VSB m h ■ ftw hBhI HHhvG ■ ■'..,.■■.' H „ 1 ■:•,■... ■■./■'■■■■ :: V ■■■,■-.■■■■■ JVwIOtKl BKll : ; ' JH11 ■■■'■• ^"' ■■'"■■'• •'■'■ ■ iBHarMm n a^*wQ*&&* ■ MSB ISfin 3 >> ...•••.. c^ ^o VV «J^a'. o - o w o ■ • ^ *♦ •! • 4 IP %'*?&'#+ > a* ♦ Wa\ ^ a** /fl&\ ^ a* ->Wa\ >o >"» - v*^ * v ; °*0 4*^ Aiat-X y-^>» y.-^i-x y ..;&&% y V * ^ ^> "oho .^ % ♦•O'' A° ^ *o, *^HT* A < "« "^o <^ V . i i * <>. o V o « o * .v ^ o ^: : %P^ Bureau of Mines Report of Investigations/1986 Evaluation of Bearing Plates Installed on Full-Column Resin-Grouted Bolts By Stephen C. Tadolini and Bryan F. Ulrich UNITED STATES DEPARTMENT OF THE INTERIOR Report of Investigations 9044 Evaluation of Bearing Plates Installed on Full-Column Resin-Grouted Bolts By Stephen C. Tadolini and Bryan F. Ulrich UNITED STATES DEPARTMENT OF THE INTERIOR Donald Paul Hodel, Secretary BUREAU OF MINES Robert C. Horton, Director Ol, Library of Congress Cataloging in Publication Data: Tadolini, Stephen C Evaluation of bearing plates installed on full-column resin-grouted bolts. (Report of investigations; 9044) Bibliography: p. 12. Supt. of Docs, no.: I 28.23:9044. 1. Mine roof bolts, Resin. 2. Coal mines and mining -Colorado. 3. Plates (Engineering), I. Ulrich, Bryan F. II. Title. III. Series: Report of investigations (United States. Bureau of Mines); 9044. TN23.U43 [TN289.3] 622 s [622'.28] 86-600185 CONTENTS Page Abstract 1 Introduction 2 Acknowledgments 2 General considerations 2 Instrumentation 4 Field investigation — Roadside Mine 6 Field investigation — Bear No. 3 Mine 8 Conclusions 12 References 12 ILLUSTRATIONS 1. Approximate location of the Roadside and Bear No. 3 Mines 3 2. Generalized stratigraphic column of the Roadside Mine 3 3. Generalized stratigraphic column of the Bear No. 3 Mine 4 4. Compression pad diagram '. 4 5. Hydraulic U-cell plates and bladder 5 6. Vertical displacement gauge 6 7. Roadside Mine test site location 6 8. Roadside Mine load contours 3 days and 72 days after test site installation 7 9. Roof spalling in Roadside Mine test site 8 10. Final Roadside Mine load contours 205 days after test site installation 9 11. Bear No. 3 Mine test site location 9 12. Instrumentation layout in the Bear No. 3 Mine 10 13. Bear No. 3 Mine load and roof displacement contours 128 days and 301 days after test site installation 11 UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT ft foot in inch ft 2 square foot lb pound ft' lbf foot-pound (force) psi pound per square inch EVALUATION OF BEARING PLATES INSTALLED ON FULL-COLUMN RESIN-GROUTED BOLTS By Stephen C. Tadolini 1 and Bryan F. Ulrich 1 ABSTRACT The Bureau of Mines conducted field investigations in two underground mines to determine the actual loads to which bearing plates i were sub- jected when installed in conjunction with full-column resin-grouted bolts and the roof movements generated by the applied loads. Measured loads indicate that the bearing plate is an integral part of the support system. Vertical displacement gauges installed to monitor roof dis- placements in the test sites show that the highest degrees of loading occur in conjunction with the largest amounts of movement. Mining engineer, Denver Research Center, Bureau of Mines, Denver, CO. INTRODUCTION Resin bolting systems continue to gain popularity in underground mines through- out the United States, and their general success under a wide range of geological and operational conditions is well docu- mented. However, many questions dealing with basic support mechanisms and the in- fluence of various factors on effective- ness in situ remain only partly answered. The Bureau of Mines conducted a major re- search project to investigate the useful- ness of bearing plates installed at the bottom (collar) of a full-column, resin- grouted bolt. The use of full-column, resin-grouted bolts to stabilize underground mine roofs necessitates the ability to determine support characteristics and