above guidelines, OMSHR developed an example design for an effective RA 

stopping that consists of a conventional rebar-reinforced concrete wall 12 in thick. The complete 

design calculation for this example is included as Appendix C.37

37 Additionally, Foster-Miller [1975] also presents some examples of RA stopping designs that would meet 

the proposed design criteria.

The example RA stopping 

design is presented to illustrate the application of the proposed design guidelines in preparing a 

design submittal to MSHA District Managers for approval. Significantly different RA 

stopping/door systems for BIP RAs have already been approved by MSHA for use, and OMSHR

believes that if BIP RAs become more commonly used in U.S. coal mines, then many more 

acceptable and economically viable stopping/door designs that meet the 15 psi static pressure 

criteria will be developed.

The design example presented here was developed by adapting recognized protective 

structure design principles in the Unified Facilities Criteria (UFC): Structures to Resist the 

Effects of Accidental Explosions [UFC 3-340-02 2008]. The design method follows three 

general steps: (1) design inputs where the design load and material properties are specified, (2) 

foundation design where an anchorage system for the stopping structure is designed, and (3) 

 
31 

design of the stopping structure itself. In all cases, the design pressure-time curve is 15 psi with a 

0.1-second rise time, 0.1-second fall time, and 0.2-second duration. The equivalent static design 

pressure is 15 psi, and the design remains in the linear elastic range under the design pressure. 

The entry dimensions for this example design are 20 ft wide by 8 ft high. The design example 

below follows conservative protective design principles used by the military. This example is not 

to limit alternative designs that may be more aggressive and economical, but only to convey the 

need to meet established performance standards and follow protective structure design principles. 

This rebar-reinforced concrete wall design is 12 in thick. Steel reinforcement within the RA 

stopping wall consists of 5/8-in-diam (#5 bar) vertical and horizontal reinforcement bars on 12-in 

centers on both sides of the RA stopping wall. Shear reinforcement consists of 1⁄2-in-diam (#4 

bar) stirrups at each intersection of vertical and horizontal reinforcement bars. The RA stopping 

wall is anchored to the surrounding rock with 5/8-in-diam (#5 bar) rock bolt anchors spaced 

every 1.5 ft around the RA stopping perimeter. The 5-ft-long anchors have 3 ft grouted into 

surrounding rock and 2 ft embedded in the RA stopping structure. The total number of rock bolt 

anchors required is 40. The yield strength for reinforcement steel is 60,000 psi. The compressive 

strength of the concrete is 3,000 psi. Figure 5 shows this rebar-reinforced concrete RA stopping 

design. 

Figure 5. Design example for a 15-psi RA stopping using

conventional rebar-reinforced concrete.

Concrete—12 in thick

Rock bolt anchors—5/8-in-diam, 1.5-ft centers,

3 ft into rock, 2 ft into wall

Steel reinforcement—5/8-in-diam,

12-in centers, vertical and horizontal
32 

Delivering a Reliable Supply of Clean, Breathable Air to a BIP RA

Most of the advantages of a BIP RA disappear if the RA is not guaranteed a constant and 

highly reliable supply of clean, breathable air. A borehole from the surface to the BIP RA is the 

most highly desirable approach. Several breathable air supply units to be used in conjunction 

with a borehole directly into a BIP RA have been approved by MSHA. These consist of either air 

supply units permanently mounted at the borehole or of trailer-mounted units which can be 

brought from some central storage location to the mouth of the borehole in an hour or less in the 

event of a mine disaster. Both fan /blower type systems and compressed air type systems have 

been approved and are currently in use to provide air to outby BIP RAs. The compressed air

units have dryers and oil and water separators to ensure the delivery of clean breathable air to the 

miners.

For some mining operations it will not be practical to employ boreholes to deliver air to BIP 

RAs due to drilling costs, difficult terrain, and surface rights issues. An alternative approach may 

be to provide air to the BIP RA via a protected compressed air line carrying clean, breathable air.

Some work has already been done to address this issue. MSHA has already granted approval 

to a compressed air supply system, including a protected compressed air line for use with BIP 

RAs.

38

38 The Hubble Breathable Air Units (Models HBA75, HBA 100, and HBA 250) have been approved by 

MSHA (Approval No. 07- LCA110001) for use in providing breathable air to an RA via a protected 

compressed air line. The main components of the Hubble Breathable Air (HBA) unit are the surface air 

supply unit and a compressed air pipeline system which carries air underground to a BIP RA. These 

components of the system have MSHA approval and are intended to be employed in conjunction with an 

approved BIP RA. The HBA unit consists of a rotary screw air compressor, a continuous flow breathing air 

purifier system, and a diesel-powered generator unit to provide backup power. There are three approved 

versions of the surface HBA unit (HBA 75, HBA 100, and HBA 250) which, respectively, supply 274 cfm 

(12.5 cfm per person = 21 people), 336 cfm (12.5 cfm per person = 26 people), and 977 cfm (12.5 cfm 

per person = 78 people). A continuous supply of breathable air can be delivered to the RA through a 2-in 

pipeline which is either buried in soft floor along the rib or anchored to hard floor along the rib and 

covered with waste material. A regulator valve, manifold valve system, flow measurement system, and a 

silencer are all located at the BIP RA.

This approval opens the door for providing a continuous supply of clean, breathable air to 

a BIP RA.

In order to guarantee a supply of clean, breathable air to BIP RAs in an emergency situation, 

a number of issues need to be considered with respect to the reliability and availability of the 

surface compressor station and the protected compressed air line. Considerations for the surface 

compressor station are as follows: 

1. Accessibility to surface location.

2. Protection from weather and elements.

3. Whether the compressor station should be treated like a main mine fan—i.e., if the 

compressor is down more than 15 minutes, what steps would be taken. 

4. The requirements for a backup compressor and power supply. 

 
33 

5. Sizing requirements—i.e., the amount of air that is needed to be supplied to the RA, 

whether a single compressor system would provide air to one or more RAs. 

6. Maintaining discharge air quality in terms of oil, moisture, filtration, etc.

Considerations for the protected compressed air line39

39 Through additional research and testing, it will be necessary to better define the requirements for the 

construction and survivability of compressed air lines for BIP refuge alternatives. As part of this process, 

the specifications, installation procedures, “best practices” for required maintenance and examinations, 

and testing criteria will be developed to establish the guidelines for use of these compressed air line 

systems OMSHR has already funded on-going contract research in this area.

are as follows: 

1. Type of material to be used—Carbon steel pipe: ASTM A106, A53, ASTM A179, 

ASTM A192, DIN17175 ST45.8 ST35.8, Alloy steel pipe: ASTM A335, ASTM 

A213, Stainless steel pipe: ASTM A312, ASTM A213, Line pipe and cashing pipe: 

API5L, API 5CT. 

2. Bending strength requirements of the compressed air line. 

3. Anchoring or protection of air line in the event of a mine explosion. The air line must 

survive 15 psi over 0.2 seconds with a 0.1-second rise and 0.1-second fall. 

a. Whether it should be buried and to what depth. 

b. If not buried, how much it should be anchored. Whether it can be covered 

with debris. If covered with debris, what type, how much, and how deep. 

c. The air line’s ability to survive an explosion and not be damaged by flying 

projectiles that would result from an explosion (concrete blocks, bolts, mine 

supplies, pieces of equipment, etc.). 

4. Drainage of water in long lines due to condensation. 

5. Whether additional filtration is needed at the RA.

6. Procedures for extending and purging the air line. 

a. While the air line is being extended, the original RA will need to be 

maintained. 

b. The time requirement for the advancement window. 

c. The time requirement for a “hardened line.”

d. Specifications/quality control on tightening connections. 

7. Distribution air manifold requirements at the RA location. 

8. Valve requirements of the air line at various locations including check valves, shutoff 

valves, and testing for leaks.

9. Whether a continuous low volume of air can be provided to a BIP RA to keep it under 

positive pressure ~ 0.1 in water gauge. If so, how much would be needed [FosterMiller 1983]. 

10. How the air line is transitioned into the BIP RA itself and kept protected. 

 
34 

11. Where best to locate protected compressed air lines under various mining scenarios 

(longwall development, continuous mining section, retreating longwall 

considerations, etc.).

12. Air delivery limits—i.e., how far air can be carried through a compressed air line 

before pressure losses become too great, how often a new borehole would be needed 

to keep the air supply at an acceptable distance from the RAs.

To assess how industry can meet the above requirements, research needs to be conducted to 

develop specific guidance. 

Considerations for Facilitating the Use of BIP RAs 

The NIOSH OMSHR 2007 report to Congress on refuge alternatives [NIOSH 2007] 

concluded that built-in-place refuge alternatives offer substantial advantages over mobile RAs, 

and that the strengths of BIP RAs compared to mobile RAs are so significant that consideration 

should be given to allowing extended distances from the face for the placement of BIP RAs. This 

Report of Investigations has argued that point more expansively, by first summarizing the 

potential advantages of BIP RAs, discussing three approaches to locating them further from the 

face, outlining design and approval guidelines for RA stoppings, and analyzing the issues 

involved in delivering a reliable supply of clean, breathable air to a BIP RA. Building on the 

above material, here OMSHR sets forth and analyzes several considerations for facilitating the 

use of BIP RAs in underground mines. 

