STRESS MEASUREMENTS FOR SAFETY DECISIONS IN LONGWALL COAL 

E. G. Zahl, J. P. Dunford, M. K. Larson, and T.M. Brady,
 

Spokane Research Laboratory, National Institute for Occupational Safety and Health, Spokane, WA
 

and
 

J. Chen
 

RAG American Coal Co., Waynesburg, PA
 
ABSTRACT 

Researchers at the Spokane Research Laboratory of the National 

Institute for Occupational Safety and Health, Spokane, WA, have 

collaborated with three Western longwall coal mines in an ongoing 

effort to develop technologies that will aid in providing safe and 

stable working areas. The goal of the research described here is to 

develop a stress monitoring system that will provide immediate 

information to mine managers for making daily safety decisions as 

areas of poor ground are mined through. 

Initial work has focused on answering preliminary questions 

regarding the reliability and use of stress change patterns. Research 

is concentrated on monitoring horizontal stress because horizontal 

stress is transmitted over long distances through stiff strata, thus 

allowing an extended length of entry to be monitored. 

This paper presents an explanation of the concept, key results 

from field tests at two mine sites, and a proposed process for implementing a monitoring system. System design layouts, instrument 

use, data collection and interpretation methods, and processes to 

present findings to mine staff are described. Additional validation 

and correlation with actual failure mechanisms are required before 

this approach can be recommended at ongoing operations. However, initial results indicate that this approach shows promise in 

mines prone to bumps and roof falls associated with large stress 

changes. In particular, analyses of measured changes in horizontal 

stresses appear useful. 

BACKGROUND 

Researchers from Spokane Research Laboratory (SRL) of the 

National Institute for Occupational Safety and Health (NIOSH), 

Spokane, WA, have been developing an approach to better assess 

the stability of underground openings and reduce the risk of ground 

falls to miners. Information from the accident database compiled 

by the Mine Safety and Health Administration (MSHA) (figure 1) 

reveals that for the 5-year period of 1996 to 2000, ground falls 

caused 48% of the 143 fatalities in underground mines. 

Figure 1.—Distribution of fatalities by accident class, 

underground mining 1996-2000 (MSHA database). 

Injuries 

from falls of ground for the same period accounted for 16% of the 

22,437 lost-time injuries. 

It is well understood that underground mining conditions will 

become more difficult and will present a greater risk to workers as 

companies are forced to mine at greater depths in less stable ground. 

Mines that now operate in good conditions are likely to encounter 

areas where roof conditions are poor and be faced with additional 

risk that cannot be explained by geology or calculated stress 

conditions. To reduce the risk to longwall miners, SRL researchers 

are developing a system that will continuously monitor stress 

changes in work areas as mining progresses. The system is being 

designed to provide mine personnel with additional information for 

making safety decisions, particularly when mining through difficult 

ground control conditions. 

MONITORING SYSTEM CONCEPT 

The long-term goal of this research is to develop monitoring 

systems to measure changes in stress and displacement in the 

surrounding rock to acquire information on the stability of high-use 

work areas near the longwall face. Data trends from these systems 

can then be used to quantify the overall level and nature of stress 

increases to determine if preventative safety interventions are 

warranted as mining progresses and to better understand stress 

change dynamics during mining. 
Development of the concept described in this paper was 

inspired by advances in measurement technologies not previously 

exploited for use in coal, notably the biaxial stressmeter (1-3) and 

increased understanding of the role that horizontal stress plays in 

coal mine stability (4-6). Based on these developments and the 

continued need for safety improvements, researchers at SRL are 

developing stress monitoring systems (figure 2) that utilize 

measurements of both displacement and stress changes in the roof, 

rib, and floor along entries near an active panel. 

