PLEASE 00 NaT REMOVE FRCM LIBRARY IRII 9009

Bureau of Mines Report of Investigations/1986

Geotechnology In Slate Quarry Operations

By Noel N. Moebs, Gary P. Sames, and Thomas E. Marshall

UNITED STATES DEPARTMENT OF THE INTERIOR Report of Investigations 9009

Geotechnology In Slate Quarry Operations

By Noel N. Moebs. Gary P. Sames, and Thomas E. Marshall

UNITED STATES DEPARTMENT OF THE INTERIOR Donald Paul Hodel, Secretary

BUREAU OF MINES Robert C. Horton, Director Ij

Library of Congress Cataloging in Publication Data:

Moebs, Noel N Geotechnology in slate quarry operations.

(Bureau of Mines report of investigations. 9009) Bibliography: p.35-37. Supt. of Docs. no.: 128.23: 9009. I. Ground control (Mining). 2. Slate-Pennsylvania-Pen Argyl Region . 3. Quarries and quarrying-Pennsylvania-Pen Argyl Region. t. Sames, Gary P. II. Marshall, Thomas E. III. Title. IV. Series: Report of inve s tigations (United States. Bureau of Mine s) ; 9009.

TN23. U43 (TN288] 622s [622'.354] 85-600255 CONTENTS

Abstract ...... 1 Introduction ...... •...... •...... 2 Ac knowl e dgmen t s •••••••••••••••••••••.••••••••••••••.•.••••••••••.•••••••••••••• 4 Qtlarrying methods •.•.•.••.•.••..•...••••.•.••...... ••..••..••••••..•.....•... 4 Slate production •.•.....•.....•...... •.. 6 Geologic setting ...... 10 Quarry safety ...... ••...... •.....•.•.•...... 12 Qttarry hazards •••••••••.•.••..••..••...••••..••.•..•..••.•....•...... •.••..... 14 Rock falls •••• 14 Ice falls ••••• 16 High stresses. 17 Barrier failure ...... •...... •...... 20 Equipment .•..••••...••...... ••.....•...... 21 Geologic studies for hazard detection ••••• 22 Mapping ...... •...... •.•...•.....•...... 22 Resistivity ...... •...... •...... •...... 24 Co red rill i ng. • . . . . . • ...... • . . . . . • ...... 28 Stress measurements ...... •...... 28 Strain gauge ins trllID.entation ...... •.....••..•. 30 Ro ck hoI ting tes tinge ...... •...... 30 Summary and conclusions ....•...... •.•...... •...... •.....•... 32 Re f erences ...... •...... e 35 Appendix.--Glossary of quarry workers' terms ••••••••••••••••••••••••••••••••••• 38

ILLUSTRATIONS

1. Major slate-producing districts in the United States •••••••••••••••••••••• 3 2. Method of benching used to quarry slate when cleavage is nearly horizontal and bedding is steeply dipping ...... •...... • 5 3. Method of hoisting slate slabs to surface ••••••••••••••••••••.••••••••••••• 7 4. Installing standard and sheave for wire rock-sawing operation ••••••••••••• 8 5. Dollar value of slate produced in the United States ••••••••••••••••••••••• 8 6. Production of roofing slate in squares •••••••••••••••••••••••••••••••••••• 9 7. Location of Pen Argyl slate district •••••••••••••••••••••••••••••••••••••• 10 8. Generalized columnar section of rocks in Pen Argyl slate district ••••••••• 10 9. Areal geology of bedrock in Pen Argyl slate district •••••••••••••••••••••• 11 10. Section of drill core showing bedding, cleavage, and joint •••••••••••••••• 11 11. Weathered bedding plane fault (":t:"otten ri bbon") in quarry highwall. ••••••• 12 12. Gash fractures in lower portion of bedding plane fault •••••••••••••••••••• 13 13. Joints in upper portion of quarry highwall •••••••••••••••••••••••••••••••• 13 14. Schematic of a "rotten ribbon" or weathered bedding plane fault. •••••••••• 14 15. Collapse of quarry highwall adjacent to operating section ••••••••••••••••• 15 16. Early stages of ice formation on quarry highwall •••••••••••••••••••••••••• 17 17. Advanced ice buildup on upper levels of quarry highwall ••••••••••••••••••• 18 18. Winter conditions in quarry bottom •••••••••••••••••••••••••••••••••••••••• 19 19. Pressure break across cleavage plane caused by lateral thrust of highwall against bench ...•.....•...... •...... •...... 19 20. Barrier separating flooded from active quarry ••••••••••••••••••••••••••••• 20 21. Layout of some active quarries in the Pen Argyl slate district show trend of faulting ••..•...... •...... 23 22. Profile of quarry highwall showing complex geologic structures •••••••••••• 23 23. Sketch showing bedding slip fault near brink of quarry •••••••••••••••••••• 24 ii

ILLUSTRATIONS--Continued

24. Location of resistivity traverse between two active quarries...... 25 25. Resistivity sounding curve, cumulative plot and linear plot...... 26 26. Resistivity sounding curve, double log plot...... 26 27. Apparent resistivity traverse plot ...... o...... 27 28. Profile showing relation of resistivity anomalies to buried fault traces.. 27 29. Diagram of quarry stresses...... 29 30. Common method of securing fractured rock on quarry high~·Tall with steel

pins ...- ...... 0 • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 3 1 31. Use of anchored mechanical bolts in tension to secure jointed rock...... 31 32. Use of anchored mechanical bolts in tension to secure fractured rock in quarry highwall...... 32 33. Setup for rockbolt pull test...... 32 34. Measuring rockbolt tension bleedoff...... 33 35. Representative load-displacement curves for mechanical bolts...... 34 36. Tension bleedoff graph for four mechanical bolts...... 34 37. Representative load-displacement curves for full-column resin-anchored bolts, using pull tests...... 34 TABLE 1. Physical properties of slate...... 4

UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT ft foot m meter ft2 square foot MN/m2 meganewton per square meter h hour MPa megapascal in inch ' Slm ohm meter in2 square inch psi pound (force) per square inch in/oF inch per degree Fahrenheit V volt lb pound V/in volt per inch Ib/ft3 pound per cubic foot W watt GEOTECHNOLOGY IN SLATE QUARRY OPERATIONS

By Noel N. Moebs, 1 Gary P. Sames, 1 and Thomas E. Marsha 112

ABSTRACT

This report summarizes a Bureau of Mines study on the use of geotech­ nology to identify and reduce ground control hazards at slate quarry operations in eastern Pennsylvania. The major ground control hazard is falling rock, attributed to weathered bedding plane faults, which weaken quarry highwalls. It was demonstrated that these faults can be detected outside of the quarry perimeter using surface resistivity measurements. Falls of ice from quarry highwalls constitute a second major hazard, which requires water diversion as a basic control method. In situ stresses seemingly cause only minor problems in slate extraction al­ though they could interact with developed stresses and lead to failure of barriers between quarries. Pull tests showed that rockbolts anchor firmly in slate and should prove effective in securing loose rock-