Locating BIP RAs at Greater Distances from the Working Face

Based on the three approaches undertaken by OMSHR researchers to determine how far from 

the face area miners could travel given the breathing time afforded by their available SCSRs, and 

combined with the travel distances to RA locations based on entry height and the distances 

between SCSR cache locations as outlined in Table 4, OMSHR believes that mines could locate 

built-in-place refuge alternatives at distances from the working face based on the guidance 

provided in Table 4.

Allowing mines to locate BIP RAs at greater distances from the working face introduces a 

number of advantages: 

1. RAs located further from the face have a higher likelihood of avoiding damage from 

both primary and secondary explosions which occur at the face area of the mine. 

While the current 30 CFR 75.1506(c) regulation requires that RAs be located within 

1,000 feet of the working face, in practice, mining logistics generally result in RAs 

being located much closer than the 1,000-feet requirement, making them and their 

occupants easily subject to effects from explosions at the face. Locations further from 

the face might also increase the likelihood that the communication system to the RA 

survives a disaster.

2. Locating BIP RAs further from the face would reduce the number of BIP RAs that 

would need to be constructed.
 

 

 

 

 

 

 

 

35
 

3.	 Locating BIP RAs further from the face and thus reducing the frequency of RA 

moves will make it more practical to introduce a wider variety of BIP RA designs, 

which could improve the safety as well as the psychological and physiological 

comfort and mental well-being of confined miners, significantly improving the 

likelihood of survival. 

Table 6 presents a summary of the three approaches. Using conservative rounded-down 

distances and combining the data from Table 4 results in potential maximum BIP RA distances 

from the face, which vary with entry height. 

Table 6. Maximum distances of BIP RAs from the face 

Entry height 

Approach 1: 

Based on SCSRmandated 

storage cache 

locations 

Approach 2: 

Based on 

worst-case 

SCSR usage 

times 

Approach 3: Based 

on NIOSH and 

BOM established 

travel times and 

escape probabilities 

Maximum BIP 

RA distance from 

the face 

<40 inches 2,200 feet 2,640 feet NA 2,000 feet 

>40–<50 inches 3,300 feet 3,960 feet NA 3,000 feet 

>50–<65 inches 4,400 feet 5,280 feet 6,000 feet 4,000 feet 

>65 inches 5,700 feet 6,480 feet 6,500–7,000 feet 5,000 feet 

Locating BIP RAs further from the face requires a trade-off of the advantages of locating an 

RA close to the face—such as the ability to easily assist injured miners in reaching the RA 

location and a shorter travel distance for miners to the RA—against the significant potential 

advantages gained from the use of a BIP RA. This argument is consistent with OMSHR’s 

position in its 2007 Report to Congress on refuge alternatives [NIOSH 2007]. Therefore, 

consideration should be given to allowing mines to locate RAs further from the face only if they 

employ new RA technologies that meet the following three criteria: 

1.	 Provide a constant supply of air to the RA either via a protected compressed air line 

into the RA or a borehole from the surface into the RA. 

2.	 Provide additional RA space per occupant. The original minimal space requirement of 

85 ft3

 per occupant as noted in the NIOSH report to Congress on refuge alternatives 

[NIOSH 2007] is recommended. 

3.	 Maintain the RA interior atmosphere under positive pressure when not in use to 

ensure that the RA contains breathable air immediately upon entry and to keep 

contaminated air from entering the RA with miner entry. It would be advantageous to 

have a differential pressure gauge on the RA which could be inspected regularly to 

ensure that the positive differential pressure is maintained. However, if it is not 

feasible to maintain the RA under positive pressure when not occupied, additional 

SCSRs must be available in the RA to sustain miners until the RA can be adequately 

purged of contaminated air by the air supply to the RA. 
 

 

Importantly, it is possible that a protected compressed air line could be run to a mobile RA. 

This would eliminate the need for compressed air and O2 cylinders in the RA itself, which could 

result in more occupancy space in the RA. 

Designing RA Stoppings for MSHA Approval 

Previous sections of this report have detailed structural design criteria for RA stoppings that 

could meet with MSHA approval, including an example design for an RA stopping constructed 

from a conventional rebar-reinforced concrete wall. OMSHR believes that the guidelines set 

forth in this report could be used by industry when submitting stopping design applications for 

approval by MSHA. 

Based on findings from this Report of Investigations, the recommended design pressure-time 

curve for an RA stopping has a magnitude of 15 psi, a rise time of 0.1 seconds, a fall time of 0.1 

seconds, and a duration of 0.2 seconds.40 

40 These criteria are specified in 30 CFR 7.505 and further clarified in the April 29, 2009, section of the 

document, “Questions and Answers: MSHA’s Refuge Alternatives Requirements.” 

Because of the slow rise time, this pressure-time curve 

always has a dynamic load factor of 1.0 and an equivalent static design pressure of 15 psi. This 

design pressure-time curve is representative of the pressure-time history close to an explosion 

source. It is similar to observed explosion pressures in recent underground coal mine disasters, 

and is commensurate with the blast pressure tolerance of the human body. This curve is identical 

to that developed in OMSHR’s 2007 report to Congress on refuge alternatives [NIOSH 2007]. 

Delivering Clean, Breathable Air to a BIP RA 

As noted in this report, most of the advantages of a BIP RA disappear if the RA is not 

guaranteed a constant and highly reliable supply of clean, breathable air. A borehole from the 

surface directly to the BIP RA is the most advantageous and reliable option. However, a 

borehole from the surface to the BIP RA is often impractical due to such factors as drilling costs 

and surface rights issues. Using a protected compressed air line carrying clean, breathable air to a 

BIP RA is emerging as a practical and achievable goal.41

41 The Hubble Breathable Air Units (Models HBA75, HBA 100, and HBA 250) have already been 

approved by MSHA (Approval No. 07-LCA110001) for use in providing breathable air to an RA via a 

protected compressed air line. 

OMSHR believes that a constant 

supply of clean, breathable air can be provided to BIP RAs either from a borehole to the surface 

or through a protected compressed air line system. Several efforts to develop and test such 

systems are already being undertaken by private companies. 

 

 

 

 

 

36 

Considering Economic Drivers when Designing New BIP RAs

Currently, there are approximately 30 BIP RAs with a constant air supply (all via borehole to 

the surface) and MSHA-approved stopping and door systems in U.S. underground coal mines. 

These BIP RAs are all located outby the face areas. These RAs range in volume from 1,800 ft3 to 

10,000 ft3 and are designed to house from 20 to over 100 occupants given the requirement of 
37 

12.5 ft3

/min of breathable air per occupant in 30 CFR 7.506 and the current space requirement of 

15 square feet of floor space per person in 30 CFR 75.1506. Each of these BIP RAs provides 

more than the 85 ft3 of space per occupant recommended in the NIOSH report to Congress on 

refuge alternatives [NIOSH 2007]. 

The costs of these existing BIP RAs range from $50,000 to $150,000, depending mainly on 

stopping and door system costs and borehole costs. Given these costs, it does not appear to be 

economically viable to provide these types of RAs within 1,000 feet of the working face of 

mines. However, if BIP RAs that advance with the face become an accepted practice, it is likely 

that much more cost-effective stopping and door systems, which could be disassembled and 

advanced or be inexpensive enough to abandon in place, will be developed and made available. 

Some work in this area is already being undertaken by private companies.

In the context of economic drivers, design needs for new BIP RAs that can be advanced with 

the face are as follows:

1. BIP RA designs are needed to employ stopping and door systems that are inexpensive 

enough to build and leave in place or stopping and door systems that are designed to 

be quickly and easily disassembled, moved, and reassembled. 

2. Novel BIP RA concepts that provide more space and breathable air per occupant and 

can be easily moved with the face are needed. An example would be a series of boltor clamp-together steel box modules that could be moved with the face and easily 

assembled into a single larger steel box (modular design). The smaller individual 

boxes could be moved easily in the mine (possibly with a scoop) and assembled into 

the larger unit in a crosscut or cutout. This would allow for an RA that could provide 

significantly more space per occupant.

3. Breathable air would most likely be provided to any of the above RA designs through 

a protected compressed air line. Therefore, there is an economic driver to provide a 

protected compressed air line that supplies breathable air to the RA occupants and can 

be advanced with the face. The compressed air source could be located on the surface 

and distributed to the RA via a borehole and a protected underground compressed air 

line.

Another economic consideration is to develop ways to reduce RA stopping and door costs by 

requiring only one stopping and door system per RA. This could be done by locating RAs in 

crosscuts that are not completely cut through or by locating them in cutouts in long pillars that 

originate halfway through a crosscut. The development of improved air supply and cooling 

systems employing technologies such as liquid O2 systems, or cryogenics could also reduce the 

cost of advancing more habitable RA designs with the face.

If there is a willingness to accept new BIP RA design concepts for advancement with the 

face, competition could bring new economically viable BIP RA designs into the marketplace.
38 

Acknowledgments

The authors thank the many U.S. underground coal mining companies who provided 

information on their current BIP RAs, and especially those companies who allowed NIOSH 

OMSHR personnel to visit their mines to observe their BIP RAs. We also thank Mr. Harold 

Aker, President, Hubble Mining Company, and John G. Blackburn, Manager of Contract Mines, 

TECO Coal, for the information they provided on the Hubble Breathable Air Units.