Figure 2.—Cutaway view of longwall panel and gate roads illustrating the concept of 

measuring stress change and displacement in the roof and rib ahead of mining 

These systems are 

designed to collect data from an array of instruments during mining 

operations. The data are then plotted and, along with underground 

observations and information from other data collection systems, 

made available for daily safety decisions. Results may also help in 

the interpretation of large seismic events and gas outbursts. 

Researchers are developing ways to capitalize on previous 

experience with biaxial stressmeters (BSM’s). Such experience 

suggests that mining-induced stress changes may be detected over 

500 m (1650 ft) away (7-8). A key element is the measurement of 

horizontal stress changes using BSM’s installed in the stiffest rock 

horizons above or below the coal seam. Experience has shown that 

horizontal stress changes are preferentially carried in zones with the 

highest modulus of elasticity (9). Plots of stress-versus-time from 

these horizons are analyzed and compared to actual events to 

determine if recognizable patterns of stress change exist that may 

be indirect indicators of roof and rib failure. The idea is that rapid 

and/or large changes in horizontal stress have a cause-and-effect 

relationship with failures. 

Once researchers establish that these monitoring systems can 

yield recognizable patterns, the goal will be to determine if this 

approach is reliable enough to assist in making decisions to take 

preventative action. To increase reliability, measurements from 

other instruments in the array and observations from miners are 

used to determine the source of anomalous BSM patterns. Analysts 

need to be able to identify patterns that mimic those indicating an 

increased risk of failure, but that are actually caused by events not 

affecting safety (e.g., normal pillar yield, roof sag near instruments, 

or electronic interference from repositioned transformers). 

While evaluating these stress change plots, it must be 

understood that the values of p, q, and 2 are not absolute, but are 

measured from a zero datum that was established when the BSM 

came to equilibrium after it was installed in a pre-existing stress 

field. BSM stresses are measured in the horizontal plane; p is the 

change in major principal stress; q is the change in minor principal 

stress; and 2 is the angle measured from the line of longwall 

advance counterclockwise to the direction of p (in plan view). 

It was unreasonable to expect to find a mining operation in 

which the number of roof and rib failures were sufficient to test this 

concept. Hence, researchers tested the concept in operations in 

which conditions and events would produce large and/or rapid 

changes in stress, even if such changes created little safety risk. 

With this approach we could (1) determine if the patterns of stress 

changes associated with these major events had characteristics 

necessary for use as indicators, (2) evaluate the performance of 

instruments and data collection systems, adapting them as needed, 

and (3) develop effective ways of integrating the whole process into 

daily longwall operations. 

Development of this system has made it clear that using these 

instruments for this application is significantly different from their 

traditional use in studies to better understand rock mechanics 

principles. Designing the system to conform with the rigors of daily 

operations has required researchers to develop a monitoring system 

configuration and information delivery process at the same time the 

capabilities of these new systems are being explored. The 
expectations of what these types of monitoring systems can offer 

has changed as research progressed, but is based on the following 

design requirements. 

1. Results should provide useful information to assist in making 

safety decisions. Types of decisions could include adding support, 

restricting access to specific areas during high-risk times, destressing critical areas, altering face advance rate, and evaluating 

support types and configurations and effects of panel orientation. 

2. The data delivery process should allow time for mine foremen 

to implement preventative actions. 

3. Installation and operation should cause a minimum disturbance 

to coal production processes. 

4. The system should be robust with built-in redundancy. 

5. Simplicity and reliability of results should make it cost 

effective. 

6. Mine coordinate system, time reference, and software formats 

should allow engineers, geologists, and foremen to compare results 

with data from other operational systems at the mine. 

FIELD TESTING 

Initial development of a stress monitoring system (8) was conducted at a longwall operation in mine A (figure 3). 

Figure 3.—Plan view of monitoring system with instrument layout with BSM 3, mine A 

Subsequent field 

tests were conducted at two other Western longwall operations 

(mine B and mine C). At mine C, researchers tested and evaluated 

grouting and placement methods for installing BSM’s; however, no 

installations were deemed fit for collecting data, and results are not 

presented here. 