1 Geologist. 2Engineering technician. Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. 2

INTRODUCTION

During the 1920's, when production of using various methods and equipment for roofing slate in the United States had this purpose. While this study was con­ declined to low levels, the Bureau of ducted chiefly in the Pen Argyl district Mines conducted studies to help the slate of Pennsylvania, some of the findings industry adopt more economical methods should be applicable to other slate dis­ and use the most efficient labor-saving tricts or other types of quarries. equipment. It was anticipated that this A brief reconnaissance was conducted of would promote economy and the reduction quarries in the Vermont-New York slate of waste in the industry. The princi­ district, although no studies were at­ pal investigator for the Bureau, Oliver tempted by the authors. These quarries Bowles, succeeded in encouraging industry are operated chiefly as unsymmetrical to adopt the then innovative wire saw, open pits using explosives for extrac­ which greatly facilitated the quarrying tion, whereas in Pen Argyl the quarries of slate and minimized waste. Bowles (1- utilize wire saws to achieve a relatively 7)3 reported on several technical aspects smooth-walled rectangular opening. of the industry, particularly in the Any discussion of the U.S. slate indus­ Pen Argyl district of Pennsylvania, and try should include at least a passing Thoenen (8) authored a supplementary re­ reference to Wales, where most practices port on the wire saw in 1928. in slate quarrying and processing had Between 1928 and 1980, the slate indus­ their origin, and from whence many of the try received little attention because quarrymen emigrated to America in the priorities and national interests had 19th century. As an example of the im­ turned to a depressed economy, strategic portance of the Welsh slate industry, one minerals in times of war, and environmen­ quarry alone, the Dinorwic, employed tal protection. However, in recent years nearly_ 3-t-0_OO _men in -L900, and its work­ there has been a renewed interest in pro­ shop buildings now form the North Wales moting health and safety technology Quarrying Museum, demonstrating all the through research. The Bureau, which is stages in the production of slate [Butt responsible for a broad spectrum of pro­ and Donnachie (10)]. Slate production grams for improving technology to make from both the United States and Wales has production and processing of minerals diminished greatly, however, from pre­ more efficient, safer, and more health­ vious high levels near the turn of the ful, once again recognized that the slate century. industry was in need of more up-to-date The quarrying of slate in the United safety technology and production methods. States began in the 18th century and by In response, the Bureau first conducted the mid-19th century was well established an overall study of the slate industry at several localities. By 1880, Penn­ of the Vermont-New York district [Watson, sylvania had become the major center of Ohlsson, Shorey, Miller, and Whittier production, although western Vermont fol­ (9)], and second, initiated an investiga­ lowed closely (11). Pennsylvania con­ tion in the Pen Argyl district to iden­ tinues to ·oe the leading producer because tify quarrying hazards and to develop of the active quarries located in the methods for reducing these hazards. This Pen Argyl district, along with some in report describes the efforts in the Pen nearby Slatedale, Lehigh County, also a Argyl district to apply geotechnology to part of the "soft (slate) belt." the reduction of safety hazards in slate In the United States, siate is produced quarrying and to test the practicality of by the open pit or quarry method of min­ ing except at one locality at Monson, ME, 3Underlined numbers in parentheses re­ where it is mined underground. In the fer to items in the list of references Pennsylvania slate district, the quar­ preceding the appendix at the end of this ries tend to be deep and tiArrow because, report. for the most part, beds of high-grade 3

slate are closely folded and steeply dip­ nonpolluting, important properties in an ping, and the working of such beds down environmentally sensitive society. Slate dip, rather than along strike, results in once was very popular for tombstones be­ a quarry commonly several hundred feet cause it does not weather easily. deep. The major slate-producing districts in Underground mining of slate in Pennsyl­ the United States (fig. 1) are the vania was attempted at one time by work­ Vermont-New York district near Poultney, ing laterally from the bottom level of VT, and Whitehall, NY, and the Pen Argyl an active quarry. This slate mine is no district in Northampton County, PA. Some longer in operation despite certain ad­ slate also is produced in Maine, Vir­ vantages of working underground, such as ginia, North Carolina, Georgia, and a minimum disturbance of the surface en­ Arkansas. vironment and freedom from severe weather conditions. Although more producers of crushed stone are considering underground mining, according to Robertson (12), dimension stone production is a special case be­ cause careful extraction and handling are essential. Some abandoned quarries have been used for solid waste disposal and petroleum product storage, and under­ ground mining for dimension stone also could lead to end uses of the resulting space, which can constitute a source of income after closure. Many factors would have to be investigated thoroughly before planning underground dimension stone mining. Slate is a fine-grained, low-grade metamorphic rock with a very pronounced cleavage, enabling it to be split readily into thin, smooth sheets. Its physical properties are shown in table 1. Most 0 200 400 slate originally consisted of a clay or I I I mud. It is composed chiefly of sericite, Scale, miles chlorite , and quartz, and occurs in a variety of colors. Red, green, and pur­ Number of Number of ple slates are common in the Vermont-New State Product quarries employees York slate district, while most slate in ME ••• 1 8 Dimension the Pen Argyl district of Pennsylvania is stone. gray ~lith thin beds colorec nearly black NY-VT 33 312 Do . by carbonaceous materia l . PA .. . 6 330 Do. Because of its cleavage , ease of shap­ VA ... 6 163 Dimension ing, and stability, slate i s useful for stone , the manufacturing of a wide variety of crushe d pLoducts, including roof tiles, floor stone. tiles, burial vaults, billiard table NC ... 2 10 Do. tops , and architectural stock. The high GA ••• 2 25 Crushed dielectric strength of slate ( 7 , 000-V s tone. minimum per I-in thickness) makes it AR ... 1 20 Do . h ighly valuable f or switchboards and s i m­ ilar applications. Slate is relatively FIGURE 1. - Major slate-producing districts in the inert ,> virtually nonradioactive , and United States. 4

TABLE 1. - Physical properties of slate

Specific gravity ••••••••••••••••••••••. 2.79-2.83 Weight ••••••••••••••••••••••••• lb/ft3 •• 175 Rockwell .. c .. hardness •••••••••••••••••• 35

Shore hardness ...... 0 • fI •• 31-46 Brinnell hardness ••••••••.• , ••••.•.••.• 320 Compressive strength •••••••••••.•• psi •• 15,500 Tensile strength •••••••••••••••••• psi •• 2,500 Modulus of elasticity ••••••••.•••• psi •• 1.42 x 10 6 Poisson's ratio •••••••.•••••••••••••••• 0.158 Coefficient of expansion •••••• ••in/oF •• O. 5-1. 0 x 10-5 Point load •••••••••••••••••• •• •• MN/m 2 •• 2.26-6.29 Dielectric strength ••••••••.•.••• "'.!/in •• 7 ,, 000

The Pen Argyl district, subject of this (15), Mullen and Stickler (li), Poyt report, is designated as being part of (17), and Miller, Fraser, and Miller the "soft belt" of slate, a versatile, (18). Detailed geologic maps on seLected high-grade slate. Geologic stI'ucture in areas of Pennsylvania slate belt have the Pen Argyl district is complicated by been prepared by Davis, Drake, and Ep­ numerous folds, overturned bedding, and stein (19) and Epstein (20), while Ep­ faults, which have been the major factors stein has also mapped the- location of in determining the shape and size of quarries and dumps in the Stroudsburg quarries in the area. Much of the sur­ quadrangle, northeast of Pen Argyl, to face is covered by glacial deposits of show their environmental significance ground and terminal moraine, in general (21). Most of the literature, however, less than 15 ft thick. is- largely devoid of any discussion of Slate in Pennsylvania has been the s-a.fet=-y -t=-e-ehnology; it is hoped that- some subject of several comprehensive reports of the material in this report will con­ that fully describe the geology, quarry­ tribute to an improved understanding of ing, processing, and marketing of this quarry hazards and the means to investi­ material, including reports by Behre gate and minimize them. (ll-~), Stickler, Mullen, and Bitner

ACKNOWLEDGMENTS

The authours wish to express their ap­ helpful information on the quarrying and preciation to Anthony Dally and Sons, processing of slate, for which the au­ Inc., for providing material, for assist­ thors are grateful. Also, thanks are due ance in test procedures, and for provid­ to Charles Ratte, Vermont State Geolo­ ing unlimited access to quarry operations gist, for conducting the authors on a during the course of this investigation. reconnaissance tour of the Vermont-New The Pen Big Bed Slate Co. and Williams York slate district. and Sons Slate and Tile, Inc., provided

QUARRYING METHODS

The details of dimension slate extrac­ Slare in Pennsylvania is obtained from tion cannot be described fully here be­ open quarries, where narrow, deep pits cause of the diversity of slate cleavage are the rule because beds of quality and bedding, and the variety of local slate tend to be steeply dipping (fig. mining techniques that are tradition 2). Working down dip, sometimes to 900 bound, but a general description is es­ ft, is more advantageous than quarrying sential to understanding the slate quarry along strike, because at shallow depths operation. the stone is weathered and stained, and 5

