TN295 .U4 No. 9179 LIBRARY OF CONGRESS DQQD1D4H217 ^«? *oV ^cr .0 c ° ° » ^> A* v< v ■ay ^ •» -wro^r* f - ^ cr c o " ° * **b a* . l • » .. <*^ o^ , o « e , ^ ^ *o . » v-o T -of :iSf^'- '"^o*" °<^»" "o^ :,iM^~ '^o* •■ o > **'% "oV" 'Pa A^ ^-^M^" .-&> «. *„^J&.«. «P^ A* ^J^W^k." «> A, »- ft ^^,'„ "£*, A^ * ^^ ^ <& *is a> V ' ."«., ^^ ' > \ y v-->* v^V v^V <, '" "W <, *'..** ,G -5°-*. ,"^ w *bV ^„ r V >"., ^O i\ \ „** ^. o * ~o' .0' 5* ^r .^ ^.^ : 'V,^ A v -^ - ^ r ^ * ** . sPr, ^0 X ^•0^ ft*. • y- o^ 0w o' •» o *b^ r * ^c,- • A V *^ * .^ •^-o* o. * '^••- '*- ■■•' - ,j Bureau of Mines Information Circular/1988 Probe-Hole Drilling: High-Stress Detection in Coal By John P. McDonnell and Khamis Y. Haramy UNITED STATES DEPARTMENT OF THE INTERIOR Information Circular 9179 Probe-Hole Drilling: High-Stress Detection in Coal By John P. McDonnell and Khamis Y. Haramy UNITED STATES DEPARTMENT OF THE INTERIOR Donald Paul Hodel, Secretary BUREAU OF MINES T S Ary, Director Tltas Library of Congress Cataloging in Publication Data: McDonnell, John P. Probe-hole drilling. (Information circular/United States Department of the Interior, Bureau of Mines ; 9 1 79) Bibliography: p. 11. 1. Rock bursts. 2. Coal mines and mining — Safety measures. 3. Boring. I. Haramy, Khamis Y. II. Title, in. Series: Information circular (United States. Bureau of Mines) ; 9179. TN295.U4 [TN317] 622 s [622 '.8] 88-600003 CONTENTS Page Abstract 1 Introduction 2 Probe-hole drilling 2 Theory 2 Background 3 Case study 3 Mine 3 Geology 3 General background 3 In-mine probe-hole drilling. . . 5 Procedure 5 Results 5 Laboratory tests 7 Procedure 8 Results 9 Conclusions 10 References 11 ILLUSTRATIONS 1. Boundary stress concentration for circular opening 2 2. General layout of the mine and drilling test area 4 3. Sample field data form 6 4. Drilling yield field results 7 5. Longwall face area showing probe-hole locations 8 6. Laboratory equipment setup for drilling yield tests 9 7. Relationship between applied load and cuttings volume 10 8. Drilling yield laboratory results 10 TABLES 1. Coalcrete physical property data 9 2. Drilling yield laboratory test data 10 UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT ft foot in cubic inch gal gallon lb pound gal/ft gallon per foot mL milliliter h hour psi pound per square inch in inch vol P< 2t volume percent PROBE-HOLE DRILLING: HIGH-STRESS DETECTION IN COAL By John P. McDonnell 1 and Khamis Y. Haramy 1 ABSTRACT Coal mine bounces and bursts are major problems facing U.S. mine oper- ators. Bounces and bursts have the potential to inflict severe injury on mining personnel, damage equipment, and cause mine closures. High- stress conditions, at or near the working face, are the common denomina- tor in the burst problem. If mine operators can locate high-stress and potentially burst-prone zones, they can then use stress-relief methods to control the burst condition. One method of locating the high-stress zone is the probe-hole-drilling or drilling-yield method. Probe-hole drilling is used frequently in Europe, the U.S.S.R., and Japan as a means for locating potential burst zones. Consequently, the Bureau of Mines performed tests in the labo- ratory and in a deep, burst-prone western mine to analyze probe-hole drilling. The in-mine method, very simply, involves drilling a hole into the coal seam and measuring the volume of cuttings obtained. A certain volume of cuttings can be expected from a certain diameter and length drill hole. A significant increase in the volume of cuttings means the zone around that particular hole is highly stressed. In-mine use of the drilling yield method has shown it to be a useful tool for locating highly stressed and potential burst zones. Results from laboratory testing confirm that high stress applied tri- axially to a cube specimen will cause a significant increase in the vol- ume of cuttings from a small-diameter drill hole in the specimen. 