behavioral patterns. These investigations have taken two forms in recent research. The first form involves investigating support systems and individual bolts by an exact theoretical solution, or modeling. These methods analyze the state of stress in and around the bolts in three phases. The first phase analyzes the initial loading of the bolt-grout-rock. The sec- ond phase considers subsequent move- ments along the bolt and in the immediate area. The third and final phase involves the analysis of discontinuous rock move- ments along normal bedding planes. The results from these types of investiga- tions generally conclude that the load transfer mechanism in grouted bolts dictates that all movements, and thus generated loads, are controlled along the bolt axis and interbed slips. This implies that the bearing plates' effec- tiveness is limited to helping retain loose material at the mine roof (1-3). 2 The second form of solution involves field investigations designed to analyze actual phenomena witnessed in underground mines. Bearing plates and, consequently, bolts appear to be subjected to large amounts of load, which cause plates to bend, bolts to elongate, and bolt ends to fail completely. When bearing plates at the head of a full-column, untensioned, properly installed grouted bolt are sub- jected to significant loads, they can be considered to be contributing to the sup- port of the mine roof. Therefore, the assumption has been made that load on the bearing plate indicates that the plate may be an important part of the support system. To verify this assumption, full- column resin-grouted bolts were installed in coal mine roofs equipped with devices to measure the load applied during in- stallation and subsequent roof loading carried by the bearing plates. To deter- mine if the loads are related to the underground stability, roof deflection measurements were recorded. The loads were then compared with roof movements to determine if a correlation exists between the two parameters. ACKNOWLEDGMENTS The authors would like to thank the mine operators and all who contributed to the success of this effort. Special thanks are given to Jim Diamanti, mine manager, Powderhorn Coal Co. , and to Bill Bear, president, Bear Coal Co., for their continued support and the use of their personnel and equipment in this study. GENERAL CONSIDERATIONS The study includes field test results from two mines: the Roadside Mine and the Bear No. 3 Mine (fig. 1). Both test sites were chosen because of thick coal seams in which the mines are located and the geological characteristics of the rock above the coal seam. The first phase of this investigation was conducted in the Roadside Mine, owned and operated by the Powderhorn Coal Co. The Roadside Mine is located in the Book Cliffs Coal- field of the Uinta coal region. The ^Underlined numbers in parentheses re- fer to items in the list of references at the end of this report. FIGURE 1.- Approximate location of the Roadside and Bear No. 3 Mines. Q) O O c c t 3 ° o x o £ 2 E ° ,o a. ^ "S "O g .0) CP c Massive sandstone Carbonero coal zone 15 -32 ft Cameo coal zone 2 - 18 ft Rollins Sandstone Sandstone and shale Moncos tongue Cozzette coal zone-thickness varies Cozzette Sandstone Corcoran coal zone -thickness varies Corcoran Sandstone Palisade coal zone Upper Sego Sandstone Anchor coal zone Lower Sego Sandstone Sandstone Shale FIGURE 2.