Mine A 

The coal seam being mined was contained in a 200-m- (660-ft-) 

thick Cretaceous formation consisting of a heterogeneous sequence 

of thin, lenticular mudstones, siltstones, sandstones, shales, and 

numerous other subeconomic coal seams. Ground control was a 

primary concern because of the history of coal bumps in the mining 

district, a very stiff upper stratum, and very steep topography with 

overburden varying from 450 to 900 m (1500 to 3000 ft). Coal was 

mined in a two-entry configuration where panels were oriented to 

minimize ground control failures from high horizontal stresses. 

Upper strata contained a very strong, 180- to 200-m- (590- to 660­

ft-) thick sandstone formation that created a condition where caving 

behind the face shields was significantly delayed. Because caving 

was incomplete, overburden loads normally transferred through the 

gob into the floor were transferred onto adjacent abutments and forward into rock surrounding active work areas. Mine planners were 

concerned that these stress buildups could not be relieved safely and 

could possibly result in violent coal bumps that could kill workers 

or trigger the release of large 

volumes of explosive methane gases. 

A stress monitoring system was 

designed and installed where instruments were clustered in two panels, 

a yield pillar, and the immediate roof 

and floor (figure 3). Data were 

gathered continuously for 6 months 

from a variety of stress- and displacement-measuring instruments as 

longwall mining proceeded from the 

start-up room 700 m (2300 ft) away 

and moved toward and past the instrument site. With the aid of mine 

staff, researchers started developing 

methods to integrate the whole process into daily longwall operations. 

Following are activities that were 

done in that context. 

• Developed a method for placing 

BSM’s into a roof for measuring 

horizontal stresses. 

• Tested and evaluated instruments 

and placement strategies in relation 

to the operating face. 

• Performed numerical analyses to 

evaluate placement strategies. 

• Acquired geotechnical information (e.g., rock stiffness, in situ stress 

measurements). 

• Tested and evaluated methods to 

interpret and disseminate results 

among mine staff. 
 During the initial tests, researchers wanted to determine if 

stress change patterns emerged that could be useful in decision 

making. Some form of stress change anomoly was anticipated surrounding events. Hence researchers used the monitoring system to 

determine if magnitudes and profiles were unique enough to warrant 

further development and if stress change patterns could be associated with actual events. To accomplish this, plots of stress-versustime were evaluated after the fact by comparing them with events 

noted by researchers and recorded in a foreman’s log and with a 

NIOSH seismic system operating at the site. Figure 4 shows such 

a plot and illustrates stress change patterns surrounding three events 

(identified at the top of the plot and on Figure 3). 

Figure 4.— Horizontal stress change measured in roof of 

gate roads by BSM 3 during days before and after three 

events, mine A. Symbols indicate lines, not events. 

In the first event, 

note the correspondingly large drop in q as the longwall face passes 

the instrument site. Stress in that direction could have been quickly 

relieved as mining cut through the nearby stress field, but then built 

up again as gob formed. The second event was the roof fall on the 

stage loader corresponding to the dramatic drop in stress during day 

221. This event could have caused a simultaneous decoupling of 

the BSM from the stress field, which would also explain why a 

nearby 4.2-magnitude seismic event (third event) created such a 

small stress change pattern at day 222. 

Note that in this plot, stresses leading up to the first two events 

rose quickly by many megapascals for a few days. Stresses then 

leveled off for a few more days, 

suggesting rock yielding mechanisms 

at work, before a dramatic drop. In 

this field test, the events happened 

when the face was so close to the 

instrument site that local effects were 

added to the stress profile, thus 

complicating the effects of the global 

stress changes the monitoring system 

was targeting. The field test at Mine 

B was designed to eliminate this 

condition. 

