Development of Stress Measurements and Instrument Placement 

Techniques for Longwall Coal 

E. G. Zahl, J. P. Dunford, D. R. Tesarik, M. K. Larson 

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

Spokane, Washington, USA 

J. Chen 

RAG American Coal Company, Waynesburg, Pennsylvania, USA 

ABSTRACT: Western coal mines are operating under increasingly challenging ground conditions. Researchers 

at the Spokane Research Laboratory of the National Institute for Occupational Safety and Health are cooperating 

with mines to evaluate how these conditions affect mine opening stability and related miner safety. A system 

was developed to monitor rock and coal stresses during mining of an entire longwall panel. The system was 

designed to generate data in near real time to evaluate rock behavior in roof strata and the onset of hazardous 

conditions as overburden stress was redistributed over working entries during gob formation. Computer 

simulations and in-mine evaluations were used to optimize instrument placement. A prototype packer assembly 

was designed and tested for installing sensitive instruments in a mine roof. This paper provides background 

information on the stress monitoring concept and focuses on the development of instrument placement techniques 

and the prototype packer assembly. 

1 BACKGROUND 

1.1 Stress monitoring system concept 

Researchers at the Spokane Research Laboratory 

(SRL) of the National Institute for Occupational 

Safety and Health (NIOSH), in cooperation with a 

mining company, have developed and continue to 

refine a stress monitoring and reporting system. The 

initial objective was to determine if stress monitorng instruments could gather sufficient data to recogize the conditions leading to coal bumps so these 

conditions could be mitigated with engineering controls before a catastrophic event. During use of the 

monitoring system, it became apparent that the 

system could also be a valuable research tool in 

eval-uating rock behavior surrounding mine 

openngs. The system could also be used to study the 

technologies designed to maintain safe openings in 

more chal-lenging ground conditions. Below is a 

description of the system developed to meet these 

objectives. 

The stress monitoring system is comprised of a 

cluster of instruments (figure 3) placed strategically 

to detect changes in stress and displacement at a preselected site in the headgate of an operating longwall. These stress changes are initiated near the 

longwall face as load is redistributed following coal 

removal and gob formation. Stresses are transferred 

through roof strata from the area of active mining 

back to the instrument site. A data acquisition system retrieves data from these instruments at preselected time intervals. Raw data are then processed 

and reduced and used to generate graphs. These 

graphs illustrate trends that may be used to evaluate 

(1) behavior of roof strata above mine openings, (2) 

imminent catastrophic events, (3) effects such as 

typical yield pillar movement that are not a safety 

concern but may distort needed information, and (4) 

placement and type of additional support or other 

safety intervention. 

1.2 Phase 1 research 

In the first phase, a monitoring system was developed and used to monitor stresses during the mining of 

an entire longwall panel (Zahl et al. 2000). Instruments were clustered in two panels, a yield pillar, 

and the immediate roof and floor of a two-entry 

longwall gateroad. 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 cluster. 
Figure 1.—Computer-generated cutaway showing examples of instrument 

placement. 

Researchers and the mine’s technical staff evaluated 

these data with respect to bounces, bumps, and bump 

mechanisms. Selected graphs were also presented to 

mine foremen on a periodic basis to test the graphs’ 

usefulness in identifying imminent bump conditions. 

One of these graphs was used to compare vertical 

biaxial stressmeter measurements from the stress 

monitoring system with bounce or bump events 

reported by miners (table 1) and seismic events from 

a microseismic system (figure 2). This graph also 

illustrates the level of detail that data trends provided, although it was apparent that more research 

was necessary to interpret the meaning of these types 

of data trends fully. Evaluation of many data trends 

indicated there was a potential for using this type of 

system as an assessment or hazard recognition tool 

and hence to enter a second phase of research. A 

sequence was developed for installing and using a 

stress monitoring system, as well as providing a 

basis for development of specific components (table 

2). 