FIGURE 2.· Method of benching used to quarry slate when cleavage is neady horizontal and bedding is steeply dipping . 6

a large amount of both loose overburden the cleavage plane. Both methods had and weathered rock must be stripped away disadvantages. Drilling and blasting re­ before production of usable slate can be­ sulted in excessive fracturing and waste gin. Working down dip exposes smaller and ragged angular rock surfaces. Chan­ areas of slate to freezing; once soft neling was a slow process requiring con­ slate is frozen it does not split proper­ siderable time to drill and to broach the ly. In addition, lateral quarry develop­ slate between the holes. ment necessitates the frequent relocation Shortly after 1926, the Bureau assisted rJ of aerial trams and other fac.ilities, a the slate industry in introducing the I slow and costly operation. For similar wire saw to facilitate slate extrac­ I reasons, new operations seldom are opened tion. The wire saw consists of an end­ I, in virgin ground. Instead, old quarries less three-strand wire rope about 1/4 in. are dewatered and reactivated. in diameter, which is carried on pulleys Slate usually is raised from the quarry from the driving wheel located on the floor to the surface by means of an aer­ surface near the edge of the quarry to a ial tram (fig. 3), which also is used to quarry bench; there it is guided again by transport workers and equipment into and anchored pulleys against the rock where out of the pit. a slot in the quarry bench is to be cut Prior to 1926, slate quarries in Penn­ (fig. 4). The wire rope is placed in sylvania extracted large slabs and blocks tension, set in motion, and fed with a of slate either by drilling and blasting slurry of sand and water. Compared with or by a channeling process. With the older quarrying methods, the wire saw channeling process, a row of closely conserved power and labor, lowered costs, spaced parallel holes was first drilled reduced waste, and permitted the quarry in the rock bench and then the interven­ walls to be cut more smoothly, thereby ing rib of rock between each pair of reducing the hazard of rock falls. The holes was broken away to form a contin­ wire saw method continues to be the prin­ uous slot. The rock then could be freed cipal manner- by which slate is extracted entirely from the bench by wedging along from the quarries in Pennsylvania.

SLATE PRODUCTION

Roofing continues to constitute the ma­ 1980 shows an overall incr~asing trend jor market for quarried slate, although (fig. 5), reaching a peak in 1980. Roof­ mill stock, which includes billiard table ing slate, as measured in the number tops, grave vaults, and architectural of squares produced from 1879 through items, is an important and growing prod­ 1980, reached a peak output during 1900- uct. Roofing tiles are used principally 13. The output generally has declined in the repair of existing buildings or in since then (fig. 6) but shows some signs the construction of institutional build­ of stabilizing in the last few years ings and expensive dwellings. The reno­ reported. vation of historic structures also is an It has been estimated that almost 85 important use of slate roofing tile. The pct of the slate quarried in Pennsylvania Parsons Brothers Slate Co. has published is waste, and most of this material goes a handbook fully describing all aspects to make up the large waste piles that of roofing slate, roofing alterations, characterize the slate belt. Some waste and reroofing (22). Architectural slate has been consumed in the manufacturing of consists chiefly--of floor tile and fac­ lightweight aggregate, roofing granules, ing used for institutional or commercial fillers, fiberglass, rock wool, and glass buildings. Box tile, a slate flooring ceramics, but the overall consumption of somewhat\thinner gauge than architectural waste for these items remains small. A floor tile, is used primarily for resi­ promising use for waste slate in the man­ dential patios and entrance areas. ufacturing of a substitute for asbestos The dollar value of all slate produced was recently investigated by Mackenzie in the United States from 1890 through ell), and the results were summarized 7

FIGURE 3. - Method of hoisting slate slabs to surface. 8

I.,

FIGURE 4. - Install ing standard and sheave for wire rock-sawing operation.

28

26

24

22

20

18

!:A- I.!) ...... 0

l.JJ => -I « >

2

OL-__L- __L- __L- __L- __L- __~~ __~ __~ __~ __~ __~ __~ __~ __~ __~ __~ __~ - 1890 1900 1910 1940 1960 1970 1980 FIGURE 5. - Dollar value of slate produced in the United States. 9

1,500

1,400

1,300

1,200

1,100

1,000

VI 900 ~ 0 ::l CT VI rt'l 800 0 >- t- 700 i= z 1 a 600

500

400

300

200

100

0 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 FIGURE 6. - Producti on of roofing slate in squares. 10

in Chemical and Engineering News (~). lack of expansion in production facili­ Chrysotile asbestos, used in manufactur­ ties and investment capital, and a resur­ ing asbestos cement pipe, asbestos cement gence in the use of natural materials. sheet, and roofing and flooring products, Dickson (26) reviewed the production and is largely imported from foreign sup­ marketing--of slate, including some new pliers and, further, is suspected of be­ uses, and also reported that interest in ing a cancer-causing agent, lending ur­ higher priced slate products, such as gency to the search for a substitute raw roofing and architectural stock, is in­ material. creasing again. Some of this increase in As recently as 1981, the overall demand demand can be attributed to the renova­ for slate, except for waste, was greater tion of slate roofs on older homes and than the supply [Meade (~)] because of a historic structures.

GEOLOGIC SETTING

The Pen Argyl slate district is located Cambro-Ordovician limestone of the Lehigh in Northampton County, Pa, about 60 miles Valley. The slate is more resistant than north of Philadelphia. The slate occurs the limestone and forms a low terrace of in a northeast-trending belt some 6 miles rolling topography between Blue Mountain wide that lies along the southeastern and the limestone valley. The Martins­ foot of Blue Mountain (fig. 7), a north­ burg Formation originally consisted of east-trending ridge of resistant Shawan­ clay and silt sediments. According to gunk conglomerate of Silurian age. The McBride (27), the formation was deposited rocks in the slate belt consist of the in relatively deep water below wave base, Martinsburg Formation of Ordovician age as indicated by the absence of crossbed­ and are bounded on the southeast by the ding, channeling, and wave-ripple marks. The clay minerals in the sediments were recrystallized during regional metamor­ phism and deformation into sericite, chlorite, and quartz. Some carbonate and carbonaceous material also is present in Pen Argyl slate.

Description

Shawongunk Sandstone and Silurian System conglomerate Bethlehe~ Pen Argyl Member, upper soft slate member, thick-bedded ~Allentown claystone slate, 3,000 fl Ihick c: g 5 E Ramseyburg Member, middle NJ ~ hard slate member, thin- to thick­ PA Ordovician Syslem bedded claystone slale, siltstone and graywaCke, 2,800 ft thick

Bushkill Member, lower hard slate member, Ihin-bedded claystane slate and siltstone, 4,000 ft thick

Cambro-Ordovician Jacksonburg Limestone 10 20 System I I Scale, miles FIGURE 8. - Generalized columnar section of rocks FIGURE 7. - Locotion of Pen Argyl slate district. in Pen Argyl slote district. 11

Drake and Epstein (28) have divided the high-grade slate are deep and narrow. Martinsburg Formation--into three members Continuing to follow a steeply dipping (fig. 8) to facilitate geologic mapping run of slate and quarrying to greater of the area and to fix the stratigraphic depths eliminates the necessity of remov­ and areal distribution of the commercial ing overburden and weathered shallow slate in the formation (fig. 9). The up­ slate for lateral expansion of a pit. per member of the formation, designated A nearly perfect cleavage unrelated to as the Pen Argyl Member, includes the bedding (fig. 10) was formed in the slate principal beds or runs of high-grade com­ belt as a result of the deformation. mercial§late in the district. The Pen Faulting also occurred in many directions Argyl Member consists chiefly of thick­ but is principally parallel to bedding bedded gray claystone slate with occa­ where exposed by quarry openings. These sional thin beds or "ribbons" of nearly bedding slip faults sometimes are weath­ black carbonaceous slate. The upper por­ ered and clayfilled at shallow depths tion of the middle or Ramseyburg Member (fig. 11) and are then deSignated as of the 'Martinsburg Fo-rma-tion also has "rotten rloDons" by quarriers. Many yielded some commercial high-grade slate, faults are filled with vein quartz and but rocks occurring lower in the forma­ tion are inferior in quality because of interbedded hard layers of siltstone. The Martinsburg Formation in this area Joint has been deformed by tectonic forces into highly complicated folds, the formation largely being thrust northwestward into Bedding a sequence of overturned to recumbent (ribbon) folds. Beds are inclined at many differ­ ent angles ranging from horizontal to 0 0 90 , and more than 90 or overturned and, therefore , upside down. Quarries developed on steeply dipping beds of

