W * -V ** ^ **** ^°«* 'o • » * .*\ 9 •!••- *> < v O ' . . o ,, ^ . c«*..i^:.% ,« K * K f A': V •••«^ V* rfltt: S^ wy ++ 'W- : /\ : -w- ** v % # /\ : -w- : A ' v » ' * "' *> V % » » * * ' o o^ V *> ^ v^ v 4 , ...^, v ^/ BfeX^ Jam IC 9053 Bureau of Mines Information Circular/1985 Ground Control Instrumentation A Manual for the Mining Industry By Eric R. Bauer UNITED STATES DEPARTMENT OF THE INTERIOR CD C S m > c 7& tolHES 75TH AV^ (f^x^^ N Information Circular 9053 Ground Control Instrumentation A Manual for the Mining Industry By Eric R. Bauer UNITED STATES DEPARTMENT OF THE INTERIOR Donald Paul Hodel, Secretary BUREAU OF MINES Robert C. Horton, Director 1 n 5 W Library of Congress Cataloging in Publication Data: Bauer, Eric R Ground control instrumentation. (Bureau of Mines information circular ; 9053) Bibliography. Supt. of Docs, no.: I 28.27: 9053. 1. Ground control (Mining)— Instruments. 2. Rock deformation— Mea- surement. I. United States. Bureau of Mines. II. Title. III. Series: Infor- mation circular (United States. Bureau of Mines) ; 9053. TN295.U4 [TN288] 622s [622'. 2] 85-600140 ^ CONTENTS Page i Abstract 1 c Introduction 2 Acknowledgment 2 Instrument selection guidelines 2 Chapter 1. — Convergence measurements..... 4 Introduction 4 Ground movement indicators 4 Instrument selection 4 Description of individual instruments 6 Glowlarm 6 Guardian Angel 7 Horizontal roof strain indicator 9 Infrared scanner 10 Infrared thermometer 11 Plumb bob 12 Spider roof monitor 13 SRC closure rate instrument 14 Tape extensometer 15 Tube extensometer 16 Visual roof sag bolt 17 References 19 Chapter 2. — Strata separation measurements 20 Introduction 20 Ground movement indicators 20 Instrument selection 20 Description of individual instruments 21 Borehole extensometer 21 Simple weighted bed separation indicator 22 Stratascope 23 References 25 Chapter 3. — Lateral roof movement measurements 25 Introduction 25 Ground movement indicators 25 Instrument selection 25 Description of individual instruments 26 Plumb bob 26 Stratascope 27 Chapter 4. — Stress measurements 27 Introduction 27 Ground movement indicators 28 Instrument selection 28 Description of individual instruments 29 Borehole deformation gauge 29 Borehole inclusion stressmeter 31 Borehole-mount strain gauge 33 CSIR strain gauge strain cell (Doorstopper) 34 CSIR triaxial strain cell 35 CSIR0 hollow inclusion stress cell 36 Cylindrical borehole pressure cell 37 Flat borehole pressure cell 38 Flatjack 40 Mechanical strain gauge 41 11 CONTENTS — Continued Page Surface-mount photoelastic gauge 42 Surface-mount strain gauge 43 Surface rosette undercoring 45 Vibrating wire stressmeter 46 References 47 Chapter 5. — Support load measurements 48 Introduction 48 Ground movement indicators 48 Instrument selection 48 Description of individual instruments 49 Gloetzl pressure cell 49 Powered-support pressure recorder 50 Prop load cell 51 Roof bolt load cell 53 Roof bolt U-cell 54 Surface-mount photoelastic gauge 55 Surface-mount strain gauge 55 Torque wrench 55 References 56 Discussion 57 Bibliography 58 Appendix A. — Case studies 60 Appendix B. — Instrument suppliers 66 ILLUSTRATIONS 1. Instrument selection worksheet for ground control measurements 3 2. Type of movement measurable by each convergence measuring instrument 5 3. Cost range of purchase or fabrication of convergence measuring instruments 5 4. Range of technical ability required for installation, monitoring, and data interpretation of convergence measuring instruments 5 5. Type of measured data obtained from convergence measuring instruments.... 6 6. Installed Glowlarm 6 7. Method for measuring bend of Glowlarm at installation 7 8. Guardian Angel 8 9 . Cutaway view of roof and installed Guardian Angel 8 10. Cross section of a typical coal mine opening illustrating the HORSI principle 9 11. Horizontal strain measuring apparatus 10 12. Infrared scanner 11 13. Typical digital-readout infrared thermometer 12 ,14. Plumb bob setup for roof convergence measurement 13 15. The Spider roof monitor 14 16. Installation diagram for the Spider roof monitor 14 17. Side view of tape extensometer 15 18. Top view of tape extensometer 15 19. Index mark alignment for correct reading of tape extensometer 16 20. Dial-gauge tube extensometer 17 21. Visual roof sag bolt at installation 18 22. Visual roof sag bolt after convergence movements 19 ILLUSTRATIONS— Continued 111 Page 23. Cost range of purchase or fabrication of strata separation measuring instruments 21 24. Range of technical ability required for installation, monitoring, and data interpretation of strata separation measuring instruments 21 25. Type of measured data obtained from strata separation measuring instruments 21 26. Borehole extensometer 21 27. Simple weighted bed separation indicator 23 28. Fiberoptic stratascope, battery pack, camera, and attachment 24 29. Stratascope setup for roof observation 24 30. Cost range of purchase or fabrication of lateral roof movement measuring instruments 26 31. Range of technical ability required for installation, monitoring, and data interpretation of lateral roof movement measuring instruments 26 32. Type of measured data obtained from lateral roof movement measuring instruments 26 33. Plumb bob setup for detecting lateral roof movement 27 34. Cost range of purchase or fabrication of stress measuring instruments.... 28 35. Range of technical ability required for installation, monitoring, and data interpretation of stress measuring instruments 29 36. Type of measured data obtained from stress measuring instruments 29 37. Three-component borehole deformation gauge 30 38. Cross section through a borehole showing borehole deformation gauge after overcoring 31 39. Recommended borehole configurations for complete, three-dimensional, state-of-stress determination 31 40. Cross section of stressmeter 32 41. Stressmeter and tapered sleeve into which it fits 32 42. Cross-sectional view of a borehole showing installed photoelastic stressmeter 32 43. Typical installation of a cylindrical borehole pressure cell 37 44. Steps in fabrication of an encapsulated flat borehole pressure cell 39 45. Flatjack pressure cell 40 46. Mechanical strain gauge 41 47. Typical measurement setup for mechanical strain gauge showing measuring points and stress relief holes 41 48. Example of photoelastic fringe patterns displayed by a surface-mount photoelastic gauge 43 49. Vibrating wire surface-mount strain gauge 44 50. Surface-mount strain gauges showing stress relief using large overcoring bit 44 51 . Vibrating wire stressmeter 46 52. Cost range of purchase or fabrication of support load measuring instruments 48 53. Range of technical ability required for installation, monitoring, and data interpretation of support load measuring instruments 49 54. Type of measured data obtained from support load measuring instruments... 49 55. Powered-support pressure recorder 50 56. Chart of hydraulic pressures in longwall roof supports during normal operation 51 57. Prop load cell (strain gauge design) 52 IV ILLUSTRATIONS — Cont inued Page 58. Photoelastic prop load cell and readout equipment 52 59. Typical roof bolt load cells (strain gauge design) 54 60. Cross-sectional view of a spring-and-disk roof bolt load cell 54 6 1 . Steps in fabrication of a roof bolt U-cell 55 62. Torque wrench 56 A-l. Typical gauge station 61 A-2. Average deflection, by location 61 A-3. Instrumentation plan for each array 62 A-4. Amount and direction of lateral roof movement found in holes 20 through 29 63 A-5. Shortwall section 7 right 64 A-6. Pressure changes recorded in pillars and shortwall panels during mining. . 65 | UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT °F degree Fahrenheit min minute ft foot yin microinch ft-lbf foot pound yin/in microinch per inch h hour pet percent in inch psi pound per square inch in2 in 3 in/in square inch cubic inch inch per inch psi/min V pound per square inch per minute volt lb pound GROUND CONTROL INSTRUMENTATION A Manual for the Mining Industry By Eric R. Bauer 1 ABSTRACT This Bureau of Mines manual is intended to provide a better under- standing of ground movement and the technology available for measuring it. The manual deals with convergence, strata separation, lateral roof movement, stress, and support load in underground mines. The instru- ments that measure these ground control parameters are described in de- tail. Step-by-step procedures for selecting the appropriate instrument are presented, which consider such factors as approximate cost, instal- lation procedures, data collection, and data interpretation; and an in- strument selection worksheet is provided to facilitate the instrument selection process. Actual instrument case studies and a list of instru- ment suppliers are presented in the appendixes. 'Mining Engineer, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. INTRODUCTION Ground control is the control of the immediate area surrounding an underground excavation, including the stability of the mine roof, ribs, and floor. Since ground instability can result in unsafe mining conditions, it is vitally impor- tant that mine operators have effective means of controlling the mine area. To apply ground control techniques effec- tively, it is essential to ascertain the type of movement involved and the extent of the hazard created. Each instrument available for measuring ground movements has particular charac- teristics, and the instrument to be used in each situation must be carefully se- lected to achieve optimum results. This manual is designed to assist mine opera- tors in making such selections. Opera- tional features of the instruments are included only as an aid to selection. Manufacturers of the devices should be consulted for instructions in their use. Additional information can be obtained from instrument suppliers and from sources listed in the references and bib- liography for effective instrumentation use. It is hoped that this instrumentation manual will improve the understanding of ground control movements and provide the information needed for efficient instru- ment selection. ACKNOWLEDGMENT The author wishes to express his appre- Pittsburgh Research Center, for his pre- ciation to Steven C. Stehney, mining liminary input and his assistance during engineer, now at Jim Walter Resources, the initial literature search, and formerly with the Bureau of Mines, INSTRUMENT SELECTION GUIDELINES Efficient instrument selection depends on the mine operator's (user's) ability to define the requirements and parameters of each ground control problem. At times this may be rather difficult, but even an educated estimate can provide adequate guidelines for choosing the appropriate instruments. As the requirements and parameters are defined, they should be listed on an instrument selection work- sheet (fig. 1). The first step is to identify the ground control movement that is occurring or needs investigation. This can be con- vergence (roof sag, rib failure, floor heave), separation of mine roof strata, lateral roof movement, stress, or support load. When the movement has been iden- tified, it should be listed in the "move- ment occurring" column. The next step is to determine the controlling parameters, namely cost, technical aspects, and data requirements. The cost parameter is the amount of money available for instrument purchases. The technical aspects parameter deals with the expertise of the people who will in- stall and monitor the instrument and ana- lyze the data. This expertise is desig- nated as slight (no technical background or previous instrumentation use), moder- ate (some technical or engineering back- ground and/or previous instrumentation use) , or extensive (engineering degree and/or extensive instrumentation use). The data requirements parameter is the type and detail of data desired. Data can be either visual, simple numerical, detailed numerical, or a combination of these. As these controlling parameters are defined, they should be listed on the worksheet in the "desired elements" column. The steps given in each chapter should then be followed to make a preliminary choice of the instrument(s ) best suited to the ground control problem being investigated. To assist mine operators to make the final instrument selection, individual instruments are discussed in terms of principle and application, availability, description, installation and operation, data collection, and data interpretation. 2 "2 Not all of the instruments described are commercially available at this time. For those that are, suppliers are listed in appendix B. For the others, instruc- tions for in-house assembly are given where possible. INSTRUMENT SELECTION WORKSHEET Requirements Movement occurring or measurement desired Instruments available 1. 2. Type of ground control 3. 4. problem 5. 6. 7. Controlling parameters Desired elements Instruments available Cost 1. 2. 3. Technical aspects 1. 2. 3. Data required 1. 2. 3. Instruments satisfying all the above requirements and parameters: 2. 3. FIGURE 1. - Instrument selection worksheet for ground control measurements. CHAPTER 1.