We further thank the following NIOSH OMSHR personnel without whose contributions this 

work could not have been accomplished: Candace A. Wolf, Technical Writer-Editor, Brittney 

Warnick Bragg, Health Communications Fellow, and Eric P. Brown, Contractor, for their 

contributions to the technical editing, formatting, and 508 compliance preparation of this RI; 

Audrey Podlesny, Statistical Assistant, for her assistance with the MSHA coal mine accident, 

illness, injury, and employment database; and Kenneth W. Strunk, Visual Information Specialist, 

for his drafting contributions. 

All photographs in this report were taken by Eric R. Bauer or Niccolle LaBranche of NIOSH 

OMSHR.
39 

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43 

Appendix A—An Overview of Built-in-place Refuge Alternatives in 

U.S. Underground Coal Mines

Introduction

This Appendix discusses the current use of BIP RAs in U.S. underground coal mines. This 

includes statistics on the number of BIP RAs currently in the Nation’s underground coal mines, 

the number of coal companies and mines employing the BIP RAs, how BIP RAs are currently 

used, and their role as part of a mine’s comprehensive escape and rescue plan. The MSHA 

regulatory requirements for design and provision of BIP RAs are presented. This Appendix also 

provides a review of site-specific information obtained during mine visits by OMSHR 

researchers to view BIP RAs. The site-specific BIP RA information includes construction, 

location, air supply, and RA provisions and inventories. The characteristics of an ideal BIP RA 

are assessed, in addition to other acceptable but less desirable attributes. 

Review of Refuge Alternatives

RAs are intended to provide a post-disaster safe place of refuge for miners that have 

exhausted all attempts at escaping. In the face area they replace barricading which traditionally 

was the method miners used to isolate themselves from the post-disaster noxious gases. There is 

a history of miners successfully utilizing barricades to survive post-disaster, but most of the 

successful barricading occurred in distant years past. In fact, since 1958, only 11 miners have 

survived by building a barricade. Table A1 illustrates the success and failures of barricading 

from 1900 to 1998. It does not include the Sago miners who perished behind a barricade. 

Table A1. Historical review of mine barricading efforts as a

result of fires and explosions in coal mines

Outcome for miners 1900–1958* 1958–1998** 

Fatally injured 4,084 421 

Escaped 2,835 588 

Rescued 381 23 

Survived after barricading 880 11 

Died after barricading 189 22 

Sources: *BOM Bulletin 586; **Misc. BOM Information Circulars

In outby mine areas, RAs can serve as places of refuge for outby workers if escape is cut off 

or as waystations as miners escape for short-term rest and regrouping before continuing escape. 

RAs provide miners with the basic necessities to survive including a breathable atmosphere 

(oxygen, compressed air, and/or carbon dioxide scrubbing) and food and water. They can be of 

two types, either mobile RAs or BIP RAs. Table A2 presents a comparison of the characteristics 

of the two different RA concepts. 
44 

Table A2. Comparison of characteristics for mobile RAs and BIP RAs 

Characteristic Mobile Tent-Type 

Inflatable Mobile Rigid Steel BIP RA

Size Smaller than mine 

opening 

Smaller than mine 

opening Utilizes mined opening 

Number of 

occupants 36 maximum 20–24 maximum 

Limited only by size and 

space/volume 

requirements of 30 CFR 

75.1506 (b)(1)

Breathable air 

supply Oxygen bottles Oxygen bottles 

Borehole compressed 

air, borehole blower 

system, compressed air 

line

Amount of air 1.32 ft3

/hr O2 per person 1.32 ft3/hr O2 per person 12.5 cfm air per person 

Designed duration 

of occupancy (hr) 96 96 96 + 

Carbon dioxide 

scrubbing 

Lithium hydroxide or 

soda lime curtains or 

canisters

Lithium hydroxide or 

soda lime curtains or 

canisters

None 

Airlock Yes, required Yes, required Probably not required 

Food, water, 

supplies 

Minimum as required by 

law

Minimum as required by 

law

Generally more than 

required by law

Approximate cost* $100K–$200K $100K–$200K $80K–$150K *

It should be noted that the mobile RAs (both inflatable and rigid) are reusable. 

Mobile RAs

Mobile RAs are designed to be moved frequently as the mining face advances, and by 

regulation, are to be kept within 1,000 feet of the active mining face. This frequent moving 

requirement necessitates that they “fit” within the mined openings; thus their dimensions are less 

than the height and width of the opening. The result is limited space, both for occupants and 

supplies. There are two types of mobile RAs, inflatable tent-type or rigid steel. The tent-type 

RAs are comprised of a steel container that protects their contents which include the tent, 

inflation, purge and breathing air bottles, and all supplies. To deploy this type of RA, a door on 

one end is opened, the tent is manually rolled out, and then inflation is initiated by pulling or 

turning an activation valve. Inflation air, contained in high-pressure compressed air bottles, is 

released into the tent’s internal supporting structure. Tent-type RAs can accommodate up to 36 

occupants. A rigid steel RA is simply a structurally reinforced box constructed from steel with 

one or more entrance/exit doors. The breathing air, purge air, and all supplies are stored inside 

the RA. Rigid steel RAs typically have a lower occupancy, at most up to 24 miners. Most mobile 

RAs are equipped with an airlock that must be purged of contaminated air (primarily carbon 

monoxide (CO) and methane (CH4)) before miners can enter the main RA chamber. A few are 

designed for the entire main RA chamber to be purged after miners enter. In addition, both types 

of mobile RAs must provide breathable air to sustain each occupant for 96 hrs. Information 
45 

obtained from MSHA [Shumaker 2014] places the approximate number of mobile RAs 

manufactured and shipped as of February 2012 at 1,660 units, of which nearly 80% were 

inflatable tent-type and the remaining 20% rigid steel. Currently it is believed that there are over 

1,700 mobile RAs at the Nation’s underground coal mines, with about 1,400 of them in active 

usage.

BIP RAs

BIP RAs are known by many names including waystations, safe rooms, safe havens, refuge 

rooms, bulkhead-based refuge stations, and in-place shelters. BIP RAs use the mine structure as 

the refuge area where miners can retreat to when escape is not possible. Typically, BIP RAs are 

hardened rooms located in a crosscut or stub (butt) heading cut into a coal or barrier pillar within 

the coal seam, with the resulting room serving as a protected secure space with an isolated 

atmosphere. The mine ribs serve as two or three of the walls of the BIP RA, and one or two 

explosion-resistant stoppings are constructed to isolate the area from the rest of the mine. 

Entrance doors are provided in one or both of the stoppings. In most cases, additional roof and 

rib support is provided to strengthen the roof and ribs. Spraying of the mine area with a cementbased sealant such as shotcrete is common as well as setting timbers against the ribs in higher 

seams. All currently employed BIP RAs provide fresh air to the miners through a borehole from 

the surface, as well as food, water, and other supplies the miners will need inside the RA. The 

main advantage of a BIP RA is space. Because the whole mined opening is utilized, and the 

length can be virtually as long as desired or practical based on crosscut length, considerably 

more space can be provided for each occupant as compared to a portable RA. In addition, 

depending on the breathable air supply system employed, the number of occupants can be much 

greater. From unofficial information obtained by OMSHR from all MSHA Districts, there are 

approximately 30 BIP RAs in U.S. underground coal mines, operated by six mining companies. 

All of these BIP RAs employ boreholes to the surface. 

Current Role of BIP RAs

Currently, all BIP RAs are located outby the active mining faces and are designed and 

approved for use by the outby workers only when seeking refuge in case escape is not possible. 

Face workers are expected to use the mobile RAs on the sections if they decide to refuge. The 

BIP RAs located outby have occupancies that are selected to “accommodate persons reasonably 

expected to use it” according to 30 CFR 75.1506(b)(3). Most of the mines visited by OMSHR 

researchers as part of this study allowed for escaping face workers to utilize the outby BIP RAs. 

Built-in-place Shelters as Part of a Comprehensive Escape and Rescue Plan

RAs can be extremely useful to facilitate escape from the mine (when the RA is used as a 

waystation) as well as to serve as a safe haven of last resort. Such an approach would be far 

superior to one in which RAs are simply placed into the mine to comply with a regulation 

[NIOSH 2007]. As part of a comprehensive escape and rescue plan, BIP RAs could be used in a 

number of specific situations including those detailed below. 

1. As a rest stop or waystation during escape for miners to take a short break, get a drink 

of water, communicate with the outside, review maps and escape routes, plan their 

next escape path, change out their self-contained self-rescuers (SCSRs), then resume 

escape.
46 

2. Give aid to injured workers, then resume escape or to refuge and wait for rescue if 

non-injured miners have lost their strength and ability to continue escaping, and help 

injured miners escape, or if continued escape would negatively impact the chance of 

survival for both the injured and non-injured workers. 

3. As a place to retreat to and to seek refuge if further escape becomes impossible.

4. For use as a possible advanced fresh-air base by mine rescue teams trying to reach 

unaccounted-for or refuged miners.

5. As originally intended—to serve as a place of refuge for outby miners who have 

exhausted all attempts to escape.