1.3 Phase 2 research objectives 

Hundreds of stress graphs were generated in the first 

phase. Those using horizontal stress changes 

measured by vertically installed biaxial stressmeters 

appeared to be the most sensitive to events farthest 

away. For example, horizontal stress changes measured by these stressmeters became evident when the 

working face was 518 m (1700 ft) away from the 

instrument site. This was significantly farther than 

the hundred meters or so typical of borehole pressure 

cells in coal. It is known that horizontal stresses are 

an integral factor of roof failures. For these reasons 

and others (Maleki 1995), SRL researchers believed 

that use of horizontal stress data showed the most 

promise for assessing rock behavior and recognizing 

hazards. Evaluation of research needs at the close of 

first phase indicated that top priority items were (1) 

selecting optimal locations to place the monitoring 

instruments and (2) a more reliable system for installing biaxial stressmeters vertically into the mine 

roof. The remainder of this paper describes these 

approaches, explains the equipment and processes, 

and discusses the application of these developments. 

2 PREINSTALLATION TESTING 

2.1 Approach for testing 

To determine where instruments should be located 

more effectively, researchers evaluated a number of 

techniques that could be employed both in and out of 

the field before the instruments were placed. 

Preinstallation tests resulted in a new approach and 

new tools to the site selection step in the installation 

process outlined in section 1.2. The most helpful 

preinstallation tests were (1) two-dimensional FLAC 

modeling analyses, (2) evaluation of roof borehole 

logs for material properties and the extent of roof 

strata, and (3) measurement of rock stiffness at 

selected roof horizons based on borehole logs and 

FLAC modeling. 
Table 1.—Observed bump-related events near instrument site 

Event Date Face location Event Comments 

No. 

1 . . . . . 2/5/00 60-80 shield Coal blowout Crews avoid this area because of conditions. 

2 . . . . . 2/7/00 45-50 shield Shield yielding 

3 . . . . . 2/85/00 28-38 shield Coal blowout with bump Strong bounce reported; downtime to put pan line 

back together. 

4 . . . . . 2/9/00 32-45 shield Coal blowout with bump Larger chunks blown into shields; power knocked 

off to shearer, which was in area headed toward 

headgate. 

5 . . . . . 2/9/00 10 shield Coal blowout with bump Not as severe as earlier bump, but did blow coal 

into shields. Floor heaved and power killed to 

shearer. This shearer also in area headed toward 

tailgate. 

6 . . . . . 2/9/00 40 shield Coal blowout with bump Fine coal chunks blown into shields; killed power to 

shearer. Shields also yielding in this area. 

7 . . . . . 2/9/00 40 shield Coal blowout with injury [6:35] Face blew out, sending large chunks of coal 

into shields. Employee was struck, resulting in 

fractured leg. 

8 . . . . . 2/17/00 Tailgate Coal blowout with bump [10:00] Bounce in tailgate; shearer was in area. 

Coal blowout knocked lights off of two shields. 

Equipment damage was reported. 

9 . . . . . 2/18/00 NA Minor bouncing, no major events reported. 

10 . . . . 2/24/00 No. 2 entry Bounce [21:00] Big bounce felt, no damage reported. 

headgate NIOSH system reported bounce, as did Univ. of 

Utah system. 

11 . . . . 2/24/00 Headgate and Bump Bumps in headgate and tailgate areas when 

tailgate shearer pulled into area. 

12 . . . . 3/5/00 Stage loader Roof fall [0:30] Roof fall over stage loader. Longwall shut 

down to reestablish walkway along stage loader. 

13 . . . . 3/7/00 [19:15] Earthquake triggered several roof falls and 

rib sloughs. Magnitude 4.1 on national and state 

seismographs. 

14 . . . . 3/17/00 Headgate Roof fall [13:45] Roof fall over stage loader shut down 

longwall. 

15 . . . . 3/29/00 Headgate Roof fall [12:20] Roof fall over stage loader. Production 

stopped until later in afternoon. 

Table 2.—Stress monitoring system process 

Step Task Objectives 

1 . . . Site selection Determine most hazardous locations, identify zones away from surface 

sloughage. 

2 . . . Instrument selection Select instruments proven capable of measuring expected stresses, determine 

amount of redundancy required, and ensure constraints posed by the mine 

and drilling are met. 

3 . . . Instrument installation Meet instrument installation requirements, schedule activities to minimize 

interference with ongoing mining operations. 

4 . . . Install data acquisition system Determine monitoring intervals, develop a data retrieval plan 

5 . . . Data management Create reduction matrices for raw data, create time- and location-dependent 

graphs for stress changes. 