— CONVERGENCE MEASUREMENTS INTRODUCTION Convergence is the vertical closure be- tween roof and floor. It is also the horizontal closure between two parallel ribs. It involves three distinct move- ments: roof sag, rib failure, and floor heave. Roof sag, the downward movement of the immediate roof, occurs after the coal is mined and is due to the weights of the immediate roof and overburden. Rib failure — the spalling, lateral expan- sion, or shear failure of the pillars — is due to excessive loading of insufficient- size pillars by the overburden. Floor heave, the upward movement of the floor, results from the combination of small pillars and soft floor; overburden weight pushes the pillars downward and this pushes the soft floor sideways and upward. Convergence measurements are an impor- tant investigative tool that permits op- erators to detect rib, roof, and floor movements before they become major ground control problems, to analyze ground move- ments and prevent further occurrences, and to plan changes in mine design. Early detection of hazardous ground con- ditions can result in increased safety and production. GROUND MOVEMENT INDICATORS Ground movements have specific charac- teristics that serve as indicators. Un- fortunately, some indicators are common to several types of movement. In some cases , determining which movement is oc- curring can be difficult, amounting at times to no more than an educated esti- mate. The following list of convergence movements and their indicators can help to identify the type of movement that is occurring. Roof sag indicators: • Visible closure or convergence of entry. • Tension cracks in middle of roof span. • Increase in water dripping from cracks in the roof. • Bent or broken roof supports. • Falls of large blocks of rock. • Shear cutters in root along rib line. Rib failure indicators: • Sloughing of ribs at top, center, or bottom. • Cracks developing in ribs. • Bumps or bursts from ribs. Floor heave indicators: • Fractures developing in floor along center line of roadway. • Visible closure of entry. • Unevenness developing in floor. • Excessive seepage of water from floor. • Sloughing of pillars at floor line only. • Fractures developing in floor along rib line. INSTRUMENT SELECTION As requirements and parameters are defined, they should be listed on an instrument selection worksheet (fig. 1). The convergence movement (roof sag, rib failure, or floor heave) that is occurring or needs investiga- tion should be listed in the "move- ment occurring" column. The control- ling parameters, as defined in the sec- tion "Instrument Selection Guidelines," should be listed in the "desired ele- ments" column. Once the requirements and parame- ters have been defined and listed on the worksheet, the user should refer to figures 2 through 5 and follow steps 1 through 6 to make a preliminary choice of the instrument(s) best suited to the ground control problem being investigated. Step 1: From figure 2, choose the instruments that satisfy the "movement occurring" requirement of the work- sheet. List these instruments on the worksheet under "instruments availa- ble," across from the "movement occur- ring" parameter. Instrument Type of convergence measurable Roof sag Rib failure Floor heave Glowlarm • o o Guardian Angel • o Horizontal roof strain indicator o o Infrared scanner o o Infrared thermometer o o Plumb bob • o Spider roof monitor o SRC closure rate instrument o o Tape extensometer ^B ^B o Tube extensometer • o Visual roof ^^k sag bolt ^B o Car KEY Best suited to measure FIGURE 2. - Type of movement measurable by each convergence measuring instrument. Step 2: From figure 3, choose the in- struments that satisfy the cost parameter previously determined. List them on the worksheet under "instruments available," across from the cost parameter. Step 3: From figure 4, choose the in- struments that satisfy the technical as- pects parameter. List them under "in- struments available," across from the technical aspects parameter. Step 4: From figure 5, choose the in- struments that satisfy the data require- ment parameter listed on the worksheet. List them under "instruments availa- ble," across from the data requirement parameter. Step 5: Determine from the "instru- ments available" column of the worksheet the instruments that satisfy all of the requirements and parameters. List these instruments at the bottom of the Instrument Cost of purchase or fabrication Dollars 10 50 100 250 500 1,000 2,000 10,000 1 . . . 1 . 1 . . 1 . . 1 , . , , 1 , 1 ,1 Glowlarm □ Guardian Angel □ Horizontal roof strain indicator □ Infrared scanner Infrared Thermometer , 1 1 Plumb bob 1 1 Spider roof monitor □ SRC closure rate instrument d Z] Tape extensometer 1 1 Tube extensometer 1 1 Visual roof sag bolt □ c KEY Cost range IGURE 3. - Cost range of purchase or fabrica- tion of convergence measuring instruments. Instrument Range of technical ability required Slight Moderate Extensive Glowlarm ^w^^ Guardian Angel ^^w^^ K ww\ \ww\\ Horizontal roof strain indicator ^^ mm .www \\S\\VJ Infrared scanner Infrared thermometer 1 l\\ W sWWJ Plumb bob a^^wt Spider roof monitor ^R^wr SRC closure rate instrument ^R^^w Tape extensometer Tube extensometer Visual roof sag bolt i^X\\.\\\\\\yk\Y\\> 3 Installation KEY Monitoring iwwi Data interpretation FIGURE 4. - Range of technical ability required for installation, monitoring, and data interpretation of convergence measuring instruments. Instrument Type of measured data obtained Visual Simple numerical Detailed numerical Glowlarm o o Guardian Angel o o Horizontal roof strain indicator o Infrared scanner o Infrared thermometer o Plumb bob o o Spider roof monitor o SRC closure rate instrument o o Tape extensometer o Tube extensometer o o Visual roof sag bolt o KEY O Data obtainable FIGURE 5. - Type of measured data obtained from convergence measuring instruments. Scale, in FIGURE 6. - Installed Glowlarm (1). worksheet. They represent the best pos- sible choices, relative to need, for the ground control problem being investi- gated. If no instruments satisfy all the requirements and parameters, it may be necessary to change the parameters or to choose the instruments that satisfy the most requirements and parameters. Step 6: At this point, it is up to the user to make the final decision as to the most suitable instrument(s) , based on the following detailed descriptions. DESCRIPTION OF INDIVIDUAL INSTRUMENTS Glowlarm^ Principle and Application The Glowlarm is a device that gives visible warning of impending failure of roof, rib, or floor (1_). 4 It detects roof sag, rib expansion, and floor heave by "lighting up" when movement occurs. Availability It is available from Glowlarm Rock Fall Warning Devices. Cost is approximately $10 per unit. 5 Description The Glowlarm warning device consists of a flexible see-through plastic tube about 6 in long and 0.5 in. in diam, which is held in place by an anchor and stainless steel wire (fig. 6). It contains two liquids separated by an inner glass tube. It is the mixing of these two liquids, after the glass tube breaks because of ground movement, that produces the bright yellow warning light. This light per- sists for 24 h. ^Reference to specific products does not imply endorsement by the Bureau of Mines. ^Underlined numbers in parentheses re- fer to items in the list of references at the end of this chapter. ^Instrumentation costs are the best available estimates based on manufac- turers' information at the time the re- search was completed. Installation Step 1: Drill a hole in the roof or rib to a stable zone of rock at a depth greater than the anchorage zone of the 'roof bolts being used. Step 2: Push the anchor, with wire attached, to the end of the hole and set by rapping it sharply with a stick. Step 3: Wrap wire around the Glowlarm and tighten it so that the Glowlarm has the desired bend. The amount of bend at installation determines the amount of movement to be detected. The larger the bend the less movement needed to break the glass tube, allowing the instrument to light up. The bend can be measured with a straightedge and rule (fig. 7). Step 4: Trim off the excess wire. The Glowlarm is now ready to monitor ground movements . These observations should be made dur- ing routine travel throughout the mine. A glowing instrument should be report- ed immediately to the appropriate mine personnel, who should initiate corrective action as soon as possible. Data Interpretation A glowing instrument means that some convergence has taken place; however, the amount of movement that indicates unsta- ble conditions varies from mine to mine. Only through experimentation with this instrument can mine personnel become knowledgeable as to the amount of bend required at installation. Guardian Angel Principle and Application Data Collection The Glowlarm should be observed at least once every 24 h, since this is the life span of the warning light. R lie Scole. in FIGURE 7. - Method for measuring bend of Glowlarm at installation. The Guardian Angel is a device for mea- suring and monitoring roof sag (2^). A reflector flag drops when the preset amount of movement is reached, to signal that movement has occurred. The instru- ment is also available with a graduated scale that measures the amount of move- ment numerically. This instrument can be used where trav- el must not be impeded and over permanent installations, such as belt lines, haul- ageways, etc., where other instruments are impractical. Availability The Guardian Angel is available from Conkle, Inc. The price is $30 per unit. Description The Guardian Angel consists of anchor clips, threaded rod, reference head, trigger mechanism, and reflector flag (fig. 8). Installation Step 1: Drill a 1-in-diam roof bolt hole to a stable zone of rock and deeper than the anchor horizon of the roof bolts in the test area. Step 2: Connect the desired lengths of rods, making sure to tighten the connect- ing locknuts. Scale, in FIGURE 8. - Guardian Angel. Step 3: Connect two anchor clips to the rod, making sure to tighten the lock- ing nuts. Step 4: Install the rods by manually pushing them upwards into the hole. Step 5: Slide the monitor assembly on- to the rod, then screw on the adjusting wingnut until the assembly is in contact with the roof (fig. 9). Step 6: To set the monitor for the de- sired amount of movement detection, latch the flag in the "up" position (as shown in figure 9). Turn in the adjusting nut until the flag drops. Back the adjusting nut off a few turns, relatch the flag, then turn the adjusting nut back in until it just unlatches the flag. Reset the flag by slightly backing off the adjust- ing nut. Using the adjusting nut wings for reference, back the nut off to the desired amount of movement to be detected ( 3) : Each quarter turn equals 0.015 in, etc. Scale, in FIGURE 9. - Cutaway view of roof and in- stalled Guardian Angel (2). Data Collection This instrument is designed to give a visual indication of roof movement. The flag drops when the preset amount of movement has occurred. The amount of movement can be determined by either of the following methods (2). Method 1: If it is desirable to know the amount of roof sag prior to the flag's dropping, screw in the adjusting nut while counting the number of turns until the flag drops. Subtract the inch- es of deflection corresponding to the number of turns from the preset inches of deflection to obtain the amount of move- ment that has occurred. The system must now be reset to the previous, or a new, preset amount of movement to be detected (step 6 of installation). Method 2: If the flag has dropped, back off the adjusting nut while counting the turns until the flag just resets. Screw the nut in while counting the turns until the flag unlatches. Subtract the second count from the first, and add the corresponding inches of deflection to the original preset amount of movement to ob- tain the total amount of movement that has occurred. Again, the instrument must be reset if additional movement detection is desired. Each instrument can be observed as often as desired. For short-term instal- lations or potential movement areas, observations can be made each shift, daily, or weekly. For long-term, stable areas , observations can be made weekly or monthly. Data Interpretation The Guardian Angel detects a preset amount of roof sag. The preset amount is arbitrary. Allowable movement limits must be determined for each mine, based on prior experience and/or ongoing in- strumentation. Values for safe movement, movement indicating a need for caution, and movement indicating unstable roof must be established. The process of determining these values can be time con- suming and can require hundreds of mea- surements. From these values, the appro- priate presetting for movement detection can be determined. The values, however, may not be constant throughout the mine area. Horizontal Roof Strain Indicator Principle and Application The horizontal roof strain indicator (HORSI) is a device that measures the horizontal roof strain that accompanies roof sag preceding a roof fall (4). It is based on the principle that the hori- zontal distance between two roof bolts will increase as the roof sags, producing a measurable strain difference that can indicate stable or unstable roof condi- tions (fig. 10). Availability The HORSI is not yet commercially available. Consequently, it must be fab- ricated in-house. Cost of fabrication and materials is approximately $50; an additional $35 to $50 is needed to pur- chase the dial gauge. Roof bolts - ;//;;;;//;;/////// \j; ;/;?;??;; s- y . '■'■'■' •'.'.'. '■ ' . , . ■.'.'.' /.■/.' I STATIFIED ROOF BEFORE SAG ,'{'.'/{ i '.i '.'. '77 Compression / / 1 '.'. j {j ■ ^m^^^^^^j^^^^m^,. no " STATIFIED ROOF AFTER SAG KEY L Original distance between bolts a l Distance change of left bolt £p Distance change of right bolt Horizontal strain ■ ^L-t-^R FIGURE 10. - Cross section of a typical coal mine opening i llustrating the HORSI principle (4-6). 