LEGEND S5· Showongunk Sondslone Omp Pen Argyl Slate Omr Romseybvr9 S IOl6 Omt! Bushkill Slate (-OJ Joc!l.sonburg Umestone """'-"Thrusl foul! ScoIe , mites ~ Ac rive slal1~ quarry Scale, in ;tlUr Town

FIGURE 9 0 , Areal geology ot bedrock in Pen Argyl FIGURE 10. - Section of drill core showing bedding, slate district. cleavage, and joint. 12

plane is steep but dips nearly parallel to the beds on one limb of a compressed fold. He also describes the gash veins that tend to occur in both walls of the faults. These descriptions compare closely with most of the bedding plane faults observed in the slate quarries. The direction of movement along these faults, but not the amount, is determined by the gash veins. It seems probable that the faults formed while the beds were nearly horizontal and then were com­ pressed further into close folds along with the beds. Joint sets in the area tend to be lo­ cally developed (fig. 13) and for the most part are different from one quarry to the next, probably because of the in­ tense folding, refolding, and localized response of the slate to stresses. There is a tendency toward a northeast strike parallel with the regional structure, while dips are highly variable. General­ ly, the presence of joints is a disadvan­ tage in quarrying because sufficiently large blocks of high-grade slate cannot be quarried. Some areas of- th e slate district are covered by a thin veneer of glacial de­ posits of both ground and terminal mo­ raine of Pleistocene age and consisting of a mixture of unconsolidated boulders, gravel, sand, and clay. This material is FIGURE 11. Weathered bedding plane fault ("rot- reported to reach a thickness of 50 ft in ten ribbon") in quarry highwall. places, but it seldom exceeds 15 ft in the vicinity of the Pen Argyl quarries calcite. Subsidiary gash veins filled where it grades into the underlying with quartz and calcite occur adjacent broken and weathered slate. Resistivity to maj or bedding slip faults (fig. 12). soundings, reported in a later section, Behre (29) described a special case of "Geologic Studies for Hazard Detection," bedding:plane faults in which the thrust confirm this estimate.

QUARRY SAFETY

A study of operational safety in the mining, while falls of face, rib, and stone, sand, and gravel industries was highwall occur most commonly in surface conducted under a Bureau contract by stone mining. It is noteworthy that Yegulalp and Boshkov (30) for the purpose none of the companies quarrying surface of defining factors affecting safety in crushed stone that were surveyed in this various areas of ground control. Only study bolted the pit wall on a regular five injuries were reported for slate di­ basis to prevent falling rock, a l though mension stone quarries for 1975-78. The a number of dimension s tone quarri es d id study further revealed that roof fall bolt on a spot basis when necessary . accidents are the most common ground con­ Accident statistics relating specif­ trol accidents in underground stone ically to slate quarries are available, 13

FIGURE 12. - Gash fractures in lower portion of bedding plane fault.

FIGURE 13. - Joints in upper portion of quarry highwolf. 14

but because of the small number of quar­ ries, they do not provide a statistical base concerning causes of accidents. For .\ 1978, 44 slate quarries were operational Weathered portion of in the United States, with 312 employees. faullzone (rollen ribbon) ~----+----, / ~ , I The Mine Safety and Health Administration , i i --r-----I -;-- / ~ I I (MSHA) (31) reports that, in 1982, there I I ! 1---1 were -s-:l::x--i-njurr es and- no fatalities re­ I - j -f--f--+- lated to ground control in dimension ,':bbOO', . slate quarries. Accident and fatality ~ Bedding plane fault with _ ~ reports for earlier decades are incom­ gash vein s and spar filling I ~ I plete, but occasional fatalities occurred (loo se ribbon) -f---L ;-----;- f i :----i­ in the Pennsylvania slate quarries. For ~ I ; : : ;----t­ example, in 1939, rock falling from a :-V---I ' highwall resulted in two fatalities. In ---i--- ' Cleaya~e ---t-- I I I 1969, three more deaths resulted from a -+----L: i- similar accident. In both of these inc_i­ i~ ~I ~ dents, the cause of the rock fall was at­ I <------L tributed to a rotten ribbon or weathered ~!\ i ---r------+-- bedding plane fault (fig. 14) located ~ near the top of a quarry highwall. In ------+- I \ \ \ I r----'r~ 1979, a similar large rock fall occurred r------L \ I , \\\~ \ unexpectedly from a quarry highwall near r--L-.-.\ I \ o 50 I 0 0 I I \ \ \ \ \ Pen Argyl (fig. 15). The fall occurred ,-' ---::--:-'-'---;,---" ~ I I \ Scale, fl i \ ----r-----L I \ ,~ on a weekend and thus no workers were en­ ~ \ \ \ dangered. It is suspected that the f _a11 was caused by the intersection of a rot­ FIGURE 14. -- Schematic of a "rotten ribbon" or ten ribbon and a weathered cross joint or weathered bedding plane fa ult. transverse fault, although these struc­ tures were obscured by mud, rubble, and quarries, but this has not entirely pre­ vegetation. Falls such as these, which vented rock falls. Currently, little re­ occur without any advance warning, are search in ground control technology is evidence that much is yet to be learned being conducted by the dimension stone about the exact cause of rock falls from industry, especially the slate industry, vertical quarry highwalls and methods by which is struggling to regain what ap­ which they can be prevented. While it pears to be a market recovering from many seems clear enough that geologic defects depressed years. weaken highwalls, methods have yet to be Specific ground control hazards that developed to assess the hazard potential are especially troublesome in Pennsyl­ of these defects adequately. Precau­ vania slate quarries are described in the tions such as periodic examination, pin­ following section. ning, and scaling are practiced at some

QUARRY HAZARDS

ROCK FALLS breakage. After several decades of wire saw quarrying on a selected "run" of The hazards encountered in Pen Argyl­ beds, a slate quarry resembles a large type slate quarries are somewhat differ­ shaft more than a conventional quarry. ent from those in conventional, shallow Unlike a shaft, however, the vertical rock quarry operations. No blasting is walls of the slate quarry are accessible performed, which eliminates the common for examina-t-i-on or scaling only th rock fall problem resulting from highwall great difficulty, and any weakness or 15

FIGURE 15. - Collapse of quarry highwall adjacent to operating section. geologic discontinuity in the walls is partially supported and is usually weak­ difficult to detect and constitutes a ma­ ened by fractures, thereby becoming vir­ jor hazard to those working on the lim­ tually freestanding. The forces that ited floor space below. For this reason eventually cause the highwall to hinge and because of the fatalities that have outward, buckle, or collapse can include occurred from time to time, rock falls in situ rock pressures (described in the from the quarry walls can be designated section "High Stresses"), a hydraulic ram as the principal ground control hazard in effect from fluid pressure in an open slate quarry operations. Most accidents water-filled fault fissure, the forma­ and fatalities have resulted from rather tion and expansion of ice in highwall large collapses of a quarry wall r ather fractures, and the lubrication effect of than the dislodging of a single slab or percolating ground water on the shear wedge or rock, and most can be attributed strength of any geologic discontinuity . to the presence of a major geologic dis­ To a large extent, the engineering behav­ continuity such as a weathered fault ior of slate as a rock mass is governed zone. The conditions along a fault zone more by faults or joints than by the that trigger a highwall collapse can cleavage in the Pen Argyl district, which be attributed to several sources. As a dips at a very low angle. Laboratory quarry is deepened, more of the slate tests show only a relatively small dif­ that buttresses the highwalls is removed, ference in physical properties at right and the remaining rock slab between the angles and parallel to cleavage. Cleav­ highwall and fault (fig. 14) is only age, hmolever, constitutes a potential 16