Mine B 

Mine B is in a coal seam 3.5 to 4.5 m 

(12 to15 ft) thick. The overburden 

consists of soft carbonaceous mudstones and siltstones. Three-entry gate 

roads are developed to mine coal 

panels under 100 to 120 m (325 to 

400 ft) of overburden in the vicinity of 

the instrument site. The immediate 

floor is composed of mudstones and 

siltstones with a 2- to 3-m- (7- to 10­

ft-) thick sandstone member located 2 

to 3 m (7 to 10 ft) below the coal 

seam. This sandstone was identified 

as the stiffest strata above or below 

the coal seam and was best suited for 

instrumentation. Three BSM’s were 

installed in this formation, and a twopoint sag station was installed in the 

roof directly over each BSM to evaluate local changes affecting stress 

measurements (figure 5). 

Figure 5.— Plan view and vertical section of monitoring system with instrument layout and 

datalogger, mine B.
The monitoring system was designed to evaluate system 

response to a single event, the initial cave. Instruments were placed 

at various distances behind the start-up room to provide redundancy 

as well as evaluate horizontal stress change patterns and the 

effectiveness of instrument placement. A comparison of figures 6, 

7, and 8 shows how stress change patterns from instruments placed 

further within the abutment became attenuated. 

Figure 6.—Horizontal stress change and direction versus 

time as measured by BSM 1, mine B. Theta zero direction is 

direction of longwall advance, and counterclockwise is 

positive in plan view. 

Figure 7.—Horizontal stress change and direction versus 

time as measured by BSM 2, mine B. Theta zero direction is 

direction of longwall advance, and counterclockwise is 

positive in plan view. 

Figure 8.— Horizontal stress change and direction versus 

time as measured by BSM 3, mine B. Theta zero direction is 

direction of longwall advance, and counterclockwise is 

positive in plan view. 

Stress changes at 

the time of the caving event were greatest at BSM 1, which was 15 

m (50 ft) from the start-up room and gob. As expected, the stress 

change dropped off according to how far away the BSM’s were 

installed from the start-up room. (BSM 2 was 45 m [150 ft] and 

BSM 3 was 65 m [210 ft] from the start-up room.) However, the 

stress change was nearly zero at BSM 3. Such readings provide 

researchers with information on the sensitivity of instrument 

placement. It is also notable that the stress change pattern from 

BSM 2 varied significantly from the pattern obtained from BSM 1, 

possibly indicating the complexity of stress change dynamics. 

Analysis of figure 9 (a close-up of data surrounding the firstcave event from BSM 1) provides insight on stress change patterns 

and whether this type of monitoring has any fatal flaws. Most 

significantly, the stress change patterns are similar to those in mine 

A, with the features of having a few days of sharply rising stresses 

followed by a few days of leveling off just prior to the cave event. 

Note that stresses remained somewhat constant for many days while 

mining was progressing up to the headgate position at 28 m (93 ft), 

but changes occurred quickly after that. This type of response 

pattern would be easy to identify before an event. Further evaluation of p, q, and 2 provides quantitative measurements from which 

caving dynamics can be deduced that could be helpful in 

operational safety decisions. Such types of patterns encouraged 

researchers in their belief that measurements can be identifiable and 

can be recorded long enough before an event to allow preventative 

actions to be taken. 

INFORMATION-GATHERING PROCESS 

Implementation of a successful monitoring system will require 

attention to integrating many details into the daily operations at a 

mine. It is anticipated that each monitoring system will vary 
depending on the ground control hazards of a specific operation. 

The details of installing and using a monitoring system will become 

more explicit on further development and experience in different 

mine settings. 

While testing various reporting methods at these mine sites, the 

authors became aware that communicating the basic elements of 

this concept to those who will be using it on a daily basis is critical. 

