NEW TOOLS FOR ROOF SUPPORT EVALUATION AND DESIGN 

Steve Signer and Carl Sunderman 

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

ABSTRACT 

 Researchers at the National Institute for Occupational Safety and 

Health (NIOSH) have developed several new tools for evaluating roof 

support performance. A miniature data acquisition system (MIDAS) 

was developed that can collect readings from up to 16 strain gauges at 

regular time intervals and store the readings for later retrieval. Three 

LED lights change from green to yellow, then red, based on the 

reading levels and/or rates of change of rock movement. This feature 

can be used to warn miners of rock instabilities. The system can be 

used with any strain-gauged support (resin bolt, friction bolt, cable 

bolt) or with a newly developed rock strain strip (ROSS) developed 

by NIOSH to measure rock strain. A ROSS is grouted in a hole 

drilled adjacent to the rock support to be evaluated; the deformed 

shape of the ROSS provides anchorage on either side of the strain 

gauges. Data from both laboratory tests and field evaluations are 

presented that describe the capabilities of both the MIDAS and ROSS 

instruments. 

BACKGROUND 

 Statistics from the Mine Safety and Health Administration (MSHA) 

indicate that falls of roof remain one of the major hazards in 

underground mines. From 1997 to 2001, 26 percent of all 

underground injuries were caused by roof falls. Bauer and Dolinar 

(2000) indicate that between 1995 and 1998, 98% of roof fall injuries 

in underground coal mines were caused by sloughing or spalling of 

opening surfaces (the “skin”), and 77% of roof fall fatalities were 

caused by massive roof failures. 

 Many factors cause these failures. Skin failure can be due to 

weathering or it can be caused by roof bending, compression failure, 

tensile stresses, shear, etc. Massive failures are generally caused by 

roof bolt failure or loss of roof bolt anchorage. The anchorage 

capacity of roof bolts can be significantly lower in weaker rock, 

which leads to bolt pull-out failures and may be a significant factor in 

massive roof falls in weak ground. 

 Researchers have worked for many years to develop theories and 

design methods for the selection of roof supports. An excellent 

review is given by Choquet and Hadjigeorgiou (1993). Studies have 

shown that bolt loading is often much higher than the predicted 

designs. Field measurements of 92 instrumented bolts at eight coal 

mines indicated that 75% of the bolts reached the yield point of the 

steel, and 50% exceeded the yield point (Signer 2000). These values 

were two to three times the design prediction of dead weight loading. 

This shows that application of design methods must be combined with 

field measurements to ensure proper support selection. 

 Although studies conducted by researchers from the former U.S. 

Bureau of Mines and NIOSH have demonstrated that geotechnical 

instruments can be used effectively to identify and monitor ground 

control hazards, modern rock mechanics instruments are rarely used 

by the mining industry in the United States. Data are collected by 

either taking readings with hand-held instruments or installing 

electronic data acquisition systems. Hand readings are very labor 

intensive and tend to be collected infrequently. Typical continuous 

electronic data acquisition systems are large, difficult to use, and 

expensive. Many mines, especially smaller operations, do not have 

the expertise or resources available to design an effective instrumentation plan, properly install and monitor a variety of instruments, and 

analyze and interpret the electronic readings obtained from the 

instruments. 

 NIOSH developed a miniature data acquisition system (MIDAS) 

that addresses these problems. This system has the potential to make 

the use of geotechnical instruments in underground mines more 

feasible. Another instrument developed by NIOSH is a “rock strain 

strip” (ROSS) that, in conjunction with MIDAS, can measure rock 

movement more accurately. The purpose of this paper is to provide 

an overview of this new instrument, its capabilities, and its use in 

improving evaluations of roof support. 