Regulatory Requirements for BIP RAs

MSHA uses the term refuge alternative (RA) to refer to both BIP RAs and portable RAs. For 

use in underground coal mines they both must satisfy the MSHA requirements for refuge 

alternatives as listed in 30 CFR 7 and 30 CFR 75. There are two key issues with respect to 

MSHA allowing the use of BIP RAs, as detailed below. 

1. BIP shelters can have boreholes to the surface or an MSHA-approved protected 

compressed air line as the source of the RA’s continuous supply of fresh breathing 

and purging air, or the RA can be built without either boreholes to the surface or a 

compressed air line supply and employ the same type of purging air, breathing air, 

and carbon dioxide scrubbing components as portable RAs employ. Depending on the 

method of breathable air and purge air supply, the BIP RA may or may not be 

required to have an airlock. When boreholes or compressed air lines are used and a 

positive pressure is created within the RA, no airlock is required as per 30 CFR 

7.505(a)(3). If the BIP RA relies on oxygen bottles for breathable air and compressed 

air tanks for purging, an airlock is required according to 30 CFR 7.505(a)(3). In both 

cases, harmful gas removal requirements that must be satisfied are found in 30 CFR 

7.508. 

2. The stopping walls and door of a BIP RA must meet the MSHA explosion criteria of 

withstanding a 15-psi overpressure for 0.2 seconds as stated in 30 CFR 7.505(a)(4). 

This is basically a 15-psi static pressure and is identical to what a portable RA must 

withstand. In addition, stopping materials must meet the non-combustibility criteria 

codified in 30 CFR 75.333 Subpart D, based on the ASTM Standard E119-14 [2014] 

fire endurance test. This endurance test consists of a one-hour exposure of up to ~ 

1,652°F, whereas the portable RA must pass exposure to a flash fire consisting of a 

three-second exposure at 300°F. Mining companies or stopping vendors requesting 

approval for their designs must submit the designs to the MSHA District Manager in 

the district where the BIP RA will be built. The District Manager generally submits 

the design to MSHA Technical Support for its analysis of the design. In general, 

submitters must design their own RA stopping and door plan, do the engineering 

calculations to show they meet MSHA’s 15-psi overpressure criteria, and have the 

design certified by a professional engineer.
47 

Summary of Mine Visits 

General Information on BIP RAs Observed

OMSHR researchers visited underground coal mines to view BIP RAs and discuss with mine 

officials why they employ BIP RAs. Information concerning the construction, location, capacity, 

air supply, and provisioning was obtained. Mines visited were selected to obtain a cross section 

of the industry including high and low coal and eastern and western coal mines. The mines were 

located in the following states: Colorado, Kentucky, Montana, and New Mexico. Table A3 

summarizes the information obtained by OMSHR researchers, while the sections that follow 

include details of the specific BIP RA characteristics observed.
48 

Table A3. Summary of in-mine observations of BIP RAs

*Distance apart refers to the actual distance the outby BIP RAs are from each other or—in the case of Mine 3, which

has only one BIP RA—the distance selected by the mine operator for placement of a second BIP RA in the future.

Characteristic Mine 1 Mine 2 Mine 3 Mine 4

State CO KY MT NM

Number of BIP RAs 2 5 1 8 

Distance apart (ft)* Less than 6,000 8,000 5,700 6,000 

Area (sq ft) 715; 850 1,000 360 1,000 

Mining height (in) 104; 132 63 120 105 

Volume (ft) 6,197; 9,350 5,250 3,600 8,750 

Design number of 

occupants based on 

supplies provided 

50 6 15 20 

Potential number of 

occupants based on 

volume of air supplied 

62; 68 120 104 100 

Maximum number of 

occupants based on mining 

height and unrestricted 

volume requirement of 

30 CFR 75.1506(b)(1) 

103; 156 87 60 146 

Stopping and door MICON MICON MICON Mine-designed

Number of doors 2 1 2 2 

Door locations Front and back 

stopping Front stopping Front and

back stopping 

Front and back 

stopping 

Breathable air supply

Borehole, 

permanent 

blower 

Borehole, 

portable 

blower 

Borehole, 

permanent 

compressed 

air

Borehole, 

portable 

compressed air

Amount of air (cfm) 770; 850 1,493 1,300 250 to 1,500 

Borehole depth (ft) Less than 600 Less than 450 220 Less than 900

Borehole diameter (in) 10 6 6 7 

Carbon dioxide scrubbing None None None None 

Airlock No No No No

Approximate cost $150K $100K $80K $80K 

Separate toilet area, size 

(sq ft)

Yes, 22.5 and 

90 No

No, brattice 

cloth 

provided, 

must be hung 

Brattice cloth in 

place for privacy
49 

BIP RA Stopping Construction

The stopping and door systems used for BIP RAs were of two types, either a MICON42-

designed and MSHA Technical Support-approved RA stopping and door system (Figure A1) or a 

mine-developed and MSHA-District Manager-approved43 RA stopping and door design (Figure 

A2). To date, MICON provides the only MSHA Technical Support-approved RA stopping and 

door system for BIP RAs. Stopping design and certification is performed by MICON for each 

BIP RA. Initially, a mine provides MICON with a location for the BIP RA, and then MICON 

checks the shear strength of the surrounding strata and measures the cleaned opening. Once the 

opening is cleaned and measured, MICON uses the design chart to determine the thickness of the 

stopping necessary to withstand 15 psi overpressure, with a safety factor of 2. MICON RA 

stoppings can be built up to 14 ft high and 28 ft wide. The steel door is designed to withstand a 

50-psi explosion pressure. 

Photo by NIOSH

42 MICON is a company that provides specialty ground control and contracting services to the mining 

industry.

43 This specific design was approved by the MSHA district manager for use only in one specific mine in 

that district.

Figure A1. Example of a MICON built-in-place refuge alternative stopping and door system. 
50 

Photo by NIOSH

Figure A2. Mine-designed and MSHA District Manager-approved built-in-place

refuge alternative stopping and door system. 

The MICON BIP RA stoppings are constructed with concrete block and polyurethane 

bonding adhesive that is applied to both sides of the blocks, between blocks as they are laid, and 

on the perimeter between the blocks and the roof and ribs. The perimeter expands upon curing, 

leaving an impermeable membrane with virtually no leakage. The blocks are a concrete masonry 

unit (CMU) with a minimum of 1,800 psi compressive strength which meets the C140 ASTM 

standard [ASTM 2012]. MICON BIP RA stoppings are typically 2 to 5 blocks thick. They can be 

flush mounted at the edge of the crosscut, but are typically installed a foot or two from the 

corners for an added safety factor. The stoppings are designed for a higher safety factor than 

what is required by regulation to account for the degradation over time. The doors are usually 

installed on top of the first layer of block, and steel expansion pieces are used if the stopping is 

more than a few rows thick. Each mine decides on the whether a door will be installed in each 

stopping. Many mines have only one door to reduce the cost. Construction by MICON personnel 

can take as little as two days in lower coal seams and up to a week in the thicker seams.

There is presently only one mine-designed and MSHA District Manager-approved BIP RA 

stopping and door design. This RA stopping is built using 16-in-thick solid blocks dry stacked 

with each row laid at 90° from the previous (the first row of blocks is laid with the long side 

perpendicular to the ribs and the next row is laid with the long side parallel to the ribs). The 

stopping’s perimeter is pressurized using packsetter pressurized pre-stressing bags between the 

blocks and the roof and ribs. These bags are of a heavy impermeable material and are filled with 
51 

a quick-setting grout under pressure of about 50 psi. The grout slightly expands causing a tight 

fit around the perimeter and filling all voids. This loads the stopping and keeps it airtight. 

Finally, both sides of the stopping are sealed with trowel grade cement plaster.

Supplemental Support

The roof and ribs in the BIP RAs are normally provided with some additional support simply 

because the RAs have an anticipated long life and must remain stable and safe for the occupants. 

Most mines visited by OMSHR researchers installed wire mesh on the roof to prevent small 

rocks from falling. In the higher seams, the ribs were usually wire meshed and bolted and then 

covered with a pneumatically applied concrete-based sealant for long-term stability. The mine 

roof was sprayed in this manner as well in many of the shelters. Also in the higher seams, posts 

or jacks were installed along both ribs to help eliminate rib roll problems (Figure A3). 

Figure A3. Posts and chain link fence placed to prevent rib

rolls in a higher seam built-in-place refuge alternative.

Photo by NIOSH
52 

Location (Distance Apart) for BIP RAs

The location for BIP RAs, which were all built outby the face areas, was determined by 

actual walking trials. 30 CFR 75.1506(c)(2) states that RAs shall be “spaced within one-hour 

travel distances in outby areas where persons work such that persons in outby areas are never 

more than a 30-minute travel distance from a refuge alternative or safe exit.” Mine operators 

determined the appropriate distance through simple walking trials under normal mine conditions. 

Depending on the seam height, mine conditions, slope or gradient of the entries, etc., one-hour 

walking distances ranged from 5,000 to 15,000 ft. The mine operators elected to build their BIP 

RAs at distances less than the maximum walking distances determined. Several factors 

influenced location selection including in-mine site conditions, surface availability, and of 

course, walking distance. 

