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IC 
 
 9058 
 
 Bureau of Mines Information Circular/1986 
 
 Anchorage Capacities in Thick Coal Roofs 
 
 By Stephen C. Tadolini 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 
Information Circular 9058 
 
 Anchorage Capacities in Thick Coal Roofs 
 
 By Stephen C. Tadolini 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 
 Donald Paul Hodel, Secretary 
 
 BUREAU OF MINES 
 Robert C. Horton, Director 
 
Trims 
 •ft 
 
 no. 9dsx 
 
 Library of Congress Cataloging in Publication Data: 
 
 Tadolini, Stephen C 
 
 Anchorage capacities in thick coal roofs. 
 
 (Information circular / United States Department of the Interior, 
 Bureau of Mines ; 9058) 
 
 Bibliography: p. 11-12. 
 
 Supt. of Docs, no.: I 28.27: 9058. 
 
 1. Coal mines and mining— Safety measures. 2. Mine roof bolting. 
 I. Title. II. Series: Information circular (United States. Bureau of 
 Mines) ; 9058. 
 
 TN295.U4 622s [622'. 334] 85-600220 
 
tf- 
 
 *3- 
 
 J) CONTENTS 
 
 "^ Page 
 
 Abstract 1 
 
 Introduction 1 
 
 Acknowledgments 2 
 
 Pull test: A method for determining the strength of a rock bolt anchor 2 
 
 Characteristics of roof bolting systems 2 
 
 Atlas Copco Swellex system 2 
 
 Combination system 3 
 
 Mechanical system 4 
 
 Fully resin-grouted system 4 
 
 Strain gauge system 5 
 
 Pull test results 5 
 
 Atlas Copco Swellex bolts 6 
 
 Combination bolts 6 
 
 Mechanical expansion anchor bolts 7 
 
 Resin-grouted bolts 7 
 
 Strain-gauge bolts 7 
 
 Representation of pull test by finite element model 9 
 
 Results of finite element analysis 10 
 
 Conclusions 11 
 
 References 11 
 
 Appendix. — Torque-tension ratio tests.. 13 
 
 ILLUSTRATIONS 
 
 1. Tested roof bolts V 3 
 
 2. Atlas Copco Swellex bolt manufacturing sequence 4 
 
 3 . Atlas Copco Swellex bolt expansion sequence 4 
 
 4. Strain-gauge bolts 5 
 
 5. Pull test apparatus 6 
 
 6. Swellex bolt pull test results 6 
 
 7. Combination bolt pull test results 7 
 
 8. Mechanical expansion anchor bolt pull test results 7 
 
 9. Resin-grouted bolt pull test results 8 
 
 10. Strain-gauge locations and resin columns in strain-gauge bolts 8 
 
 11. Load-strain curve for strain-gauge bolt with 24-in resin column 9 
 
 12. Load-strain curve for strain-gauge bolt with 36-in resin column 9 
 
 13. Finite element model 10 
 
 14. Typical plot of percent of applied load versus bolt distance 10 
 
 TABLE 
 
 1. Physical properties of materials used in finite element model 9 
 
 <4» 
 
 
 o 
 —ft 
 

 UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 
 
 ft 
 
 foot lb/in 3 pound per cubic inch 
 
 in 
 
 inch yin microinch 
 
 lb 
 
 pound psi pound per square inch 
 
 lbf-ft 
 
 pound (force) foot 
 
ANCHORAGE CAPACITIES IN THICK COAL ROOFS 
 
 By Stephen C. Tadolini 
 
 ABSTRACT 
 
 The anchorage capacity of four types of bolting systems was investi- 
 gated in two mines with thick coal roofs. In conjunction with the in- 
 mine tests, a simple axisymmetric finite element computer program was 
 developed. Both the field and computer results indicated that thick 
 coal roofs can be effectively supported entirely by the coal member. 
 
 INTRODUCTION 
 
 Many coal mines in the Western United States are located in thick coal 
 seams that often leave the mining operators several feet of coal to form 
 the mine roof. Information is needed concerning effective roof support 
 in such mines. In this investigation of bolt anchorage capacities in 
 thick coal roof, the following parameters were investigated: 
 
 1. Type of anchorage — mechanical or chemical, either fully or par- 
 tially bonded, using resin or cement as the bonding agent. 
 