10 Description Data Collection The HORSI consists of two caps that attach to two adjacent boltheads with setscrews, a length of piano wire, a spring-tensioned plunger, a fitted col- lar, and a standard dial gauge indicator (fig. ID- Installation If installation is to be in the face area, the two bolts should be instrument- ed immediately after installation, be- cause the strain usually stabilizes with- in 2 days, unless the roof is trending to an unstable condition. In previously mined and bolted areas , the HORSI can be installed whenever desired (_5 ) . Step 1: Attach one of the bolt caps (anchor cap) to the head of one of the roof bolts, using the setscrews to secure it. Step 2: Attach the spring-loaded plunger cap (measuring cap) to the other roof bolt head, using the setscrews to secure it. The removable dial gauge is connected to this cap by a fitted collar. Step 3: Attach the piano wire to the spring plunger. Use a wire clamp to fas- ten it securely. Step 4: Connect the free end of the piano wire to the other bolt cap. The wire must be lightly tensioned, then se- cured with the setscrews. Step 5: Use the dial gauge indicator to take an initial reading, which will serve as the reference point for all ad- ditional readings. Roof bolts- fitted collar Spring-tensioned _,. , plunger Removable dial indicator 9 Music wire Scale, In FIGURE 11. - Horizontal strain measuring apparatus (4-6). After the initial reading, the dial gauge should be read on each shift for the first 2 days after installation, then once a week if the readings have stabilized. If the readings have not stabilized, they should continue to be taken on each shift until stabilization or roof failure occurs. In old workings, which may be considered stabilized, read- ings can be taken daily or weekly, as de- sired. Any unusual changes should be re- ported to the appropriate mine personnel immediately. Data Interpretatio n Previous experiments by the instrument developers have shown that a 0.0001-in/in strain within 2 days could indicate un- stable roof conditions (4, 7-8). How- ever, this may not be the case in all mines. Mine operators must determine, through experiments and use of this in- strument, what amount of strain will in- dicate unstable roof conditions for a particular mine. Infrared Scanner Principle and Application The infrared scanner senses temperature differences between loose rocks and their background and expresses those differ- ences as a visual image (9) . Temperature differences appear as light and dark ar- eas on the visual image. The infrared scanner is one of two instruments that detect loose rock from a safe distance. (The other is the infrared thermometer.) Availability This instrument is still in the experi- mental stage with respect to use under- ground. The Bureau of Mines has a pro- totype that may soon be commercially available. Scanners approved by the U.S. Mine Safety and Health Administration may soon be ayailable from Hughes Aircraft Co.; Seco, Standard Equipment Co.; and Wahl Instruments, Inc. The approximate 11 cost of the infrared scanner ranges from $8,000 to $9,000. Description The infrared scanner is a handheld, 6- lb instrument, powered by a 6-V recharge- able battery (fig. 12). It produces a visual image of the heat waves from the material viewed. Installation No installation is needed, except to connect the battery to the scanner (some have internal batteries). Data Collection Turn the scanner on, point it toward the area to be observed, look through the eyepiece at the image, focus if neces- sary. No numerical data are obtained, only a visual picture of the viewed area, which cannot be recorded for future reference. The scanner should be used at the face prior to mining or roof bolting, at roof falls before cleanup begins, in air- courses where loose rock is likely to de- velop, and any place where roof can be tested only from a distance. Scale, in Data Interpretation The visual image will show light areas for warmer temperatures and dark areas for colder temperatures. Generally, loose rock will be colder and will pro- duce the darker image, but this does not hold true 100 pet of the time. Care must be taken to accurately determine which rocks are loose before attempting to cor- rect the hazardous conditions. Also, ex- ternal heat sources such as mining equip- ment may cause false readings. Infrared Thermometer Principle and Application Infrared thermometers are designed for temperature measurements where direct contact is impossible or impractical. They may be useful in mines to detect temperature differences between loose and stable roof rock; however, it has yet to be definitely established that they can perform this function. Availability Infrared thermometers are available from the following suppliers: Barnes Engineering Co.; Extech International Corp.; Industrial Products Co.; Mikron Instrument Co., Inc.; Raytek, Inc.; Seco, Standard Equipment Co.; and Wahl Instru- ments, Inc. The approximate cost ranges from $400 to $1,715. Description The infrared thermometer is handheld, with the gauge on the back facing the operator. It is shaped like a pistol with barrel and pistol grip. Infrared thermometers are made with different tem- perature ranges, either Fahrenheit or Celsius. Most are autocalibrating; some have sights for locating the object to be measured (fig. 13). Installation FIGURE 12. - Infrared scanner MO). No installation is necessary, but the thermometer must be calibrated, either manually or automatically. 12 SIDE VIEW 3ACK VIEW >/ 2 ~in LCD display Ernissivity Connection for battery charger FIGURE 13. - Typical digital-readout infrared thermometer (Y\). Data Collection Point the instrument at the roof area to be measured. Distance from object should be 20 ft or less. This means that the area can be monitored while the per- son monitoring remains under supported roof. Slowly scan the area to detect any significant changes in temperature. Loose rock will be either warmer or cold- er, depending on whether the mine air is warmer or colder than the solid rock. There is no specific time span for data collection. Possible collection could be at the face before any mining operations begin. This instrument could also be used in airways, escapeways, etc., when the need to detect loose roof arises. It might also be used at roof falls before cleanup begins, to detect any remaining loose rock. Data Interpretation Loose rock will register either as warmer or colder. Temperature differ- ences will be minute, on the order of 0.36° to 0.9° F (9). Such small dif- ferences may not be detectable by the present infrared thermometers, which are sensitive only to 1° F. External heat sources may also affect the accuracy of this instrument. Plumb Bob Principle and Application A plumb bob is a pointed weight that is suspended by a string. It can be used for imprecise measurement of entry con- vergence. This is accomplished by sus- pending the plumb bob from a point on the roof line and measuring its movement with respect to a reference pin in the floor (12). A plumb bob alone cannot dif- ferentiate between roof sag and floor heave. Availability Plumb bobs and associated materials are available from most hardware stores. The approximate cost ranges from $10 to $20. Description Instrumentation consists of a plumb bob, string, an eyebolt (which fastens to the roof bolt head) , and a reference pin that is grouted into the mine floor (fig. 14). Installation Step 1: Drill and tap the head of a roof bolt to accept the eyebolt. Step 2: Screw the eyebolt into the roof bolt and tighten. Step 3: Using the plumb bob as guide (hanging from the eyebolt), drill or chip a hole into the floor vertically below the eyebolt. Step 4: Grout the reference pin in the floor hole. This pin should be flat with a slight indentation at the center to accept the point of the plumb bob. Step 5: When the grout has hardened (15 to 30 min) , hang the plumb bob so that its point just touches the center of the floor reference pin. 13 n / ^ &« Eyebolt -String Mine floor \ Not to scale Plumb bob Reference pin Grout FIGURE 14. - Plumb bob setup for roof con- vergence measurement. Data Collection When the plumb bob starts to lean, the entry has begun to converge. If numerical values are desired, the plumb bob should be hung a specified dis- tance above the floor reference pin. The amount of movement is the change in dis- tance from plumb bob to pin. A plumb bob installation should be checked as often as considered neces- sary. The frequency of observations should increase if substantial movement is detected. Data Interpretation The amount of movement indicating un- stable roof or floor conditions var- ies from mine to mine. A continuing ground control instrumentation program can define this movement. In addition, humidity will cause slight variations in string length, resulting in reduced accuracy. Spider Roof Monitor Principle and Application The Spider roof monitor is designed to signal roof movement (roof sag) (13). It attaches to an existing roof bolt in min- utes and requires no special tools or drilling. It gives a visual display when a specified amount of movement has oc- curred, and can be reset to measure addi- tional movement. Availability The Spider is available from The Spider Inc. The approximate cost is $30. Description The Spider consists of a plastic hous- ing, latching mechanism, pin, reflective canister, roof-contact actuating arms, and mounting screws (fig. 15). Roof movement moves the actuating arms , which release the latching mechanism, which lets the reflective canister drop. Installation sen the setscrew on the ac- and let the arms swing sen the bolthead-connecting tall the Spider on a roof secure by tightening the is recommended that the ailed on a bolt, minus the which is anchored 12 in ting bolts and extends 1 or roof line (fig. 16). Step 1: Loo tuating arms freely. Step 2: Loo setscrews . Step 3: Ins bolt head and setscrews . It Spider be inst bearing pi ate , above the exis 2 in below the 14 SciSe, in FIGURE 15. - The Spicier roof monitor (13). 12 in above existing bolts Remove existing bolt plate ( < AY //.// ^<^/r<<< i \ /// y ,< V y// //A]r y U . «■*! m- Not to scale FIGURE 16. - Installation diagram for the Spicier roof monitor (13). Step 4: Set the actuating arms against the roof and secure them in posit* on by tightening the setscrews. The amount of movement detected will vary according to the location of the actuating arms (the installation pressure against the roof). Step 5: Latch the reflective drum by pushing it and the pin upward until they latch. If they will not latch, the actu- ating arms are applying too much pressure against the roof and must be readjusted. Data Collection If sufficient roof movement has oc- curred to trip the mechanism, the reflec- tive canister can be seen. This instru- ment can be viewed on a regular basis (weekly, monthly, etc.) or during regu- lar travel throughout the mine. If the instrument has tripped, it should be re- ported to the appropriate mine authority immediately. The detection of movement must be in- dividually analyzed for each mine, pos- sibly for each specific installation. The amount of movement that indicates potentially hazardous roof will vary from site to site. Basically, this instrument detects movement only; it does not re- veal the subsequent effect of the move- ment. Once the Spider has tripped, and the reflective canister is visible, it should be assumed that a hazardous roof condition has developed, until proven otherwise. SRC Closure Rate Instrument Principle and Application The SRC closure rate instrument pro- vides roof-to-floor closure rate measure- ments during underground mining, espe- cially retreat mining of coal pillars (14) . Change in resistance resulting from closure along a simple potentio- metric extensometer is relayed to a read- out box by means of a long cable. The instrument is designed to be pulled out of place and dragged to a safer location when closure reaches a predetermined rate. Availability This instrument can be purchased from Serata Geomechanics , Inc. Expected cost range is $1,000 to $2,000. Description The closure rate instrument system con- sists of a rugged telescoping potentio- metric extensometer and a digital readout and control box. The extensometer is de- signed to accommodate a height of from 4.6 to 12.1 ft, with a measurement range of 6 in. It is spring loaded over this range. Long cables (98 to 125 ft) con- nect the extensometer to the readout box, permitting the operator to remain in a safe, supported area. A breakaway fea- ture on each extensometer allows it to be pulled from the fall area by its elec- trical cable (15). Installation The instrument is placed between the roof and floor in the desired measurement area, while the readout box is kept in a safe area. 15 mechanism, and two snaphooks (figs. 17- 18). Anchoring stations are usually rods, grouted in place, with an eyebolt for connection to the instrument. Installation Data Collection The operator watches the digital read- out on the control box during pillar min- ing. When a predetermined, critical clo- sure rate is reached, an alarm light and horn are activated, and the operator re- trieves the extensometer and signals the miner operator to pull back. Data Interpretation Initially, the mine must determine a closure rate that indicates a roof fall is about to occur. This is the rate at which the alarms should be set to be ac- tivated. In many cases, a rate at which most falls will occur within 2 min of signalling can be determined. Tape Extensometer Principle and Application When measuring roof-to-floor conver- gence: Step 1: Drill and tap roof bolt head. Screw the eyebolt into the roof bolt head. Step 2: Directly below the roof sta- tion, drill a floor hole, and grout the floor station in place (eyebolt already attached) . When measuring rib-to-rib closure: Step 1: At the desired location, drill a horizontal hole into the rib, then grout the anchor station in place. Step 2: Repeat step 1 for the remain- ing rib anchor station. Rib and floor stations should be made from a minimum 12-in rod, which should be completely grouted into the hole to en- sure that the station is permanent. The tape ex convergence or the change in d manent stations (closure) or gence) . The s much as 100 ft tage this inst extensometer. tensometer detects entry rib failure by measuring istance between two per- , either from rib to rib roof to floor (conver- tations can be located as apart, which is an advan- rument has over the tube Tape extensometer housing p=M3> Steel tape FIGURE 17. - Side view of tape extensometer (16). Availability Tape extensometers are available from the following suppliers: Geokon, Inc.; Irad Gage; Roctest, Inc.; Sinco, Slope Indicator Co.; and Soiltest, Inc. The approximate cost ranges from $550 to $1,500. Description A tape extensometer consists of a steel engineers' tape, a dial-tensioning _ , .... . ... Tensioning /Thrust beoring Spring plunger Window with screw \ index marks Spring housing^ Compression^/ ^ ' g Dial qougey housing - 2 i ■ Scale, in Rotating / shaft/ FIGURE 18. - Top view of tape extensometer (16). 