'Mining engineer, Bureau of Mines, Denver Research Center, Denver, CO. INTRODUCTION Bounces and bursts have become a common occurrence in U.S. coal mines. Between 1978 and 1984 there were 73 accidents attributed to bounces and/or bursts. As coal mine operators attempt to mine deep- er reserves, the problems could increase. Common to the burst problem is the oc- currence of high-stress zones in the area of the active face. Such high-stress and potential burst zones can be attrib- uted to many factors; much research has been conducted in attempts to precisely determine causes of bounces and bursts. High-stress zones, regardless of their causes, significantly contribute to burst occurrences. Locating the high-stress zones in the working face is important to eventual burst control. When the high- stress zones are located, they can be de- stressed. As a result, the burst poten- tial can be controlled. This paper describes the analysis of the probe-hole-drilling or drilling-yield method, which has been used in several countries as a practical tool to detect high-stress zones at the working face in underground coal mines. The method in- volves drilling a certain diameter and length hole into the coal, measuring the volume of cuttings obtained, and compar- ing the volume of cuttings to the volume of the hole. Laboratory tests were con- ducted to determine if there is a cor- relation between the magnitude of the stress and the volume of cuttings ob- tained. A coal mine in western Colorado, which regularly experiences high-stress conditions, was used as a field site for probe-hole drilling. This paper de- scribes the laboratory and in-mine probe- hole-drilling results. PROBE-HOLE DRILLING THEORY The theory behind probe-hole drilling comes from the concept of stress around a circular opening. The magnitude and dis- tribution of the stresses around a sin- gle underground opening in massive rock, e.g. , a drill hole in a thick coal seam, have been determined analytically and from laboratory model studies (_1_). The stress concentrations around a circular opening in a bidirectional stress field are shown in figure 1. This figure shows that when Poisson's ratio for a material equals 0.25, the boundary stress concen- trations around the circular opening can be obtained. For example, if the applied stress, Sv, equals 1,000 psi, then the boundary stress at point A on the circu- lar opening equals 1,000 X 2.6, or 2,600 psi. . ^Bounce, as used in this paper, refers to a sudden, forceful impact or jolt, which may be accompanied by face or rib sloughage; a burst involves an explosive release of material. •^Underlined numbers in parentheses re- fer to items in the list of references at the end of the report. When the boundary stress exceeds the strength of the material, the hole begins to deform and fail. When the applied stress is high to begin with, as is the case with forward abutments ahead of longwall faces, the coal around the hole fails and behaves plastically. The coal around the drill hole in the plastic zone flows into the hole and results in large volumes of cuttings. a> 3 1. "3 M 1 A / 1 — 1 1 S 11 1 1 ) t 1 / / \ 1 1 / 3* / / / f * / / / / / / KEY M Ratio of S h /S v S v Vertical stress S n Horizontal stress °9 Stress at angle 9 FIGURE 1.— Boundary stress concentration for circular opening. BACKGROUND 2. The auger steel becomes stuck. Probe-hole drilling has been used in Germany since the 1960's, in the U.S.S.R. since the 1950's, in Poland since 1965, and in Yugoslavia since 1968 (2^3) • i n the United States, Talman and Shroder (4_) reported experiences with large-diameter, 6-in, auger-drilled holes that were used as a stress-relief method in the 1950's. The large-diameter holes actually trig- gered coal bursts. The in-mine drilling-yield method in- volves drilling small-diameter holes into the coal panel or pillar, usually with an auger drill, and recording certain infor- mation while drilling, such as the volume of drill cuttings obtained from certain depths in the hole. A certain volume of cuttings can be expected from a drill hole of a particular diameter and length. If the actual volume of cuttings gener- ated exceeds the volume of the hole by a significant amount, the seam in that location is determined to be highly stressed and potentially burst prone. The high-stress zone is determined by the following criteria: 1. The ratio of volume of drill cut- tings obtained to the volume of the hole. If the ratio greatly exceeds 1, the zone is potentially burst prone. 