-Generallzed stratlgraphic column of the Road- side Mine. geology consists of interf ingering sand- stones and shales of Upper Cretaceous age with several important coal zones. Bed- ding generally dips about 5° northeast away from the Uncorapaghre Uplift toward the southwest rim of the Piceance Creek Basin. A generalized stratigraphic col- umn of the Roadside Mine area is shown in figure 2. The oldest exposed rock, unit in the area is the Mancos Shale, a black to dark-gray soft shale with occasional thin sandstone beds. The Mancos grades upward into the Sego Sandstone, a fine-grained, buff to light-gray sandstone with some gray shale. The Sego is divided into an upper and lower member by the Anchor coal zone, a tongue of the Mancos Shale, which has been mined in some parts of the area. The Mount Garfield Formation consists of buff and gray, medium-fine-grained sandstone interbedded with gray shale. There are five economically important coal zones in the area. The Palisade zone forms the base of the Mount Garfield Formation. The Corcoran coal zone and the Cozzette coal zone overlie the Cor- coran and Cozzette Sandstone Members, re- spectively. The Cameo coal zone overlies the Rollins Sandstone Member. This zone produces most of the coal from the Book Cliffs Coalfield. The Roadside Mine pro- duces from the Cameo B seam, which is 4.4 to 9.4 ft thick. The uppermost coal zone is the Carbonera. The uppermost unit in the area is the Hunter Canyon Formation. The Hunter Can- yon is a medium-coarse-grained, buff and gray, massive cliff-forming sandstone with small beds of gray to greenish-gray shale. There are no coal deposits in this formation (4_"5)« The second phase of the investigation was performed in the Bear No. 3 Mine, owned and operated by the Bear Coal Co. The mine is located in the Somerset coal- field near Somerset, CO. The oldest ex- posed rock unit in the Somerset area is the Upper Cretaceous Mancos Shale (fig. 3). This unit consists of 2,000 to 3,000 ft of black or dark-gray soft shale with the thin sandstone beds. Overlying Man- cos is the Mesaverde Formation, also of Upper Cretaceous age, here composed of four members. The basal member, the Rollins Sandstone, is a 150- to 200-ft- thick, massive, cliff-forming, white to light-yellow-brown sandstone. The lower coal (Bowie Shale) member is an inter- bedded and lenticular sandstone, silt- stone, and shale sequence 250 to 300 ft thick; it contains three important coal seams. The A seam forms the base of the member and is to 5 ft thick. The B seam was mined previously directly below the Bear No. 3 Mine in the Edwards Mine. The C seam, 7 to 9 ft thick, is the seam presently being mined. The seams are all separated by 33 to 40 ft. The upper coal (Paonia Shale) member is lithologically very similar to the lower coal member but is more lenticular. Up to 400 ft thick in some areas, this contains two major coal seams, the D and E seams. A similar but non-coal-bearing member, the Barren Member, overlies the upper coal member, bringing the entire mine cover to approximately 1,000 ft. There are numerous igneous intrusives of post-Eocene age throughout the area, some of which appear in mine. The coal- beds dip north and northeast at 0° to 6°. Faults and other fractures occur throughout the area with stratigraphic displacements of 2 to 20 ft (6). INSTRUMENTATION Several types of instrumentation were used in this investigation to measure plate loading and roof movements. Com- pression pads and hydraulic U-cells were used to directly measure the loads applied to bearing plates after bolt 800 (Approi) O =■£ 3 & || 1 s 140- 200' 150- 200' 2,000- 3,000' Interbedded sandstone, siltstone, and shale E seam 4-8 ft D seam 0- 7 ft Interbedded sandstone, siltstone, shale, and coal Interbedded sandstone, siltstone, shale, and C seam 7 -9 ft co q| B seam 10-17 ft A seam 0-5 ft Massive, cliff-forming sandstone Shale and sandstone FIGURE 3. -Generalized stratigraphic column of the Bear No. 3 Mine. installation. Vertical displacement gauges were installed to measure differ- ential roof displacements during test site monitoring and support operations. In addition, observation holes were drilled throughout the test areas to mon- itor with a stratascope the locations and widths of roof separations. Each compression pad (fig. 4) consists of a rubber membrane placed between two steel plates. The compression pads have a working load limit of 32,000 lb with