BOLT LOAD CHANGES DURING INITIAL FACE ADVANCE AND CROSS-CUT BREAKTHROUGH 

Steve Signer, Mining Engineer 

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

James Pile, Geotechnical Engineer, and Steve Bessinger, Engineering Manager 

BHP Billiton San Juan Coal Company, Waterflow, New Mexico, USA 
ABSTRACT 

 The San Juan Mine and the National Institute for Occupational 

Safety and Health conducted a study to measure how development 

mining affected bolt loads. Twelve fully grouted, instrumented roof 

bolts were installed at two three-way intersections as part of the 

standard bolting pattern. Newly developed miniature data 

acquisition systems (MIDAS) were used to measure bolt load 

changes during initial face advance and cross-cut breakthrough. 

The effects of cut placement and depth on roof bolt loads were 

studied. 

 This test showed how bolt loads increased at five positions along 

the bolt length during initial mining. Both entry advance and crosscut breakthrough produced a similar percentage of increase in bolt 

loads. Geologic differences between the test sites were probably 

responsible for the differences in amounts of bolt loading. The test 

site with more top coal and a higher rock quality designation 

(RQD) had lower bolt loads. 

BACKGROUND 

 The operator of a continuous miner cannot go inby permanent 

support. MSHA regulations limited cut depth to 6 m (20 ft) until 

the development of remote-controlled miners, which permitted cut 

depth to be extended up to 12 m (40 ft). At this depth, an operator 

of a shuttle car is just outby the last row of supports. Extended-cut 

mining began in the 1980's and has become very popular because 

this method increases productivity. However, not every ground 

condition is suitable for extended-cut mining. To address safety 

concerns, studies were done by the U.S. Bureau of Mines and the 

National Institute for Occupational Safety and Health (NIOSH). 

Some studies looked at the accidents statistics of extended-cut 

mining versus cuts of normal depth (Bauer et al., 1993; Grau et al., 

1997). Mark (1999) correlated Coal Mine Roof Rating (CMRR), 

entry width, and overburden to roof standup time and determined 

that CMRR values less than 38 indicated that extended-cut mining 

was not feasible. Thus, while this method is excellent as an overall 

design guideline, it may not be practical where ground conditions 

are marginal nor address questions about how deep a cut can be 

taken under various mining conditions. 

 One way to evaluate site-specific conditions is to monitor roof 

behavior. Strain-gauged bolts have proven to be invaluable for 

monitoring bolt loads in many different mine situations. Previous 

studies at eight different coal mine sites (Signer, 2000) where 92 

strain-gauged bolts were installed showed that 75% of the bolts 

reached the yield point of the steel (0.2% strain) and 50% of the 

bolts exceeded the yield point. Thus the use of strain-gauged bolts 

could indicate roof behavior during extended-cut mining. However, 

this type of instrument has never measured bolt load during mining 

because attaching a data acquisition system would interfere with 

mining. It is important to note that loading on a bolt only measures 

movement in adjacent rock and may not give a complete picture of 

roof behavior at some distance away. 

 Researchers at NIOSH have developed a miniature data 

acquisition system (MIDAS) designed to withstand the harsh 

environments of underground mines (Sunderman et al., 2003). 

MIDAS was designed to measure strain from resistive sensors, such 

as strain-gauged bolts, cable bolts, CSIRO stress-measuring gauges, 

pressure transducers, and string pots. MIDAS is attached directly to 

the instruments 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 onboard clock sets the scan rate for readings in real time and timestamps each scan. A thermistor records temperature during each data 

scan, and a RS232/RS485/IRdA port can communicate with any 

computer. 

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

battery will provide enough power to take daily readings on all 

channels and run 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 (30.5 mm in diameter by 216 mm long 

[1.2 by 8.5 in]) makes the MIDAS adaptable for almost any location. 

Because it is so small and self-contained, it can monitor instruments 

while a continuous miner is cutting coal. The instrument has an 

experimental MSHA permissibility approval, which allows 

monitoring instruments to be operated when methane is present, so 

the bolts can be monitored during the initial phase of mining. 
t sites is 

shown in figure 3. The first cut began on the left side and is marked 

A. This cut advanced 4.6 m (15 ft), then the miner changed sides to 

B. This was repeated for cuts C and D, which were advanced 

approximately 3.6 m (12 ft). E is the time that entry mining was 

completed. The cross-cut breakthrough is marked F. 

TYPES OF BOLT LOADING 

 Bolt loads can be axial, bending, and/or shear. Axial loading is 

generally the primary force on a steel bolt, although under some 

conditions, combinations of axial, bending, and/or shear forces can 

cause bolt failure. Shear loads are impossible to estimate with this 

type of instrument because of the nature of the loading mechanisms 

and the uncertainties of load locations. However, when joint 

movement is present, shear loading can be critical when designing 

bolt systems, and additional research is required for a better 

understanding of this loading mechanism. 

