UNITED STATES DEPARTMENT OF THE INTERIOR I 'x % 

w 

i r%'-d I 
Report of Investigations 9301 

Field Verification of Load Transfer 

Mechanics of Fully Grouted Roof 

Bolts 

By S. P. Signer 
 by 

 Title. 

. 

CIP 
CONTENTS 

Abstract ........................................................................... Introduction ........................................................................ Acknowledgments .................................................................... Load transfer mechanics ............................................................... Testprocedures ..................................................................... Boltloading ...................................................................... Instrumentation ................................................................... Testsites ........................................................................ Testresults ........................................................................ Elastictests ...................................................................... Postyieldingtests .................................................................. Conclusions ........................................................................ References ......................................................................... Appendix-Calibration procedure and data reduction .......................................... 

ILLUSTRATIONS 

Pulltestgeararrangement ....................................................... Gauge locations on instrumented bolts .............................................. Average field test results ......................................................... Comparisonofresults ........................................................... Average results from mine 1 ...................................................... R

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FIELD VERIFICATION OF LOAD-TRANSFER MECHANICS 

OF FULLY GROUTED ROOF BOLTS 

By S. P. Signer 

ABSTRACT 

The Bureau of Mines conducted a series of field tests to improve the understanding of the support 

interaction mechanics between fully grouted bolts and coal mine roofs and to help lay the foundation 

for improved design and evaluation techniques. Strain gauges were installed on 14 fully grouted bolts 

placed in shale roof rock at four mines in Colorado, Illinois, and Pennsylvania to determine how load 

was transferred between the bolts and the rock. The results of field tests on elastic bolt behavior 

compared well with previous laboratory work and numerical models. The field tests showed that the 

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bolts have been formulated (111); numerical models of 

grouted bolt systems have been developed (12-13), and 

empirical approaches based on rock mass identification 

and classification have been proposed (14). Laboratory 

models have been created to study the effects of shear 

resistance of grouted bolts and to determine how shear 

resistance produces beam building in mine roofs (15-17). 

Bolts have been instrumented with strain gauges and 

studied both in the laboratory and in situ (18-25). 

However, the mechanics of interaction among parts of the 

Results from an axisymmetric finiteelement numerical 

model was compared with these test results and is detailed 

in reference 1. 

To determine if the results fro^ the laboratory tests 

could be applied to conditions encountered in mine rock, 

similar tests were performed at four different coal mines 

in Colorado, Illinois, and Pennsylvania. The comparisons 

between the laboratory tests and field tests are detailed in 

this report. 

ACKNOWLEDGMENTS 

The author would like to thank mine personnel at the the Galatia Mine, Harrisburg, IL; and the Warwick Mine, 

Eagle Mine, Craig, CO; the Wabash Mine, Keensburg, IL; Greensboro, PA, for their assistance and cooperation 

during installation of the instruments. 'italic numbers in parentheses refer to items in the list of references 

preceding the appendix at the end of this report. 
LOAD TRANSFER MECHANICS of the installation; the smoothness OF the drill hole; and 

wssiblv other factors such as mout annulus. Weaker 

The redistribution of forces along a bolt is the result of 

movement in the roof strata. This movement may be 

vertical (strata separation) or horizontal (strata slippage). 

One mechanism that retards strata slippage is a doweling 

effect created when the grout and bolt completely fill the 

hole. Strata separation is resisted by the axial strength of 

the roof bolt. This study will examine only the axial response of the bolt system. 

Load is transferred between the bolt and the rock by 

shear resistance in the grout. This resistance could be the 

result of adhesion and/or mechanical interlocking. Adhesion is an actual bonding or gluing among the grout, the 

steel, and the rock; and mechanical interlock is a keying 

effect created when grout fills irregularities between the 

bolt and the rock. Adhesion is considered by some 

researchers to be the primary means of shear resistance in 

a grouted bolt system. However, test bolts were examined 

and showed no adhesion between the grout-bolt or groutrock interfaces. 

Mechanical interlock will transfer load between the 

.2 

grout and/or rock may require longer anchorage lengths 

because of reduced shear strength. Proper installation of 

the bolt system is critical to its performance. If the grout 

is inadequately mixed, is overspun, or is glove fingered, 

then the capacity of the grout to provide mechanical interlock is severely impaired. Glove fingering occurs when 

parts of the plastic wrapper of the resin cartridge is not 

shredded during installation. 

Various types of axial failure can occur when using 

grouted bolts. Failure can take place in the bolt, the 

grout, the rock, or at the bolt-grout or grout-rock interfaces. The type of axial failure depends on the characteristics of the system and the material properties of individual 

elements. 