plane of weakness and must be given seri­ thawing or breaking off under load con­ ous consideration in assessing rock fall stitute a serious handicap during winter hazards. operations and a hazard to quarry workers Slope stability studies at the 492-ft from large masses of falling ice. (150~) deep Old Delabole Slate Quarry The ice forms chiefly from water that near Tinagel in North Cornwall, Wales, seeps through cracks in the wall rock by Brown, Richards, and Barr (32) also near the brink of the quarries. The ice point to the importance of discontinu­ coats virtually every surface in the ities such as cleavage, joints, and quarries, the sloping floor becomes slip­ faults in quarry operations . In partic­ pery and footing is uncertain, cables of ular, a series of failures that occurred various kinds are pulled from the walls in the face of the quarry prompted an by the weight of accumulated ice, and investigation of shear strength of dis-­ large masses of ice sometimes weighing continuities, with resulting data on several hundred pounds form on the quarry the influence of water and surface walls (figs. 16-18). These ice stalac­ roughness. tites are scaled, shattered with shotgun . r In a related study at the Dinorwic slugs, or sometimes dynamited to avert slate quarry, Paterson and Arthur (33) the danger of their becoming dislodged presented portal design and construction and falling onto the quarry floor, par­ methods, including rock bolting, dowe~s, ticularly during the early spring thaw. steel mesh, and sprayed concrete, based Since slate dumps and the glacial over­ on geotechnical information. Joints were burden together constitute aquifers with identified as major potential planes of a sizable storage capacity and since they failure, and on one occasion they con­ have been removed to only a few yards tributed to the complete collapse of a from the quarry brinks, water diversion tunnel portal. At the time of the col­ is not eaSily accomplished. At one quar­ lapse, all the relevant geological infor­ ry, three shallow dewatering wells are mation was known, but the interaction of located on the side of the quarry and all the adverse factors had not been ap­ -downsrope from- some large s late dumps. preciated fully. These wells, however, divert only a small A preliminary study of slate quarry op­ portion of the ground water reaching the erating experience and the geologic con­ upper levels of the pit, and ice accumu­ ditions in the Pen Argyl district indi­ lation in the winter continues to be a cates that, in addition to routine visual severe problem. examination and scaling of rock on quarry Ice control or prevention methods that highwalls, some established geologic and have been suggested for evaluation in­ engineering methods might be used to de­ clude grouting of the quarry walls to re­ tect potential rock fall conditions. duce seepage, drainage drill holes, the These methods include geologic mapping of use of a wax emulsion coating on the rock discontinuities and the use of strain walls to prevent the adherence of ice, gauges or tiltmeters to detect incipient and suspended movable chains to break up movement of large wedges of rock near accumulations of ice at critical areas. faults and joints. Extended time will be To date, only one of these methods, wax required to assess the practicality of emulsion, has been assessed by the Bu­ these methods, as quarry development pro­ reau. ~e wax emulsion failed to sub­ gresses slowly; however, a brief sum­ stantially reduce ice adhesion in prelim­ mary of the methods employed and the re­ lnary ~aboratory tests. Another method, sults obtained could lead to further grouting of quarry walls to reduce seep­ improvement in quarry safety and hazard age, could have the adverse effect of in­ detection. creasing to an excessive level the hy­ draulic head in a water-filled joint or ICE FALLS fault. At the present time , ice buildup during the winter continues to be a seri­ The accumulation of ice on quarry high­ ous problem. walls and its dislodgement because of 17

FIGURE 16. - Early stages of ice formation on quarry highwall.

HIGH STRESSES floors in the Appalachian plateaus de­ scribed by Ferguson (38-39). The closure of 1/4-in vertical rock saw Adams (34), using quarry and buckle di­ slots and the occurrence 0: pressure mensions and an adopted value for Young's breaks (fig. 19) in the floor of several modulus, estimated that the upper limit active Pen Argyl quarries are indications of the maximum compressive stress in the of high horizontal stre.sses. While slot McFarland Quarry, Ottowa, Canada, ranges closures can be controlled by wedging and from 1,400 to 4,300 psi (10 to 30 MPa) seldom occur near benches " stresses in­ for the limestone floor after removal of creasing with depth of operations could 45 ft of overburden. lead to severe buckles or failure of Although no buckles were reported, highwalls and barrier pillars As used Anderson, Arthur, and Powell (40) con­ here, " buckles" (popups) describe the ducted measurements of in situ-Stresses heave of flat-lying strata or rock with a in a tunnel complex being excavated in pronounced horizontal cleavage f ascribed the main Cambrian slate belt of Dinor­ to load removal from rocks under higrl wic, Wales, because of the relevance of horizontal stress. Quarry buckles in On­ stresses on the stability of excavations. tario have been described by Adams (34) The results showed a considerable degree and Coates (35). Smith (36) and Sbar and of scatter but indicated that the major Sykes (37) -mention buckles at several principal stress of 2,500 psi (17.5 MPa) quarries--in upstate New York . These was about double the vertical or overbur­ buckles are not entirely unlike the pre­ den stress and normal to cleavage, which historic stress relief phenomena that oc­ dips 70 0 southeastward. curred because of the unloading of valley 18

FIGURE 17 •• Advanced ice buildup on upper levels of quarry highwall. 19 ;1"

FIGURE 18< - Winter conditions in quarry bottom.

FIGURE 19. - Pressure break across cleavage plane caused by lateral thrust of highwall against bench. 20

Measurements of rock saw sLot cLosures were attempted at a Pen Argyl quarry for the purpose of estimating stresses and evaluating their role in floor and high­ wall stability. The maximum slot closure measured was only 0.035 in. This value represents only a small portion of the usual 0.25-in slot closure in the first slot cut, because of interference from nearby wedges in the slot and an existing parallel slot about 72 ft away . Further­ more, the quarry is operated on succes­ sive benching levels , and even the sink , or lowest level, is not always fully con­ fined laterally. The closure measure­ ments were obtained by installing a dial­ type mechanical strain gauge on the quarry floor astride the wire saw slot and taking progr essive r ead ings of clo­ sure as tht saw was operated.

BARRIER F AlLURE

In the Pen Argyl s l ate distr i c t, some actlve quarries have been developed a dja­ cent to abandoned flooded quarries , from which they are separat e d by a ba r r ier pillar (fig. 20). Some guideline s f or designing coal mine barri e r pillars h ave FIGURE 20. - Barrier separating flooded from active been available for many years , but they quarry. would not be satisfactory for other types of deposits. Existing slate quarry ba r­ riers probably were designed on an even the actual thickness, may be in empirical basis and for limited develop­ doubt and should be a serious concern as ment only. These should be subject to the depth of operations increases and the minute inspection as to adequate size, barrier is subject to growing pressures strength, stress conditions, and geologic from abutments and possible impounded defects that could lead to failure . Bar­ water. rier failure could result in both massive The integrity of an existing bar­ rock falls and water surges into the ac­ rier can be explored using many avail­ tive quarry. Simila~ barriers exist in able methods, including core drilling, limestone quarries located in the cement geologic mapping, stress measurements, district of the Lehigh Valley , although referral to design equations, or geophys­ quarry depth generally is not as great as ical probes such as radar or sonic de­ in the slate belt. vices. All of these methods are limited While it is a general practice to lower somewhat when only a portion of the ex­ the water level in a flooded quarry t o isting barrier is accessible, as in sev­ that of an adjacent active quarry , it i8 eral quarries of the slate belt" not always feasible to inspect an eXlst­ Some preliminary measurements were con­ ing barrier from the abandoned quarry, ducted on the barrier shown in figure 20, especially when an accumulation of rub­ using the borehole deformation gauge bish, metal junk, rotten vegetation, mud, and overcoring method , in an effort to and broken rock obscure the barrier faceo estimate the developed stress conditions Thus, the integrity of the barrier '0 and in the vicinity of the barrier abutment. 21