16 Data Collection Tube Extensometer Readings are taken as follows (fig. 19): Step 1: Connect the free end of the tape to one of the stations. Step 2: Connect extensometer (dial gauge) end to the remaining station. This should be the station at which the dial gauge is most easily read. Step 3: Tension the tape by turning the handle, then connect the extensometer body to the tape by inserting the hook into one of the holes in the tape. Step 4: Adjust the system to the zero mark on the extensometer spring plunger. Step 5: Measurement is obtained by adding the tape distance (where the hook is in the hole) to the reading on the dial gauge. Normally one reading per week is ade- quate, but large changes require more frequent readings, while small changes require less frequent readings. It should be noted that high air velocity will affect accuracy because of tape flutter (1_7). Data Interpretation The amount of measured movement indi- cating unstable conditions depends on the mine. Only through experimentation can this value be determined. Principle and Application A tube extensometer detects entry con- vergence (roof sag and/or floor heave) by measuring the change in distance between pairs of permanent stations anchored in the roof and floor of a mine (18). The readout system is either a dial gauge, a sonic probe, or a continuous drum. Availability Tube extensometers are available from the following suppliers: Geokon, Inc.; Irad Gage; Sinco, Slope Indicator Co.; and Soiltest, Inc. The approximate cost of a tube extensometer and readout system is from $690 to $3,000. Description A tube extensometer consists of a se- ries of telescoping tubes of Invar steel (19) (or other suitable metal alloy) , an internal spring that provides tension against the reference stations, two ref- erence stations, and a readout system (dial gauge, sonic probe, or continuous drum) (fig. 20). The sonic probe has an accompanying readout box and electrical connection cable. The continuous readout has a rotating-drum strip chart, and a lever arm and pen, which records all movements. Index marks not aligned Index marks aligned Scale, in FIGURE 19. - Index mark alignment for correct reading of tape extensometer (26). 17 o //xxv^/it Kkwy/xw "W//T Compression spring Dial indicator (2-in range) Not to scale ^/avw/awv/ Installation To anchor the reference stations: Step 1: Drill and tap a roof bolt head to accept the roof reference pin. Step 2: Use a plumb bob to mark the floor directly below the bolt, then drill a hole 12 in deep into the floor. Step 3: Grout the floor reference sta- tion into the floor hole. Step 4: Screw the roof station into the roof bolt head and tighten securely. Step 5: When the grout has hardened, connect the appropriate number of tubes together, place the extensometer between the reference stations and record the first reading; this reading serves as the reference for all subsequent readings. Data Collection The collection procedure depends on the type of tube extensometer readout being used. For the continuous-recording type, all that is required is to observe the readings recorded on the strip chart and change the chart periodically. The dial gauge only needs to be read. The sonic probe readout requires connecting the readout box to the extensometer using the electrical cable, then reading the de- flection as displayed. Data should be collected once a week unless conditions warrant otherwise. Data Interpretation Interpretation of data depends on the mine in which the data are collected. Unacceptable movement must be deter- mined for each mine through continuing experimentation. Vi sual Roof Sag Bolt Principle and Application FIGURE 20. - Dial-gauge tube extensometer (20). As its name displays roof implies, the instrument movements visually, and 18 gives no numerical data. This is not a precise measuring instrument, but it is useful for detecting impending roof fail- ure, at minimal cost, in time to provide additional support (21) . Visual roof sag bolts are intended for installation in the face area but can be installed mine- wide. They do not replace pattern bolts since they have no supporting capability. Availability Visual roof sag bolts are not commer- cially available, but the materials to fabricate them are. Since a standard roof bolt is used, the only material to be purchased is the reflective tape or paint. Total cost will range from $5 to $15. Description FIGURE 21. - Visual roof sag bolt at installation (19). A visual roof sag bolt system consists of a standard mechanical anchor bolt, three bands of reflective tape or paint (green, yellow, and red), and a poly- styrene foam (such as Styrofoam) plug (fig. 21). The roof bolt can have the head left on or cut off and should be at least as long as (preferably longer than) the pattern bolts in the area being monitored. The movement-indicating bands (approximately 0.5 in wide) are placed on the bolt with the green nearest the an- chor, followed by the yellow and then the red, going toward the head of the bolt. A polystyrene foam plug, cut to slide over the bolt and into the hole, serves as the reference level indicator. Step 3: Replace the anchor, slide the bolt into the hole, and seat the foam plug. Step 4: Tighten the bolt so that all three color bands just show. Data Collection No numerical data are collected. A quick glance tells mine personnel if the roof has moved. The bolts can be checked either randomly in passing, or on a regu- lar schedule. The frequency of observa- tion depends on the amount of movement occurring. The appropriate mine person- nel must be notified of any significant movements as soon as possible. Installation The bolts should be installed in the middle of the entry or at intersections where, theoretically, the most movement will occur. Step 1: Drill a standard roof bolt hole the length of the visual roof sag bolt. The bolt should anchor above the anchorage horizon of the surrounding pat- tern bolts in a stable zone of rock. Step 2: Remove the anchor and slide the foam plug down over the bolt to the bolt head. Data Interpretation The amount of movement (indicated by the disappearance of the color bands) will vary from mine to mine. Each mine must determine the rate of movement that indicates unstable roof conditions. The color bands are interpreted as follows: 1. All three colors showing: No move- ment, stable roof conditions (fig. 21). 2. Green disappearing: Initial (slight) movement, caution, possible unstable roof conditions developing (fig. 22.4). 19 Mine roof __ Foam plugs Scale, in B © FIGURE 22. - Visual roof sag bolt after convergence movements. A, Initial (slight); B, moderate; C, substantial. 3. Yellow disappearing: Moderate move- ment, caution, unstable roof conditions developing (fig. 225) . 4. Red disappearing: Substantial move- ment, warning, unstable roof conditions (fig. 22C). REFERENCES 1. Glowlarra (White Pine, MI.). Rock Fall Warning Devices. Brochure, 1979, 1 p. 2. Conkle Inc. (Paonia, CO.). The Guardian Angel. Brochure, 1979, 2 pp. 3. Guccione, E. Conkle' s Warning Mon- itor: The Miners' Guardian Angel. Coal Min. & Process., v. 15, No. 9, 1978, pp. 106-108. 4. Chironis, N. P. (ed.). Homemade Roof-Strain Indicator Helps Judge Safety of Bolted Coal Mine Roof. Sec. in Coal Age Operating Handbook of Underground Mining. McGraw-Hill, v. 1, 1977, p. 218. 5. U.S. Bureau of Mines. Horizontal Roof Strain Indicator (HORSI). Technol- ogy News, No. 2, 1974, 2 pp. 6. Panek, L. A. Evaluation of Roof Stability From Measurements of Horizontal Roof Strain. Paper in Ground Control As- pects of Coal Mine Design. Proceedings: Bureau of Mines Technology Transfer Seminar; Lexington, Ky.; March 6, 197 3, comp. by Staff, Mining Research. BuMines IC 8630, 1974, pp. 92-96. 7. McDowell, C. D. Coal Safety. Min. Eng. (N.Y.), v. 25, No. 2, 1973, p. 97. 8. Radcliffe, D. E. , and R. M. State- ham. Effects of Time Between Exposure and Support on Mine Roof Stability, Bear Coal Mine, Somerset, Colo. BuMines RI 8298, 1978, 13 pp. 9. Chironis, N. P. (ed.). New Infra- red Scanner Helps Spot Hazardous Condi- tions in Mines. Sec. in Coal Age Oper- ating Handbook of Underground Mining. McGraw-Hill, v. 1, 197 7, pp. 212-216. 10. Hughes Aircraft Co. (Carlsbad, CA) . Probeye Infrared Viewers. Bull. SL 2491, 1980, 4 pp. 11. Mikron Instrument Co., Inc. (Ridgewood, NJ) . Digital Infrared Ther- mometers. Brochure M80-879-10M, 1980, 8 pp. 12. Shepherd, R. , and D. P. Ashwin. Measurement and Interpretation of Strata Behavior on Mechanized Faces. Colliery Guardian, v. 216, No. 12, 1968, pp. 795- 800. 13. The Spider Inc. (St. Louis, MO). The Spider. Brochure, 1980, 1 p. 20 14. U.S. Bureau of Mines. Roof to Floor Closure Rate Instrument for Under- ground Mines. Technology News, No. 136, 1982, 2 pp. 15. McVey, J. R. , and W. L. Howie. New Closure Rate Instrument for Retreat Mining Operations. Min. Eng. (N.Y.), v. 33, No. 12, 1981, pp. 1699-1700. 16. Terrametrics (Golden, CO). In- struction Manual, Tape Extensometer. Un- dated, 6 pp. 17. Mann, C. D. , and J. J. Reed. St. Joe Builds Practical Rock Mechanics Tools. Eng. and Min. J., v. 162, No. 3, 1961, pp. 100-106. 18. Wang, C. Survey of Tools and Techniques for Roof Control Studies in Underground Coal Mines. BuMines, PMSRC Interim Report, Mar. 1972, 43 pp.; avail- able upon request from E. R. Bauer, Bu- reau of Mines, Pittsburgh, PA. 19. Parker, J. How Convergence Mea- surements Can Save Money. Eng. and Min. J., v. 174, No. 8, 1973, pp. 92-97. 20. Bauer, E. R. , and G. J. Chekan. Convergence Measurements for Squeeze Mon- itoring: Instrumentation and Results. BuMines TPR 113, 1981, 9 pp. 21. Barry, A. J., and J. A. McCormick. Spotlight on Roof Control. Coal Min. & Process., v. 3, No. 2, 1966, pp. 21-22. CHAPTER 2.— STRATA SEPARATION MEASUREMENTS INTRODUCTION Strata separation is the differential downward separation of distinct roof strata layers. It results from a low co- efficient of friction between roof strata layers and the weight of the immediate roof and overburden. Strata separation measurements allow mine operators to detect movement be- fore roof control problems occur, to analyze and prevent further occurrences, and to make changes in mine plans. Early detection of developing hazardous roof conditions can improve safety and production. GROUND MOVEMENT INDICATORS Each ground movement is characterized by specific indicators. However, since some indicators are common to several types of movement, recognizing the move- ment that is occurring can be difficult, and at times may be just an educated estimate. The following indicators of strata separation can help to identify this movement: • Cracks developing in middle of roof span. • Distinct layers of roof falling. • Succession of falls of distinct layers in the same area. • Increase in dripping of water from roof. • Visible entry closure or convergence. INSTRUMENT SELECTION As requirements and parameters are de- fined, they should be listed on an in- strument selection worksheet (fig. 1). In this case, the user should list strata separation in the "movement occurring" column. The controlling parameters, as defined in the section "Instrument Selec- tion Guidelines," should be listed in the "desired elements" column. Next, the user should refer to figures 23 through 25 and follow steps 1 through 6 to make a preliminary choice as to which instrument(s) best suits the ground control problem being investigated. Step 1: List the instruments available for monitoring strata separation on the worksheet under "instruments available" across from "movement occurring. " All three instruments described in this chap- ter will monitor strata separation. Step 2: From figure 23, choose the instruments that satisfy the cost param- eter selected and list them under "in- struments available," across from the cost parameter. Step 3: From figure 24, choose the in- struments that satisfy the technical as- pects parameter and list them under "in- struments available," across from the technical aspects parameter. Step 4: From figure 25, choose the in- struments that satisfy the data require- ment parameter and list them under "in- struments available," across from the data requirement parameter. 21 Step 5: Examine the "instruments available" column of the worksheet to find the instruments that satisfy all re- quirements and parameters, and list them at the bottom of the worksheet. If no instruments satisfy all the requirements and parameters, it may be necessary to change the parameters or to choose the instrument that satisfies the most re- quirements and parameters. Step 6: At this point, the user must make a decision as to the instrument or instruments to be used, based on the fol- lowing detailed descriptions. DESCRIPTION OF INDIVIDUAL INSTRUMENTS Instrument Cost of purchase or fabrication Dollars 50 100 250 500 IJOOO 2,000 10,000 i . , , i . i . . i , . i , . . r i , i i Borehole extensometer i Simple weighted bed seporotion indicator a Strotoscope ! 1 KEY 3 Cost range FIGURE 23. • Cost range of purchase or fabrica- tion of strata separation measuring instruments. Instrument Borehole extensometer Simple weighted bed separation indicotor Range of technical ability required Slight i^WW^ Moderate Extensive \WV\>^W\VJ T' 5SS . 3 Strotoscope UK AUUv,v^ KEY Installation Monitoring 13 Dato interpretation FIGURE 24. - Range of technical ability required for installation, monitoring, and data interpretation of strata separation measuring instruments. Instrument Type of measured data obtained Visual Simple numerical Detailed numerical j Borehole -IOmeter o o l-rtt *e -.'•--. bed o Strotoscope o <| f Q Data obtainable FIGURE 25. - Type of measured data obtained from strata separation measuring instruments. Borehole Extensometer Principle and Application Borehole extensometers are in-hole mea- suring devices that detect strata separa- tion movements at various horizons of the roof. Movement is detected by the change in distance between anchors at vari- ous depths and the reference head at the roof line ( 1_) . 