The following discussion of activities and rationale was developed 

to illustrate a proposed process of instrument installation, data 

delivery, analysis, and decision making. Mine engineers, geologists, 

longwall foremen, and other selected mine staff can use their 

knowledge of site-specific conditions to install and utilize such a 

monitoring system successfully . Nothing is suggested here that has 

not been tested in field trials. In today’s competitive coal markets, 

mining operations are continually testing and implementing improvements. Hence, the process described here is proposed for 

purposes of encouraging discussion for future research and 

application. However, in no way can the authors state that the 

monitoring system has been validated or that it is recommended for 

use at any other longwall operation. 

Figure 9.—Horizontal stress change measured by BSM 1 in 

floor near start-up room of a longwall panel in days before 

and after first major cave of roof, mine B. Symbols identify 

lines, not events. 

Installation 

The first activity is that of identifying a potentially hazardous 

segment of the coal panel. Typically, this would include areas 

where geologic anomalies, ground conditions, or previous experience indicate that it is appropriate to use this type of monitoring 

system. The system monitors stress changes over an interval of a 

few hundred meters of entry, where large stress changes are associated with coal bumps, roof falls, or other safety-related ground 

control problems. 

Once the potentially hazardous area has been identified, a 

monitoring site is established immediately outby that area. This is 

done to take advantage of monitoring the entry when it is used both 

as a headgate and a tailgate. Experience has also shown that stress 

changes are not detected as well in the roof after the face has passed 

the instruments. This is most likely a result of normal caving, 

which can create a free (or strain-relieved) face parallel to the panel 

(along the line of caving, parallel to the gateroad entry) through 

which stress changes are not easily transferred. Results from 

existing or additional physical property tests are used to determine 

which strata in the immediate roof or floor are stiffest and which are 

most continuous between the instrument and the face. 

An instrumentation site is designed with the key being instrument selection and placement. Strata directly above or below the 

coal seam with the highest modulus of elasticity and the greatest 

continuity between the site and the operating face are identified for 

placement of BSM’s. Not having BSM’s in the right zone can 

severely compromise the ability of the system to detect stress 

changes. Numerical modeling can be used to determine zones 

where stress changes are likely to be greatest. Additional tests to 

determine the stiffest zones can be done with a Goodman jack. 

Installing a number of BSM’s about 15 to 45 m (50 to 150 ft) apart 

provides redundancy and allows interpretation of local stress 

changes. Additional instruments are used to help determine 

whether stress changes are due to normal yielding in the vicinity of 

the site, which should not be considered a hazard. These instruments include displacement-measuring devices, such as sagmeters 

and extensometers, as well as stress-measuring devices for coal, 

such as borehole pressure cells and flatjacks. 

Once the instrument array and site configuration have been 

determined, the instruments and datalogger are installed. Usually 

the site is located in the roadway used in ongoing operations, so 

coordination between miners and installation crews is essential. 

Data Delivery 

Designing, maintaining, and upgrading a data delivery system 

requires special considerations. Decisions must be made on the frequency that instrument readings are to be taken. Reading frequency 

can be changed as desired when conditions change. Some considerations are anticipated rates of stress change for the monitoring 

period, level of redundancy needed, level of concern about conditions or need for the data, concerns about clogging data analysis 

with too much data, and the drain on a datalogger’s battery power. 

Once the datalogger has been designed, installed, and connected 

to the instruments, rigorous tests need to be performed to ensure the 

data are reliable. Good testing limits problems with instruments, 

wiring, components, programming, or external interference, such as 

high-voltage lines, motors, and nearby mobile equipment operations. 

Analysis and Decision Making 

Teamwork and communication are critical because this process is 

dynamic and includes people with varying specialties. Initially, a 

selected team needs to define routine tasks, determine who is 

responsible for each task, and schedule tasks so the data collection 

and delivery process becomes operational. In teams or in individual 

efforts, the process needs to be reviewed continually and changed 
to ensure that the generated graphs and reports are best designed to 

aid in making sound decisions. These changes will reflect the staff’s 

need for information, which changes as mining progresses through 

the area being monitored. 