INSTRUMENTS 

Miniature Data Acquisition System (MIDAS) 

 MIDAS (figure 1) is designed to collect strain measurements from 

resistive sensors, such as strain-gauged bolts, cable bolts, ROSS’s, 

and string pots. MIDAS is attached directly to a ROSS so no long 

lead wires are required. Up to 16 monitoring channels can be selected. A 125-kbyte, on-board flash memory can store 2,192 data scans if 

all 16 channels are read. An on-board clock is used to set the scan 

rate for readings in real time, and time-stamps each scan. A thermistor records temperature with each data scan, and a RS232/RS485 port 
Figure 1.─MIDAS demonstration model 

Figure 2.─MIDAS LED lights and serial connector

can communicate with any computer. MIDAS has an experimental

MSHA approval rating for use in return air. 

The MIDAS has an amplifier gain and frequency response that can be

adjusted for the requirements of various instruments. The gain has seven 

settings that vary the measurement range from ±35 mV to ±2.25 V. 

Using strain gauges with a gauge factor of 2, the gain setting used for 

large strain tests has a measurement range of ±562.5 mV (250,000 

microstrain), which has a resolution of approximately 0.067 mV (0.03 

microstrain). A computer program was written to set up MIDAS, plot 

data, convert the information to engineering units, and write to a 

spreadsheet. This software makes MIDAS set-up and data processing 

easier.

The low power requirement enables long-term testing. A 9-V battery

will provide enough power to take daily readings on 16 channels and 

power the light-emitting diode (LED) lights for 6 months. If the battery 

voltage does run low, the data are still available for downloading. The

small size (17 mm wide by 74 mm long by 17 mm high [0.67 by 2.90 by

0.67 in]) makes the MIDAS adaptable for use in almost any location.

Because the MIDAS is so small and self contained, it can monitor

instruments while a continuous miner is cutting coal. 

Strain is measured by the MIDAS’s datalogger, stored in memory

for later retrieval, and activates the LED lights (figure 2). These 

lights change from green to yellow to red to warn miners of hazardous

conditions. The threshold levels to change the LED colors can be set

by the end user to adjust for different types of ground conditions and 

instruments.

Figure 3.─Rock strain strip

Rock Strain Strip (ROSS) 

The ROSS (figure 3) is made of thin (1.5-mm [0.060-in]) stainlesssteel plate that has been cut and twisted at every foot along the 

ROSS’s length. Type 304 stainless steel was selected for the ROSS 

because it has high ductility (70%). (The ductility of typical steel

bolts used for roof support is between 6% and 16%.) This means that 

the ROSS will not break prior to tensile failure in typical roof 

supports. This type of stainless steel is also corrosion resistant, which

helps to preserve the instrument in long-term tests.

The shape of the ROSS is critical. This shape is similar to that of a

dog bone between two adjacent sets of teeth. At each set of teeth, the 

ROSS is twisted 60°, which allows the ROSS to be installed in a hole

that has not been drilled perfectly straight. The twist also centers the 

ROSS in the hole. This shape causes the ROSS to stretch at the

thinnest points, which is also where the strain gauges are positioned. 

The size and number of teeth are important to anchoring the ROSS 

into the grout when the ROSS is placed in the hole. The tooth area in

contact with the grout must balance the force-deflection relationship

of both the grout and the stainless steel. 

High-elongation strain gauges rated for 20% strain are also attached

every foot along the length of the ROSS, but between the twists. The 

surface is sand blasted so that the gauges will remain attached at high

strain levels. The glue is rated for both high elongation and minimal 

amounts of long-term creep. Two sealants are applied to the strain

gauges to protect them against oxidation. After the lead wires are 

applied, electrical tape is used to keep the lead wires from becoming 

embedded in the grout after installation. The ROSS is attached to a

pipe with a T where the wires from the strain gauges attach to an

electrical connector, as shown in figure 4. 

The ROSS was designed to be installed in a 38-mm (1.5-in) diam 

hole. Just prior to installation, a small (6 mm outer diameter) air vent 

is attached to the ROSS and run along the length of the instrument. 

The ROSS is placed in the hole, and the edges of the plastic pipe are 

sealed against the hole wall with two-part, fast-setting epoxy. A 

flexible hose is attached to the threaded connection, and grout is 

pumped to fill the hole and encase the instrument in the mine roof.