 2. Length of anchor — short (4 ft or less), medium (4 to 6 ft), or 
 long bolts (6 ft or more). 
 
 3. Diameter of anchor — small (less than 0.5 in), medium (from 0.5 to 
 1.0 in), or large (greater than 1.0 in). 
 
 To establish basic support criteria for mines with thick top coal, 
 various types of roof bolts were investigated in small-scale in-mine 
 tests. The bolts were pull tested to determine the average amount of 
 support capacity that could be expected. In conjunction with this field 
 investigation, a simple axisymmetric finite element model was devel- 
 oped to simulate pull tests. The model simulated a 3/4-in-diam bolt 
 installed in a 1-in-diam hole in a thick coal roof. To ensure equilib- 
 rium and compatibility, deformation lengths were predetermined using 
 strain-gauge bolts. The information obtained from the pull tests was 
 compared with data derived from the finite element model to provide a 
 fundamental approach for designing an adequate support system for thick 
 coal roofs. 
 
 ^ Mining engineer, Denver Research Center, Bureau of Mines, Denver, CO. 
 
ACKNOWLEDGMENTS 
 
 The author expresses gratitude to the 
 Consolidation Coal Co. and Valley Camp 
 of Utah Inc. for their continued support 
 of ground control research. Special 
 thanks are extended to Steven Jaccaud, 
 superintendent of Consolidated 1 s Emery 
 
 No. 50 Mine, Emery, UT, and Virgil Lamb, 
 superintendent of Valley Camp's Belina 
 No. 1 Mine, Scofield, UT, who made avail- 
 able personnel and equipment for this 
 study. 
 
 PULL TEST: A METHOD FOR DETERMINING THE STRENGTH OF A ROCK BOLT ANCHOR 
 
 Pull tests were conducted in two mines. 
 In the first mine, a total of 40 bolts 
 of four different types were tested. The 
 four types were — 
 
 1. Atlas Copco Swellex, 2 a bolt that 
 is expanded by high water pressure, 
 
 2. Combination resin-grouted and me- 
 chanical bolts, 
 
 3. Mechanical expansion anchor bolts, 
 and 
 
 4. Resin-grouted bolts. 
 
 The bolts were installed in a roof that 
 consisted of 8 ft of coal as predeter- 
 mined with a fiber-optic stratascope. 
 Various torque and expansion pressures 
 were used, except that these parameters 
 did not apply to the resin-grouted bolt 
 installations. In the second mine, only 
 fully resin-grouted bolts were tested, 
 again in a roof with thick top coal. 
 
 Pull tests were performed on each bolt 
 to determine the bolting effectiveness of 
 each individual bolt type. 
 
 The pull test is intended to measure 
 the strength of a rock bolt anchorage 
 system under field conditions. This is 
 based on the premise that the best way 
 to compare data obtained from anchorage 
 tests is to analyze the load-deformation 
 curve produced from plotting field infor- 
 mation. However, information obtained 
 from pull tests performed on full-column 
 grouted bolts reveals very little about 
 anchorage capacity. Tests on such bolts 
 usually determine only the strength of 
 the steel used to manufacture the bolt. 
 This makes it imperative that shorter 
 grout columns be investigated to deter- 
 mine actual anchorage capacities. 
 
 CHARACTERISTICS OF ROOF BOLTING SYSTEMS 
 
 Roof bolts have become an essential 
 means of support in all types of under- 
 ground openings. Numerous types and con- 
 figurations of bolts are presently uti- 
 lized to support excavations under all 
 types of conditions. 
 
 Previous investigations have involved 
 both the developmental and evaluation 
 stages of virtually all roof support de- 
 vices (l_-4). 3 In this investigation, 
 pull tests were conducted on the four 
 
 — _ 
 
 ^Reference to specific products does 
 not imply endorsement by the Bureau of 
 Mines . 
 
 3 Underlined numbers in parentheses re- 
 fer to items in the list of references 
 preceding the appendix. 
 
 types of bolts previously mentioned. 
 These bolt types are illustrated in 
 figure 1. Strain-gauge bolts were also 
 pull tested. The bolt types tested are 
 not the only types deemed to be accept- 
 able; they were, however, the bolts being 
 considered by mine operators in the area 
 where the tests were conducted at the 
 time of the investigation. 
 
 All bolt characteristics discussed in 
 the following sections are minimum labo- 
 ratory values. In some cases, the char- 
 acteristics vary from bolt to bolt. 
 