3. The probe-hole drilling triggers a bump (noise) or a bounce or burst. The high-stress-zone criteria can in- clude zones of gas, cuttings size, and other driller experiences; for example, the auger steel being drawn into the hole, similar to the action of a wood screw, indicates the intersection of the high-stress zone. Any combination of the criteria can indicate a potential burst zone. The experience of the drillers at each operation will dictate what criteria work for that operation or that partic- ular site. Well-disciplined probe-hole drillers try to prevent triggering bursts by taking precautions such as selecting proper auger size, using proper drilling techniques, and paying close attention to the actions of the hole being drilled. Probe-hole drilling does not give the absolute stress magnitude of the coal seam. The purpose of the laboratory tests was to confirm the theory that high stress will produce large volumes of drill cuttings. The absolute magnitude of the stress is unimportant for this method. What is critical is the ability to locate the high-stress zone before mining into it. CASE STUDY MINE The case study mine is located in west- ern Colorado. The mine workings are un- der approximately 3,000 ft of overburden. The mine utilizes the advancing longwall method. Figure 2 shows the general lay- out of the mine and the test area. The coal seam dips to the southeast at ap- proximately 12° with the longwall panels oriented fairly parallel to the strike of the seam. GEOLOGY The coal seam, approximately 10 ft thick, is surrounded by hard roof and floor material. The immediate roof, ap- proximately 5 ft thick, is composed main- ly of strong siltstone, shales, and sandstone layers. The immediate roof is overlain by a 9-ft competent sandstone layer that does not readily fracture. The mine floor consists of a strong shale-sandstone layer 4 to 10 ft thick, and a lower coalbed beneath the shale- sandstone floor which consistently mea- sures 8 to 10 ft thick. The compressive strength of the coal is 2,440 psi, while Young's modulus is 5.2 x 10 psi. GENERAL BACKGROUND Owing to the thick overburden and the existence of strong roof and floor mate- rial, bounces and bursts have occurred in this mine in both room-and-pillar and longwall sections. Longwall panel A ex- perienced six significant bursts as the face advanced to 620 ft from the starting room of the panel. Scale, ft Room-and-pillar gob areas Longwall gob areas Fault Probe-hole— dri Ming test area FIGURE 2.— General layout of the mine and drilling test area. After a burst, when the face was at 620 ft, mine management decided to incor- porate a burst-control plan into the daily operations. The program consisted of detecting high-stress areas in the coal face using probe-hole drilling and then destressing high-stress areas using volley firing. Volley firing consists of fracturing certain zones of the face using explosives. Where the face is blasted and fractured, no stress can build up; the face is destressed. After the stress detection program was incorpo- rated into the mining cycle, the longwall panel was mined to completion, nearly 3,900 ft, with essentially no uncon- trolled burst occurrences. IN-MINE PROBE-HOLE DRILLING PROCEDURE Probe-hole drilling at the study mine was used to locate high-stress zones, which were then destressed. Drilling op- erations utilized a Turmag 4 model hand- held, air-powered auger drill (5_~6) • Auger flights, which permitted either air or water flushing of the hole, were gen- erally 3 to 5 ft long. Along with the auger flights, a two-wing, 2-in-diam, carbide-insert drag bit was used to cut and clean the hole. Drilling operations consisted of a two-man crew, one driller and one