6 Anchors are connected to the reference head by rod or wire connection systems. Borehole extensom- eters are available as single-position (one horizon detection) (fig. 26A) and "Underlined numbers in parentheses re- fer to items in the list of references at the end of this chapter. -Deep C-anchor -Connecting rod Scale, in ■Roof level anchor -End cap -Deep C-anchor Ring magnets -Anchor tube Scale, in -Magnet ■Roof level shel anchor FIGURE 26. - Borehole extensometer. A, Single point; B, multipoint (2). 22 multiple-position (multiple horizon de- tection) instruments (fig. 26B) . Read- out systems include flexible and rigid sonic probes, dial gauge, and spring cantilever. Availability Borehole extensometers can be purchased from the following suppliers: Geokon, Inc.; Irad Gage; Roctest, Inc.; Sinco, Slope Indicator Co.; and Soiltest, Inc. Approximate cost for the extensometer and readout system ranges from $100 to $1,800. Description Borehole extensometers consist of an- chors, a reference head, wire or rod con- necting assemblies, and a readout system. Anchors are of many different types: rock bolt expansion shell, double-wedge expansion shell, flat sliding wedge, wedge-split ring, screw-activated shoe, hydraulic anchor, cement-grouted anchor, C-anchor, cam anchor, and expansion shoe anchor. The readout system can be a dial gauge, depth gauge, dial microm- eter, continuous-drive mechanical chart, spring cantilever, potentiometer, or son- ic probe. Installation Installation procedures are very gen- eral, because each type of anchor has a specific installation sequence that can- not be adequately described here. In- formation on special tools or methods needed to install the connecting rods or wires can be obtained from the instrument suppliers. Step 1: Drill anchor holes to appro- priate diameter and depth (just past the last horizon of desired monitoring). Step 2: Set anchors in the hole at de- sired horizons. Step 3: Install reference head in the hole at the roof line. Step 4: Connect anchors to reference head with wires or rods (depending on the type of extensometer being installed). Step 5: Connect the readout system to reference head and take the initial read- ings, which will serve as the baseline for all subsequent readings. Data Collection Data are collected using the readout system appropriate to the extensometer selected. A total of eight horizons in 15 ft of strata can be monitored for their change of location, to determine bed separation. Borehole extensometers should be read once a week unless (1) no movement is oc- curring (readings can be stopped) or (2) large movements are occurring (frequency of readings should increase) . Data Interpretation The critical factor is the amount of bed separation at each horizon monitored and the total roof sag created. The strata will sag differently from mine to mine; therefore, there is no degree of deflection that can be taken as an industry-wide warning of developing un- stable roof conditions. Each mine must determine allowable separation for the roof strata being supported. Simple Weighted Bed Separation Indicator Principle and Application The simple weighted bed separation indicator (also known as a vertical dis- placement gauge) can be adapted to mea- sure one or several horizons of separa- tion within the same hole. Movement is measured by the change in length of wires with reference to a brass plug at the roof line. Availability This instrument is not commercially available. It can be fabricated in-house or by a local machine shop. Cost for each installation is approximately $5 to $15. 23 Description Components include spring clip an- chors, lightweight steel wires, ring- tongue solderless terminals, a brass ref- erence plug, and a weight. Measurements are made with a vernier caliper or gradu- ated scale , depending on the precision required. Installation Step 1: Braze wires to anchors before installation, attaching the wires to cen- ter or sides of anchors as needed. Step 2: Drill 3-in-diam hole into roof to a depth greater than the zones of sus- pected separation. Step 3: Install anchors one at a time, making sure not to tangle the wires. To do this , compress the anchor in a hollow cylinder, position it at the desired depth, then eject it from the cylinder by pushing a rod through the cylinder. Step 4: Slide wires through appropri- ate holes in the brass plug, then secure the plug in the hole opening. Step 5: Clip wires and install ring- tongue solderless terminals (fig. 27), to which the weight will be attached. Step 6: Attach the weight to one wire at a time and take an initial reading to serve as the reference for all subsequent readings. Data Collection Readings are taken by hanging the weight on the wire, then measuring the distance from the solderless terminal to the reference plug. A vernier caliper works well, but a graduated scale will also work. A change in wire length indi- cates a separation of strata. Data Interpretation How much each zone has moved is shown by the amount of movement recorded in comparison with the reference reading. Subtracting the movement measured from the reference reading will give the total movement from the time of installation. Spring clip anchor Measuring wires — Scale, in Spring clip anchor 6 -Spring clip anchor 6 Movement indicated by | change in length ®*. — Weight attached here FIGURE 27. - Simple weighted bed separation indicator. Interpretation of the data will depend on the mine or area being monitored. The amount of movement that indicates unsta- ble roof conditions must be determined for each mine. Stratascope Principle and Application The stratascope (borescope) is an opti- cal viewing instrument designed to permit visual or photographic observations with- in a drilled hole. It is used to detect cracks, separations, geologic makeup of mine strata, and lateral roof movements. Recently developed models use flexible f iberoptics. 24 Availability Stratascopes can be purchased from the following suppliers: American Op- tical Corp.; Baltimore Instrument Co., Inc.; Eder Instrument Co., Inc.; Expand- ed Optics Co., Inc.; Instrument Technol- ogy, Inc.; Lenox Instrument Co., Inc.; Olympus Corp of America; Soiltest, Inc.; and Welch Allyn, Inc. Approximate cost ranges from $390 to $10,000. Description The stratascope is basically a peri- scope, either handheld or tripod mount- ed. It consists of a light, a protec- tive outer housing, a lense, and mirrors or fiberoptics (fig. 28). The light is powered by a battery pack. If pic- tures are to be taken, tripods are need- ed for both the stratascope and camera (fig. 29). FIGURE 28. - Fiberoptic stratascope, battery pack, camera, and attachment (3). Installation Step 1: Drill a hole to the desired depth (the diameter depends on the stratascope used). Step 2: Assemble necessary extensions for viewing at the desired depth. Step 3: Connect the battery pack to the stratascope using electrical cable. Step 4: If photographs are to be ta- ken, assemble the stratascope tripod directly below the hole. Data Collection Visual observation: Slowly slide the stratascope into the hole, stopping to view the hole where desired, by looking through the eyepiece. By rotating the stratascope, the entire circumference of the hole can be observed. Photographic observation: Slide the stratascope into the hole, then clamp it to the tripod. Move the tripod-strata- scope assembly until the stratascope is centered in the hole. Set up the camera tripod and attach the camera to tripod and to stratascope. Set the stratascope to the borehole area to be photographed and take the picture. FIGURE 29. - Stratascope setup for roof observation. 25 There are no specific guidelines as to how often a hole should be viewed. Using the stratascope in newly mined areas to determine if the roof strata characteris- tics have changed or in previously mined areas to check for developing cracks and separations will determine the frequency of viewing. Data Interpretation Since no two mines are the same, the significance of any irregularities ob- served will depend on the particular mine. Irregularities such as clay veins, mud seams, cracks, fractured zones, and slips will create different problems for each mine. The important fact is that early detection of these ground conditions will enable mine personnel to initiate the appropriate action to con- trol them. REFERENCES 1. International Society for Rock Me- chanics. Suggested Methods for Monitor- ing Rock Movements Using Borehole Exten- someters. Int. J. Rock Mech. and Min. Sci. and Geomech. Abstr. , v. 15, No. 6, 1978, pp. 307-317. 2. Irad Gage (Lebanon, NH). Geotech- nical Instrumentation. Catalog, 1980, 40 pp. 3. FitzSimmons, J. R. , R. M. Stateham, and D. E. Radcliffe. Flexible, Fiber- optic Stratascope for Mining Applica- tions. BuMines RI 8345, 1979, 12 pp. CHAPTER 3.— LATERAL ROOF MOVEMENT MEASUREMENTS INTRODUCTION Lateral roof movement is the differen- tial horizontal displacement (sliding) of distinct roof layers. It is a result of a low coefficient of friction between roof layers and a high horizontal stress within the roof. Lateral roof movement measurements pro- vide mine operators with a means of de- tecting such movements before roof con- trol problems occur, analyzing their cause and preventing further occurrences, and making necessary changes in mine de- sign. Early detection of developing haz- ardous roof conditions can result in in- creased safety and production. GROUND MOVEMENT INDICATORS Like other ground movements, lateral roof movement has specific indicators that characterize it. Unfortunately, some indicators are common to sever- al movements. Therefore, differentiating among movements and determining which movement is occurring can be difficult, and at times may be just an educated estimate. The following indicators of lateral roof movement should help in rec- ognizing its occurrence: • Offsets developing in holes drilled in the roof. • Falls where many bolts are bent in in the same direction. • Bolts shearing and falling out of holes. • Tension cracks in roof at one rib line, compression cracks in roof at opposite rib line. INSTRUMENT SELECTION As requirements and parameters are defined, they should be listed on an instrument selection worksheet (fig. 1). Since this chapter deals with lateral roof movement only, "lateral roof move- ment" should be written in the "move- ment occurring" column. The controlling parameters as defined in the section "In- strument Selection Guidelines," should be listed in the "desired elements" column. Next, the user should refer to fig- ures 30 through 32 and follow steps 1 through 6 to make a preliminary choice as to the most suitable instrument(s) for the ground control problem being investigated. Step 1: Both of the instruments in this chapter satisfy the "movement occur- ring" requirement previously entered on the worksheet. List them under "instru- ments available," across from the "move- ment occurring" parameter. 26 Step 2: From figure 30, choose the in- strument that satisfies the cost parame- ter selected and list it under "instru- ments available," across from the cost parameter. Step 3: From figure 31, choose the instrument that satisfies the technical aspects selected and list it under "in- struments available," across from the technical aspects parameter. Step 4: Figure 32 shows that both in- struments provide visual and simple nu- merical data. Neither provides detailed Instrument Cost of purchase or fabrication Dollars 10 50 100 250 500 1,000 2,000 10,000 1 , , , 1 , 1 , , 1 , , 1 , . . , 1 , 1 1 Plumb bob 1 1 Stratascope 1 1 KEY 3 Cost range FIGURE 30. - Cost range of purchase or fabrica- tion of lateral roof movement measuring instruments. Instrument Range of technical ability required Slight Moderate Extensive Plumb bob ^^^^^W Stratascope skkkkkkkkkkkkkkwww KEY 1 Installation I Monitoring K\\\\l Data interpretation FIGURE 31. - Range of technical ability required for installation, monitoring, and data interpretation of lateral roof movement measuring instruments. Instrument Type of measured data obtained Visual Simple numerical Detailed numerical Plumb bob o o Stratascope o o KEY O Da 'a obtainable FIGURE 32. - Type of measured data obtained from lateral roof movement measuring instruments. data. List both or neither on the work- sheet depending on the type of data required. Step 5: From the "instruments availa- ble" column, determine which instrument satisfies all requirements and parame- ters. Enter this instrument at the bot- tom of the worksheet. If neither instru- ment satisfies all the requirements and parameters, it may be necessary to change the parameters or to choose the instru- ment that satisfies the most requirements and parameters. Step 6: At this point, the user must decide which instrument should be se- lected, based on the following detailed descriptions. DESCRIPTION OF INDIVIDUAL INSTRUMENTS Plumb Bob Principle and Application A plumb bob is a pointed weight sus- pended by a string. It can be used for imprecise measurement of lateral roof movement, by being suspended from a point on or in the roof and its movement mea- sured with respect to a reference pin in the floor. ' Observed plumb bob move- ments, coupled with visual observation of the holes drilled into the roof will be sufficient for detection of lateral move- ment of roof strata. Availability Plumb bobs and associated materials are available from most hardware stores. The approximate cost ranges from $10 to $20. Description The assembly consists of a plumb bob, string, a spring clip anchor, and a ref- erence pin grouted into the mine floor (fig. 33). 'Shepherd, R., and D. P. Ashwin. Mea- surement and Interpretation of Strata Be- havior on Mechanized Faces. Colliery Guardian, v. 216, No. 12, 1968, pp. 795. 