Location (Physical/Actual) of BIP RAs

The actual physical location of the BIP RAs, which were all built outby the face areas, varied 

from mine to mine. Most were located in a crosscut off the primary (intake) escapeway and were 

attached by a branch lifeline to the main lifeline. Others were located in a crosscut between the 

intake and return air course. The locations were somewhat dictated by the availability of locating

the borehole—i.e. terrain, surface rights, power availability, etc.

Breathable Air Supply Systems for BIP RAs

Breathable air was supplied to the BIP RAs through a borehole from a permanent 

compressed air supply or blower (Figures A4 and A5) or through a borehole from a portable 

compressed air supply or blower (Figure A6). Most boreholes were from 6 to 10 inches in 

diameter, cased, and equipped with a muffler for noise reduction or diffuser for improved airflow 

at the termination end inside the BIP RA. Some boreholes were just a hole in the roof as shown 

in Figure A7. 30 CFR 7.506(c), which requires that a minimum flow rate of 12.5 cfm of clean air 

per person be supplied when breathable air is supplied via compressed air from fans or 

compressors. All borehole air systems visited by OMSHR researchers exceeded this standard. 

All systems must satisfy 30 CFR 7.506(a), which states that “only uncontaminated breathable air 

shall be supplied to the refuge alternative.” 
53 

Figure A4. Permanent blower air supply system. 

Photo by NIOSH
54 

Figure A5. Connection to borehole for a permanent air supply system. 

Photo by NIOSH
55 

Figure A6. Portable, diesel-powered blowers for borehole air supply systems. 

Photo by NIOSH
56 

Figure A7. Breathable air supply borehole opening into a built-in-place

refuge alternative with no collar. 

Photo by NIOSH

Occupancy/Capacity

All of the BIP RAs visited by OMSHR were located in outby areas of the mines. 30 CFR 

75.1506(b)(2) states that “each refuge alternative for outby areas shall accommodate persons 

reasonably expected to use it.” This is interpreted as the outby BIP shelters only needing to be 

sized to accommodate the number of outby persons reasonably expected to use the RA, since 

face workers deciding to refuge are expected to use the portable RAs located on the active 

mining sections. Occupancy is limited by the amount of breathable air provided, the size of the 

RA, and/or the amount of food and water available. The design occupancy of the BIP RAs 

examined was limited by the supplies provided and ranged from 6 to 50 occupants. Table A3 

also shows that based solely on the volume of breathable air supplied, the occupancy could be as

many as 120 occupants. Finally, BIP RAs must also meet the mining-height-based unrestricted

volume requirement of the table presented in 30 CFR 75.1506(b)(1). Table A3 shows that the 

design occupancy of all the BIP RAs visited as part of this research satisfies this regulation and 

could be in the range of 60 to 156 occupants. 

RA Supplies

The supplies provided varied widely from one BIP RA to the next. All the BIP RAs observed

during the mine visits provide food and water that satisfies the requirements of 30 CFR 

75.1507(d)(1) for the number of persons expected to use the RA. In many instances extra food 

and water was provided. Other basic supplies included were those required by regulation such as 

a first aid kit (30 CFR 75.1507(d)(4)), fire extinguisher (30 CFR 75.1506(j)), communications 

(30 CFR 75.1600-3(a)), repair materials and tools (30 CFR § 75.1507(d)(3)), air monitoring 

equipment (39 CFR 7.507), and lighting sufficient for persons to perform tasks (30 CFR 75.1507

(a)(10)). Generally, the quantity of supplies in the BIP RAs was greater than in portable RAs 

because of the extra space provided by the BIP RAs. Additional items not normally found in 

portable RAs that have been provided in BIP RAs include extra first-aid supplies and stretchers, 
57 

foldable cots, blankets, face wash, emergency eye wash stations, hand sanitizer, disposable 

coveralls, hand and foot warmers, and self-contained self-rescuers (SCSRs). The borehole used 

for the breathable air supply delivery can also serve as a point of introducing supplies, 

communications, medicines, etc., if the size of the borehole is adequate and is configured such 

that supplies can be introduced without jeopardizing the airflow. 

Waste (Toilet) Facilities in BIP RAs

A means to effectively contain human waste and minimize objectionable odors must be 

provided as specified in 30 CFR 7.504(c)(3). A range of waste facilities/areas was observed that 

are designed to comply with this standard. In the larger BIP RAs, a folding toilet and disposable 

toilet bags were one option supplied. Waste bag disposal was normally accomplished by placing 

the bag in a trash can and tightly closing the lid. Other options included camp toilets, or marinetype flushable toilets (Figure A8) that expel the waste outside of the RA. One mine actually built 

urinals out of polyvinyl chloride (PVC) pipe and shutoff (obviously with only male miners in 

mind) that would allow miners to urinate, then open the shutoff and let the fluid flow out the 

pipe, through the stopping and onto the mine floor outside the back of the RA. Neither of these 

last two systems are the most sanitary systems, but they do eliminate bags full of human waste. 

Most of the BIP RAs had an area within the RA that was designated as the “toilet area.” Some 

RAs required the occupants to hang brattice curtain at first use; others had the curtain already in 

place; some had concrete block walls or walls constructed from metal stoppings and a door to 

isolate the toilet area (Figure A9).
58 

Figure A8: A marine-type flushable toilet. 

Photo by NIOSH
59 

Figure A9: A mine-constructed isolation room for a toilet. 

Photo by NIOSH

Internal Facilities 

A range of internal facilities was observed. These included benches (Figure A10), picnic 

tables, folding chairs, and storage shelves (Figure A11). The supplies were stored in the original 

manufacturer’s cardboard boxes and/or various types of plastic storage boxes (Figure A12). 

Some mines experienced long-term problems with cardboard boxes and wood shelving 

developing mold, which necessitated using plastic storage boxes and non-wood shelving, 

benches, chairs, and tables.
60 

Figure A10. Metal benches in a built-in-place refuge alternative.

Photo by NIOSH
61 

Figure A11. Storage shelves in a built-in-place refuge alternative.

Photo by NIOSH
62 

Figure A12. Typical storage boxes in a built-in-place refuge alternative.

Photo by NIOSH
63 

Appendix B—Statistical Analysis of Times to Complete Needed 

Actions in a Mine Emergency

Approach

As part of this Report of Investigations, estimates were made of the minimum and maximum 

time in minutes miners would need to complete the following activities required to travel to and 

access a BIP RA: 

1. SCSR Donning (1 min to 5 min) 

2. Face Travel Time, Assembly, and Decisionmaking (5 min to 15 min) 

3. SCSR Changeover (2 min to 5 min) 

4. RA Deployment, RA Entry and Purging (2 min to 5 min) 

It is reasonable to assume that the distribution of time to complete each activity would be 

approximately normal. 

The specifications for the four activities are shown in Table B1 below. Based on the 

symmetric property of the normal distribution, the mean time was posited to be midpoint 

between the minimum and maximum time. The values of the standard deviation were derived on 

the basis of a rule of thumb which states that dividing the range by 4 produces a reasonable 

estimate of the standard deviation. 

Table B1. Specifications for generation of simulated data for four

activities required to travel to and access a refuge alternative

Phase Minimum Maximum Mean Range SD

SCSR donning 1 min 5 min 3.00 4 1.00 

Face travel time, assembly,

decisionmaking 5 min 15 min 10.00 10 2.50 

SCSR changeover 2 min 5 min 3.50 3 0.75 

RA deployment, RA entry, and 

purging 2 min 5 min 3.50 3 0.75 

It was decided that a sample of 500 cases per phase would be adequate. The STREAMINIT 

and RAND procedures in SAS [Wicklin, 2013] were used to generate samples of 530 random 

cases from each of four normal distributions, respectively, with the specifications listed in Table 

B1. The 30 additional cases were generated to allow for exclusion of the small proportion of 

cases that fell below or above the minimum time or maximum time specified. It is known that 

normal distributions encompass extreme values although such values rarely occur.
64 

As the next step, the random values generated by SAS for each sample were written to Excel 

files. Public domain software accessed through the Research Randomizer Web page44

44 The Research Randomizer is a computer software package (version 4.0) developed by Geoffrey C. 

Urbaniak and Scott Plous. Retrieved on July 19, 2013, from http://www.randomizer.org/about.htm.

was used 

to determine which observations from each sample to exclude in order to arrive at the target 

sample size of 500. Observations were identified by the sequence in which they were 

generated—e.g., 1 for the first observation, 2 for the second observation, etc. The four columns 

of data (observations 1–500) from the four data files (a file for each of the four phases) were 

combined into a single master data file. Table B2 shows the first 12 cases from the master data 

file as an illustration. The four values in each row of data were summed to compute a total time 

requirement.

Table B2. First 12 cases in master data file that correspond to data from Table B1* 

*Rounded to 2 significant figures.