After the fast-setting grout hardens, the teeth cut into the metal 

ROSS keep the instrument anchored when the rock begins to move. 

The strength of the grout must be sufficient to develop the ultimate 

strength of the ROSS. It should be slightly expansive to avoid

shrinkage cracks, able to maintain its strength if it is submersed in

water, easy to apply, and inexpensive. The grout selected was 
Figure 4.─End connection on ROSS

Thorogrip.1

 It is a portland cement-based material with a water-togrout ratio of 0.17 and can be easily pumped. It sets in 15 minutes, is 

slightly expansive, and achieves an unconfined compressive strength 

over 2,000 psi in 1 hour. 

The gauges measure strain in the steel caused as the rock moves

between the two adjacent sets of teeth. The size of the ROSS and the

number of sets of teeth can be adjusted for different types of 

applications and must be correctly engineered on the basis of the 

strength of the grout and the strength of the rock. Thermal expansion

is 17 microstrain per degree Celsius, which is close to the thermal

expansion of readily available commercial strain gauges. Yield 

strength is 215 MPa (31,200 psi), and ultimate tensile strength is 505

MPa (73,200 psi). The instrument can measure up to 170,000 microstrain of rock movement. 

SUPPORT EVALUATIONS 

The ROSS measures strain (rock movement) in a different manner 

than do instrumented rock bolts (figure 5). The cross-sectional area of 

a twist section is twice as large as the area where a strain gauge is 

positioned. So, when the rock moves, that movement is distributed

over the length between the twists. 

The definition of strain is─

 ε = ∆L/L 

where ε = strain, 

 ∆L = change in length

and L = gauge length.

∆L equals rock movement in the roof. The gauge length for the 

ROSS used in these tests is approximately 200 mm (8 in). Rock 

movement is then equal to instrument strain times gauge length

(figure 6). This calculation ignores the small amount of friction

between the side of the ROSS and the grout and the strain relaxation 

in the ROSS caused by elastic deformation of the grout at the contact 

point of the teeth.

 

1 Mention of specific products and manufacturers does not imply endorsement by the National Institute for Occupational Safety and Health. 

Figure 5.─Strain behavior of ROSS and instrumented bolt 

Figure 6.─Strain on ROSS

The advantage of k movement is that 

e ROSS can measure 1 microstrain without significant anchor

l

d

olt in response to rock movement. Strain is transferred to the bolt 

i

using a ROSS to measure roc

th

s ippage. The accuracy of other instruments varies from 0.025 to 1 

mm (0.001 to 0.04 in). If anchor spacings are 300 mm (1 ft) apart, 

then strain over these lengths would range from 83 to 3,300

microstrain. These figures do not include measurements of anchor 

slippage. Thus, many types of rock monitoring instruments are totally

ineffective for monitoring strain changes in typical roof supports. 

Figure 5 shows the strain distribution along the length of a groute

b

v a mechanical interlock at the interfaces between the resin grout and 

the rock and the resin grout and the bolt. The rock movement causes 

shear in the grout and at both the resin-rock and resin-bolt interfaces. 

After the peak shear strength at these interfaces is reached, additional

movement or slip takes place that transfers the strain farther from the 

point of the initial rock movement. Resin grout shrinks slightly after 

it hardens, which creates small voids that allow the steel bar to slip 

before it becomes locked in the hole. These slippages result in lower 
strain levels in a grouted bolt than would be generated in a ROSS. An

excellent study by Aziz et al. (2000) showed the effects of bolt and 

rib deformation on shear stress at the bolt-grout interface. Peak shear

in their test samples was reached with 5 mm (0.2 in) of movement,

after which peak shear decayed to residual shear. 

Figure 5 shows the difference in how a grouted bolt and a ROSS 

respond to rock movement. Laboratory tests showed that the ROSS 

captures 98% of the rock movement between one set of teeth, whereas 

the grouted bolt dissipates rock movement over a much longer length. 