27 Suspected glide zone- Mine roof -Spring clip anchor Mine roof -Wire K- Entry -• — String = :] Ring-tongue solderless terminal FRib ■- — Plumb bob -Reference pin Mine floor Not to scale 12 -Grout Mine floor FIGURE 33. - Plumb bob setup for detecting lateral roof movement. Installation Step 1: Braze a long piece of steel wire (longer than depth of hole) to the center of a spring clip anchor. Step 2: Drill a minimum 2-in-diam hole into the roof strata layer that is thought to be moving. Step 3: Install the spring clip anchor in the hole by compressing the anchor in a hollow cylinder, pushing this cylinder and anchor to the desired location, and then pushing the anchor out of the cylin- der and against the strata using a rod slid up through the cylinder. Step 4: Remove the cylinder and rod from the hole. Step 5: Clip the wire several inches below the hole, then clamp a ring-tongue solderless terminal to the end of the wire. Step 6: Using the suspended plumb bob as a guide, drill or chip a hole into the mine floor directly below the plumb bob. Step 7: Grout the reference pin in the floor so that its point is directly under the plumb bob point. Step 8: When the grout has hardened, adjust the plumb bob so that it is just barely above the reference pin. Data Collection Any roof movement will be indicated by a change in horizontal distance between the reference pin and the plumb bob. A plumb bob installation should be checked as often as experience indicates is necessary. The frequency of obser- vations should increase if substantial movement is detected. Data Interpretation The amount of movement that indicates unstable roof conditions will vary from mine to mine. Only by experience and continuing measurements can this movement be determined. Once this is known, de- veloping unstable roof can be detected in time for corrective action. Stratascope See chapter 2 for a complete descrip- tion of this instrument. CHAPTER 4. —STRESS MEASUREMENTS INTRODUCTION Ground stress is area distributed There are two type in situ (produced tion (resulting f stress measurement design. Based on stress around an orientation of ent the force per unit about an excavation, s of stress, either by nature) or extrac- rom mining). Ground s are used for mine the distribution of excavation, the best ries and the optimum pillar size can be determined, thus re- ducing or eliminating ground control problems. Stress measurements provide mine opera- tors with a means of detecting ground (roof, rib, and floor) control problems before and after they occur, analyzing their causes and preventing further oc- currences, and making necessary changes in mine design. 28 GROUND MOVEMENT INDICATORS Like other ground movements, ground stress has specific indicators that char- acterize it. Unfortunately, some indi- cators are common to several movements. Therefore, differentiating among sever- al movements can be difficult, and at times may be just an educated estimate. The following indicators of excessive stress can help miners to recognize its occurrence: • Excessive number of roof falls. • Numerous roof falls with same orientation. • Excessive rib sloughing. • Floor heave. • Roof sag. • Entry closure (squeeze). INSTRUMENT SELECTION As requirements and parameters are de- fined they should be listed on an instru- ment selection worksheet (fig. 1). Since this chapter deals with stress measure- ments only, stress should be listed under the "measurement desired" column. The controlling parameters, as defined in the section "Instrument Selection Guide- lines," should be listed in the "desired elements" column. Next, the user should refer to fig- ures 34 through 36 and follow steps 1 through 6 to make a preliminary choice as to the most suitable instrument(s) for the ground control problem being investigated. Step 1: All of the instruments in this chapter satisfy the "measurement desired" requirement (stress) previously listed on the worksheet. List these instruments under "instruments available," across from the "movement occurring" parameter. Step 2: From figure 34, choose the in- struments that satisfy the cost parameter previously selected and list them under "instruments available," across from the cost parameter. Step 3: From figure 35, choose the instruments that satisfy the technical aspects parameter and list them under Instrument Cost of purchase or fabrication Dollars 10 50 100 250 500 1,000 1 .'. . 1 . 1 , . 1 . . 1 . . . . 1 2,000 10,000 1 ' 1 Borehole defor- mation gauge 1 1 Borehole inclusion stressmeter 1 1 Borehole- mount strain gauge 1 1 CSIR strain gauge strain cell 1 1 CSIR triaxial strain cell a CSIRO hollow inclusion stress cell i i Cylindrical borehole pressure cell a Flat borehole pressure cell □ Flatjack i i Mechanical strain gauge D Surface-mount photoelasticgauge i i Surface-mount strain gauge i l Surface rosette undercoring a Vibrating wire stressmeter i i KEY in Cost range FIGURE 34. - Cost range of purchase or fabri- cation of stress measuring instruments. "instruments available," across from the technical aspects parameter. Step 4: From figure 36, choose the in- struments that satisfy the data require- ment parameter and list them under "in- struments available," across from the data requirement parameter. Step 5: From the "instruments availa- ble" column, determine the instruments that satisfy all requirements and parame- ters. List these at the bottom of the worksheet. If no instruments satisfy all of the requirements and parameters, it may be necessary to change the parameters or to choose the instruments that satisfy the most requirements and parameters. Step 6: At this point, the user must select the most suitable instrument(s) . The final decision can be made from the following detailed descriptions. 29 k\\\\1 Data interpretation FIGURE 35. - Range of technical ability required for installation, monitoring, and data interpretation of stress measuring instruments. DESCRIPTION OF INDIVIDUAL INSTRUMENTS Borehole Deformation Gauge Principle and Application Borehole deformation gauges are de- signed to measure diametral deformations of a borehole during the overcoring process of stress relief (l). 8 These de- formation measurements provide informa- tion to calculate the state of stress in the plane normal to the borehole. This technique determines the absolute field stress and is not easy to use, especially in coal measure rocks. The gauges are either single component (one "Underlined numbers in parentheses re- fer to items in the list of references at the end of this chapter. Instrument ~>pe of measured data oDtained Visual Simple numerical Detailed numerical Borehole deformation gouge o Borehole inclusion stressmeter o Borehole-mount strain gauge o CS 1 R strain gauge strain cell o CSI R tnaxial stram cell o CSIRO hollow inclusion stress cell o Cylindrical borehole pressure cell o Flat borehole pressure cell o Flatjack o Mechanical strain gauge o o Surface-mount photoe la stic gouge o o Surface-mount strain gauge o Surface rosette undercoring o Vibrating wire stressmeter o o KEY O Data obtainable FIGURE 36. - Type of measured data obtained from stress measuring instruments. point of contact and one direction of measurement) or three component (three points of contact and three directions of measurement) . Availability Borehole deformation gauges are avail- able from the following suppliers: Geokon, Inc.; Irad Gage; Rogers Arms and Machine Co.; Sinco, Slope Indicator Co.; and Soiltest, Inc. The approximate cost for the gauge and readout system ranges from $1,300 to $3,600. 30 Description Borehole cylinders, tilever st matched pai tached (fi gauge has cantilevers has three These gaug in-diam hoi diam core electronic for data co deformation gauges are round 1.4 by 12 in, that house can- rain transducers to which rs of strain gauges are at- g. 37). The single-component one pair of oppositely placed ; the three-component gauge pairs of such cantilevers, es are designed for a 1.5- e and overcoring by a 6-in- drill (fig. 38). A standard strain gauge readout is needed llection. Installation and Data Collection The installation procedure is basi- cally the same for the single- and three-component gauges, but the single- component gauge must be overcored three times within the same hole at 120° orientations . Step 1: Drill a 1.5-in-diam hole to the desired depth. Step 2: Position the gauge in the hole. Step 3: Connect the gauge lead wire to the strain readout box. Step 4: Read the instrument. Be sure to read out all three components when a three-component gauge is used. Step 5: Overcore the gauge with the 6-in overcore drill while simultaneously recording the deformations and depth of overcore. Step 6: When the gauge is completely overcored, remove the core drill, and read and remove the gauge. (For single-component gauges, repeat above installation and data collection steps two more times, rotating the gauge 120° each time.) Step 7: Remove the core and store se- curely if the core is to be tested in the laboratory. Scale, in KEY / Lug to engage placement tool 2 Sleeve for placement tool 3 Cap for cable clamp 4 Rubber grommet 5 Body of gauge 6 0-ring seals 7 Clamp block 8 Transducer strip 9 Tungsten carbide wear button 10 Piston cap // Shim washers 12 Piston base 13 Cover of gauge LONGITUDINUAL SECTION A- A -12 PISTON ASSEMBLY DETAIL '/ 2 I i I SECTION Scale, in FIGURE 37. - Three-component borehole deformation gauge (]_). 31 FIGURE 38. - Cross section through a borehole showing borehole deformation gauge after over- coring (2). Data Interpretation Stress in the plane normal to the bore- hole can be calculated using the follow- ing equations (1): P + Q = 3d(1 - y2) (u 1+ u 2 +u 3 ) t P - Q = E /2 6d 5 i L f(u - U ) 2 (1 - y 2) KU 1 V and where and + (U 2 - U 3 )2 + ( U]L - U 3 )2]l/2, Tan 29 ■ - ^ (U 2- U 3^ , 2Ui - U 2 - U 3 ' D lf U 2 , and U 3 = the three borehole de- formation measure- ments, uin/in, E = Young's modulus of rock, psi, y = Poisson ratio of rock, d = diameter of the pilot borehole, in, P = maximum secondary prin- cipal stress, psi, Q = minimum secondary prin- cipal stress, psi, = angle from U^ to P. If a three-dimensional state of stress is desired, deformation measurements must be made in three nonparallel boreholes. Some of the recommended borehole config- urations are shown in figure 39. FIGURE 39. - Recommended borehole configurations for complete, three-dimensional, state-of-stress deter- mination. These various configurations will all reveal the same state of stress {]). {D is diameter of mine opening.) Borehole Inclusion Stressmeter Principle and Application Borehole inclusion stressmeters are rigid devices with an elastic modulus greater than that of the material to be tested (2^ pp. 363-383). When insert- ed into a borehole, a stressmeter mea- sures directly the stress changes of the surrounding rock. It is secured in the borehole by hydraulic pressure or with grout and can be left in place for long-term stress monitoring. Stress is detected by pressure changes on a diaphragm, which are transformed by a transducer. The transducer can be an electrical or foil resistance strain gauge or an electrical coil imped- ance, photoelastic, or magnetostriction device. Availability The availability of this instrument is limited. The cost ranges from $800 to $3,000. Description The borehole inclusion stressmeter consists of a rigid outer shell, 32 fluid-filled diaphragm or magnetic coil, transducer, and readout box. Stress- meters have been developed by many in- dividuals and organizations (3_) (figs. 40-42). Installation Step 1: Drill a 1.5-in-diam hole to the desired depth. Step 2: Install the stressmeter in the hole. If a hydraulic stressmeter is used, pump pressure up to that speci- fied by the manufacturer. If a grouted stressmeter is used, fill the hole with grout, then slide the stressmeter to the desired depth. Step 3: After the stressmeter is set, take an initial reading, which will serve as reference for all subsequent readings. KEY / Blade or core 2 Slit 3 Tapered sleeve 4 Slots 5 Diaphragm 6 Electric resistance strain gauge 7 Insulated sealing disk 8 Lead wire SECTION A- A Scole.in FIGURE 40. - Cross section of stressmeter (3). FIGURE 41. - Stressmeter and tapered sleeve into which it fits (3). Cement Glass inclusion Polarizing material I , Reflecting paint ■^-wave plate Lead through hole in inclusion to light source Borehole diameter '8 j Light source cast in plastic FIGURE 42. - Cross-sectional view of a borehole showing installed photoelastic stressmeter (3). 33 Data Collection For nonphotoelastic stressmeters , read- ings are taken by connecting the lead wires from the stressmeter to a read-out box. The photoelastic type of stress- meter is read with a polarizer light, which illuminates the stress fringe pat- terns. Data should be collected on a regular basis, preferably once a week. Data Interpretation Stress can be determined from the re- corded strain, using Hooke's law: where and 6 = eE, 6 = stress, psi, e = strain, in/in, E = Young's modulus of rock being tested, psi. 9 This stress value can be compared with the laboratory experimental stress determined from a sample of the mate- rial to see if the material is near failure. Borehole-Mount Strain Gauge Principle and Application This instrument is mounted to the flat end of a borehole, then overcored and removed to relieve the strain (stress). It can be of the electric-resistance or photoelastic type. This gauge is used primarily to measure strain in pillars but can also be used in mine roof and floor. Availability This gauge can be purchased from many suppliers, including Micro-Measurements. The approximate cost of the gauge and readout system ranges from $30 to $1,900. ^Average values for E are Coal: 0.1*10 6 < E < 1.5x10 6 psi. Description This instrument is simply a single strain gauge or rosette that is glued to the flattened end of a borehole. In addition to the gauge, the user needs setting glue, setting tools, overcore drill, flat-grinding drill, and readout equipment. Installation 2.0*10 6 < E < 15*10 6 psi. ? o ~> : Steel: E * 30x10° psi, Step 1: Drill a 1.75-in-diam hole to the desired measuring depth (usually in a pillar) . Step 2: Grind the bottom end of the hole flat. Step 3: Read the gauge before installation. Step 4: Glue gauge to the flat surface of the hole. Orient gauge so that one component is axially lined up with the suspected direction of principal stress (strain) . Step 5: Overcore gauge 6 to 12 in, then remove core drill and core. Step 6: Read the gauge. Data Collection Data are collected twice during the in- stallation process: The gauge is read just prior to being overcored and after it has been overcored and the core and gauge have been removed from the hole. These readings, as well as the date, time, installation location, depth of gauge in hole, and person(s) taking the readings and involved in the installation process should be recorded in a field book. Data Interpretation The strain in the pillar is found by subtracting the original reading from the overcored reading. Stress is calculated using Hooke's law: 6 = eE. Since stress is measured inside the pillar, it can be compared with the yield strength of the pillar to deter- mine if the pillar is near failure. 