6 . . . Data evaluation Correlate data trends to observed events and other relevant data collection 

systems, discuss and interpret results with people having differing expertise. 
Figure 2.—Change in stress versus time for a vertical biaxial stressmeter. 

Data trends leading to actual events are shown. 

Figure 3.—Cross section of mine at the instrument site. 

2.2 Finite-difference modeling 

A two-dimensional, finite-difference model (FLAC) 

was used to simulate stresses in a two-dimensional 

cross section of a coal panel, entry 1, yield pillar, 

and entry 2 (Itasca 1998) (figure 3). The model was 

used to identify areas in the mine roof above entry 2 

that would undergo the largest horizontal stress 

changes as a result of mining. These results, along 

with practical considerations such as borehole depth 

and observable host rock degradation around the 

entry, were used to target a location for the biaxial 

stressmeter. 

The two gateroads, longwall panel, and surrounding mine structure were modeled in plane strain with 

a 102- by 102-m (336- by 336-ft) vertical cross section oriented perpendicular to the gateroads. Strata 

dipped 6° in this plane. Based on drill hole logs 

provided by the mine staff, each stratigraphic layer 

greater than 0.3 m (1 ft) thick was represented in the 

28,224-element finite-difference mesh. Elastic properties and tensile strengths used in the model were 

obtained from material test results that were also 

provided by the mine staff. Because the values for 

cohesion and angle of internal friction were unreasonably large when they were derived from strength 

values, average textbook values were used (table 3). 

Secondary horizontal principal stresses obtained 

from overcore measurements were rotated into the 

model’s plane for the initial horizontal stress field, 

and vertical stress in the model was assumed to be 

-19.3 MPa (-2800 psi), based on the force of gravity 

on the overburden material (table 4). The vertical 

boundaries of the finite-difference mesh were fixed 

in the horizontal direction and the horizontal 

boundaries were fixed in the vertical direction. 
Table 3.—Material properties used in finite-difference model 

Young’s modulus Poisson’s Cohesion Angle of Tensile strength 

ratio internal 

friction, 

GPa 10^6

 psi MPa psi deg MPa psi 

Roof sandstone . . 23.4 3.40 0.26 27.20 3945 30 7.52 1091 

Roof siltstone . . . . 19.3 2.80 0.23 22.81 3309 30 8.91 1293 

Roof mudstone . . . 45.2 6.55 0.42 20.72 3005 26 5.03 730 

Coal . . . . . . . . . . . 2.96 0.43 0.38 7.73 1122 30 1.93 280 

Floor mudstone . . . 23.0 3.34 0.25 26.73 3877 30 26.73 3877 

Floor sandstone . . 51.9 7.53 0.21 21.49 3117 30 24.56 3562 

Table 4—Initial stresses used in finite-difference 

model 

Direction Magnitude 

MPa psi 

Sxx ...... -22.041 -31961 

Syy ...... -19.31 -2800 

Szz ...... -13.16 -1908 

Sxy ...... 5.31 770 

1

 Compression is negative 

To identify the areas of highest horizontal stress 

change in the mine roof above entry 2 induced by 

longwall mining, the excavation of a large room 

approximately 12 m (40 ft) wide and 3 m (10 ft) 

high was simulated southwest of entry 1. The entire 

longwall was not modeled to reduce boundary 

effects on stress changes around entry 2. Results 

from the numeric model indicated that the location 

of the highest horizontal stress increase was above 

the yield pillar between entries 1 and 2. In general, 

the magnitude of these stress changes decreased as 

distance from the excavated longwall increased. 

The different deformation moduli of the roof strata 

was reflected in the magnitude of stress change. 

Hori-zontal stress changes in mudstone in layers 

from 1.2 to 3.7 m (4 to 12 ft) above entry 2 was 1.6 

to 2.7 times larger than in adjacent sandstone layers. 

The mudstone modulus used in this analysis was 1.9 

times larger than the sandstone modulus. 

These numerical modeling results were used as 

a guide for placement of a biaxial stressmeter in the 

mine roof above entry 2. The highest calculated 

horizontal stress change was above the yield pillar. 