 ATLAS COPCO SWELLEX SYSTEM 
 
 The Swellex water-pressure expansion 
 bolt is manufactured from steel pipes 
 
1 '■ ■ ■ 
 
 *D 
 
 FIGURE 1. - Tested roof bolts. A, Atlas Copco Swellex; B, combination; C, mechanical expansion 
 anchor; D, resin grouted. 
 
 with a 1.61-in OD and a 0.08-in wall 
 thickness. The pipes are reshaped to the 
 Swellex profile with a 1.00-in OD as 
 shown in figure 2. The bolt ends are 
 strengthened with short support sleeves 
 and sealed with welds. A small hole 
 in the outer end of the bolt allows a 
 high-pressure pump to inject water into 
 the bolt, which then expands in the bore- 
 hole, as schematically shown in figure 3. 
 The bolt starts to expand at 880 to 1,180 
 psi. This expansion pressure causes the 
 shell of the tube to deform plasti- 
 cally around irregularities in the bore- 
 hole wall, thus creating a strong mechan- 
 ical bond between the bolt and the 
 rock. 
 
 The Swellex bolt also works in smooth 
 and straight (diamond-drilled) boreholes. 
 Anchoring is achieved as a result of the 
 high water pressure being transmitted to 
 the borehole wall and causing the bolt 
 and rock to expand elastically a few hun- 
 dredths of an inch. After installation, 
 when the water pressure is released (and 
 the water flows out of the injection hole 
 in the outer end of the bolt) , rock re- 
 laxation is prevented by the expanded 
 
 bolt, resulting in a residual pressure 
 between the bolt and the borehole wall. 
 The configuration of the expanded bolt 
 combined with its material properties 
 creates a spring action. 
 
 An additional support characteristic 
 of the bolt is its ability to provide 
 active support. As the bolt is expanded, 
 a small reduction in length occurs in the 
 lower portion of the bolt. This creates 
 tension in the bolt, which holds the 
 bearing plate firmly against the rock. 
 However, the resultant pressures can 
 fracture brittle rock. This fracturing 
 can be prevented by sliding a short tube 
 over the bolt to prevent expansion of the 
 bolt near the collar of the hole (5). 
 
 COMBINATION SYSTEM 
 
 The combination bolt utilizes resin- 
 grouted rods for anchorage and mechanical 
 bolts for tensioning the bolting system. 
 The combination bolts used in this study 
 consisted of a 2-ft length of 7/8-in-diam 
 rebar coupled with a 3-ft length of 5/8- 
 in-diam grade 55 steel (12,400 lb yield 
 and 19,200 lb tensile strength). The 
 
FIGURE 2. - Atlas Copco Swellex bolt manu- 
 facturing sequence. 
 
 physical distinction of the combination 
 system is the length of the resin 
 anchor — 24 in or greater. The anchor en- 
 cases a threaded joint in which the 
 mechanical bolt can be inserted. The 
 resin-grouted portion of the combination 
 system not only provides anchorage for 
 the device, but also reinforces the 
 strata like a fully grouted bolt. The 
 mechanical portion of the system can be 
 adjusted to exert a desired tension on 
 the bolt. This force is traditionally 
 measured in torque and can be varied to 
 limit deformation or prohibit deformation 
 entirely. 
 
 
 ililiiiiliilii; 3 wmzM* 
 
 FIGURE 3. - Atlas Copco Swellex bolt ex- 
 pansion sequence. 
 
 MECHANICAL SYSTEM 
 
 Mechanical bolting systems utilize a 
 split-block, wedge-type expansion shell 
 to create anchorage and support loads 
 generated through the tensioned portion 
 of the bolt. The mechanical bolts used 
 in this study were 5/8-in-diam grade 55 
 steel (12,400 lb yield and 19,200 lb ten- 
 sile strength) . 
 
 The success of a mechanical system is a 
 function of its anchorage capacity. Tra- 
 ditionally, clamping loads have been re- 
 stricted to one-half of the yield point 
 or anchorage capacity of the material in 
 order to preserve both the integrity of 
 anchorages and the bolt itself. To 
 ensure that proper torque levels were 
 used in the thick top coal, torque- 
 tension ratio tests were performed. (See 
 appendix. ) 
 
 FULLY RESIN-GROUTED SYSTEM 
 
 Fully grouted bolts rely on the mechan- 
 ical interlock of the resin and inter- 
 stices within the confines of the bore- 
 hole wall and along the surfaces of the 
 
reinforcing rod to bind roof strata to- 
 gether. The bolts used in this study 
 were comprised of 3/4-in-diam reinforc- 
 ing steel rod, grade 40 (17,600 lb yield 
 and 30,000 lb tensile strength) , grouted 
 in a 1-in-diam hole. No lateral forces 
 are generated with fully grouted bolts 
 at the time of installation, but high 
 levels of anchorage can be expected. The 
 horizontal reinforcement characteristics 
 of the bolt permit it to carry high 
 sheer-resistance values, with moderate 
 stiffness resistance (resistance to 
 bending) . 
 