Previous studies (Serbousek and Signer 1987; Littlejohn 1993) have

shown that the bolt anchorage length can range from 0.3 to 1 m (1 to 

3.3 ft). Thus, if only one rock separation occurred in a mine roof, the 

ROSS would show the effect at only one gauge position, whereas the 

instrumented bolt would show effects at several positions. This 

suggests that the ROSS would better define rock movement at a 

specific location. Furthermore, with an instrumented bolt, it is not 

possible to tell if bolt anchor slippage occurs because the strain level 

drops to zero at the end of the bolt farthest from the roof. Installing a 

ROSS next to an instrumented bolt would allow researchers to 

determine if anchor slippage has actually occurred. 

TEST RESULTS

Figures 7 and 8 comp a ROSS and a No. 7,

rade 60,strain-gauged rebar bolt installed with slow-setting resin in a 

figure 7 resulted from roof

ovement as the cut was made. Readings were then taken 

(at

05 mm) (figure 7A) indicate low initial strain increases of 100 

are the behavior of

g

mine where the immediate roof consisted of carbonaceous shale and

mudstone rock interbedded with coal layers. Note that the y-axis 

scale of figure 7 varies. Both instruments were 2.4 m (8 ft) long and 

were installed within 250 mm (10 in) of each other at the edge of a 

cross-cut in an intersection before the cross-cut was extended into the 

intersection. The instrumented bolt had five pairs of strain gauges 

positioned at equal distances apart along its length, and the ROSS had

eight pairs of equally spaced gauges. 

The initial strain increase shown in

m

continuously for the next 7 weeks. The greatest amount of movement

on the ROSS occurred at the midpoint and is shown in figure 7C. The 

strain level of the ROSS was approximately four times greater than

the strain level in the instrumented bolt. The strain measurement of 

8,000 microstrain converts to approximately 1.6 mm (0.064 in) of

movement. This amount of rock movement produced strain in the 

bolt that was transferred along the length of the bolt. After the initial 

movement, however, the strain in the bolt stopped increasing. 

Both the first position on the bolt (at 406 mm) and the ROSS

3

microstrain for the bolt and -300 microstrain for the ROSS. As the 

continuous miner cut through the entry, it pressed up on the vent tube, 

which pushed up on the ROSS (but not on the adjacent instrumented

bolt) and caused the initial position on the ROSS to show negative 

strain. The rate of increase after this initial movement showed that the 

rate of change for these two gauges was similar. The strain level at

the second position (figure 7B) on the instrumented bolt compared 

closely with that on the ROSS. On the bolt, the strain level was likely 

to have been affected by the much-larger amount of rock movement at 

the third position (figure 7C). The amount of increase in strain after

the initial movement was higher for the bolt than for the ROSS and 

was most likely caused by strain redistribution from the third position. 

A similar ratio of strain on the ROSS to strain on the bolt during 

initial rock movement is seen between positions 3 (figure 7C) and 4 

(figure 7D), but was much larger at position 5 (figure 7E). The rate of

Figure 7.─Comparisons of strain over time on instrumented bolt

and ROSS at five positions. 

Figure 8.──Comparison of strain on instrumented bolt and RO

crease in strain after initial movement follows the same pattern, with 

e largest increase at position 5. Table 1 shows the ratio of ROSS 

SS

in

th

strain to bolt strain for the five strain gauge positions at three different

times. 
22nd International Conference on Ground Control in Mining 

Table 1.─Ratio of strain on ROSS to strain on instrumented bolt 

Strain gauge position Date, time A B C D E 

1/14/03, 0:00 -5.73 1.16 4.46 4.74 8.53 

1/14/03, 15:41 -3.88 0.83 4.80 4.81 9.68 

2/25/03, 15:16 -1.65 0.77 5.51 5.88 11.98 

 The ratio of strain shown in table 1 indicates a decrease with time at 

positions A and B, a steady level with time at positions C and D, and a 

significant increase with time at position E. The ratio values indicate 

a similar pattern where position A and B are low, position C and D are 

medium, and position E is high. The negative value of position A was 

caused by mining equipment pushing up on the instruments. 