 A design engineer should consider several factors when 

calculating bending moments as measured by instrumented bolts. 

Maximum bending moments can be localized and may not be 

measured accurately. Bending is measured in only one plane, but 

can take place in other directions, especially if high horizontal 

stress fields are present. Bending moments can also be caused by 

joint movement, large-block rotation, and/or differential loading in 

mats and meshes. 

INSTRUMENTS 

 The strain-gauged bolts were made by milling a slot 6.4 mm (1/4 

in) wide and 3.2 mm (1/8 in) deep and attaching five strain gauges 

on both sides. The stain gauges were spaced at 406-mm (16-in) 

intervals from the head of the bolt. After the bolts were slotted, 

yield load was 149 kN (33,500 lb), ultimate load was 249 kN 

(55,900 lb), and cross-sectional area was 3.61 cm2

 (0.56 in2). Prior 

to slotting, yield load had been 160 kN (36,000 lb), ultimate load 

269 kN (60,500 lb), and cross-sectional area 3.87 cm2

 (0.60 in2). 

The after-slotting figures represent a reduction in capacity of 

approximately 7%. Spacing of the instrumented bolts was reduced 

by 10% to account for the reduced capacity. The bolts were 

installed in a 35-mm (1-3/8-in) diameter hole using slow-setting 

resin. During installation, all bolts were oriented with the strain 

gauges parallel to the entry ribs. 

 Each strain-gauged roof bolt had a MIDAS unit attached. The 

MIDAS amplifier gain and frequency response were set to measure 

over a range of ±562.5 mV (250,000 microstrain) with a resolution of 

approximately 0.067 mV (0.03 microstrain). Variations in the roof 

bolts and strain gauges produced more noise than the resolution of 

the MIDAS, which made it a perfect choice for measuring straingauged bolts. 

 Bolt strain was calculated directly from the voltage readings 

based on the equation—

, = 4 )V

 (GF)(EV) 

Figure 1.—Test sites 
Figure 2.—Roof geology at A, cross-cut 17, and B, cross-cut 52 
Figure 3.—Cut sequence 

where ε = Strain, in/in, 

V = Change, V 

 GF = Gauge factor, 

and EV = Excitation voltage. 

 Bolt load was calculated by the formula—

where E = Modulus of elasticity 

and A = Bolt area. 

 If bolt strain exceeded the yield point of the steel, then the 

nonlinear stress-strain curve derived from laboratory tests was used 

to calculate bolt load from strain. Axial loading was calculated by 

averaging the load on each side of the bolt at each gauge position. 

Figure 4.—Axial load on bolt 62 from time A to time E 

RESULTS 

 The electrical connector for bolt 65 was damaged during 

installation, so no data from that instrument are presented. Three 

gauges (at the 0.8-, 1.6-, and 2-m [32-, 64-, and 80-in] positions) on 

bolt 68 had open circuits after installation. Axial load at each of these 

positions was calculated using only one gauge, and bending strain is 

not shown. Figure 4 shows the change in axial load for bolt 62 as the 

continuous miner made the four advance cuts. Points A thru D 

represent the time when mining began on each cut. Point E represents 

the time during which mining stopped on the last cut. Each curve 

represents the change in axial load at that bolt section. The highest 

load occurred at the 0.8- and 1.2-m (32- and 80-ft) sections. In 

general, this was true for most of the bolts. 

 Figure 5 compares axial load at the 0.8-m (32-in) position for all 

bolts at both test sites during entry advance, and figure 6 shows the 

same information during the cross-cut breakthrough. The highest 

loads occurred at the bolts in the center of the entry during entry 

advance. When the cross-cut was broken through, the bolts in the 

center of the span loaded to almost the same levels as the bolts in the 

center of the entry. Figures 7 and 8 show a representation of axial 

load in kilonewtons on all bolts at three time intervals. Bolts 61 and 

68 are in both rows, so are shown twice. Time A is the load before 

any mining activity, E is after entry development, and F is after the 

cross-cut breakthrough. Yield load for the instrumented bolts is 149 

kN, and only two bolt sections reached that load, both on bolt 62. 

Maximum axial load for each bolt was averaged with respect to test 

section, location (entry or cross-cut), and time (table 1). 

 Figures 9 and 10 show bending strain for the bolts in cross-cuts 17 

and 52. The highest bending load changes occurred during the crosscut breakthrough. Bending strains for the bolts in cross-cut 52 were 

significantly lower than for cross-cut 17. Bolt yield occurred at 

approximately 2,000 microstrain. One bolt section reached that strain 

level; all other bending values were significantly less than yield. The 

position that showed the most consistent bending was at 0.8 m (32 

in). 

P = E A ,
Figure 5.—Comparison of axial load during entry advance at gauge 

position 0.8 m 

DISCUSSION 

 Bolts at the center of the entry loaded faster and to higher values 

when the entry cut was directly in front of the bolts, and the bolts 

farthest away loaded to lower values. This is what was expected. 