34 The pillar follows: strength is determined as where S = estimated strength of pillar, psi, T = seam thickness, in, L = least lateral pillar dimen- sion, in, K = S L /W[, psi, W L = cubic size of lab specimen, in 3 , and S L = strength of lab specimen, psi. CSIR Strain Gauge Strain Cell (Doorstopper) Principle and Application The Doorstopper Cell, developed by the Council for Scientific and Industrial Re- search (CSIR), is designed to determine the absolute stress in rock using an overcoring stress-relieving technique. It may, however, be used to measure changes in stress, provided the glue used to bond the cell possesses sufficiently stable characteristics. The cell mea- sures the major principal stress where its direction and those of the two other principal stresses are known or can be assumed. It is also possible to deter- mine the complete state of stress using Doorstoppers by taking measurements in three boreholes drilled in any three dif- ferent and known directions (4). angles, molded into a rubber casting that fills a plastic shell. The gauges are connected by lead wires to a strain- indicating instrument. The cell can be used in either a BW (2.4-in nominal diam) or NW (3.0-in nominal diam) diamond- drilled borehole. The rubber and plastic shell serve to protect the strain gauges from damage and from water during the overcoring operations (4). Installation and Data Collection Step 1: A temperature compensation dummy cell with a 1/2-in length of BX (1.5-in nominal diam) core attached must be installed in the installation tool prior to use in the mine. Step 2: Drill a borehole to the de- sired measurement zone. Grind flat and polish the end of the borehole. Step 3: Plug a Doorstopper Cell into the installation tool, cover cell with glue, then push cell to back of the bore- hole and orient cell as indicated by ori- entation device. Step 4: Push cell against rock until correct pressure is obtained. Wedge the rods in place to maintain this pressure until glue hardens. Step 5: After the glue has hardened, read out the strain in the cell, using a standard strain gauge indicator unit, and then remove the installation tool from the borehole. Step 6: Overcore cell with the same size core barrel to a minimum depth of 6 in. Break off core and remove from the borehole. Step 7: Plug stallation tool relieved readings, laboratory testing. cell back into the in- and take the stress- s . Save the core for Availability The Doorstopper Cell is available from Roctest, Inc. , at a cost of approximately $56 per cell. A complete system will cost approximately $7,500. Description The Doorstopper consists of four strain gauges at -45°, 0°, +45°, and +90° Data Interpretation The magnitude and direction of stress can be calculated from the data using the following equations (40: Magnitude: ' a l = - y 2 (e l + ye 2 ) 35 and where a„ = 2 1 - u 2 (e 2 + pej), o± = magnitude of maximum horizontal stress at end of borehole, psi, 0"2 = magnitude of minimum horizontal stress at end of borehole, psi, E = Young's modulus of rock, psi, \i = Poisson ratio of rock, e^ = maximum principal strain, pin/in, e 2 = minimum principal strain, yin/in, 1,2 " I/? |(e H + e v ) ±/[2e 45 -(e H + e v )] 2 + (e H -e v ) 2 ], and e H> e 4 5- and difference in strain readings in the horizontal, 45°, and vertical directions before and after overcoring, re- spectively, yin/in. r, = 1/1.53 a[ 2 ~ 1/1.53 a 2 » where a^ = magnitude of maximum horizon- tal stress, psi, and o"2 = magnitude of minimum horizon- tal stress, psi. Direction: Tan e, - , 2 (£l "V V 2e 45 " ( e H + e v ) 6? = 2 (e 2 - e H ) 2 '" 2e 45 - (e H + e v )' where Q± is the angle measured anticlock- wise from the horizontal (e H ) direction. The determination of the stress is im- portant in planning of mine design, layout of panels, and analysis of proba- ble areas of ground control problems. CSIR Triaxial Strain Cell Principle and Application The CSIR triaxial strain cell is de- signed to obtain the complete state of stress in rock in a single borehole (5). The change in strain associated with the overcoring stress-relief method is de- tected by the strain gauges mounted in the instrument body. Availability This instrument is available from Roc- test, Inc. Cost of the cell, installa- tion equipment, and readout boxes is ap- proximately $6,000. Description The cell consists of a plastic housing containing three strain gauge rosettes mounted on pistons, which are subsequent- ly glued to the borehole walls. The pistons are actuated by air pressure. Strain changes are read using a standard strain gauge indicator unit. Installation and Data Collection Step 1: Drill a 3.5-in-diam hole to the depth at which the stress is to be determined. Step 2: Drill a 1.5-in-diam hole for 18 in into the end of the borehole. Step 3: Cover the three strain gauge rosettes with glue, insert the cell into the hole, then turn on the air pressure to actuate the pistons. Step 4: After the glue has set, turn off the air pressure and take the inital strain readings using a standard strain gauge indictor. Step 5: Overcore the cell with a 3.5- in-diam core barrel. Step 6: Remove the core, plug back in- to the installation tool, then take a second set of strain readings. 36 Data Interpretation The complete state of stress is deter- mined from the strain readings using the following equations: Magnitude: E e A + e C B + e A e B T+IT and °B AB e A + e B _ e A ~ e B 2 L l - y 1 + y J 2e c - (e A + e B ) where a A = normal stress in A direction, psi, a B = normal stress in B direction, psi, T AB = tangential stress, psi, E = Young's modulus of rock, psi, y = Poisson ratio of rock, e A = measured strain in A direc- tion (X-axis), yin/in, leasured strain in B di tion (Y-axis), yin/in, e B = measured strain in B direc- and e c = measured strain in C direc- tion (45° to A and B), yin/in. Direction: Tan A = and Tan 9 B = 2(e, - e A ) 2e c - (e A + e B ) 2(e 2 - e A ) 2e c - (e A + e B )' where 9 A is the angle measured anticlock- wise from the horizontal direction (X- axis) , and e, >2 = 111 J (e A + e B ) ± /(e A - e B ) 2 + [2e c -(e A + e B )] 2 |. CSIRO Hollow Inclusion Stress Cell Principle and Application The CSIRO cell, developed by the Commonwealth Scientific and Industrial Research Organization (CSIRO), provides a method of determining the three- dimensional stress state in rock or coal (60. Strain gauges mounted in the in- strument measure the change in strain as the rock "relaxes" after overcoring. The cell can also be left in place for long- term monitoring of stress changes. Availability This instrument is available from Geokon, Inc. , at a cost of approximately $500 per cell, $6,000 for a complete system. Description The CSIRO cell consists of a fully encapsulated array of nine strain gauges mounted in the instrument body. The cell is 0.4 in long and is grouted into a 1.5-in-diam hole. It is constructed from epoxy pipe with the gauges precise- ly oriented at 120° angles along the circumference. Installation and Data Collection Step 1: Drill a 1.5-in-diam hole to the desired measurement zone. Step 2: Fill the hole with grout, then insert cell and push to back of hole, extruding the grout. Step 3: Allow the grout sufficient time to harden. Step 4: Readout the strain on all the gauges. Step 5: Overcore the cell with a 6- in-diam core barrel. Monitor the strain response during overcoring. Step 6: Read out the new strain on all the gauges. Data on the strain in the rock as indi- cated by cell readings are obtained using a nine-channel switch box and quarter bridge strain indicator. 37 Data Interpretation A data reduction program supplied by the manufacturer analyzes the stress tensor using the overcore strain data and biaxial pressurization results, giv- ing the stress state in both principal form and oriented along coordinate axes (6). The stress information can be used in mine design studies and geotectonic studies. Cylindrical Borehole Pressure Cell Principle and Application This device determines the modulus of rigidity of rock or coal by direct measurement inside a small-diameter bore- hole. It measures the change in volume of the borehole with respect to the ap- plied pressure, which is interpreted by thick-wall cylinder equations for an elastic body (_7_) . Availability Cylindrical borehole pressure cells can be made in-house, or purchased from Sinco, Slope Indicator Co. The approxi- mate cost is $350. Description The cylindrical borehole pressure cell consists of a steel core and copper jack- et. Overall length is 8 in. The cell is installed in a 1.5-in-diam hole without grout. In-mine tests require a hydraulic pump, fluid reservoir, and gauge. A cal- ibration cylinder is needed to calibrate the cell before installation. Installation and Data Collection Step 1: Drill a hole to the desired depth. The hole must be 1.5 in, at the measuring zone, and must not have open joints or cracks wide enough for the cop- per shell to extrude into them. Step 2: Slide cell into hole. No spe- cific orientation is required because cell is equally sensitive to changes in all directions. Step 3: Connect pressure gauge and fluid pump to cell and fluid reservoir (fig. 43). / Cylindrical borehole pressure cell 2 Hydraulic tubing 3 Wedges 4 Fluid pump 5 Pressure gauge Scale, ft FIGURE 43. - Typical installation of a cylindrical borehole pressure cell (]_). 38 Step 4: Completely fill cell with flu- id, then bleed off any air in the system. Only slight pressure is needed. Step 5: Cell is ready for pressure cy- cling, consisting of loading, unloading, and reloading at a rate of 200 psi/min, with readings taken at 1-min intervals. The measurements taken are the volume of fluid injected into the cell versus the cell pressure reading. The test for the modulus of rigidity is complete after the second loading. Maximum pressure per loading is 3,500 psi. Step 6: Cell can now be removed by re- leasing pressure. Measurements can be repeated at another horizon within the hole. Cell can also be left in place to continue monitoring. In this case, pres- sure in cell must be continued by closing valve located between cell and hydraulic pump. Pump and fluid reservoir can now be removed ( 1 ) . Readings need to be taken only during the test procedure described in step 5 above. If the cell is left in place, it should be checked once a week. 2. Insert the cell into the calibra- tion cylinder and determine the slope of the experimental pressure-volume curve (M m ). This is done by pressure cycling as described in the installation proce- dures, step 5. The system stiffness (M s ) is calculated by M = s M„ - M„ 3. cell the late (M r ) from the test data: Perform pressure cycling with the in the borehole and determine pressure-volume curve (M t ). Calcu- the pressure-volume relationship M t M s M = r M + - M c 4. The modulus of rigidity for the rock is calculated from G = M r tt &r; 2 Flat Borehole Pressure Cell Data Interpretation Principle and Application The modulus of rigidity (G r ) can be de- termined as follows (1_) : 1. Calculate M c for a calibration cyl- inder of known properties using M c = vG TT IVy 1 + B - 2 vB 1 - B A flat borehole pressure cell is a flatjack, 2 in wide, 8 to 10 in long, and 0.125 to 0.25 in thick ( _7) , designed for permanent (long-term) installation. It measures the changes in pressure from continued mining. It can be installed in the roof, floor, and ribs and is either grouted in place or preencapsulated. where v = volume per turn of pressure generator, in 3 , I = effective length of pressure cell, in, and B = = — L , where r } and r Q are the inner and outer radii of the cylinder, G = modulus of rigidity of cali- bration cylinder, psi. Availability This instrument can be made in-house or purchased from Geokon, Inc. , or Sinco, Slope Indicator Co. The approximate cost ranges from $195 to $230. Description The flat borehole pressure cell con- sists of a thin-walled, fluid-filled met- al bladder, hydraulic connection lines, and a pressure gauge (fig. 44). It is 39 FIGURE 44. - Steps in fabrication of an encapsulated flat borehole pressure cell. A, Copper tube cut to length; B, tube flattened to 1 4-in opening; C, top half of ends cut; J), top half of ends re- moved; E, bottom of ends folded up and brazed to top; F, fluid-filling and pressuring and gauge tubes attached; 0, cell encapsulated in plaster. either preencapsulated with a cement mix- ture or grouted in place. A hydraulic hand-operated pump is required to set the instrument at the correct insertion pressure. Installation The procedure depends on the type of cell used. Preencapsulated cells require a precise hole but can be installed, pressurized, and read within 30 to 60 min. Grouted cells do not require a pre- cise hole, but pressurization and moni- toring cannot begin until the grout has set. Step 1: Drill a hole to desired depth. Hole diameter is about 2.25 in. Step 2: Insert cell and orient it within the hole. Grout in place if nec- essary. Cell measures pressure perpen- dicular to its flat side. Step 3: Attach pressure gauge and pump to cell. A shutoff valve must be located between the pressure gauge and pump. Create a pressure in the cell equal to the known or estimated pressure of the surroundings. Hold this pressure by closing the shutoff valve. Step 4: Record this pressure as read from the gauge. Step 5: Take readings every few min- utes for the first hour to check for pressure leaks or other problems. 