 STRAIN-GAUGE SYSTEM 
 
 Strain-gauge bolts (fig. 4) were con- 
 strucetd to be pull tested in conjunction 
 
 with the other bolting systems. The 
 strain-gauge bolts used were 1-in-diam 
 rolled rebar bolts, each with a hole ap- 
 proximately 0.125 in in diameter along 
 the axis at the centerline. Six sets of 
 strain gauges were placed on 5-ft bolts 
 at three levels. The three gauge levels, 
 as measured from the top of the threaded 
 end, were 6 in, 27 in, and 57 in, re- 
 spectively. A total of 10 bolts were in- 
 stalled with 2 to 3 ft of resin grout, 
 enabling the strain gauges at the 27-in 
 level to be monitored with and without 
 resin grout. The bolts were pull tested 
 to obtain the traditional load-displace- 
 ment curve, as well as the strains mea- 
 sured by the gauges at 1,000-lb incre- 
 ments of load (6). 
 
 PULL TEST RESULTS 
 
 The bolts were installed in the test 
 sites and pull tested approximately 24 h 
 later. This ensured an adequate resin 
 cure and permitted retorquing of the com- 
 bination and mechanical bolts. To ensure 
 proper anchorage, the diameters of the 
 drilled holes were measured with a flexi- 
 ble hole gauge. Every fifth hole was 
 measured at 1-ft intervals, with a maxi- 
 mum fluctuation of 1/16 in in diameter 
 being deemed acceptable. The drill-hole 
 lengths were also corrected to compensate 
 
 for the 1-in pull collar that was needed 
 at the head of each bolt. For example, a 
 60-in bolt was placed in a hole drilled 
 to a depth of 59 in. Because all the 
 support systems tested require the same 
 pull-ring, no corrections were necessary 
 for the subsequent analysis. The bolts 
 were pull tested in accordance with the 
 methods recommended by the International 
 Society for Rock Mechanics (7_) . The pull 
 test apparatus used in the investigation 
 is represented in figure 5. 
 
 FIGURE 4. - Strain-gauge bolts. 
 
Mine roof 
 
 Roof bolt 
 
 and 
 bearing plate 
 
 Pulling collar 
 
 Aluminum 
 housing 
 
 Hydraulic piston 
 assembly 
 
 Deformation gauge 
 
 " To mine floor 
 FIGURE 5. - Pull test apparatus. 
 
 ATLAS COPCO SWELLEX BOLTS 
 
 The Swellex bolts were expanded using 
 two installation pressures — 3,087 and 
 4,410 psi, respectively. The results 
 varied, with the yield point for the 
 bolts installed at 3,087 psi (fig. 6) 
 
 o.i 
 
 0.2 0.1 0.2 
 
 DEFORMATION, in 
 
 FIGURE 6. - Swellex bolt pull test results. 
 5-ft bolts, 1.375-in-diam holes, at installation 
 pressures shown. 
 
 being reached at an average value of 9.5 
 tons. The deformation at this value 
 was <0.1 in. The bolts installed with 
 4,410 psi of installation pressure 
 yielded at an average load of approxi- 
 mately 11 tons. 4 The deformation at this 
 point was also <0.1 in. 
 
 COMBINATION BOLTS 
 
 The combination bolts were installed 
 using two different torque loads. The 
 results of the pull tests showed that 
 when the bolt was installed with 200 lbf* 
 ft of torque (fig. 7) , the average yield 
 point was 10.5 tons with displacements of 
 <0.1 in. The bolts installed with 150 
 lbf»ft of torque failed completely or 
 just met the testing criteria selected 
 for this study (8 tons of load with no 
 more than 0.2 in displacement). Because 
 pull tests are primarily concerned with 
 the vertical movements related to the 
 anchor of the bolt, these results might 
 appear to be unreasonable upon cursory 
 examination. All of the combination 
 bolts were installed with identical an- 
 chors consisting of 2 ft of resin grout. 
 