a calculated accuracy of ±200 lb. Readings of the compression pads are monitored with a special calibrated ring that measures the change in circumference of the rubber membrane as it loads and un- loads. Laboratory tests on the ring in- dicate that when loads exceed 30,000 lb, the accuracy drops to ±1000 lb. The 8.00" diom- FIGURE 4. -Compression pad diagram. nature of the pad is to act as a spring between the bolthead and the roof. In laboratory investigations the pad, after being subjected to high loads, failed to rebound to its specified unloaded circum- ference. The rubber tends to permanently deform after continued loading. In this investigation only positive loading was recorded, eliminating the possibility of inaccurate readings. The hydraulic U-cells are U-shaped, fluid-filled, flat jack-type load cells used to measure relative loads between the bearing plate and the installed roof bolt (fig. 5). The cell and accompanying platens are designed to fit "horseshoe" fashion about the bolthead for easy in- stallation and retrieval. Each U-cell was individually calibrated in a stiff testing machine to allow the measurement of cell pressure. The U-cells can mea- sure loads to 30,000 lb with measured ac- curacies of ±250 lb. Resin bolt applica- tions required that the bolthead be threaded to facilitate installation and removal. The vertical-displacement gauge (fig. 6) consists of four spring clips used to anchor high-strength, stainless steel prestretched wire at selected depths in a 1-3/8-in-diam hole drilled in the mine roof. The uppermost spring clip is placed in a stable layer to be used as a base reference for measured displace- ments. For this investigation, a hole depth of 7 ft was used. The remaining three spring clips are placed at 5 ft, 3 ft, and 1 ft away from the bolthead. The wires from the four spring clips run through a 10-in-long tube anchored in the collar of the drill hole. The wires go through numbered holes in the copper cap on the end of the tube and have small brass fittings that are used as reference points. A loop is made at the end of the wires so that a 3-lb weight can be at- tached to maintain a constant tension on the wire while readings are taken. Read- ings are made with a dial indicator placed between the cap and the reference point on each wire. The bearing plates used in the inves- tigation were laboratory tested. The ASTM standard requires that the plate be ^^r ^ o 4 6 -j Scale, in FIGURE 5. -Hydraulic U-cell plates and bladder. preloaded to 6,000 fflbf when measuring displacements, to within 0.001 in, of the axial movement of the bolthead. The load is then increased to 15,000 ft'lbf, and the axial displacement is read. The max- imum permissible deflection between the 6,000- and 15,000-f f lbf loads is 0.120 Pipe cap ""' Brass reference point — . ^H hr Holes for wires Center hole for gauge "- Dial gauge ft l' \*r- 3— lb weight FIGURE 6.-Vertlcal displacement gauge. FIGURE 7. -Roadside Mine test site location. in. The plate is then loaded to 20,000 fflbf, and again the axial displacement is measured. The maximum permissible de- flection between the 6,000- and 20,000- fflbf loads is 0.250 in (7_). Six bearing plates were randomly se- lected and tested from a purchased lot of 250. All plates exceeded the ASTM stan- dards by a minimum of 15%. FIELD INVESTIGATION — ROADSIDE MINE The first phase of this investigation was conducted in the Roadside Mine. The test site was established in a develop- ment panel in the No. 2 East Mains, 1st East section in one of the deepest areas of the mine (fig. 7). The test site instrumentation was located approximately midway in an 80-ft room and included 44 compression pads and 6 vertical displace- ment gauges. This combination of instru- mentation made possible the measurement of both the loads on the bearing plates and the separations in the immediate roof. The test site instrumentation was read and