As the third and fourth cuts were taken, loads on the bolts increased 

at a slower rate and stabilized until the cross-cut was broken 

through. Then most of the load increase was on the cross-cut bolts, 

with load in the center increasing the fastest. The highest loads 

occurred just below the midsection bolt, which was consistent with 

previous studies where the roof was not subjected to high horizontal stresses. Bolt yield only occurred on one bolt, and that was 

after the cross-cut had been broken through. 

 The largest percentage of increase in bolt load resulting from 

entry development occurred in the lower midsection of the bolt, 

where load was already highest. When the cross-cut was broken 

through, the largest percentage of increase was at the ends of the 

bolts in the cross-cut. The highest amount of loading in these bolts 

was farther up in the roof and indicates more stress was being 

applied to the bolt anchorage. One reason the percentage of 

increase was higher at the end of the bolts was that load levels at 

this position were initially low. 

Figure 6.—Comparisons of axial load during cross-cut breakthrough at gauge position at 0.8 m 

Table 1. Average of maximum axial loads, kN 

Cross-cut 17 Cross-cut 52 Time 

All Entry Cross-cut All Entry Cross-cut 

A 18 21 19 20 23 18 

B 30 40 24 34 42 20 

C 51 73 26 40 51 21 

D 55 79 28 42 54 21 

E 60 87 29 43 56 22 

F 119 123 116 71 79 58 

 Geological differences between test sites were probably the 

biggest factor affecting bolt loads. The additional top coal at crosscut 52 combined with more competent rock strata produced bolt 

loads significantly lower than at cross-cut 17. The results indicate 

that a bedding plane was present in the cross-cut 17 test site and 

produced some movement during initial mining. The movement 

may have resulted from the influence of the anomalous geology, as 

this bending behavior was not evident at all at cross-cut 52. 

Movement could have also contributed to roof degradation. 

TIME, DAYS

0 1 2

BOLT LOAD, KN

160 

140 

120 

100 

80 

60 

40 

20 

0 

59

60

61

62

64

 Crosscut 17

KEY

F

TIME, DAYS

0 1 2

BOLT LOAD, KN

160 

140 

120 

100 

80 

60 

40 

20 

0 

66

67

68

69

70

71

 Crosscut 52

KEY F
CONCLUSIONS 

 The highest bolt loads during entry advance were in the center of 

the entry at the cross-cut 17 test site. The largest increase in bolt 

load occurred when the cross-cut was broken through. The entry 

face advance increased the average of maximum bolt loads by 75% 

at cross-cut 17 and 59% at cross-cut 52. When the cross-cut was 

broken through, the average maximum bolt load increased by 75% 

and 62%, respectively. 

 The largest increase in loading on the entry bolts occurred when 

both sides of the initial advance cuts were taken. The second set of 

cuts produced less bolt loading. Geological differences between the 

test sites were probably the biggest factor in these differences. The 

additional top coal at cross-cut 52, combined with more competent 

rock strata, produced bolt loads significantly lower than in crosscut 17. The results indicate that a bedding plane in the cross-cut 17 

test site produced some movement during initial mining. 

 The MIDAS datalogger provided the ability to measure bolt 

loads during initial mining. Because the strain-gauged bolts only 

measure localized rock movement, the question of whether this 

type of instrument can be used to determine the depth of cut for 

extended cuts remains unanswered. This test is a good case study of 

how bolts load quickly during initial mining around a section and 

the effects of geological conditions on bolt loading. 

REFERENCES 

Bauer, E.R., G.J. Chekan, and L.J. Steiner. 1997. Stability 

Evaluation of Extended Cut Mining in Underground Coal Mines. In 

Proceedings, 36th U.S. Rock Mechanics Symposium. June 29-July 

2, 1997, Columbia University., NY. International Journal Rock 

Mechanics & Mining Science 34:3-4, Paper 302. 

Grau, R.H., and E.R. Bauer. 1997. Ground Control Worker Safety 

During Extended Cut Mining. In Proceedings, 15th International 

Conference on Ground Control in Mining (Morgantown, WV). 

West Virginia University, pp. 283-288. 

Mark, C. 1999. Application of Coal Mine Roof Rating (CMRR) to 

Extended Cuts. Mining Engineering, April, pp. 52-56 

Signer, S.P. 2000. Load Behavior of Grouted Bolts in Sedimentary 

Rock. 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 

Sunderman, C., S. Signer, and J.A. Johnson. 2003. A Miniature 

Data Acquisition System with LED Warning Lights. 4th 

International Conference on Computer Applications in the Mineral 

Industry. Published on CD-ROM. 

Figure 7.—Axial bolt loads at times A, E, and F for cross-cut 17, kilonewtons 
Figure 8.—Axial bolt loads at times A, E, and F for cross-cut 52, kilonewtons. 

 

Figure 9.—Bending at times E and F at cross-cut 17, microstrain 
Figure 10.—Bending at times E and F at cross-cut 52, microstrain