Seq SCSRdon Trvl/Dec SCSRchgovr RAdeploy

1 3.81 min 11.41 min 3.87 min 3.39 min

2 1.96 min 6.30 min 3.48 min 4.51 min

3 1.94 min 5.07 min 3.31 min 4.01 min

4 1.25 min 10.51 min 3.13 min 3.71 min

5 3.57 min 14.66 min 2.44 min 2.58 min

6 1.54 min 11.88 min 2.76 min 3.35 min

7 3.26 min 10.46 min 3.63 min 2.33 min

8 2.35 min 10.19 min 2.70 min 4.78 min

9 3.42 min 9.00 min 3.36 min 4.40 min

10 2.94 min 8.63 min 2.70 min 2.65 min

11 1.17 min 5.20 min 3.02 min 3.42 min

12 2.93 min 8.31 min 3.48 min 4.82 min

 
65 

Results

Descriptive statistics on the simulated data values (time in minutes) for each of the four 

phases are shown in Table B3 below. It can be seen in Table B3 that the means, minimum 

values, and maximum values are very close to the specified values shown in Table B1. The 

standard deviations for all phases are smaller than the specified value, but this can be explained 

by the exclusion of extreme values.

Descriptive statistics for total time are shown in Table B4. Table B5 presents a frequency 

table for total time rounded to the nearest minute, and Figure B1 displays the frequencies in a 

histogram.

As shown in Table B4, the average observed value for total time was approximately 19.92 

minutes, and the maximum observed value for total time was 26.49 minutes. Review of the 

“Cumulative Percent” column in Table B5 shows that 99.6% of the values were 25 minutes or 

less, and 100% of the values were 26 minutes or less.

Table B3. Descriptive statistics on simulated values for

four phases required to travel to and access an RA

Statistic SCSR 

donning

Face travel 

time

SCSR 

changeover 

RA 

deployment

Mean 3.00 min 9.93 min 3.51 min 3.49 min

Lower bound, 

95% CI for mean 

2.93 min 9.74 min 3.45 min 3.43 min

Upper bound, 

95% CI for mean

3.08 min 10.12 min 3.56 min 3.55 min

5% trimmed mean 3.00 min 9.93 min 3.51 min 3.49 min

Median 2.96 min 9.92 min 3.50 min 3.45 min

Variance 0.72 min 4.80 min 0.43 min 0.47 min

Std. deviation 0.85 min 2.19 min 0.66 min 0.69 min

Minimum 1.06 min 5.02 min 2.06 min 2.01 min

Maximum 4.98 min 14.66 min 4.93 min 4.99 min

Range 3.91 min 9.65 min 2.87 min 2.98 min

Interquartile range 1.22 min 3.26 min 0.96 min 0.99 min

Skewness 0.01 0.02 -0.06 0.05 

Kurtosis -0.55 -0.66 -0.66 -0.66 
66 

Table B4. Descriptive statistics on simulated values for

total time to travel to and access an RA

Mean 19.92 min

Lower bound, 95% CI for mean 19.70 min

Upper bound, 95% CI for mean 20.15 min

5% trimmed mean 19.93 min

Median 19.85 min

Variance 6.41 min

Std. deviation 2.53 min

Minimum 13.53 min

Maximum 26.49 min

Range 12.96 min

Interquartile range 3.69 min

Skewness -0.03 

Kurtosis -0.57 
67 

Table B5. Frequency distribution for total time to travel to and access an RA* 

*Rounded to the nearest minute. 

Time

(min)

Frequency Percentage 

(%)

Cumulative 

Percentage 

(%)

14 9 1.8 1.8 

15 7 1.4 3.2 

16 29 5.8 9.0 

17 46 9.2 18.2 

18 56 11.2 29.4 

19 76 15.2 44.6 

20 64 12.8 57.4 

21 66 13.2 70.6 

22 59 11.8 82.4 

23 44 8.8 91.2 

24 30 6.0 97.2 

25 12 2.4 99.6 

26 2 0.4 100.0 
68 

Figure B1. Histogram of simulated values for total time to travel to and

access an RA (rounded to the nearest minute).

0

10

20

30

40

50

60

70

80

14 15 16 17 18 19 20 21 22 23 24 25 26

Frequency

Time (min)

To validate the results described above, the simulation was repeated with different starting 

values for random number generation. Results were similar to those for the original simulation. 

For example, the mean total time was 20.15 minutes in the second simulation compared to19.92 

minutes in the first simulation. In the dataset generated by the second simulation, 97.8% of the 

values were less than 25 minutes, and 99.6% of the values were less than 26 minutes. For easy 

comparison, in Figure B2, a histogram for the second simulation is juxtaposed to the histogram 

for the first simulation.
69 

Figure B2. Comparison of frequency distributions for total time to travel to

and access an RA in first and second simulation runs. 

0

10

20

30

40

50

60

70

80

13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

Frequency

Time (min)

1st run

2nd run

To achieve a larger sample size, and hence a more precise estimate of the population 

proportion, the two simulated datasets of 500 cases each were combined into a single dataset of 

1,000 cases, and a one-sided 95% confidence interval for the population proportion was 

constructed. If the process of simulating 1,000 cases were repeated an infinite number of times, 

there is a 95% probability that at least 99.6% of the values would be less than or equal to 26 

minutes. Therefore, it is highly unlikely that a miner would require more than 26 minutes to 

access the refuge alternative.

71 

Appendix C—Design Example for a 15-psi RA Stopping Using RebarReinforced Concrete

The following design example for an RA stopping using rebar-reinforced concrete follows 

recognized protective structure design principles in the Unified Facilities Criteria (UFC): 

Structures to Resist the Effects of Accidental Explosions [UFC 3-340-02 2008]. The design 

method follows three general steps: (1) design inputs where the design load and material 

properties are specified; (2) foundation design where an anchorage system for the stopping 

structure is designed; and (3) design of the stopping structure itself. 

Design Inputs

Specify design load

Peak pressure—P = 15 psi 

Rise time—0.1 sec 

Fall time—0.1 sec 

Duration—T = 0.2 sec 

Specify allowable failure

The design must remain linear elastic—“elasticity of design.” No failure or permanent 

deformation of the RA stopping structure or the RA stopping foundation is allowed. 

Specify safety factor

Φp = 1.0 

The adjusted peak dynamic pressure is 

Pd′ = Pd ∙ φp = 15 ∙ 1.0 = 15 psi (1) 

Determine dynamic load factor (DLF)

The rise time, Tr, of the design load is 100 ms. The natural period, TN, of the RA stopping 

structure is unknown currently, but will likely range from 1 to 10 ms. Because the rise time is 

likely to be much greater than the natural period, the ratio Tr/TN is also always greater than 10. 

From Figure 3-51 [UFC 3-340-02 2008, page 619], the dynamic load factor is 1.0. The 

equivalent static pressure is

Ps = Pd′ ∙ DLF = 15 ∙ 1 = 15 psi (2) 

Specify coal mine entry geometry

Height—H = 96 in (8 ft) 

Length—L = 240 in (20 ft)
72 

Determine support conditions

The RA stopping is simply supported on four sides. It can resist translation, but it cannot 

resist moment at the supports. The foundation rocks surrounding an RA stopping are invariably

weak or low strength and unlikely to provide any rotational (moment) resistance to the RA 

stopping structure. This simple support condition on all four sides is sometimes designated 

(SSSS). The RA stopping wall is a two-way wall, meaning that it can resist bending in both 

directions—rib-to-rib and roof-to-floor.

Specify material properties

The rebar-reinforced concrete RA stopping will use ordinary concrete, steel reinforcement 

bar, and steel rock bolt anchors. 

Steel yield strength—fy = 60,000 psi 

Concrete compressive strength—f’c = 3,000 psi 

Determine dynamic increase factor

The steel and concrete strengths are adjusted upwards to account for the small increase in 

strength when dynamic loads are applied (using Table 4-1 [UFC 3-340-02 2008, page 1,068]). 

Type of stress Rebar Concrete

Bending fdy.m = 1.17* 60,000 = 70,200 psi f’dc.m = 1.19* 3,000 = 3,570 psi

Direct shear fdv.s = 1.10* 60,000 = 66,000 psi f’dc.v = 1.10* 3,000 = 3,300 psi

Compression fdy.c = 1.10* 60,000 = 66,000 psi f’dc.c = 1.12* 3,000 = 3,360 psi

Foundation Design (RA Stopping Anchorage)

Determine approximate yield line location

The UFC uses yield line theory to calculate maximum shear and moment in one- and twoway walls that are either free, simple, or fixed supports on any of the four sides. For two-way 

walls with four edges supported, UFC 3-340-02 [2008] provides relations for the ultimate 

resistance of the wall in terms of its geometry and moment capacity [UFC 3-340-02 2008, page 

574]. These equations can be combined and solved for an estimate of the yield line location x*

. 

An estimate of the horizontal location of the yield line is the real root of the following cubic 

equation: 

0 = 2 ∙ x∗3 − 6 ∙ L ∙ x∗2 − 5 ∙ H2 ∙ x∗ + 15

4 ∙ H2 ∙ L (3) 

15 0 = 2 ∙ x∗3 − 6 ∙ 240 ∙ x∗2 − 5 ∙ 962 ∙ x∗ + ∙ 962 ∙ 240 (4) 4

x∗ = 63.861 in (5) 
73 

Determine approximate shear forces

The approximate shear forces per foot along the perimeter at the roof, floor, and ribs of the 

RA stopping are found using Table 3-11 [UFC 3-340-02 2008, page 582]. 