 4 In this report, "ton" indicates 2,000 
 lbf. 
 
0.2 0.3 0.1 
 
 DEFORMATION, in 
 
 0.2 
 
 FIGURE 7. - Combination bolt pull test re- 
 sults. 5-ft bolts, 0.75indiam, with 24-in res- 
 in columns, torques as shown. 
 
 However, if the pretensioning of the 
 grouted length is taken into considera- 
 tion, it would be possible to experience 
 a higher level of resistance for the same 
 increment of load for both torque lev- 
 els. The grouted portion is placed into 
 a higher degree of tension by the in- 
 creased amount of force applied at the 
 coupling. This grouted portion would, 
 therefore, show a higher yield strength 
 and, at the same time, less deformation. 
 
 MECHANICAL EXPANSION ANCHOR BOLTS 
 
 Mechanical bolts support the roof by 
 suspension, friction, and keying. The 
 effectiveness of mechanical bolting sys- 
 tems is determined by the quality of 
 their anchorage in the rock. In all 
 instances of bolt failure in the thick 
 coal roof, the strength of the bolt was 
 not approached, but the anchor failed 
 (fig. 8). Even when complete failure did 
 not occur, anchor slippage was observed. 
 The basic cause of the anchorage problem 
 was stress concentrations at the anchor- 
 age point. These concentrations can be 
 effectively dissipated by increasing the 
 contact area of the anchor by increasing 
 its length or diameter. The coal's low 
 compressive strength prevented most of 
 the keying anchorage. Failures occurred 
 at an average of 4 tons, with failure 
 
 0.2 0.3 
 DEFORMATION, in 
 
 FIGURE 8. - Mechanical expansion anchor 
 bolt pull test results. 
 
 0.5 
 
 considered as >0.2 
 displacement. 
 
 in total anchorage 
 
 RESIN-GROUTED BOLTS 
 
 The resin-grouted bolts were installed 
 using various grout lengths, starting at 
 2 ft and then increasing in 1-ft incre- 
 ments to full column length. The results 
 indicated <0.2 in bolt yield, regard- 
 less of column length, when the bolt 
 was subjected to 8 tons of load. The 
 bolts yielded at an average of 10.5 tons, 
 and displacement never exceeded 0.20 in 
 (fig. 9). The resin bolts achieved good 
 anchorage even in weak rock. One possi- 
 ble explanation could be that the grout 
 bonded all the cracks where dilation 
 was possible. This action produces a 
 suspension effect, confining stretching 
 to a short section of the bolt. 
 
 STRAIN-GAUGE BOLTS 
 
 Pull test results for the strain-gauge 
 bolts were similar to those for the 
 resin-grouted bolts. Small amounts of 
 displacement occurred as a result of bolt 
 elongation as opposed to anchor slippage. 
 The 5-ft bolts were installed using 24- 
 and 36-in-long resin columns. The gauge 
 locations and resin columns are shown 
 in figure 10. The setup enabled strains 
 
0.2 
 
 0.3 
 
 0.1 
 
 0.2 
 
 0:3 0.1 02 
 
 DEFORMATION, in 
 
 0.3 
 
 0.1 
 
 02 
 
 0.3 
 
 0.4 
 
 FIGURE 9. - Resin-grouted bolt pull test results. 5-ft bolts, 0.75 in diam, resin column lengths as 
 shown. 
 
 • 24" resin column --36" resin column 
 
 ± 
 
 £ 
 
 
 
 
 
 <e 3 "-» «J 
 
 24" 
 
 27' 
 
 ■Sf — 6"- 
 
 >*- 4 
 
 A 
 
 33"- 
 
 57"- 
 
 60 
 
 FIGURE 10. - Strain-gauge locations and resin columns in strain-gauge bolts. 
 
 to be recorded with varying amounts of 
 grout. The strain gauges indicated dis- 
 tinctive stress patterns at the three in- 
 strumented levels. 
 
 The first type of field test was under- 
 taken to determine strain gauge responses 
 along the length of the bolt. A 5-ft 
 bolt was installed with 2 ft of resin 
 grout. This placed the grout 3 in above 
 the gauge located at the 27-in level. 
 The results, shown in figure 11, indi- 
 cate that the gauges at the bottom and 
 middle positions behaved practically 
 identically. Also, the strains measured 
 by these two gauges corresponded with the 
 calculated strains predicted by the fol- 
 lowing equation: 
 
 where 
 
 P = EeA, 
 P = load, lb, 
 
 and 
 
 E = Young's modulus, psi, 
 
 e = strain, 10~ 6 in, 
 
 A = area of bolt, in 2 (the effec- 
 tive area for the strain 
 gauge bolts in 0.55 in 2 ). 
 