Results are discussed in the section An alternative t o the above , but one "Stress Measurements." requiring an entirely new design, would be the use of a track-mounted diamond EQUIPMENT saw, a reciprocating abrasive cutter bar, or a rock chain saw--ideas that have been The hazards inherent in working around proposed but not developed specifically moving equipment, heavy objects, com­ for slate. These devices could have pressed air, and overhead activity are self-contained electric or pneumatic pow­ nearly always present in a quarry opera­ er units or could be powered by a sepa­ tion. Federal safety r egulations per­ rate hydraulic unit. taining to open pit mines such as slate Any design for a rock-sawing device quarries are authorized by the Federal that would depart radically from those in Mine Safety and Health Amendment Act of use might necessitate a somewhat greater 1977 and are described in the U. S . Code off set a t t he qua rry wall on ~ ach bench of Federal Regulations, Title 30, Part 55 cut in order to provide adequate operat­ (41). Virtually every aspect of extrac­ ing space. This would entail a larger tion and ground control in a slate quarry quarry dimension at the surface to pro­ is interrelated with equipment usage. vide for a diminishing width further down Most equipment currently used in slate where the cut could not be made flush quarries in the eastern United States was with the quarry walls; however, a bench­ developed during the first half of this ing of quarry walls would provide some century and is suited to old and well­ protection from falling rock. entrenched extraction procedures. It is commonly agreed that equipment moderniza­ Standard and Sheave Hole Drill tion would vastly increase the efficiency of slate extraction and could have a ma­ Th e electric-powered drills currently jor impact on safety but would require i n use for coring a 3-ft-diameter hole in capital expenditure beyond the financial which to pl ace the standard a nd sheave capability of a small industry in which for directing the wire rope for slot cut­ future market trends are indeterminate. ting are heavy , cumb ersome, and difficult Nonetheless, some suggestions for equip­ t o move. It has been s uggested that a ment updating are outlined below, along hydraul ic-powered unit would be lighter with related modifications in quarry de­ i n weight and easier to move and control, sign and operations that might result. e specially if the standard and sheave could be reduced in size so that only an Wire Saw 18- or 24-in-diameter borehole would have to be drilled. The drive f or the wire saw currently in use is located near the quarry brink , Split tingwedges necessitating excess wire length, severai pulley arrangements, and the exposure of Large blocks of slate that have been workers to moving wire under load. This oawea free on four sides are separated situation could be corrected by moving f r om t he quarry floo r by ma nually driving the wire driving and tensioning mechanism steel wedges aiong the nearl y horizontal onto the quarry floor and incorporating cleavage , t hereby producing thick slabs the components into a more or less self­ of slate t hat a re t hen raised to the contained sawing unit. A modification o f s urf ace on an ae r i a l t r am . Sometimes, t he standards and sheaves used to direct shall ow , sma l l-d i amet er h ol es a r e first t he wire into cutting slots would be nec­ drilled along cleavage to f acilitate essary. Any advantages in reducing the splitting. A small-diamet er t ubular hy­ length of exposed wire, however, might be draul ic spl itter, a ctivat ed by a h and lost by the added cost of increased wear pump, h as been developed recently , which o n the wire. can be i nserted i nto a shallow drill hole 22

to further facilitate splitting; a few without the rigorous use of sledge hammers strokes of the pump will free the slab, and wedges .

GEOLOGIC STUDIES FOR HAZARD DETECTION

MAPPING Figure 21 illustrates the general lay­ out of a group of acti ve quarries i n Geologic mapping in the vicinity of the the Pen Argyl district, where geologic Pen Argyl slate quarries is severely l i m­ mapping of exposed quarry structures was ited by a lack of outcrop, owing to a conducted t o locate some prominent oed­ cover of slate waste piles and glacial ding plane faults , whic~ contribute moraine . Furthermor e , structure is com­ to the instability of quarry highwa1ls. plicated by in,tense folding so that great Th ese f aults were targeted for f urther care must be exercised in projecting in­ study, either by instrument monitoring of dividual structures beyond the exposures strain or by the use of resistivity to in quarry walls. Thus, any effort to map detect the e x tension of the fault beyond geologic features that might contribute the quarry brink. Figure 22 is a trans­ to large falls of rock or collapse of verse profile of the structure in a a quarry highwa11 is usually limited to selected quarry , which illustrates the quarry exposures. Figure 15 illustrates usually complex relation of cleavage, the collapse of a quarry highwa11 that bedding, and faults. The mapping of bed­ was attributed to an intersection of at ding slip faults, cause of most large least two planar geologic discontinui­ rock falls, was a high priority but was I ties, a weathered bedding slip fault and somewhat limited by the virtual inac­ a transverse joint. The bedding slip cessibility of the quarry walls and the fault is an easily recognized feature, lack of outcrop beyond the quarry brink. but the transverse joint is obscured by Nonetheless, most major bedding slip weathering and mud, and other factors are f-a-u.-lts- -we-r.e-Wea-ther.ed_ a_Lshallow depths suspected of contributing to the highwa11 and, therefore, could be identified as failure. loci of potential highwa11 instability. Similar collapses, resulting in fatali­ Figure 11 shows the weathered character ties, have occurred in two other quarries of one of the more obvious faults, al­ where a highwa11 failure was attributed though some faults showed virtually no to geologic structures that weakened the evidence of weathering and could be de­ integrity of the wall; such failures in­ tected (with difficulty) only because dicate the need to recognize, map, and of the presence of gash veins, drag, or fully assess the effects of geologic dis­ slickensides. continuities on highwa11 stability. This brief discussion of geologic map­ During this investigation, geologic ping demonstrates that the most common mapping of the trends and character of cause of highwa11 failure in the Pen major structures in several selected Argyl district, the bedding slip fault, quarries was conducted, in an attempt to is a mappable feature, especially when determine the practicality of locating weathered. The bedding slip fault there­ potentially unstable conditions on the fore provides a target for ground control quarry walls. These sites, if success­ measures, surveillance, or warning sys­ fully identified, could receive special tems based on the detection of incipient surveillance, instrument monitoring, or movement as described in the section remedial measures, in order to provide "Strain Gauge Instrumentation." While advance warning or to prevent the occur­ quarries are usually aware of the hazards rence of major falls of rock. However, of bedding slip faults during develop­ falls of small rock slabs or fragments ment, once these structures are pene­ that gradually become loosened from the trated and exposed in the upper inacces­ quarry walls require a regular regimen of sible portions of a quarry (fig. 23), scaling and are outside the scope of this their presence is easily forgotten; then report. 23

/

K 47"

LEGEND o 4--r-U Str,ke and d,p of reve rs. foull u "57" -p- Strike and di p of overturned KOr64. 40· 60. beds

~ STrIke and dip of clea vage -- - Municipality limits

°IL -'--, _ '--, '-,-'-----.JI,~OO Scale, "

FIGURE 21. - Layout of some active quarries in the Pen Argyl slate district show trend of faulting.

r------~~------_r------_,

LEGEND .:-::.~'=-~ Cleavage ~ Direction of faulting Scale, It -;::::= Bedding lines W W Calcite-filled gash fractures --- Hinge line

FIGURE 22 . - Profile of quarry highwallshowing complex geologic structures. 24

was selected for testing the ability of shallow resistivity equipment to detect Glacia l over bu rden bedding plane faults (fig. 24). Three target faults exposed in a quarry's northeast wall (fig. 22) were projected along strike into the study area. The equipment used consisted of a resistivity Quarry hig hwall meter with a power output into the earth of 24 W in short bursts, to attain a depth penetration of 100 ft or less. The Wenner configuration [Wenner (47), Tel­ ford (48)], in which four electrodes are arranged in a line at equal distances apart (designated as the A-space), was used for all sounding and traverses in this study.

Resistivity Soundings

Resistivity soundings were conducted to determine an appropriate depth for run­ ning the traverse line. Glacial deposits of unsorted sand, silt, and gravel with some associated boulders seldom exceed 15 ft in thickness in the Pen Argyl area but ca!1 vary consideral:!!Y.. Th~refor~l it was 20 necessary to conduct depth soundings to 1 Sc ale, fl determine an appropriate electrode spac­ ing for traversing. An electrode spac­ ELEVATION ing too large can mask the presence of FIGURE 23. - Sketch showing bedding sl i p fau lt faults, while an electrode spacing too near bri nk of quarry. small may not penetrate the glacial cover and reach the target depth. some method for monitoring the stability The empirical cumulative resistivity of the faulted area is essential. plot [Moore (49-50), Mooney (51)] and actual value plot-rTelford (48)-,- Mooney RESISTIVITY (51)] using both logarithmic--and linear axes were used for interpretation dur­ The use of resistivity traverse me~­ ing this study. These two methods, while surements to detect fault traces has been not the most widely accepted for accuracy tested and found successful in various among experts, are the easiest interpre­ geological settings [Stahl (42-44), Hub­ tative plots . bert and Weller (45), Lee an~ Hemberger The cumul ative and actual value plots (46).] In Pennsylvania slate quarries , can give adequate depth determinations weathered bedding plane faults are a ma­ without excessive interpretative time jor cause of catastrophic highwal l fail ­ when the user has a good understanding o~ ure; therefore , the use of resistivity the Local geology [Telford (48), Mooney traverses to detect concealed faults was (51)] . Examination of published geologic explored in that geologi c setting. The i nformation on the Pen Argyl area and di­ ability to detect concealed faul t s with rect observat ion establi shed the p res e n ce resistivity could promo te s a f er future of glacial cover, and dis cussion s with quarry developme nt. the quarry oper ator and examination of A study site with minimal s u rfa ce dis­ existing quarrie s indicate d a decreas­ turbance between two existing quarrie s i ngly weathered zone of s l ate e x tending 25