helper; the latter assisted in adding auger flights, measuring drill cuttings, and recording drilling informa- tion. Site preparation involved scaling the rib or face to provide a solid, sta- ble surface for the collar of the hole. Site preparation generally took as long as or longer than actual drilling. Ac- tual drilling operations involved con- trolling penetration rate and adding auger flights. Drilling operations were performed while the longwall face was not operating. This permitted the driller to listen to the activity caused by the drilling. Because the drill is hand-held, the driller's experiences are critical to locating the high-stress zones. The driller actually "feels" the hole squeez- ing around the auger flights. The driller controls the penetration rate to prevent sticking the auger steel in the hole. Data collection includes recording the volume of cuttings generated per given length of drill hole, cuttings Reference to specific products does not imply endorsement by the Bureau of Mines. size, presence of gas or water, hole squeeze, occurrence of bounces, etc. A sample field data form is shown in figure 3. The following criteria for locating the high-stress zone at the study mine were used: 1. Cuttings (Vc) were in excess of 5 gal per 3 ft of hole length. The ex- pected volume of cuttings (Ve) from a 3-ft-long, 2-in-diam hole is 0.5 gal. Therefore, where the ratio of actual cut- tings (Vc) to expected cuttings (Ve) was greater than or equal to 10, a high- stress zone was determined. 2. Auger became stuck. 3. Drilling generated a bump, bounce, or burst. The presence of gas blowing from the hole was also used as an indicator of a high- stress zone. RESULTS The drilling-yield program at the study mine consisted of drilling probe holes on 50- to 100-ft centers along the length of the longwall face and recording the drilling information. Probe-hole drill- ing was performed on a daily basis. A plot of the drilling-yield results shows the location of the stress abutment peak ahead of the face. Figure 4 shows typical drilling-yield data from three drill holes. In hole 1, for example, there is more than one small abutment zone within the length of the drill hole at this location. The maximum abutment zone for hole 1 occurred at a depth of Date: Hole diameter: Shift: DrillerL Hole location: DRILLING RECORD Drilling start: Drilling end: h h + Low noise O Noise can be heard 60 ftaway • Noise can be heard 300 ft away Depth ... ft 3 6 9 12 15 18 21 24 27 30 Volume of cuttings, gal + O • Remarks Depth., .ft 33 36 39 42 45 48 51 54 57 60 Volume of cuttings, gal + O • Remarks REMARKS, CODES B-Gas is blowing out of hole W— Drill cuttings are damp F- Drill cuttings are fine Z —Drill is drawn into the coal G-Drill cuttings are coarse ST- Rock ahead K-Coal pressure squeezes drill WW-Drill cuttings are wet FIGURE 3.— Sample field data form. o LU _J § CO h- o- u 10 15 20 25 30 35 40 45 DEPTH INTO HOLE, ft FIGURE 4.— Drilling yield field results. 50 55 60 53 ft. The distance from the face to the abutment zone varies from hole to hole. In hole 3, for example, the abutment zone is at a depth of only 39 ft. Figure 5 shows the longwall face area with other probe-hole locations, high- stress zones, and blowing-gas locations. This is a typical plot of the probe-hole information. On this sketch are the lo- cation and hole depth of the high-stress zone and the blowing-gas line (5_ ) . No- tice that the gas line nearly parallels the stress abutment line. At this mine, if the drilling-yield results showed the abutment zone to be approximately three times the seam height away from the face, the face was gener- ally said to be non-burst-prone. Com- bined with driller and miner experiences, the "three times" rule provided fairly good information regarding locations of high-stress areas and whether the face could be safely mined. Operators at other mines experiencing bounce and/or burst conditions need to determine high- stress criteria that particular operations, scribed here can be guidelines. apply to their The methods de- used as general LABORATORY TESTS Probe-hole drilling, as proven by the in-mine testing, can give reliable infor- mation on the general state of stress in the coal seam. The absolute magnitude of the stress is not determined, however. Also, the orientation of the stress is not and cannot be determined by probe- hole drilling. The laboratory tests were performed to quantify, to some degree, what stress magnitude would produce an excessive vol- ume of cuttings. Panel ^723) \ Stress/ t" t. •'<£ jAJ.