40 Data Collection The amount of pressure, as indicated on the gauge, should be recorded once a week. Any drastic changes should be in- vestigated and reported to the appropri- ate authorities. Data Interpretation The information obtained from flat borehole pressure cells is the pressure induced by mining. To obtain the change in pressure, the original (installation) pressure must be subtracted from the sub- sequent readings. Various ground control problems are associated with increased pressure. Knowing the allowable pressure limits could help eliminate hazards as- sociated with squeezing, floor heave, bursts, etc. Flat jack Principle and Application A flat jack is a thin-walled fluid- filled metal bladder designed to with- stand several thousand pounds per square inch when confined in a slot in rock strata (]_) . It records the pressure within the slot. The pressure needed to bring the rock back to equilibrium is the stress in the rock before the stress is relieved. The flat jack method can mea- sure rock stress directly. Availability Flat jacks are usually manufactured in- house, but can be purchased from Geokon, Inc., or Sinco, Slope Indicator Co. The approximate cost of this instrument ranges from $100 to $250. Description A flat jack consists of a metal blad- der, hydraulic connection line, and pres- sure gauge (fig. 45). For measurement purposes , a hydraulic pump and two ref- erence pins are needed. The flatjack should be sandwiched between flat steel plates or encapsulated to ensure uniform pressure distribution. FIGURE 45. - Flatjack pressure cell. Installation and Data Collection Step 1: Cement two measurement pins to rock surface or grout them in holes drilled into the rock. Spacing should be 1 to 12 in from the slot (2 to 24 in apart) . Pins should be perpendicular to the flatjack when installed. Step 2: Measure the distance between pins with as precise a measuring in- strument as is available. Record this measurement. Step 3: Cut a slot into the rock sur- face perpendicular to pins by drilling a series of overlapping holes. This re- lieves stress perpendicular to the slot. Step 4: Take and record another read- ing across the pins. Step 5: Embed flatjack in slot. The flatjack can be grouted in the slot if desired. Use hand pump to steadily increase pressure within flatjack until displacement (stress relieved) by the cut slot is cancelled. This requires frequent measurements across the pins to determine when they are the original dis- tance apart. This cancellation pressure is equal to the original stress in the area monitored. The information obtained is the can- cellation pressure needed to restore the area to its original position, which represents the original stress in the area. Usually, no additional readings are taken since the flatjack is removed when the process is completed. However, if desired, the flatjack can be left in place to continue monitoring rock stresses. 41 Data Interpretation Cancellation pressures (original stress) can be analyzed to determine several possible situations, all deal- ing with excessive pillar pressures. Excessive pressures (stress) can be the result of inadequate pillar size, pillar orientation, insufficient barrier pil- lars, etc. If it is known what pressures are acceptable, mine design can be mod- ified, resulting in safer and more stable roof and pillars. Mechanical Strain Gauge Principle and Application A mechanical strain gauge determines rock stress by the stress relief method. It measures the strain between three ref- erence points, set in the surface of the rock, after a series of relief holes are drilled. This gives surface strain only. Hooke's law is used to convert the strain to the maximum and minimum principal stresses. Availability Mechanical strain gauges are available from Soiltest, Inc. The approximate cost is $425. Description This system consists of a mechanical strain gauge (fig. 46) with an attached dial gauge and three reference (measure- ment) pins. A suitable drilling appara- tus is also required. Installation Step 1: Grout three pins into or on the rock surface, in a triangular layout. Step 2: Measure the strain between the pins with the mechanical strain gauge. Record the readings. Step 3: Drill a series of adjacent holes, 12 to 30 in deep and 1.5 to 3 in. in diam, completely around the pins. This relieves stress and allows the pins to move in any direction (fig. 47). Step 4: Remeasure and record strain between pins using mechanical strain gauge. Data Collection Strain is measured and recorded twice during the instrumentation process: (1) just after pins are installed and (2) af- ter the stress relief holes have been drilled. Data Interpretation Hooke's law is used to derive the principal stress. The strain used in the equations is the change in strain (the second measurement minus the first measurement). The allowable stresses be- fore failure occurs must be determined through observations such as this. These stresses will help determine the proper entry orientation, pillar size, etc. Dial gouge, j^^ in Moving f\ /Rigid cast / frame 10 point - — H Fixed point FIGURE 46. - Mechanical strain gauge. (8). FIGURE 47. - Typical measurement setup forme- chanical st rain gauge showing me asuringpoints(l A, 2B, and 3C) and stress relief holes (8). 42 Hooke's law (9) S = E T = E e A + e B + e c + 1 lr _ e A + e B + e c \ 2 + S e c - e B y 3 (1 ■ y) 1 + y VV A 3 / V /3 / e A + e R + e f 3 (1 - y) = 1/2 tan -1 /5 (ec " ee) e A - e A + e R + e, /3 e A _ e A + e B + e c y + where e A , e B , e c = S = T = E = strain at 60° orienta- tions, pin/in, maximum principal stress, psi, minimum principal stress, psi, Young's modulus of rock, psi, and y = Poisson ratio of rock, = angle from maximum principal stress to A-axis. Vibrating Wire Stressmeter Principle and Application A vibrating wire stressmeter is an instrument designed to monitor stress changes within mine rocks. It measures the altering period of the resonant fre- quency of a highly tensioned steel wire clamped diametrically across the gauge ( 13 , 15) . This reading is converted to pressure (pounds per square inch) using the appropriate calibration graph. Availability This instrument is available from Geokon, Inc. , or Irad Gage. The approxi- mate cost of the stressmeter and readout system ranges from $150 to $6,000. Description The vibrating wire stressmeter con- sists of a highly tensioned steel wire surrounded by a metal housing, anchorage platen, anchorage wedge, and electrical wire (fig. 51). A readout box is used to collect data. Special setting tools and rods (manual or hydraulic) are needed for installation. Installation Step 1: Drill a 1.5-in-diam hole to desired depth. Diameter range is 1.475 to 1.525 in for hard rock and 1.450 to 1.545 in for soft rock and coal. Step 2: Thread the loose end of wire from the stressmeter through the hole in setting tool. Place the stressmeter and platen in place on the end of the setting tool. Using a hydraulic pump, slowly apply pressure until the stress- meter is held in place. Take initial reading. Step 3: Slide the stressmeter and set- ting tool into hole. Align the stress- meter to desired measuring plane in hole. Connect needed setting rods onto assembly as it is slid into hole. *-»<^^» yj J USrehole 1u OOOOOOOOOODi QQQQQ QQDO borehole ,16 OEi£\Dpqpgg |U; roanaanaoo innnn nn nod fan Surface/ 2a o i_ 300 i LEGEND Clay vein Flat borehole pressure cell Roof fall FIGURE A-5. - Shortwall section 7 right. Scale, ft «3 65 o K U_ Ill Z < I o _j _i hj o — ." Q 1 1 _ K H * „■" ft v\ ... _l 1 < 2_ D a o LU z Z - 3 (- - — > □ X O < i i i i i — i — i — i — i—i — i — I i i i i — i — r~ i — i i i i i i i i i — i — j i i i i — i I i i i — n — I — i— i — i — i i — |— i — |— i — i — i — I — n — i — r— r W/,i'/A Y//A//A V/AV/A Y7^\ I2,l3>=r^ 18,19 ^ j .11 «£ C 8,9 6,7 4,5 16, 17 x-j 28,29 24,25 — - (Panel 5) 34,35 h 20,21 30,31 *= 22,23 26,27- 1,2 — (Panel 3) ,1,1, 14,15 32,33 \s^ KEY " — • Pressure cell E2ZJ Mining time of panel j i i i i L_i i i i i I i i i i i I i i i_i i l_i i i i_l I l_l i_i I Lj i_l i l I i_i l i i_ Nov Dec Jan Feb Mar Apr May June July Aug 1973 1974 DATE OF MEASUREMENT FIGURE A-6. - Pressure changes recorded in pillars and shortwall panels during mining. REFERENCES 1. Bauer, E. R. , and G. J. Chekan. Convergence Measurements for Squeeze Mon- itoring: Instrumentation and Results. BuMines TPR 113, 1981, 9 pp. 2. Radcliffe, D. E., and R. M. State- ham. Effects of Time Between Exposure and Support on Mine Roof Stability, Bear Coal Mine, Somerset, Colo. BuMines RI 8298, 1978, 13 pp. 3. Chekan, G. J., and D. R. Babich. Investigation of Longwall Gateroad Roof Support Characteristics at Powhatan No. 4 Mine. Instrumentation Plan. BuMines RI 8628, 1982, 9 pp. 4. Horino, F. G., and J. R. Aggson. Pillar Failure Analysis and In Situ Stress Determinations at the Fletcher Mine Near Bunker, Mo. BuMines RI 8278, 1978, 19 pp. 5. Moebs, N. N. , and E. A. Curth. Geologic and Ground-Control Aspects of An Experimental Shortwall Operation in the Upper Ohio Valley. BuMines RI 8112, 1976, 30 pp. 6. Hill, J. L. Ill, and E. R. Bauer. An Investigation of the Causes of Cutter Roof Failure in a Central Pennsylvania Coal Mine: A Case Study. Pres. at 25th U.S. Symp. on Rock Mechanics, Evanston, IL, June 25-27, 1984, 12 pp.; available from J. L. Hill III, BuMines, Pittsburgh, PA. 66 APPENDIX B.— INSTRUMENT SUPPLIERS 1 Ailtech 19535 East Walnut Dr. City of Industry, CA 91748 (213) 965-4911 American Optical Corp. 14 Mechanic St. Southbridge, MA 01550 (617) 765-9711 Armstrong Bros. Tool Co. 5273 W. Armstrong Ave. Chicago, IL 60646 (312) 763-3333 Baltimore Instrument Co., Inc, 4610 Harford Rd. Baltimore, MD 21214 (301) 426-3656 Barnes Engineering Co. 30 Commerce Rd. Stamford, CT 06902 (203) 348-5381 BLH Electronics 42 Fourth Ave. Waltham, MA 02154 (617) 890-6700 Bristol Division, Acco Industries Inc. 40 Bristol St. Waterbury, CT 06708 (203) 756-4451 Budd Co. Grant and Franklin Sts. Phoenixville, PA 19460 (717) 935-0225 Conkle Inc. P.O. Box 190 Paonia, CO 81428 (303) 527-4848 Eder Instrument Co., Inc. 5115 N. Ravenswood Ave. Chicago, IL 60640 (312) 769-1944 Enerpac 13000 W. Silver Spring Dr. Butler, WI 53007 (414) 781-6600 Expanded Optics Co., Inc. 14102 Willow Lane Westminster, CA 92683 (714) 894-1388 Extech International Corp. 114 State St. Boston, MA 02109 (617) 227-7090 Geokon, Inc. 7 Central Ave. West Lebanon, NH (603) 298-5064 03784 Geophysical Instrument and Supply Co. , Inc. 4665 Joliet St. Denver, CO 80239 (303) 371-1940 Glowlarm Rock Fall Warning Devices P.O. Box 465 White Pine, MI 49971 Handy Geotechnical Instruments, Inc. P.O. Box 1200, Welch Ave. Station Ames, IA 50010 Hitec Corp. Nardone Industrial Park Westford, MA 01886 (617) 692-4793 Hughes Aircraft Co. Industrial Products Division 6155 El Camino Real Carlsbad, CA 92008 (714) 438-9191 ^This list may not include all possible suppliers of ground control measuring instruments . 67 Industrial Products Co. 7445 North Oak Park Ave. P.O. Box 48022 Chicago, IL 60648 (312) 647-7855 Instrument Technology, Inc. Box 381, Mainline Dr. Westfield, MA 01085 (413) 562-5132 Irad Gage Etna Rd. Lebanon, NH 03766 (603) 448-4445 Klein Tools, Inc. 7200 McCormick Blvd. Chicago, IL 60645 (312) 677-9500 Kulite Semiconductor Products, Inc, 1039 Hoyt Ave. Ridgefield, NJ 07657 (201) 945-3000 Raytek, Inc. 325 E. Middlefield at Whisraan Mountain View, CA 94043 (415) 961-1650 Revere Corp. of America 845 N. Colony Rd . Wallingford, CT 06492 (203) 269-7701 Roc test, Inc. 7 Pond St. Plattsburgh, NY (518) 561-3300 12901 Rogers Arms and Machine Co. 1426 Ute Ave. Grand Junction, CO 81501 (303) 245-3729 Seco, Standard Equipment Co. 9240 N. 107th St. P.O. Box 23060 Milwaukee, WI 53224 (414) 355-9730 Lenox Instrument Co., Inc. Ill E. Luray St. Philadelphia, PA 19120 (215) 324-4543 Sensotec, Inc. 1200 Chesapeake Ave. Columbus, OH 43212 (614) 486-7723 Microdot, Inc. 475 Steamboat Rd . Greenwich, CT 06830 (203) 661-1200 Serata Geomechanics , Inc, 1229 Eighth St. Berkeley, CA 94710 (415) 527-6652 Micro-Measurements , Measurements Group, Vishay Intertechnology , Inc. P.O. Box 27777 Raleigh, NC 27611 (919) 365-3800 Mikron Instrument Co., Inc. P.O. Box 211 Ridgewood, NJ 07451 (201) 891-7330 Olympus Corp. of America ■evada Drive Hyde Park, NY 11040 (516) 488-3880 Sinco, Slope Indicator Co, 3668 Albion Place North Seattle, WA 98103 (206) 633-3073 Snap-on Tools Corp. 8030 E. 28th Ave. Kenosha, WI 53140 (414) 654-8681 Soiltest, Inc. 2205 Lee Street Evanston, IL 60202 (312) 869-5500 68 Spider Inc. 4001 Gratiot St. Louis, MO 63110 (314) 535-7868 Weksler Instruments Corp. 80 Mill Road Freeport, NY 11520 (516) 623-0100 Strainsert Co. 100 Union Hill Rd. Union Hill Industrial Park West Conshohocken, PA 19428 (215) 825-3310 Terra Technology Corp. 3862-T 148th Ave. , NE. Redmond, WA 98052 (206) 883-7300 Welch Allyn, Inc. 99 Jordan Rd. Skaneateles Falls, NY 13153 (315) 685-5788 Westinghouse Electric Corp. Industrial and Govt. Tube Div, Westinghouse Circle Horseheads, NY 14845 (607) 796-3211 Wahl Instruments, Inc. 5750A Hannum Ave. Culver City, CA 90230 (213) 641-6931 &U.S. CPO: 1985-605-017/20,121 INT.-BU.O F MIN ES,PGH.,P A. 28119 U.S. Department of the Interior Bureau of Mines— Prod, and Distr. Cochrans Mill Road P.O. Box 18070 Pittsburgh. Pa. 15236 AN EQUAL OPPORTUNITY EMPLOYER POSTAGE ANO FEES PAID U.S. DEPARTMENT OF THE INTERIOR INT-416 OFFICIAL0USINESS PENALTY FOR PRIVATE USE. S300 ] Do not wi sh to receive thi s material, please remove from your mailing list* ] Address change. Please correct as indicated* l / » . **i-. . ^\.A^ > .y JS A'' a** V *».»* A <> <** »^v ** c ' y a \^' V : ^*'/ \^-\/ V^^'/ %/^^V °°^" 1 4^ . > ,0* . > G a ^v 1°. ^^ A 4 o^ *4°«* "-■ ***** A o ° " ° -. "^ ■A c ° " ° « ^ r.^ « • *■ " 4 o "O > •*o v ;^ *bV" ^0* o »lvl'* v V » " * "- O. *- . ^. "" ^ :»': V** 4 Vt c?*^ ^ ^ ^r "? *- °o O > ♦ -#> V * ' ft - ^ o x V t s' »' - ; ^R : -Sail* ** oV : ^ o< ^ J ^^' #*% .•^i.°- y,i-&...v .^,^>.% > 4 \^.\ "o^ ** v % ;, /^,V' : : y ... V 7 ^.^ .... •- J 1 ... %-&*# \. '■' ,J *•*** -iife-.'%. >° >."°- ,/ • .• . •<-^^ 'u»V •P A> O. ,;■■ LIBRARY OF CONGRESS 002 955 9119