Slate dumps

Quarr y if . '.. ,:,'. ~ \:. . ... :

:,.) , 1::;";" LEGEND

Slate dumps ' ~ 56 D Strike and dip of reverse fault

~ Resistivity traverse ~2D Strike and dip of overturned beds o 400 I 12DI Strike and dip of cleavage I I OA Diamond drill hole Scale, ft a Anomaly FIGURE 24 . - Location of resistivity traverse between two active quarries. from the glacial mix and slate contact to hetween weathered and unweathered slate. a thickness of 20 to 30 ft in most areas. The three curves may be emphasizing dif­ Figure 25 shows two sounding curves ferent levels of weathering within the from the study area, with both cumulative gradually changing slate. and actual value plots of the data along Since the target faults should be de­ linear axes. Figure 26 presents the ac­ tectable within the weathered slate zone, tual value data plotted equally spaced on a clear determination of its thickness double log scales. The soundings were was unnecessary. Probing beneath the only of limited success in determining glacial cover was the only required accurately the depth of weathered bed­ certainty. With soundings indicating a rock; however, reasonably accurate esti­ maximum 10 ft of glacial cover, an elec­ mations of depth of glacial cover were trode spacing (A-spacing) of two times obtained. The cumulative curve in figure the target depth, 20 ft, was chosen for 25 breaks at approximately 10 ft and 29 traversing. ft. The actual value curve in figure 25 breaks at approximately 8 ft and 51 f t . Resistivity Traverses The double log plot of actual v alues in figure 26 breaks at approximatel y 8 f t After t he s oundings were completed, a and 45 ft. Correlation among the three traverse line through the study area was curves for depth of glacial cover (indi­ laid out. The three projected faults cated by the f irs t break) is fairly good~ fell within the traverse l ine (fig. 24). 8, 8, and 10 ft . However , a wide varia­ The traverse was begun f rom the north and tion exists in the second bre ak: 29 , 45, r an S. 40° E., parallel to the road , us­ and 51 ft . This varia t ion among the ing a 20-ft A-space and 10-ft steps. The three me thods of plotting may indicate r esults of the traverse measurements are the lack of a well-defined interface shown iil figure 27. 26

Four anomalous peaks or depressions are of hole A. The calcite spar associated evident at points a, b, c, and d. When with the anomaly a fault projection can placed on profile (fig. 28), the fault be seen in figure 22, a sketch of the projections-traverse line intersections quarry's northeast wall. and anomaly locations indicate a strong Faults indicated by resistivity anom­ correlation. The fourth anomaly point, alies at points band c are supported by d, falls outside the visible quarry corehole E. In corehole E, anomaly c is boundry in figure 28 but also indicates a indicated by a sheared, chloritized zone buried fault trace. separating highly ribboned, siliceous Structure in the traverse area and pos­ slate from normal slate. Presence of a sible fault zones indicated by corehole fault at anomaly b is supported by the data also correlate with the resistivity presence of spar in hole E approximately anomaly locations. Corehole A appears 160 ft below the surface. A projection slightly misplaced, causing an absence of spar occurrence to the surface along of evidence for fault presence at anomaly bedding intersects both the resistivity b. However, evidence that the bottom of anomaly point b and the fault projection corehole A approaches the approximate on the map view. location of the anomaly a fault is indi­ All the available evidence in this cated by the presence of massive spar and study indicates that resistivity is an disruption of the slate near the bottom effective method of detecting bedding faults in the Pennsylvania slate belt. The use of resistivity traversing to

E detect faults yielded more promising re­ Q 5,0 00 sults with less difficulty in interpreta­ ~ tion than did sounding curve interpreta­ f­ :> f= 4,000 tions in determining layering. Strong ~ If) correlation among fault projections , w oc 151 break corefiOle aata, and resistivity anomaly ~ 3,OOC w lo~ations supporc this concl usion. How­ oc r:t ever, interpretation of both the resis­ ~ 2,O OC tivity and corehole data was greatly w > 2d b,,,a:' enhanced Dy the outcrop information available in adjacent quarriesu In areas ~:J 1,000 ::i of more restricted outcrop information, :J U resistivity interpretation may be more 0'~~~1~0 --~~2~0--~~3~0~~~40~~~~--~~60 difficult, but when pronounced anomalies A- SPAC ING, II are found, similar to those in figure 27,

800 .---.--'-'-'-'-r~------,,---'--'-'-' E E q 500 CI

~ 2d break f- ~ 600 ;;: l- ~ S; 17i 400 i= w (f) a:: en 40 f- u..; x Z 1st break w 0:: a:: (t300 a.. «

2000~~~~~~~~d~o~--~~~~0--~~4 0~~~5~0~~-6~O 4 6 8 10 20 40 60 A-SPACING, ft A-SPACING, i't FIGURE 25. - Resistivity sounding curve, cumula­ FIGURE 26 . - Res ist ivity sounding curve, double tive plot (top) and linear p lot (bottom). log plot . 800r---~--~---.----~--~---'---''-

x I ~x E 600 ~

~ t:: ~ I­ ~ ffl 400 Anomaly c Anomaly d a:: I­ z w a:: rt 0.. « 200 Anomaly a Anomaly b

o 200 300 400 500 800 DISTANCE TRAVERSED, ff

FIGURE 27. - Apparent resistivity traverse plot.

LEGEND A Diamond drill hole a Anomaly Approximate position of fault .. .. Direction of faulting ( trace and resistivity anomaly bl 1°1 1 Surface lei -~~~---. \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ distorted beds \ \ o 50 \ \ \ \ \ I \ \ \ \ \ \ \ \ \ \ \ Scale, ft \ \ \ I I \ ,

FIGURE 28. - Profile showing relation of resistivity anomalies to buried fault traces. 28

fault presence should be suspected and spar, but some problems were encountered investigated further. in trying to orientate the core if it turned in pulling. Figure 28 illustrates CORE DRILLING the probable structure in the vicinity of the cored drill holes. The bedding Core drilling is virtually the only plane faults located in drill hole B oc­ practical method of directly obtaining cur nearly along strike of faults exposed subsurface information and samples of in a nearby quarry and probably would be bedrock for study purposes. According to troublesome during expansion of the ex­ Behre (14), "Core drilling is the only isting quarry. method ~y which the slate operator, no Behre (14) discusses the manner in matter how experienced, may assure himr which geologic work, and in particular self of the favorableness of strati­ core drilling, can be used to control the graphic and structural conditions before opening and operation of a slate quarry. opening." In the Slatington district, This work has almost always been con­ about 22 miles southwest of Pen Argyl, cerned only with identifying and follow­ where close folding makes the subsurface ing high-quality beds and avoiding heav­ structure very irregular, core d-rilling ily ribboned rock. In the future, core is more generally practiced than else­ drilling should also be used to assess where in the Pennsylvania slate district. the competence of wall rock in a proposed Drilling also gives an indication of the quarry site, as a more scientific ap­ thickness of unconsolidated material and proach to ground control is essential in weathered and stained bedrock that must sound quarry valuation. Roberston (52) be removed before commercial-quality also stresses the importance of adequate slate can be recovered. The sampling core drilling to define the geologic pa­ area of drill core is very small, about rameters necessary for quarry planning 3 in2 , and critical geologic structures and equi-pment selection. nearby can be entirely missed in drill­ ing. Sometimes, minor features observed STRESS MEASUREMENTS in the core can be extrapolated to extend coverage by inference, or the results of The magnitude and direction of the sec­ resistivity traverses and soundings can ondary principal stresses present in the be used both to site drill holes in the highwalls and floor of a quarry in the optimum location and to provide indirect study site were determined using the information on the subsurface between Bureau's borehole deformation gauge drill holes. [Hooker, Aggson, and Bickel (53), Merrill The core drilling equipment used in and Peterson (54), Merrill (55)]. The this study consisted of a skid-mounted quarry was adjacent to abandoned workings rig capable of drilling both vertical and approximately 540 ft below the surface. angle holes. The 2-in-diam core was ade­ The stress values measured at various quate in size for detecting geologic times in the quarry were consistent but structures and slate quality. The only were of a much lower magnitude horizon­ drilling problem encountered was due to tally than might be expected in this re­ hole collapse while an attempt was being gion, based on current literature [Sbar made to set casing through unconsolidated and Sykes (37), Hooker (56)]. glacial material and into bedrock. Figure 29-shows the measurements ob­ For the purposes of the current study , tained at depths of about 24 in beneath core drilling information was used tc lo­ exposed quarry surfaces. Theoretical cate weathered fractures and bedding values calculated for expected vertical plane faults, whicl:, severely weaken and horizonta~ stress in gravity-loaded quarry highwalls , and to confirm the ef­ rock strata are 550 and 103 psi, re­ fectiveness of the surface resistivity spectively. Stress values determined by surveys. The bedding plane faults are overcoring measurements, as shown in fig­ easily detected in drill core when they ure 29, are all well above the theoreti­ are heavily weathered or filled with cal values: an average maximum 1,203 psi 29