 The gauge at the 57-in level was encased 
 by 21 in of grout. Its response was ap- 
 proximately 6% of the strains measured 
 in the unencapsulated gauges. This gauge 
 response was highly influenced by end ef- 
 fects. The results do illustrate, howev- 
 er, how rapidly the loads dissipate when 
 the grout column is encountered. 
 
 The second type of field test analyzed 
 bolts that were installed with 3 ft of 
 resin grout. Again, the bottom gauge was 
 unencapsulated to check the system re- 
 sponse. The middle and top gauges were 
 
KEY 
 
 Gauge position: 
 a Top 
 ■ Middle 
 • Bottom 
 
 200 300 400 500 600 700 
 STRAIN, uin 
 
 FIGURE 11. - Load-strain curve for strain- 
 gauge bolt with 24-in resin column. 
 
 encapsulated in 9 in and 33 in of resin 
 grout, respectively. The results, shown 
 in figure 12, indicate that the bolt re- 
 sponse, measured by the bottom gauge, is 
 similar to the calculated response. The 
 
 - t i i r—f — i 1 
 
 4 ' / jT 
 
 \ P- 
 
 if) / /^ 
 o / / 
 
 < / / KEY 
 
 3 , J _/ Gauge position: 
 
 - 
 
 
 / / a Top 
 
 
 j / f/ ■ Middle 
 
 
 
 / • Bottom 
 
 
 '"■ / X 
 
 - 
 
 \\ ' ' ' ' ' 
 
 i 
 
 100 
 
 200 
 
 300 400 
 
 STRAIN, uin 
 
 500 
 
 600 
 
 700 
 
 FIGURE 12. - Load-strain curve for strain- 
 gauge bolt with 36-in resin column. 
 
 response of the middle gauge (encapsu- 
 lated by 9 in of grout) was approximately 
 53% of the bottom gauge response. The 
 top gauge, 33 in from the grout line, 
 showed negligible amounts of strain. The 
 test results illustrate that load trans- 
 fer in the bolt system takes place almost 
 totally within the bolt at the load lev- 
 els tested. 
 
 REPRESENTATION OF PULL TEST BY FINITE ELEMENT MODEL 
 
 To obtain a better understanding of 
 pull tests, field tests were used to ver- 
 ify the validity of a simple finite 
 element computer model that provides a 
 fundamental approach for designing rock 
 anchor support systems for thick coal 
 roofs (8-9)» The model (fig. 13) simu- 
 lates a 4-ft bolt with a 3/4-in diam, 
 installed in a 1-in-diam hole. Stresses 
 were applied to the end of the bolt in 
 100-psi increments, beginning with 100 
 psi and ending with 1,500 psi. A test 
 run was also made at 5,000 psi on the 
 bolt head. The model was three-dimen- 
 sional axisymmetric with rotation about 
 the centerline of the bolt. The prop- 
 erties of the various material compo- 
 nents used in the analysis are listed in 
 table 1. 
 
 TABLE 1. - Physical properties of 
 materials used in finite element 
 model 
 
 Property 
 
 Coal 
 
 Grout 
 
 Steel 
 
 Young's modulus (E) 
 
 
 
 
 10 6 psi.. 
 
 0.50 
 
 0.30 
 
 30 
 
 Poisson's ratio (v). 
 
 0.26 
 
 0.25 
 
 0.30 
 
 
 0.0486 
 
 0.0804 
 
 0.2604 
 
 Compressive strength 
 
 
 
 
 (C) psi. . 
 
 1,000* 
 
 5,000 
 
 20,000 
 
 Angle of internal 
 
 
 friction (<j>)....°.. 
 
 46 
 
 35 
 
 
 
 Tensile strength (t) 
 
 
 
 
 psi. . 
 
 
 
 5,000 
 
 20,000 
 
 l klso tested at 1,500, 2,000, 
 5,000, 7,500, and 10,000 psi. 
 
 3,500, 
 
10 
 
 RESULTS OF FINITE ELEMENT ANALYSIS 
 
 The materials in the pull test model 
 were assumed to be homogeneous and 
 isotropic. Two types of cases were 
 analyzed: 
 
 1. Hold all material components con- 
 stant and vary the amount of load placed 
 on the end of the bolt. 
 
 2. Keep the load on the end of the 
 bolt constant and vary the compressive 
 
 <L 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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 \ 
 
 1 
 
 
 J 
 
 63 
 
 
 
 
 
 
 
 
 
 
 
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 RH 
 
 
 
 
 
 
 
 
 
 
 
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 ID ■" 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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 ^ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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 ^ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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 10 
 
 20 
 
 25 
 
 FIGURE 13. - Finite element model (: 
 model dimensions in inches). 
 