-LT. 1 J^ . o .<=>•• A-> -.w. ■ -y , o. " !•*& ^assess liiillil ®?imm>mm 5 Probe-hole depth to stress, ft Hole clear of stress and gas to 50 ft Probe-hole depth to gas, ft FIGURE 5.— Longwall face area showing probe-hole locations. PROCEDURE Laboratory tests were conducted on cubes of coalcrete, a mixture of coal, fly ash, cement, and water in the follow- ing proportions: Material Vol pet Coal 47 Fly ash 37 Cement , 7 Water 9 Fly ash, a product of burned coal, var- ies depending on the specific coal type and coal properties. A mix containing equal amounts of class C and class F fly ash was selected for use in the tests; class C contains higher concentra- tions of alumina and cementlike materials than class F. Physical property tests on the coal- crete mixture were performed in the Bu- reau of Mines rock testing laboratory. Physical properties of the coalcrete are listed in table 1. The coalcrete exhib- its similar material properties to those of weak coal. Laboratory tests were conducted on 4-in coalcrete cubes. Figure 6 shows the com- plete equipment setup, which included a 120,000-lb-capacity hydraulic press, a steel test frame, steel flat jacks filled with hydraulic fluid, and a hand-held variable-speed drill with a 9/16-in drill bit. FIGURE 6.— Laboratory equipment setup for drilling yield tests. TABLE 1. - Coalcrete physical property data Number of samples 9 Sample dimensions, in: Length 4.23 Diameter 2.09 Lateral pressure psi.. Compressive strength, psi: Average 1,430 Range.... 1,122-2,201 Young's modulus, 10 6 psi: Average 0.35 Range 0. 28-0.48 Poisson's ratio: Average 0.28 Range 0. 17-0.42 The steel test frame was designed with the steel flat jacks to provide a means for applying variable lateral confining pressures, while the hydraulic press was used to control the vertical applied load. The test procedure involved con- fining the cube in the test frame, apply- ing lateral and axial loads, drilling the cubes, collecting the cuttings, and mea- suring the volume of cuttings. RESULTS The laboratory tests were conducted as follows : 1. The 4-in cube of coalcrete was placed in the test frame with confinement applied laterally. The lateral confine- ment was only 300 to 500 psi. Higher confining pressures were used, but this limited the ability of the coalcrete to fail when drilled. 2. A 9/16-in-diam hole was drilled through the width of the cube. The cut- tings were collected and measured. This value was used as the expected volume of cuttings from an unstressed cube, Ve. 3. Vertical stress was applied to the cube by the press in increments of 500 to 1,500 psi. 4. At each incremental load level, the cube was redrilled, and additional cuttings were collected. At low stress levels, the additional volume of cuttings 10 was very small. As the stress increased, so did the volume of cuttings until fi- nally the cube failed, and the cuttings flowed from the hole while drilling con- tinued. The cumulative volume of cut- tings from each individual hole was termed Vc. Data summarizing the drilling-yield tests are listed in table 2. The ratio, Vc/Ve, was obtained by dividing the ac- tual volume of cuttings obtained, Vc, by the volume of cuttings obtained while drilling the unstressed cube, Ve. Figure 7 shows the typical relationship between the applied vertical stress and the vol- ume of cuttings; as the stress increased, so did the volume of cuttings, until finally the cube failed. The lateral stress was maintained at 300 to 500 psi. Nineteen samples were tested. The gen- eral plot of figure 7 was consistent from cube to cube. Figure 8 shows the accumulation of the data for each incremental load value. For coalcrete cubes tested in the labora- tory and laterally confined by only 300 to 500 psi, a definite relationship ex- ists between the applied vertical stress and the ratio Vc/Ve. As the stress in- creased, so did the volume of drill cuttings. TABLE 2. - Drilling yield laboratory test data Applied stress, Psi 1,630.. 