If the bolt has sufficient length to transfer all the bolt 

load to the rock, then the bolt will fail if the ultimate 

strength of the bolt is less than what is required to support 

the rock load. Adjustments in the design of bolt spacing, 

length, diameter, and strength must be made so that the 

capacity of the bolting system is sufficient. 
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and 0.125 in deep. This configuration allowed up to six 

gauges to be placed along one side of the bolt. The 

gauges were positioned in pairs on each side of the bolt to 

account for any bending effects and to provide redundancy. 

All bolts were grade 40, No. 6 steel bars with forged 

heads, and all were from the same lot. (However, tests 

conducted on the bolts indicated a yield point comparable 

with grade 50 steel.) 

Typically, instrumented bolts measure strain, and the 

load is then calculated using the modulus of elasticity and 

the area of the bolt. This method presents problems 

because the area of the bolt cross section is not well defined, and gauge alignment is critical in obtaining accurate 

results. In this experiment, strain gauges were installed on 

the bolts and, using statistical methods, were calibrated in 

a uniaxial tension machine to correlate voltage change 

directly with load. This technique eliminated problems of 

area reduction, gauge location, and localized 

inconsistencies in the bolt and produced excellent test 

results having good repeatability. The procedure is 

presented in more detail in the appendix. 

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using calibration factors obtained from prior laboratory 

axial tension tests. At the next three test sites in l'ois 

and Pennsylvania (mines 2, 3, and 4), a port'ible data 

acquisition system collected data from the strain gauges 

and pressure transducer and stored the raw voltage readings for later computer reduction. The system provided 

5-V excitation to a full bridge configuration. Details of 

this system are also included in the appendix. 

Bolt holes at mine 1 were dried using water, while bolt 

holes at the other three mines were drilled with a dry 

vacuum system. At mine 2, the vacuum system on the 

drilling machine did not work. Therefore, the holes had to 

be brushed to remove accumulated dust. It was found that 

hole diameters at this test site were larger than normal, 

which caused two bolts to have a partial grout column and 

required using a thin wire to measure them. Test results 

from these two bolts were not averaged with the other 

results. Five bolts were tested at mine 1 and three bolts 

were tested at each of the other mines. 

TEST SITES 4~efennce to specific products docs not imply endorsement by the 

Bureau of Mines. 

Tests were performed at four different coal mines, one 

in Colorado, two in Illinois, and one in Pennsylvania; all 
0.75-in diam 

0.25" 

'A Lo., 25" 

Section A-A 

/ f 

Electrical strain gauges 
These variations in grout type and hole size had no 

statistically significant effect. An axisymmetric model was 

created to match stress distributions and bolt defections 

with those from the laboratory tests. 

Figure 44 shows a comparison of load distributions 

along the lengths of 4-ft bolts using an applied load of 

12,800 lbf for laboratory and field tests and the numerical 

model. Figure 4B compares the standard deviations 

derived from 50 bolts used in the laboratory work with 

7 bolts from the field tests. These results show that bolts 

installed in shale required slightly longer lengths to 

transfer load between the bolt and the rock compared with 

bolts used in the laboratory tests. Standard deviations 

were larger for the field results. This is to be expected 

because of geological variations. Plots of the results of all 

field tests are included in the appendix. 

The roof at mine 1 contained layers of weaker rock. 

Test results from five bolts installed and tested at this 

mine reflected the presence of these weaker layers as 

changes in the rate of load transfer. A weaker layer 

requires a longer anchorage length compared with that 

needed in stronger rock. The stiffness of the bolting 

system decreases in the weaker zones. Figure 5 shows a 

plot of data from one of the bolts tested at mine 1. Three 

bolts were installed and tested in mines 53, and 4, a total 

of nine bolts. 

0 6 12 18 24 30 36 42 

DISTANCE FROM BOLTFEAD. in 

Figure 6.-Results from tests conducted past yleld strength of 

steel bolt 

POSTYIELDING TESTS 

Comparatively small amounts of roof movement are 

required to cause a bolt to exceed the yield point of the 

steel. However, the steel used for grouted roof bolts is 

ductile and can sustain a large amount of deflection before 

it fails. After each bolt was tested in the elastic range, a 

test was conducted in which each bolt was loaded past the 

yield point of the steel. Typical test results are shown in 

figure 6, and the rest of the plots are included in the 

appendix. 