evaluated four times in a 7-month period. Bolts used in these test sites were standard 0.75-in-diam reinf orcing-steel- rod-type, grade 40 (17,600-lb yield strength and 30,000-lb tensile strength) roof bolts. The bolts were installed in 1.0-in-diam holes to the specifications of the resin manufacturer. The bolt characteristics varied from bolt to bolt in some cases; all bolt characteristics are the minimum documented laboratory values. Three days after the excavation of the opening and the installation of the in- strumentation, a distinguishable loading pattern was observed (fig. 8). The mini- mum and maximum loads measured on the bearing plates were 5,700 lb and 29,000 lb, respectively; the average load on the bearing plates in the test area was approximately 14,000 lb. The high con- centration of loads in the middle third of the entry caused the laminated shale roof to separate and fall when not con- fined by the wire mats installed in Roadside Mine 3 days after installation Roadside Mine 72 days after installation Contour irrtervol - 5,000 lb Scale, ft 10 J FIGURE 8 -Roadside Mine load contours 3 days and 72 days after test site Installation. conjunction with the bolts. The differ- ential sag stations were lost owing to their roof deterioration. The loading trends observed 72 days after installation were similar to those recorded at 3 days; however, the loads increased by 30% (fig. 9). Loads on the bearing plates ranged from 6,100 lb to 32,000 lb and averaged 18,100 lb. Visual examination of the test area revealed high degrees of roof spalling, as shown in figure 9. The measurements recorded 150 days after installation were similar to the 72-day measurements. The minimum and maximum loads were 7,400 lb and 32,000 FIGURE 9.-Roof spelling In Roadside Mine test site. lb, respectively. The average load on the 44 pressure pads was 18,300 lb, or 475 psi. These loads generated up to 41,000 psi of axial stress on the 3/4-in bolts, causing them to yield. The final measurements were recorded 205 days after the test site was estab- lished. At that time, panel development was complete and the pillar retreat line was approaching the test site (approxi- mately 300 ft away inby). The pillars were yielding, by design, resulting in high load concentrations that propagated toward the test site area. As the pil- lars adjacent to the test site yielded, the loss of rib coal to sloughing resulted in an increase of effective roof span to approximately' 30 ft. Figure 10 shows the final loading pattern. The maximum and minimum loads were 7,500 lb and 32,000 lb. The average load, mea- sured on the bearing plates, was 18,900 lb. These test results showed, uncondi- tionally, that bearing plates were sub- jected to high degrees of loading and were an important part of the total sup- port system. However, because all the vertical displacement gauges were lost and the borescope holes were closed, the roof displacements generated by these loads remained unknown. FIELD INVESTIGATION — BEAR NO. 3 MINE The second phase of the investigation was performed in the Bear No. 3 Mine. The test site was located in an entry, including both a three-way and a four-way intersection, under approximately 600 ft of overburden (fig. 11). A total of 5,600 ft^ of roof was instrumented to monitor bearing plate loads and roof Roadside Mine 205 days after installation Contour interval - 5,000 lb 10 Scale, ft FIGURE 10. -Final Roadside Mine load contours 205 days after test site Installation. WSOS a Test area ax «oc aoaaOi DDDDa a FIGURE 11. -Bear No. 3 Mine test site location. displacements. To measure the loads on the bearing plates, 51 compression pads and 26 hydraulic U-cells were placed between the mine roof and the bearing plates. Additionally, 14 vertical dis- placement sag station gauges and 7 fiber- optic boreholes were installed to monitor roof movements (fig. 12). The test site instrumentation was read and