VsVr∗ 3∙Ps∙y∙(2∙L−2∙x∗)∙12

(6∙L−2∙x∗)

3∙15∙�96

2� �∙(2∙240−2∙63.861)∙12

(6∙240−2∙63.861) = = = 6.958 kips/foot (6) 

 

VsVf∗ = 3∙Ps∙(H−y)∙(2∙L−2∙x∗)∙12

(6∙L−2∙x∗) = 3∙15∙�96−96

2� �∙(2∙240−2∙63.861)∙(12)

(6∙240−2∙63.861) = 6.958 kips/foot (7) 

VsH∗ = 3∙Ps∙x∗∙12

5 = 3∙15∙63.861∙125

= 6.897 kips/foot (8) 

The maximum shear force per foot is

maximum shear Vu

∗ = max{VsVf∗

, VsVf∗, VsH∗

} = 6.958 kips/foot (9) 

As a check, the average shear force per foot is found by dividing the total applied explosion 

force by the RA stopping perimeter.

Vave = 

Ps∙L∙H

(2∙L+2∙H) = 

15∙240∙96

(2∙20+2∙8) = 6.171 kips/foot (10) 

This simple average shear force method usually underestimates the maximum shear force 

around the perimeter by about 10% to 15% depending on the wall geometry. 

Determine number of rock bolt anchors 

The shear strength of steel is usually taken as 60% of its yield strength. The shear strength of 

a single rock bolt anchor is 

Fnv = μRB ∙ fyRB = 0.6 ∙ 60,000 = 36,000 psi (11) 

Choose rock bolt anchor diameter = 5/8 in = #5 bar and spacing SRB = 12 in. The area of a 

rock bolt anchor is 

Aθd = π∙θd

2

4 = π∙�5

8� �

2

4

= 0.307 in.

2 (12) 

The approximate number of rock bolt anchors per foot of wall is 

Nθd∗ = Vu

∗

(Fnv∙Aθd) = 6,958

(36,000∙0.307) = 0.630 (13) 

Space the rock bolt anchors no more than 12/0.630 = 19.1 in apart around the RA stopping 

perimeter. For simplicity, space the rock bolt anchors every 1.5 ft around the perimeter, 

including the roof, floor, and ribs. Use 5/8-in (#5 bar) 60 ksi steel. 
74 

RA Stopping Structure Design

Estimate reinforcing steel requirements

The horizontal and vertical positive moment capacities are [UFC 3-340-02 2008, page 574] 

MVP

∗ = MHP∗ = Ps∙x∗2

5 = 15∙63.86125 = 12,235 lb−in

in (14) 

For an initial trial design, assume that the RA stopping is 12 in thick. The reinforcement must 

have at least 2 in of concrete cover. The approximate distances of the extreme compression fiber 

to the centroid of tension reinforcement are 

dV

∗ = Tc

∗ − 2.5 in = 12 − 2.5 = 9.50 in (15) 

dH

∗ = Tc

∗ − 3.5 in = 12 − 3.5 = 8.50 in (16) 

The approximate area of vertical tension steel reinforcement per foot of section is determined 

by solving the ultimate moment capacity equations iteratively [UFC 3-340-02 2008, page 1076]. 

The minimum area for flexural reinforcement is [UFC 3-340-02 2008, page 1092] 

As = 1.875 ∙

�fc

′

fy

∙ b ∙ d = 1.875 ∙

√3,000

60,000 ∙ 12 ∙ 9.5 = 0.195 in2 (17) 

Assume ρV = 0.00200 

AVs

∗ = 

MVP

∗

(fdy∙dV

∗ ) �1−0.59∙ρV∙

fdy

f

dc

′ �

= 12,2

(70,200∙9.5) �1−0.59∙0.00200∙

70,200

3,570 �

= 0.225 in2 > 0.195 in2 OK (18) ∙b ∙ 1 35∙12 ∙ 1

ρV = 

AVs

∗

(b∙dV

∗ ) = 

0.225

(12∙9.5) = 0.00197 ≈ 0.00200 (19) 

The approximate area of horizontal tension steel reinforcement per foot of section is also 

determined iteratively.

Assume ρH = 0.00250 

AHs

∗ = 

MHP

∗ ∙b

(fdy∙dH

∗ ) ∙ 1

�1−0.59∙ρH∙

fdy

f

dc

′ �

= 12,235∙12

(70,200∙8.5) ∙ 1

�1−0.59∙0.0025070,200

3,570 �

= 0.253 in2 > 0.195 in2 OK (20) 

ρH = 

AHs

∗

(b∙dH

∗ ) = 

0.252

(12∙8.5) = 0.00248 ≈ 0.00250 (21) 

The computed areas of vertical and horizontal reinforcement are per linear foot of wall. 

Number 5 bar reinforcement (5/8-in-diam) has an area of 0.307 in2. 

Specify RA stopping wall thickness of 12 in. Specify #5 reinforcement bars (0.625-in-diam) 

for vertical and horizontal reinforcement on 12-in centers on both sides of the RA stopping wall. 
75 

Determine diagonal shear reinforcement 

The shear strength of unreinforced concrete is [UFC 3-340-02 2008, page 1079, or ACI 318-

08 2008, Section 11.11.3.1] 

vc = 2 ∙ �f′

dc = 2 ∙ √3,300 = 115 psi (22) 

vc = [1.9 ∙ �f′

dc + 2,500 ∙ ρ] ≤ 3.5 ∙ �f′dc = [1.9 ∙ √3,300 + 2,500 ∙ 0.002] = 114 psi ≤ 3.5 ∙

√3,300 = 201 psi (23) 

Choose vc = 114 psi. The shear resistance of unreinforced concrete per foot of wall is 

Vc = 12 ∙ vc ∙ Tc

∗ = 12 ∙ 114 ∙ 12 = 16,416 pounds per foot = 16.416 kips per foot (24) 

From a prior calculation, the maximum shear force per foot in the concrete is Vu* = 6.958 

kips / foot. 

Since Vu* is less than Vc, no additional shear load must be resisted by shear reinforcement. 

However, the UFC specifies minimum shear reinforcement for slabs as “at least one stirrup must 

be located at each bar intersection” [UFC 3-340-02 2008, page 1082]. Therefore, use minimum 

shear reinforcement from the front to back side of the RA stopping wall at each horizontal and 

vertical reinforcement bar intersection.

Specify # 4 reinforcement bar stirrups (0.500-in-diam) on 1-ft centers.

Determine actual vertical moment capacity

The actual area of vertical tension reinforcement steel per foot of wall is 0.307 in2 provided 

by 1 number 5 bar reinforcement (5/8-in-diam) on 12-in centers. The actual distance from the 

extreme compression fiber to the centroid of the vertical tension reinforcement is

dV = Tc − �CCw + φsw + φVw

2 � = 12 − �1.5 + 0.5 + 0.625

2 � = 9.69 in (25) 

The actual reinforcement ratio for vertical tension reinforcement steel is

ρV = AVs

(b∙dV) = 0.307

(12∙9.69) = 0.00264 (26) 

To ensure against sudden compression failures, the maximum reinforcement ratio ρ must not 

exceed 0.75 of the ratio ρb which produces balanced conditions at ultimate strength [UFC 3-340-

02 2008, page 1077]. 

The maximum allowable reinforcement ratio ρb is

K1 = 0.85 − 0.05 ∙ �

(f

′

dc−4,000)

1,000 � = 0.85 − 0.05 ∙ �

(3,750−4,000)

1,000 � = 0.8625 (27) 

ρb = �0.85 ∙ K1 ∙

f′dc

fdy � ∙ � 87,000

�87,000+fdy�

� = �0.85 ∙ 0.8625 ∙ 3,570

70,200� ∙ � 87,000

(87,000+70,200)

� = 0.02063 > 0.00264

 (28) 

The minimum vertical reinforcement ratio ρs.minV is [UFC 3-340-02 2008, page 1091] 

ρs.min.V = 1.25∙�f′dc

fdy

= 1.25∙√3,570

70,200

= 0.00106 < 0.00264 OK (29) 
76 

The actual vertical reinforcement ratio meets the minimum and maximum reinforcement 

ratio conditions. 

ρs.min.V = 0.00106 ≤ ρV = 0.00264 ≤ ρb = 0.02086 (30) 

The vertical ultimate moment capacity per foot of wall is [UFC 3-340-02 2008, page 1076] 

aV = (AVs∙fdy)

(0.85∙b∙f′

dc) = (0.307∙70,200)

(0.85∙12∙3,570) = 0.59184 in (31) 

MVu = �AVs ∙

fdy

b � ∙ �dV − aV2 � = �0.307 ∙

70,200

12 � ∙ �9.69 − 0.591842 � = 16,871 lb−inin (32) 

Determine actual horizontal moment capacity

The actual area of horizontal tension reinforcement steel per foot of wall is 0.307 in2

provided by 1 number 5 bar reinforcement (5/8-in-diam) on 12-in centers. The actual distance 

from the extreme compression fiber to the centroid of the horizontal tension reinforcement is 

dH = Tc − �CCw + φsw + φVw + φHw

2 � = 12 − �1.5 + 0.50 + 0.625 + 0.625

2 � = 9.06 in (33) 

The actual reinforcement ratio for horizontal tension reinforcement steel is

ρH = AHs

(b∙dH) = 0.307

(12∙9.06) = 0.00282 (34) 

The maximum allowable reinforcement ratio ρb is

K1 = 0.85 − 0.05 ∙ �

(f

′

dc−4,000)

1,000 � = 0.85 − 0.05 ∙ �

(3,750−4,000)

1,000 � = 0.8625

(35) 

ρb = �0.85 ∙ K1 ∙

f′dc

fdy � ∙ � 87,000

�87,000+fdy�

� = �0.85 ∙ 0.8625 ∙ 3,570

70,200� ∙ � 87,000

(87,000+70,200)

� = 0.02063 >

0.00282 OK (36) 

The minimum horizontal reinforcement ratio ρs.minH is

ρs.min.H = 1.25∙�f′dc

fdy

= 1.25∙√3,570

70,200

= 0.00106 < 0.00282 OK (37) 

The actual horizontal reinforcement ratio meets the minimum and maximum reinforcement 

ratio conditions. 