 30 33.5 
 showing 
 
 strength (shear strength) of the rock 
 (coal) encompassing the tendon. 
 
 The computer analysis provided both 
 center-point and nodal-point data for 
 each element. Postprocessing of the 
 original computer values revealed nearly 
 linear dissipations of the applied loads 
 through the length of the bolt. However, 
 linear load dissipations did not corre- 
 late with the actual field measurements. 
 To improve correlation and similitude, 
 and to ensure compatibility with the 
 original field measurements, the finite 
 element model was slightly modified (10). 
 The required modification included the 
 addition of bolt displacements. The ap- 
 proximate bolt displacements, which were 
 measured with a dial gauge during the 
 field analysis, were added to the first 
 one-third of the bolt length measured 
 from the collar of the hole. The re- 
 sults, after this modification, compared 
 favorably to the field data. 
 
 A typical representation of percent 
 load versus distance is shown in figure 
 14. The plot shows how rapidly the 
 load dissipates when applied to a fully 
 grouted bolt. At a depth of 20 in, only 
 50% of the initial load is realized in 
 
 45 
 
 Y = 62.98 exp 
 2 
 
 -0.I3I3(X) 
 
 5 20 25 30 35 
 DISTANCE, in 
 
 40 45 50 
 
 FIGURE 14. - Typical plot of percent of ap- 
 plied load versus boltdistance (r = coefficient 
 of correlation). 
 
11 
 
 the bolt. The load dissipations were 
 similar for all of the tested loads. It 
 was shown in the field tests that the 
 transfer of the load in the resin-grouted 
 bolt system took place almost totally 
 within the bolt at the load levels 
 tested. 
 
 When load is applied to the bolt, the 
 bolt stretches in the longitudinal direc- 
 tion and contracts in the traverse di- 
 rection due to Poisson's effect. The 
 contraction of the bolt will cause the 
 bond to be broken between the bolt and 
 the resin grout. If the loads are high 
 enough, the contraction will cause the 
 bolt to separate from the resin grout at 
 the end. The length of this separation 
 will increase with increasing loads. The 
 amount of contraction in the anchor de- 
 cays rapidly as the distance from the 
 bolt head increases. 
 
 At a constant stress on the head of the 
 bolt and at varying coal compressive 
 strengths of 1,000, 1,500, 2,000, 3,500, 
 5,000, 7,500, and 10,000 psi, no signifi- 
 cant changes in the stress distributions 
 were observed. This was the result of 
 the high values for the Young's modulus 
 for the bolt as opposed to the low value 
 for the coal (60:1). Safety factors were 
 calculated, based on Mohr-Coloumb failure 
 criteria, to determine the effect varying 
 compressive strength values would have on 
 the host rock. Results indicated that 
 when the compressive strength is 2,000 
 psi or less, the safety factors approach 
 critical levels. These safety factors 
 indicate that the material 4 in up from 
 the hole collar is in tension, which may 
 cause failure. 
 
 CONCLUSIONS 
 
 All of the roof bolting systems tested 
 met testing criteria required for thick 
 top coal roofs with the exception of the 
 type of expansion anchor bolts tested. 
 
 The finite element model and the 
 strain-gauge bolts provided insights on 
 load dissipation and anchorage behavior. 
 It is recognized, however, that the re- 
 sults of the finite element study were 
 dependent on the accurate determination 
 of the host rock properties. Tests on 
 
 fully instrumented pull-tested bolts in 
 the same host rock as the model provided 
 a validation of the numerical model. 
 
 The anchor load-transfer mechanism must 
 be assessed to provide a complete pic- 
 ture of the stresses and strains around a 
 fixed anchor. 
 
 It is believed that thick coal roofs 
 can be fully supported in the bottom por- 
 tion of the coal roof without anchoring 
 in the immediate host rock. 
 
 REFERENCES 
 
 1. Karabin, G. J., and W. J. Debevec. 
 Comparative Evaluation of Conventional 
 and Resin Bolting Systems. MESA (now 
 MSHA) IR 1033, 1976, 22 pp. 
 
 2. Karabin, G. J., and M. T. Hoch. 
 An Operational Analysis of Point Resin- 
 Anchored Bolting Systems, MSHA IR 1100, 
 1979, 14 pp. 
 