Monitoring strain gauges until loads pass the yield point 

of the steel presents several problems. When a steel bolt 

yields, readings from the strain gauges are inaccurate past 

5,000 microstrain. Another problem is caused as the bolthead stretches, sometimes more than 2 in. This deformation can cause the lead wires to the strain gauges to stretch 

and break. This stretch will change the resistance, which 

changes the relationship between electrical readings and 

load. 
For these reasons, the load in a bolt is represented in Readings were taken until the gauges no longer functhe plots as the applied load after yielding has occurred at tioned properly. The depth of yield along the different 

that station. Before the steel bolt yields, the grout will bolt lengths varied significantly from 3.5 in (station 1) 

reach its peak shear strength and begin to fail. However, to 21.5 in (station 4). Because of the problems already 

load is still transferred between the bolt and the rock by mentioned, the depth of yielding could be farther along the 

the residual shear strength of the grout. For this reason, bolt length, but that information was not practically 

the actual load in the bolt will be less than the applied obtainable. 

load. The straight line represented in the yield zone is not 

a true representation of load distribution. 

CONCLUSIONS 

The results from the axial elastic tests conducted on 

grouted bolts installed in shale compared well with the 

results from previous laboratory work and numerical 

modeling as detailed in reference 1. The average anchorage length for bolts installed in shale was slightly 

longer than for bolts installed in concrete blocks. The 

field results showed more variability. 

Tests conducted past the yield point of the steel indicated that the yield zone will vary from bolt to bolt and 

will translate down the length of the bolt anywhere from 

4 to 22 in. If there is sufficient length of bolt past the 

yield zone, then the load will transfer from the bolt to the 

Pull tests are routinely conducted on grouted bolts to 

evaluate anchorage capacity and installation quality. Results from this research show that the load applied during 

a standard pull test is dissipated into the rock within 24 in 

of the bolthead. However, anchorage at the end of the 

bolt, which is critical for proper support, is not being 

tested. Therefore, it is very difficult to evaluate properly 

the capacity of a grouted bolt by a standard pull test. Additional instruments, such as strain gauges, are required 

to provide a complete assessment of grouted bolts. 

These results can be used as a guide when selecting 

grouted bolts for support of coal mine roofs. However, 

APPENDIX.-CALIBRATION PROCEDURES AND DATA REDUCTION 

The calibration procedure used a linear statistical 

regression analysis to establish the relationship between 

applied load and voltage change. To ensure accuracy, data 

for the calibration of each bolt were taken from three 

loading cycles. The applied load was limited to the elastic 

range of the steel. The voltage change for each gauge was 

statistically correlated to the load to obtain a slope and an 

intercept. If variations larger than 0.5 pct were found, 

then the gauge was replaced. Calibrations for each gauge 

on each bolt were stored in a computer file and were used 

to reduce the experimental voltage readings to values for 

load automatically and to plot the results without manual 

data manipulation. Typically, the standard deviation of the 

predicted load value using a least squares linear fit was 

approximately +50 lbf. This meant that the strain gauges 

on the bolt measured the load to within 100 Ibf. This 

procedure produced excellent test results with good 

repeatability. 

components of the system were battery powered for 

portability, however, this system was not permissible and 

had to be used in fresh air. 

Test Data 

Linear 

The following plots (figs. A-1 through A-4) are the 

results of pull tests conducted in the elastic range of the 

bolts. Each test was an average of three runs. Load in a 

bolt was obtained by converting voltage readings to load by 

calibration factors obtained for each gauge. At mine 2, 

two bolts had insufficient grout to fill the hole completely, 

resulting in a partial column of grout. The test results 

from these two bolts showed similar load transfer characteristics from the start of the grout column (fig. A-2). 

Postyield 

DETAILS OF TEST EQUIPMENT 

The data acquisition system consisted of a Hewlett

The following plots (figs. A-5 through A-8) are the results of pull tests conducted past the yield point of the 
DISTANCE FROM BOLTHEAD, in 

Figure Al-Results from elastic test, mine 1. A, Bolt 1; 8, 

bolt 2; C, bolt 3; D, bolt 4; E, bait 5. 

DISTANCE FROM BOLTHEAD, in 

Figure A-2.-Result. from elantic test, mine 2. A, Bolt 1; B, 

bolt 2; C, bolt 3. 

DISTANCE FROM BOLTHEAD, in 

Flgure A-5.-Remtts from plastic test, mlne 1. A, Bolt 1; B, 

bott 2; C, bolt 3; 0, bolt 4; E, bolt 5. 

0 6 12 18 24 30 36 42 

DISTANCE FROM BOLTHEAD, in 

Figure A-6.-Resub from plastic test, mlne 2 A, Bolt 1; B, 

bolt 2; C, bolt 3.