Experience also indicates that there are only a few days from 

the time that stress change patterns appear until an event actually 

occurs. Therefore, quick data delivery and analysis are critical. For 

example, the system could be taking baseline data for weeks before 

an engineer sees a sharp rise in stress. At this time, the engineer 

could produce more-detailed graphs and reports and present them 

at the daily foreman’s shift meeting. The team could determine if 

(1) these trends indicate a need for action or are explained by some 

other cause, (2) if the geologist or foreman needs to conduct special 

inspections, (3) if reports from other data collection systems need 

to be evaluated, or (4) if conditions warrant adding secondary 

support or some other action. 

This process is quite flexible, so it can easily be adapted into 

the creative problem-solving culture prevalent at mining operations. 

Some activities that mine staff may consider are training the team 

on the capabilities of the system, determining the content of and 

scheduling routine reports for individuals or meetings, making 

special reports, and making results readily available to staff on 

dedicated monitors. 

CONCLUSIONS 

The concept of using stress monitoring systems for recognizing 

hazards during production is based on a number of research 

questions (8). The authors proceeded with the development of this 

concept by systematically testing these questions in the field. In 

general, additional research is needed to develop models for 

associated failure mechanisms and validate stress change patterns. 

• Can horizontal stress changes be measured accurately during 

a mining operation? Results from two mines show that stress 

changes associated in time with observed ground control events can 

be measured. Trends and patterns in the data peaks seam reasonable 

and give a basis on which to compare future models. Further 

validation is needed to determine if these stress changes accurately 

reflect actual stress changes between the instrument and the face 

and whether patterns in stress change are of value in making safety 

decisions. 

• Are catastrophic events initiated by poor gob caving or by 

some other mechanism that (1) develops slowly enough to be 

detected and resolved before the event, (2) is not masked by 

changing geologic properties through which stresses are 

transferred, and (3) produces horizontal stress changes at the 

instrument site? Results indicate that this system is best used for 

detecting stress change due to load redistributions during gob 

formation as the longwall advances. Results suggest catastrophic 

events may be triggered by the additional stresses transferred to 

work areas by gob formation events. They also suggest that 

measured stresses rise rapidly and then level off and indicate 

yielding before a global stress rise event such as first cave. This 

process has taken from about 3 to 7 days in the two tests to date. 

The monitoring system concept has value only to the extent 

additional tests confirm that these patterns are consistent. Three to 

seven days is reasonable for most types of preventative actions to 

be considered. However, more research is required to investigate 

differences between typical stress change response to mining and 

that which is characteristic of failure mechanisms leading to 

catastrophic events. 

• Can irrelevant factors influencing horizontal stress 

measurements be filtered out so as not to distort data trends in a 

way that does not interfere with detecting imminent catastrophic 

events? Examples of such irrelevant factors are (1) stress 

redistribution due to normal yield pillar dynamics, (2) large changes 

in ventilation air temperatures or pressures, and (3) typical 

delamination or sagging of the immediate roof strata over entries. 

Experience has shown that characteristic stress trends before a 

major event were large and unique enough that local events have 

been eclipsed. However, validation of these patterns requires 

replication through additional field testing and correlation with 

established models of failure mechanisms. 

ACKNOWLEDGMENTS 

The authors would like to thank the staff at all mines for their 

excellent support and dedication to developing a practical stress 

monitoring system. Andy Schissler, a primary mine contact, 

assisted in developing and implementing in-mine tests of the 

system, established an event-reporting system among foremen, and 

involved many of the mine staff in providing feedback to 

researchers. Charles (Rocky) Thompson, a primary mine contact, 

assisted in geologic interpretation, drilling, installation, system 

maintenance, data reduction, and interpretation, and provided 

excellent feedback. Thanks also go to SRL staff, including Paul 

Pierce and Mike Jones for drilling in challenging conditions; Rich 

Rains for datalogger wiring, installation, and troubleshooting; and 

Stephanie Ray for assisting in creating spreadsheets and reducing 

data. 

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