However, this location was not considered because 

the stress changes at this location reflect yielding 

typical of a yield pillar and would mask stress 

changes generated by the more critical loading 

between the instrument site and active mining. 

Approximately half of this pillar yielded in the 

model. Similarly, failure zones were identified 

above entry 2 that would mask critical loading. 

Observations of the roof rock in entries has shown 

separation of material close to the roof and rib. Also, 

results from the FLAC analysis indicated that instruments should be placed outside a 3.3-m (10-ft) zone 

adjacent to entry 2. Because of the higher magnitude 

of stress change in the relatively stiff mudstone 

layers and possible raveling caused by the yield 

zones, the stressmeter was targeted for installation 

in competent roof mudstone in a 8.5-m- (28-ft-) long 

borehole angled 45° upward in entry 2 (figure 3). 

2.3 Geologic interpretation and borehole logging 

Stress measurements are affected by the physical 

properties of each members through which the stress 

is transferred. For example, stress flows most 

readily through a horizontal member with the 

highest stiff-ness and the greatest continuity 

between the working face and the point of 

measurement. Geologic pro-files were plotted on 

fence diagrams created from a roof core logging 

program in gateroad entries. These fence diagrams 

allowed researchers to evaluate the continuity of the 

strata and use this information to aid in selecting a 

roof member. Unfortunately, the depositional 

character of the roof rock was so com-plex and 

discontinuous that a single member could not be 

identified that encompassed the entire stress transfer 

zone. 

2.4 In-mine determination of strata stiffness 

During prior installation of vertical biaxial stressmeters, SRL researchers were primarily concerned 
with developing placement techniques. At that time, 

instruments were placed in a competent zone at a 

horizon above roof degradation and at an angle perpendicular to bedding. Experience with roof rock 

behavior and analyses of FLAC modeling results 

confirmed that stress changes are much higher in 

layers of roof strata with a higher modulus of elasticity. Placing a biaxial stressmeter in the stiffest roof 

member resulted in obtaining measurements that 

were the most sensitive to changes farthest away 

from the instrument site. Maximizing the distance 

over which the stress monitoring system can detect 

changes is a research objective that will increase the 

utility of this tool. 

Although mine records provided physical and 

mechanical properties for typical lithological cross 

sections, large changes in the lithology of boreholes 

were observed near the instrument site. The complexity of the deposition and changes of material 

properties within the mudstones and siltstones made 

visual identification of the stiffest zones impossible. 

For this reason, it became necessary to determine 

the stiffest member of the immediate roof lithology 

so optimum horizons for placing a biaxial stressmeter could be identified. The search for a method 

led researchers to a development by Goodman 

(1980), Meyer (1974), and Heuze (1984), who built 

a NX plate-bearing device for measuring the 

modulus of elasticity in a borehole. This device is 

known as the Goodman jack. There is still debate on 

the ability of this method to provide measurements 

of Young’s modulus comparable to laboratory tests. 

However, SRL researchers agreed that this method 

could provide a reliable comparison of stiffness 

among alternative zones tested in a borehole. 

Field tests using the Goodman jack were completed on boreholes located 9 m (30 ft) from the 

preselected installation site for the biaxial stressmeter. The NX borehole was drilled, and a series of 

four tests were run, beginning at the deepest point 

(6 m [20 ft] into the roof) and continuing toward the 

collar of the hole. Test horizons were determined 

based on drilling characteristics and assessment of 

the logged core retrieved from the borehole. Results 

from the test showed a range of 4 moduli from 12.4 

GPa (1.8 million psi) at 3.4 m (11 ft) from the collar 

of the borehole to 19.3 GPa (2.8 million psi) at the 

6-m (20-ft) horizon. 

The Goodman jack is useful if reliable measures 

of the modulus of elasticity are not available for target horizons. This method used in conjunction with 

a geologic evaluation of borelog data to determine 

the extent of the stiffest members provides an 

effective way to determine where to position biaxial 

stressmeters for the stress monitoring system. 

3 INSTRUMENT PLACEMENT 

3.1 Development of instrument placement method in 

a mine roof 

The research team had previously selected biaxial 

stressmeters that used vibrating wire technology to 

monitor long-term stress changes because of the low 

electronic drift inherent with this instrument and the 

overall superior performance of biaxial stressmeters 

in previous tests (Larson et al. 2000; Seymour et al. 