ρs.min.H = 0.00106 ≤ ρH = 0.00282 ≤ ρb = 0.02086 (38) 

The horizontal ultimate moment capacity per foot of wall is [UFC 3-340-02 2008, page 

1076] 

aH = (AHs∙fdy)

(0.85∙b∙f′

dc) = (0.307∙70,200)

(0.85∙12∙3,570) = 0.59184 in (39) 

MHu = �AHs ∙

fdy

b � ∙ �dH − aH2 � = �0.307 ∙

70,200

12 � ∙ �9.06 − 0.591842 � = 15,740 lb−inin (40) 
77 

Determine static properties of the design

Specify modulus of elasticity and Poison’s ratio for steel as 29,000,000 psi and 0.30. 

The modulus of elasticity for concrete is [UFC 3-340-02 2008, page 1070] 

Ec = wc

1.5 ∙ 33 ∙ (fc′

)0.5 = 1501.5 ∙ 33 ∙ (3,000)0.5 = 3,321,000 psi (41) 

The modular ratio is [UFC 3-340-02 2008, page 1070] 

n =

Es

Ec

= 29,000,000

3,321,000

= 8.732 (42) 

The average reinforcement ratio is

ρ = (ρV+ρH)

2 = (0.00264+0.00282)2

= 0.00273 (43) 

The moment of inertia for a slab of unit width is [UFC 3-340-02 2008, page 1071] 

Ig = Tc

3

12 = 12312 = 144 in4 (44) 

Using Figure 4-12 of the UFC with ρ = 0.00273 and n = 8.732, the cracked concrete

coefficient (F) is determined [UFC 3-340-02 2008, page 1074]. 

F = 0.015 

The cracked moment of inertia is [UFC 3-340-02 2008, page 1071] 

Ic = F ∙ �

(dV+dH)

2 �

3

= 0.015 ∙ �

(9.69+9.06)

2 �

3

= 12.36 in4 (45) 

The averaged moment of inertia is 

Ia = (Ig+Ic)

2 = (144+12.36)

2 = 78.18 in4 (46) 

The flexural rigidity of the slab is [UFC 3-340-02 2008, page 528] 

D = (Ec∙Ia)

(1−ν2) = (3,321,000∙78.18)(1−0.172) = 2.674x108 lb − in (47) 

The height-to-length ratio of the RA stopping structure is

H

L = 96240

= 0.400 (48) 

Using Figure 3-36 of UFC, the moment and deflection coefficients, β1V, β1H and γ1, for a 

two-way wall with all edges simply supported are [UFC 3-340-02 2008, page 566] 

Β1V = 0.12 

β1H = 0.043

γ1 = 0.012 
78 

Determine actual yield line location and ultimate resistance

The actual moment capacities found earlier are MVp = 26,882 lb-in and MHp = 25,615 lb-in. 

The following equations are solved simultaneously for the ultimate resistance ru and the actual 

yield line location x [UFC 3-340-02 2008, page 574]. 

ru = �MVp∙(6∙L−2∙x)�

��H

2�

2

∙(3∙L−4∙x)�

 (49) 

ru = 5∙MHp

x2 (50) 

The system of equations reduces to

0 = 2 ∙ MVp ∙ x3 − 6 ∙ L ∙ MVp ∙ x2 − 5 ∙ H2 ∙ MHp ∙ x + 15

4 ∙ H2 ∙ L ∙ MHp (51) 

0 = 2 ∙ 16,871 ∙ x3 − 6 ∙ 240 ∙ 16,871 ∙ x2 − 5 ∙ 962 ∙ 15,739 ∙ x + 15

4 ∙ 962 ∙ 240 ∙ 15,739 (52) 

x = 62.070 in ≈ 63.861 in (53) 

The ultimate elastic resistance of the RA stopping structure is

ru = �MVp∙(6∙L−2∙x)�

��H

2�

2

∙(3∙L−4∙x)�

= [16,871∙(6∙240−2∙62.070)]

��96

2 �

2

∙(3∙240−4∙62.070)�

= 20.43 psi (54) 

ru = 5∙MHp

x2 = 5∙15,739

62.0702 = 20.43 psi (55) 

The elastic deflection of the RS stopping structure at ultimate elastic resistance is [UFC 3-

340-02 2008, page 528] 

Xe = γ1∙ru∙H4

D = 0.012∙20.43∙9642.674x108 = 0.078 in (56) 

The elastic stiffness of the RA stopping structure is [UFC 3-340-02 2008, page 529] 

KE = ru

Xe

= 20.43

0.078 = 261.9 psiin (57) 

Determine direct shear capacity of concrete

The direct shear capacity of the concrete near the supports is [UFC 3-340-02 2008, page 

1083] 

Vd = 0.16 ∙ f′

dc ∙ b ∙ dH = 0.16 ∙ 3,300 ∙ 12 ∙ 9.06 = 57,404 lbs/foot of wall (58) 

The actual shear forces per foot of wall at the roof, floor and ribs are [UFC 3-340-02 2008, 

page 582] 

VsVr = 3∙Ps∙y∙(2∙L−2∙x)∙12

(6∙L−2∙x) = 3∙15∙96/2∙(2∙240−2∙62.070)∙12(6∙240−2∙62.070) = 7,010 lbs/foot of wall (59) 

VsVf = 3∙Ps∙(H−y)∙(2∙L−2∙x)∙12

(6∙L−2∙x) = 3∙15∙(96−96/2)∙(2∙240−2∙62.070)∙12(6∙240−2∙62.070) = 7,010 lbs/foot of wall (60) 
79 

VsH = 3∙Ps∙x∙12

5 = 3∙15∙62.070∙125 = 6,704 lbs/foot of wall (61) 

The maximum shear force per foot of wall is 

Vu = max{VsVr, VsVf, VsH} = 7,010 lbs/foot of wall (62) 

The direct shear capacity of the concrete is greater than the shear forces in the concrete.

Vd = 57,404 ≥ Vu = 7,010 (63) 

Determine dynamic response of the RA stopping structure

The maximum dynamic displacement of the RA stopping structure and its natural period of 

vibration are determined based on the RA stopping geometry, its properties, and its ultimate 

resistance. The elastic load mass factor, KLM_elastic, is determined using Table 3-13 of UFC [UFC 

3-340-02 2008, page 595] and the RA stopping dimensions L and H. For all RA stopping 

structures, the four sides are simply supported. 

The length-to-height ratio of the RA stopping structure is

L

H = 24096

= 2.500 (64) 

From Table 3-13 of UFC, the elastic load mass factor for a two-way wall with all sides 

simply supported is [UFC 3-340-02 2008, page 595] 

KLM_e = 0.79 

The yield line location to length ratio of the RA stopping structure is

x

L = 62.070240

= 0.259 (65) 

From Figure 3-44 of UFC, the plastic load mass factor for a two-way wall with all sides 

simply supported is [UFC 3-340-02 2008, page 593] 

KLM_p = 0.595 

The effective load mass factor is the average of the elastic and plastic load factors.

KLM = �KLMe+KLMp�

2 = (0.79+0.595)

2

= 0.693 (66) 

The effective unit mass is

me = ρc∙Tc

g ∙ KLM = 150 lb

ft3∙� 1 ft

12 in�

3

∙12 in

32.2 ft

sec2∙

12 in

1 ft ∙� 1 sec1000 ms�

2 ∙ 0.693 = 1,868.2 psi−ms2

in (67) 

The natural period of vibration is [UFC 3-340-02 2008, page 591] 

TN = 2 ∙ π ∙ �me

KE

�

0.5

= 2 ∙ π ∙ �1,868.2

261.9 �

0.5

= 16.8 ms (68) 

Response chart parameters are calculated as follows.

ru

Pd′

= 20.43 psi

15 psi = 1.362 and Tr

TN

= 100 ms

16.8 ms = 5.95 (69) 
80 

From Figure 3-58 of UFC, the ratio Xm/Xe is found as 0.75 [UFC 3-340-02 2008, page 626]. 

The maximum dynamic displacement is

xm = xe ∙

Xm

Xe

= 0.078 ∙ 0.75 = 0.059 in (70) 

The maximum dynamic displacement is less than or equal to the maximum elastic 

displacement, and therefore, the design is elastic.
 

 

 

 

 

 

 

 

 

 

 

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