 3. . Contemporary Roof Support 
 
 Systems Provide a Diversified Approach 
 to Modern Ground Control. Paper in Pro- 
 ceedings, Annual Institute on Coal Mine 
 Health and Safety Meeting. VA Polytech. 
 Inst., 1980, pp. 249-268. 
 
 4. Kwitowski, A. J., and L. V. Wade. 
 Reinforcement Mechanisms of Untensioned 
 
 Full-Column Resin Bolts. BuMines RI 
 8439, 1980, 27 pp. 
 
 5. Brask, C. G., and H. Hamrin. New 
 Type of Rockbolt Simplifies Rock Rein- 
 forcement Procedures. Atlas Copco, Swe- 
 den, 1983, pp. 1-17. 
 
 6. Nitzche, R. N. , and C. J. Haas. 
 Installation Induced Stresses for Grouted 
 Roof Bolts. Int. J. Rock Mech. , Min. 
 Sci. , and Geomech. , v. 13, No. 1, 1976, 
 pp. 17-24. 
 
 7. International Society for Rock 
 Mechanics (Commission on Standardization 
 of Laboratory and Field Test, Committee 
 on Field Tests). Suggested Methods for 
 
12 
 
 Rockbolt Testing. No. 2, March 1974, 
 pp. 1-16. 
 
 8. Bathe, K. A Finite Element Program 
 for Automatic Dynamic Incremental Non- 
 linear Analysis (ADINA). Acoustics and 
 Vibrations Lab., Mech. Eng. Dep., MIT, 
 Cambridge, MA. Rep. 82448-1 Sept. 1975, 
 p. 360. 
 
 9. Yap, L. D. , and A. A. Rodger. A 
 Study of the Behavior of Vertical Rock 
 Anchors Using the Finite Element Method. 
 
 Int. J. Rock Mech., Min. Sci., and Geo- 
 mech. , v. 21, No. 2, 1984, pp. 47-61. 
 
 10. Serbousek, M. 0., and S. P. Sig- 
 ner. Load Transfer Mechanics in Fully 
 Grouted Roof Bolts. Pres. at 4th Confer- 
 ence on Ground Control in Mining, Mor- 
 gantown, WV, July 22-24, 1985, 11 pp.; 
 available from M. 0. Serbousek, U.S. Bu- 
 reau of Mines, Spokane Research Center, 
 Spokane, WA. 
 
13 
 
 APPENDIX. — TORQUE-TENSION RATIO TESTS 
 
 Torque-tension ratio tests determine 
 the amount of torque necessary for the 
 proper installation of mechanical expan- 
 sion anchor and combination bolts. The 
 test procedure requires that a calibrated 
 hydraulic U-cell be installed between the 
 bearing plate and the mine roof. The 
 bolt is then tightened by turning its 
 head with a torque wrench. The values 
 of the torque wrench are recorded in con- 
 junction with the value on the U-cell. 
 When the value on the U-cell drops, the 
 maximum anchorage capacity of the bolt 
 system has been attained. Bolts should 
 then be tensioned to 50% to 70% of the 
 yield strength of the bolt or 50% to 
 70% of the anchorage capacity of the 
 material, whichever is less. The recom- 
 mended torque-tension ratio is 50 lb ten- 
 sion per foot pound of torque for bolts 
 without hardened washers or 60 lb tension 
 per foot pound of torque with hardened 
 washers. 
 
 An example follows: Find the torque 
 range for a 5/8-in grade 55 bolt with 
 
 no hardened washer when the rock anchor- 
 age capacity is 15,000 lb. The yield 
 strength for a grade 55 bolt is 12,400 
 lb, which is less than the anchorage 
 capacity. Therefore, the yield strength 
 is used for design purposes, as shown 
 below. 
 
 Lower-limit calculation 
 
 12,400 lb 
 x 50% 
 6,200 lb 
 
 6,200 lb ... ... .. . 
 
 50 lb/lbf = 124 lbf * ft t0rqUe 
 
 Upper-limit calculation 
 
 12,400 lb 
 x 70% 
 
 8,680 lb 
 
 8,680 lb .,, ... .. . 
 
 50 Ibf/ft = 174 lbf ft t0rque 
 
 ftU.S. CPO: 1985-605-017/20,135 
 
 IN T.-BU.O F MIN ES,PGH.,P A. 28(66 
 
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 P.O. Box 18070 
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