evaluated seven times in a 10-month period. After the test site was instrumented, on traditional 20-ft mining cycles, the baseline data were recorded. The total site was instrumented in 7 days. The pillars showed no signs of yielding or sloughing. However, a high-angle, clay-filled discontinuity, spanning the entry at N 60° E, was located 8 ft north of the four-way intersection. The test site was monitored 50 days after installation. The increased load- ing on the bearing plates, of 7,000 lb, in the four-way intersection was attrib- uted to displacements associated with the clay-filled discontinuity. The vertical displacement gauges in the area recorded 0.2 to 1.0 in of total displacements. A visual observation of the roof area, with the aid of a fiber-optic stratascope, revealed a minor separation of 0.4 in at the 4-ft level. This separation occurred between layers of thinly laminated shale. Small amounts of loading, approximately 1,500 lb, developed near the three-way intersection. Only small amounts of dis- placement were recorded, with no apparent visual separations. The instrumentation was read and evalu- ated at 87 and 128 days after installa- tion. The loading pattern and roof dis- placements for the 128-day measurements are shown in figure 13. The highest de- grees of loading were recorded in the vicinity of the discontinuity near the four-way intersection. Sloughing in the 10 Crosscut 21 B A + b a a m a Crosscut 20 + © B B SB B A A a h B © A -f- A B B B A A A B B A B A © BBS © H A B + B H B A A © BBS + A A BEE + A A B + © + B a b b LEGEND □ Compression pad a U-cell + Sag station ° Fiber-optic borehole + B B B A A + © B B O) £ ■o CM 10 20 Scale, ft FIGURE "^.-Instrumentation layout In the Bear No. 3 Mine. west ribs also contributed additional loads on the bearing plates. The average load on the bearing plates was approxi- mately 5,400 lb. The displacements, measured at the 3-ft level, were as high as 1.8 in near the four-way intersection and 0.9 in in the middle of the three-way intersection. Twelve pressure pads were recording loads greater than 15,000 lb. The loading patterns and roof movements recorded at 162 and 211 days were similar to those recorded at 128 days. The test site results indicated that loads on the bearing plates in the four-way intersection were increasing at a rate of approximately 100 lb per week. The dis- placements in the roof remained rel- atively consistent with the 128-day measurements. The corners of the pillars in the three-way intersection were vis- ually inspected and observed to be begin- ning to yield, creating increased loading (fig. 13) in the immediate area. The condition of the roof appeared to be sta- ble, even with an average of 5,900 lb of load being carried by the bearing plates. Final instrumentation readings were ac- quired 311 days after the initial instal- lation. The loading patterns and dis- placements are shown in figure 13. The test site underwent considerable changes in loading pattern that were attributed to the large degree of observed pillar yielding. The effective roof span, due to this pillar yielding, had increased by 16%, or 4 ft, causing an increase in the entry's centerline roof displacement. High loads were generated along the rib- lines in certain areas owing to the ex- tended length of unsupported roof in the yield zone of the coal pillars. The min- imum and maximum loads measured on the bearing plates were 600 lb and 26,700 lb, respectively. Theoretically, this max- imum load would generate approximately 60,000 psi of pressure on a 3/4-in-diam bolt. This pressure exceeded the theo- retical yield of the bolt system. The final calculated average load on the bearing plates was approximately 8,000 lb, an increase of 38% over previous readings. The results from the test site in the Bear No. 3 Mine indicated, conclusively, that bearing plates are subjected to loading when installed in conjunction with full-column resin-grouted bolts. Roof movements and separations, monitored with vertical displacement gauges, corre- sponded closely with applied bearing plate loads. Instrumentation used in the two test sites has been shown, through past experience, to be both effective and reliable. Field data can be closely cor- related to the stabilization of the entry. 