1999). Discussions with instrument manufacturers 

indicated that no method was available to install 

vibrating wire stressmeters in an upward-oriented 

borehole, so researchers developed and tested a 

number of methods using prototype packer assemblies to meet the installation requirements for this 

instrument. 

A number of difficulties were encountered in 

laboratory and field tests while trying to grout a 

biaxial stressmeter vertically into a borehole. Primary design objectives were to (1) install the 

instrument in a borehole longer than seam height, 

(2) maintain the integrity of the grout around the 

instrument until the grout sets, and (3) control 

instrument orientation during placement. 

Although researchers had experimented with 

various designs in the first phase of research, none 

proved reliable. Particular design problems were 

that (1) assemblies were difficult to install deeper 

than seam height because of the complexity of the 

plumbing needed for grout injection and the tight 

tolerances of the assembly in the borehole, (2) 

venting tubes malfunctioned, which did not allow 

installers to determine if adequate grout had been 

delivered to the borehole, (3) the seal between the 

packer core and the outer bladder failed, (4) 

maintaining instrument orientation was difficult 

because of slipping threads in the multiple joints in 

the setting rods. 

A second-generation prototype was designed and 

tested in both the laboratory and the field (figure 4). 

The instrument appeared to have successfully 

overcome the previous design inadequacies (figure 

5). Success of the new system was based on 

changing the basic design approach so that all rigid 

components of the assembly are kept to a length of 

2 m (7 ft). This allows installers to insert the 

assembly easily within the seam height of typical 

coal mine entries. On installation, there are four 

trailing flexible lines—the instrument cable, the 
packer inflation line, the grout injection tube, and 

the vent tube. The assembly is installed with 

standard setting rods and a specially fabricated 

setting head designed to allow proper instrument 

orientation. The packer seal problem was solved by 

using a simple bicycle inner tube that requires no 

special sealing surfaces to maintain pressure. 

Venting problems were solved by increasing the size 

of the vent tube so grout could be pumped from the 

borehole through the vent line and made visible to 

the installers so they can ensure proper grout 

placement. 

Figure 4.—Cross section of final placement in a borehole 

(A) of a biaxial stressmeter (B) and packer (C). 

Figure 5.—New prototype packer showing trailing 

lines. 

3.2 Process for stress monitoring system installation 

and use 

Current plans call for further development and 

testing of this stress monitoring system. However, 

the authors would like to suggest the following 

installation process. Our aim is to evaluate the 

feasibility of using a stress monitoring system as a 

hazard recognition tool in selecting safety intervention strategies. 

1. If mine engineers and managers determine there 

may be ground control concerns for a longwall panel 

on development, they should identify entries and 

approximate locations (within a 610-m [2000-ft] 

interval) of where ground control problems are most 

likely to occur based on past experience. 

2. A stress monitoring system site would be 

selected 61 to 152 m (200 to 500 ft) outby the area 

of concern, with the data acquisition system located 

farthest outby so as to maintain it as long as possible 

as mining encroaches on the system. 

3. Some or all of the preinstrumentation tests and 

evaluations (geologic bore logs, FLAC modeling, 

and Goodman jack) would be completed to determine optimum instrument placement. 

4. A stress monitoring system should be installed 

and a monitoring program implemented that allows 

for periodic downloading of raw data, generation of 

characteristic curves, and interpretation of results by 

mine engineers. 

5. Engineers and mine managers should interpret 

the data trends continually and evaluate these data 

trends after additional support or other safety intervention steps are taken. 

6. Researchers should catalog and evaluate characteristic curves to understand the applicability of the 

system and make further developments to increase 

reliability. 
4 DISCUSSION AND CONCLUSIONS 

4.1 Application of the stress monitoring system 

Researchers have developed two uses for the stress 

monitoring system. The first is as a research tool to 

assess mining-induced rock behavior and load 

redistribution dynamics near active mining and 

understand and evaluate the effectiveness of the 

many technologies the mining industry is using to 

maintain structurally safe underground working 

areas. The need for these types of assessments will 

dramatically increase as the coal mining industry 

deals with increasingly challenging conditions as 

coal mines are developed deeper and in less stable 

ground. 