2,220.. 3,270.. 4,440.. 4,900.. 6,530.. Average Vc/Ve 1.0 1.0 1.1 1.2 1.4 1.9 Applied stress , psi 6,660... 8,160... 8,880... 11,110... 12,220... 12,780... Average Vc/Ve 1.6 2.4 2.4 2.7 3.2 3.5 10 20 30 CUMULATIVE VOLUME OF CUTTINGS, mL 40 FIGURE 7.— Relationship between applied load and cuttings volume. 0.1 0.2 0.3 0.4 0.5 0.6 Log Vc/Ve FIGURE 8.— Drilling yield laboratory results. CONCLUSIONS Probe-hole drilling, or drilling yield, is a practical method for locating high- stress and potentially burst-prone zones in the coal face. Although the method has been used in foreign countries since the 1950's, it has seen only limited use in the United States. As coal mine oper- ators mine deeper reserves and experience more burst conditions, this method of high-stress detection could become more widely used. 11 Probe-hole drilling, very simply, in- volves drilling a small-diameter hole into the coal seam and collecting in- formation while drilling, such as the volume of cuttings, behavior of the drill steel, bounces induced by drilling, gas blowing from the hole, etc. Depending on a combination of the aforementioned factors, an area of the face may be de- termined to be highly stressed. In mines with burst problems, the high-stress area is usually burst-prone. When the high- stress area has been located and deter- mined to be burst-prone, the area can be destressed. In-mine use of probe-hole drilling showed that the method can give the mine operator some idea of the relative stress level in the coal and, most importantly, can provide the location of extremely high-stress zones in the active working face. The method does not give a precise quantitative magnitude of the in situ stress, however. Tests on laboratory samples proved that there is a direct correlation between increased stress and cuttings volume. The laboratory work confirmed that probe- hole drilling can be used to locate high- stress zones in the coal seam. REFERENCES 1. Obert, L. , W. Duvall, and R. Mer- rill. Design of Underground Openings in Competent Rock. BuMines B 587, 1960, 36 pp. 2. Coeuillet, R. General Report on the Working of Outburst-Prone Mines. Pa- per in Proceedings of the Symposium on Coal and Gas Outbursts. Nimes , France, Nov. 25-27, 1964, pp. 1-23. 3. Kidybinski, A. Bursting Liability Indices of Coal. Int. J. Rock Mech. Min. Sci. & Geomech. Abstr. , v. 18, No. 4, Aug. 1981, pp. 295-304. 4. Talman, W. G. , and J. L. Shroder. Control of Mountain Bumps in the Poca- hontas No. 4 Seam. AIME, v. 211, 1958, pp. 888-891. 5. Varley, F. D. Outburst Control in Underground Coal Mines. Paper in Pro- ceedings of Fifth Conference on Ground Control in Mining. WVU, Morgantown, WV, 1986, pp. 249-256. 6. Stewart, C. L. Private communica- tion, Sept. 1983; available upon request from J. P. McDonnell, BuMines, Denver, CO. 60142 £00 U.S. GOVERNMENT PRINTING OFFICE: 1988 — 547-000/80,028 INT.-BU.0F MINES,PGH. ,PA. 28681 U.S. Department of the Interior Bureau of Mines-Prod, and Distr. Cochrans Mill Road P.O. Box 18070 Pittsburgh. Pa. 1S236 OFFICIAL BUSINESS PENALTY FOR PRIVATE USE. $300 ] Do not wi sh to recei ve thi s material, please remove from your mailing list* "2 Address change* Please correct as indicated* AN EQUAL OPPORTUNITY EMPLOYER %^ .-ate-. \/ A v c ° " ° „ t Y :^ A *f\. ,G v? *» . » " A . n • 4 c> v • - i ° . v %. * ° » o ^' ''•'if"* ° A 5 ^ .0 vO '<>.»- -v <. ' . . s - A>' 0° ♦Wfc*,'. o ,A *\^W* V 0° v-o* \/ -'Mm-, \s %f o v ^ *b^ .-. %/ :»•. \/ .-^fe-. \s .• jfe-. %/ .-^fe: %/ .-ate-- \^ % * ' * ' A & " o « o ' -0, v V.«^ °o "^«. ,A » v |^ ^ ^ -y. • .rf^VSv^'.. 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