11 2,500 Bear No. 3 Mine 128 days after installat on -<%> Bear No. 3 Mine 302 days after installation .EGEND -5.000— 2,500-lb roof load contour interval -1.2 — Roof displacement interval, in 10 20 9587 33 Scole. ft FIGURE 13. -Bear No. 3 Mine load and roof displacement contours 128 days and 301 days after test site Installation. 12 CONCLUSIONS The field data of these two sites indi- cate that bearing plates at the heads of full-column, resin-grouted bolts can be subjected to significant loads. The bearing plates not only retain the roof material but support large amounts of generated load between bolts. The loads tend to be closely related to the roof movements owing to pillar yielding and geologic anomalies. In some instances, the loads on the plates were so extreme that the ultimate strength of the No. 6 rebar, grade 40 bolts was approached and exceeded. However, overall stability was maintained, as evidenced by the retention of constant loads measured on the bearing plates and negligible vertical displace- ment measurement increases. REFERENCES 1. Coates, D. F., and Y. S. Yu. Three-Dimensional Stress Distributions Around a Cylindrical Hole and Anchor. Paper in Proceedings of the Second Con- gress of International Society for Rock Mechanics (Belgrade, Sept. 21-26, 1970). "Jaroslav Cerni" Institute for Develop- ment of Water Resources, Belgrade, 1970, pp. 175-181. 2. Haas, C. J., G. B. Clark, and R. N. Nitzsche. An Investigation of the Inter- action of Rock and Types of Rock Bolts for Selected Loading Conditions (contract HO 122 110, Univ. MO). BuMines OFR 2-77, 1974, 342 pp; NTIS PB 267673. 3. Nitzsche, R. N. , and C. J. Haas. Installation Induced Stresses for Grouted Roof Bolts. Int. J. Rock Mech. Min. Sci. and Geomech. Abstr. , v. 13, No. 1, 1976, pp. 17-24. 4. Khalsa, N. , and L. R. Ladwig (eds.). Colorado Coal Analyses 1976— 1979. CO Geol. Surv. , Inf. Ser. No. 10, 1981, 246 pp. 5. Schwochaw, S. D. (comp.). Mineral Resources Survey of Mesa County — A Model Study. CO Geol. Surv., Resour. Ser. No. 2, 1978, pp. 35-38. 6. Osterwald, F. W. , C. R. Dunrud, J. B. Bennetti, Jr., and J. 0. Maberry. Instrumentation Studies of Earth Trem- ors Related to Geology and to Min- ing at the Somerset Coal Mine, Col- orado. U.S. Geol. Surv. Prof. Paper 762, 1972, pp. 2-9. 7. American Society for Testing and Materials. Standard Specifications for Roof and Rock Bolt Accessories. F-432- 77 in Annual Book of ASTM Standards: Part 10, Bolts, Threaded Bars, Threaded Slotted Bars, Bearing and Header Plates, and All Types of Washers. Philadelphia, PA, 1977, pp. 6-7. 6 U.S. GOVERNMENT PRINTING OFFICE: 1986—605-017/40,076 INT.-BU.0F MINES,PGH.,PA . 2834 5 U.S. Department of the Interior Bureau of Mines— Prod, and Distr. Cochrans Mill Road P.O. Box 18070 Pittsburgh. Pa. 15236 OFFICIAL BUSINESS PENALTY FOR PRIVATE USE. S300 ] Do not wi sh to recei ve thi s material, please remove from your mailing list* ] Address change* Please correct as indicated* AN EQUAL OPPORTUNITY EMPLOYER v » f ' °' e» »°^J •ft. V "V^T** A f* .*!.»»*-.♦ "ft -^ ,0" ,*° ... x • s » 4 ' ♦♦"% ^ «»!***.% -ft "ft*. &y S#»^* »*> <■# *a$»^>i*- t^k a. ♦* *« » ° *^ 9 n a> ^tf • ^ A* '^ •bv? ^0^ * V ■> "ft. ^d 8 "^\ C°*.^*°o /^^/^ C ^.55«^* O o ^.titeZ-X C *.^t>o -^^ ^ *'' «>>>«/ * ^bv ♦ ^ jP-^k *b V * *£i^*\ ^^ •v < ^i»r«. ^-* ♦ f\ *bV" .* ^^ ^ « ^0^ SK :5 °^ -'ISR : ,>°^ 'SX&t k*°- ' f ^ IKES: J* : : x > >, v>- ^ .♦^ xf % ^' ^ ,0 **^ ^ l> ' " O. z* -#M> ^ . : C 0^ c'IS. ^O O v^v \"^-'y v^^>° * V; c5^ ^ ,0,. • ' A ^ '. 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