The second use is as a production tool to 

recognize hazards during mining operations. Actual 

changes in horizontal stress measurements should 

be compared with characteristic data trends to 

identify the nature and location of potential ground 

control hazards in the working entries between the 

working face and the instrument site. From this, 

mine managers may respond by adding necessary 

support or utilize some other appropriate intervention tactic to prevent ground control failure and 

subsequent injuries or fatalities. 

Extremely competitive market conditions, 

coupled with lower injury and fatality rates in recent 

years, appear to make the stress monitoring system 

too costly and complex to be implemented during 

mine production in the near term. In the future, this 

system could become practical during production if 

(1) its cost and complexity were reduced, (2) 

mining conditions became more hazardous, or (3) 

coal company profitability increases (factors that 

are all probable in the long term). If a proof of 

concept is successful, developing this tool now may 

be of value so that it is available when the mix of 

these three conditions is sufficiently met. 

4.2 Discussion of results 

The value of using this type of stress monitoring 

system as an assessment tool has been demonstrated 

throughout its development in the research program. 

Researchers and mining engineers have used results 

for assessing mining-induced rock behavior and 

load redistribution dynamics near the active mining 

face. This tool needs to be developed further by 

collecting characteristic data trends in varying 

scenarios so these trends may be correlated with 

specific failure mechanisms and load redistribution 

dynamics. Armed with this information, researchers 

could better assess the engineering controls needed 

under more hazardous conditions as mining 

proceeds deeper and into less stable ground. 

The concept of using this tool for recognizing 

hazards during production is predicated on a number 

of assumptions. This research program has taken 

these assumptions in a selected order to test them 

and from the results establish a proof of concept. 

The first assumption researchers made was that 

horizontal stresses can be accurately measured in a 

mine roof during a mining operation. To date, 

research has focused on developing a stress monitoring system by field testing various configurations 

in operating longwall headgates. The objective was 

to measure changes in horizontal stresses accurately 

and develop strategies for determining optimum 

instrument placement techniques and locations. 

Results show that NIOSH has successfully developed devices and methodologies in combination 

with existing instruments and data acquisition technologies to make useful horizontal stress measurements in roof strata. 

Another assumption was that irrelevant factors 

influencing horizontal stress measurements can be 

“filtered out” so as not to distort data trends in a way 

that interferes with detecting imminent catas-trophic 

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. In research to date, additional stress and 

displacement measuring devices have been installed 

in the roof and coal pillars and panels to monitor 

changes in the immediate area of the instrument site. 

This allows researchers to compare data trends from 

these instruments to account for possible influences 

of an irrelevant nature. However, more baseline data 

are required to determine characteristic data trends 

and their interpretation. 

Another assumption is that catastrophic events 

are initiated by poor gob caving or some other 

mechanisms that (1) develop slowly enough to be 

detected and resolved before the event, (2) are not 

masked by potentially changing geologic properties 

through which stresses are transferred, and (3) 

produce horizontal stress changes at the instrument 

site. Another way to pose this assumption is that a 

stress monitoring system can be designed to discriminate between “expected” characteristic data 

trends that indicate “target entries are safe to work 

in” as the face advances and “abnormal” characteristic data trends that indicate that failure is im­
minent and that existing support or other engineering controls are not adequate. In addition, the stress 

monitoring system must provide data that allow engineers to determine the location of the potential 

event soon enough for the planned engineering 

control to be effective. 

Researchers began addressing some of the issues 

in these underlying assumptions. The concept of 

preinstallation testing was initiated to better understand the effects of geology and mine openings 

around an instrument site. Finite-difference modeling was devised to select instrument placement sites 

to intercept maximum stress changes relevant to 

imminent catastrophic events. Goodman jack evaluations were devised to identify the stiffest strata in 

the immediate roof, which will, by definition, 

transfer the greatest amount of horizontal stress. 

Borehole log evaluations were devised to identify 

the extent of the stiffer zones through which 

horizontal stresses are transferred. Future development of characteristic data trends should focus on 

accounting for the material properties of the strata 

through which these horizontal stresses are transferred as well as the loading dynamics of the gateroad pillars and adjacent panels and the load 

transfer dynamics surrounding the active mining 

face. 

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