 Most of the rock movement occurred at 1.2 m (4 ft) from the roof 

line and caused the roof bolt to load at the 0.8- and 1.2-m (2.6- and 4-

ft) positions. Significant amounts of rock movement occurred at the 

2.1- and 2.4-m (7- and 8-ft) positions on the ROSS (figure 8) that 

were not recorded on the instrumented bolt. The ratio of strain on the 

ROSS to strain on the instrumented bolt indicates the effectiveness of 

the grout in providing anchorage for the bolt. 

SUMMARY 

 MIDAS can make the use of geotechnical instruments easier and 

cheaper and can warn miners of hazardous conditions. When used in 

combination with the ROSS, the effectiveness of roof supports can be 

evaluated to a level of detail that has not been possible before. The 

data show that the ROSS can determine the amount of rock movement 

very accurately and locate the occurrence within the distance between 

the teeth. Using both a ROSS and an instrumented bolt can lead to a 

new understanding of bolt-grout-rock interactions. The results show 

that strains recorded by the ROSS are higher than strains measured by 

instrumented bolts. The anchorage capacity of the ROSS is much 

higher and provides a method for evaluating the anchorage properties 

of bolts in situ. 

 Several ongoing tests are being conducted by NIOSH and will 

enable more comparisons of instrumented supports and ROSS’s to be 

made to study how different types of supports respond to rock 

movement. Additional research is required to study a number of 

different variables, including the effects of rock strength, roof bolthole 

properties, size of grout annulus, and grout strength. 

ACKNOWLEDGMENTS 

 Development of the ROSS was a team effort. Many different 

approaches and ideas were evaluated. I would especially like to thank 

Dennis Cox, JoAnne Johnson, Rich Rains, Tom Brady, and Richard 

Curtin for the ideas and hard work that made this instrument possible. 

REFERENCES 

Bauer, E.R., and Dolinar, R.J. 2000. Skin Failure of Roof and Rib 

and Support Techniques in Underground Coal Mines. In 

Proceedings: New Technology for Coal Mine Roof Support, ed. by 

C. Mark, D. R. Dolinar, R.J. Tuchman, T.M. Barzak, S.P. Signer, 

and P.F. Wopat. NIOSH Information Circular 9453, pp. 99-110. 

Choquet, P., and J. Hadjigeorgiou. 1993. The Design of Support for 

Underground Excavations. In: Comprehensive Rock Engineering: 

Principles, Practice, and Projects. Pergamon Press, Vol. 4, pp. 

313-345. 

Signer, S.P. Load Behavior of Grouted Bolts in Sedimentary Rock. 

2000. In Proceedings: New Technology for Coal Mine Roof Support, 

ed. by C. Mark, D. R. Dolinar, R.J. Tuchman, T.M. Barzak, S.P. 

Signer, and P.F. Wopat. NIOSH Information Circular 9453, pp. 73-

80 

Serbousek, M.O., and S.P. Signer, S.P. 1987. Linear Load Transfer 

Mechanics of Fully Grouted Bolts. Bureau of Mines Report of 

Investigations 9135, 17 pp. 

Littlejohn, S. 1993. Overview of Rock Anchorages. In: 

Comprehensive Rock Engineering: Principles, Practice, and 

Projects. Pergamon Press, Vol. 4, pp. 413-450 

Aziz, N., B. Indraratna, and Y. Wang. 1999. Study of Shear 

Loading and Bolt Load Transfer Mechanics under Constant Normal 

Stiffness Conditions. In: Proceedings, 18th International Conference 

on Ground Control in Mining, ed. by Syd S. Peng (Univ. of WV, 

Morgantown, WV, Aug. 3-5, 1999), pp. 239-247. 
NEW TOOLS FOR ROOF SUPPORT EVALUATION AND DESIGN 

Steve Signer and Carl Sunderman 

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