LEGEND Max and min secondary principal stresses

UI Min econdary " - Highwall u2 principal stress u3 Ecr: Components (indicate gauge orientation) . e<' 1,627 psi 0«' \\ l..'

"5~1 psi I 1/ u2 b- I,I~I .I ) Z PSI u ?f(3 802 psi 417PSi 394 psi u2 ul lUI u 3 u3 __~733PSi 138 psi ,, : UI ~ A 536 psi o 2 4

1 1 1 Scale, ft

FIGURE 29. - Diagram of quarry stresses. and minimum 1,008 psi, in the barrier gravity. The highwall maximum stress in highwall; an average maximum 1,627 psi a nearly vertical direction is generally and minimum 619 psi, in the quarry high­ higher than the horizontal maximum stress wall; and an average maximum 876 psi and in the floor. minimum 362 psi, in the floor. All these The st.ress measurements are affected by measured stress values are compressive. their proximity to the corner of the While there are no comparable values in quarry. As the measurement points move the literature for vertical stress mea­ outward from the corner in the floor and surements in this vicinity, horizontal in the barrier highwall, the stresses stresses have been measured. Sbar and tend to decrease in magnitude and change Sykes (37) and Hooker and Johnson (56-57) slightly in orientation. The compressive note generally high horizontal stresses strength of the slate, 17,200 psi par­ in this area. The measured average maxi­ allel to cleavage and 24,700 psi per­ mum horizontal stress in this quarry is pendicular to cleavage, indicates that not excessive (876 psi), but it does in­ in the absence of any weakening geo­ dicate pressure influence other than logic structures (i.e., faults, joints, 30 fractures) the slate should not be ap­ was quite satisfactory. However, the on­ proaching stress-induced failure. set of fall and winter months presented many problems. High winds on exposed STRAIN GAUGE INSTRUMENTATION quarry faces continually flexed the coax­ ial cables, creating breakages and af­ An electromagnetic strain gauge system fecting the small electric current flow was tested as an early warning device from sensors through the cable and con­ for hazardous highwall conditions (i.e., nections to the recorder, often creating joints, fractures, faults) by detecting spurious fluctuations in the output. Ice incipient movement. The strain gauge accumulations in winter often overcame system is capable of detecting small the strength of both the cable and con­ changes in spacing between adjacent sen­ nections, resulting in breakages and the sor pairs mounted securely on opposite loss of much important winter data. sides of a suspected plane of weakness. The use of strain gauges to monitor Strain sensor pairs were installed in highwall discontinuities that might pose three active quarries (shown in figure a hazard to workers should not be dis­ 21) to evaluate their monitoring capabil­ counted because of the problems encoun­ ities under varying geologic and weather tered during this study. Improved in­ exposure conditions. stallations and active involvement by The first pair was installed on either actual quarry operators in maintaining a side of a fault diagonally cutting the monitoring system and collecting data may top edge of a free-standing highwall. A alleviate some of the above difficulties. second pair was installed on a highwall The use of conduit to protect instrument and a subparallel slab separated from the w~r~ng and frequent inspection of the highwall by a joint. The third pair was installation by operating personnel installed above and below a fracture that should contribute to more trouble-free created on overhanging wedge on a bench monitoring. above the quarry floor. All of the sen­ During periods of accurate data collec­ sors were secured to the slate surfaces tion, no discernible rock movement was with specially designed acrylic brackets detected. Two of the sensors mounted in to maintain the most sensitive parallel full southern exposures exhibited diurnal sensor position. The gauges were wired fluctuations during continuous recording. with coaxial cable to easily accessible The gauge spacings fluctuated ±0.008 in areas for e ither continuous moni toring from the basel ine in 24-h periods, giv­ with strip chart recorders or occasional ing an indication of gauge sensitivity. direct manual readings. Changes on the order of 0.125 to 0.25 in Experience with the strain gauge moni­ were determined to indicate real move­ toring system quickly showed that t h e ment. No actual occurrence of this mag­ effects of weather on installations and nitude was detected at any of the three wiring would be difficult to control or stations. prevent. Operation during summer month s

ROCK BOLTING TESTING

The practice of using bolts to secure wedge-like expansion shell attachment on rock that is weak or may become dislodged one end (figs. 31-32) or a resin grout. is commonplace in underground mines and The mechanical bolt is placed in tension, some open pits but virtually unknown i n as are some versions of the resin­ slate quarry ing. Instead , l oose slabs i n anchored bolt , thus exerting a positive q uarry highwalls t h at cannot be scal e d stabilizi ng force against the rock s ur­ are supported by stee l pi n s i nserted i n to face, whereas the p i n is merel y a pass ive downward-sl oping holes drilled in the form o f support largely resisting the highwall (fig. 30). s h earing f orce of gravity in a highwall. In contr ast to pi nni ng, a rockbolt Because of tne extremely limited exper­ is anchored , either by a mechanical ience with rock boltIng in the slate 31

belt, several tests were conducted in a Pen Argyl quarry to determine the anchor­ age capacity and tension bleedoff of both mechanical and resin-anchored rockbolts. The results demonstrated that in slate rockbolts of both types have a high an­ chorage capacity. All bolts were tested in a quarry high­ 'Quarry highwall wall where cleavage is nearly horizontal and the bedding steeply dipping. The bolts were installed horizontally and perpendicular to bedding. After the uolls were installed, tension was applied using a pulling collar and hydraulic ram. Tension \-JaS measured both with an elec­ ,2-in-diam tronic transducer and compatible indi­ steel pin cator and with a calibrated pressure gauge on the hydraulic ram. Strain or bolthead displacement was measured di­ rectly with a mechanical dial strain gauge. The setup for these procedures is illustrated in figures 33 and 34. The mechanical bolts were 48 in long, 5/8 in. in diameter, with a bail-type four-leaf expansion shell, and were in­ Not to scale stalled in a I-3/8-in-diameter hole. The resin- anchored bolts consisted of rebar, 48 in long, 3/4 in. in diameter, anchored with 24 in of resin, and were installed ELEVATION in a I-in-diameter hole. The head of the FIGURE 30. - Common method of securing fractured resin-anchored bolt consisted of a shear rock on quarry highwall with steel pins. pin nut by which the rebar could be

Rockbolt ------

------=- CleaVage ) - - -

L ____.... ------:;,,..------______+ _ 76°

Quarry highwall --C ------Bedding

LEGEND Strike and dip of overturned bedding o 5 I Strike and dip of Scale, ft cleavage

PLAN VIEW

FIGURE 31. - Use of anchored mechanicai bolts in tension to secure iointed rock. 32

Sawed surface of quarry highwall

o 5

Scale, ft

ELEVATION FIGURE 32. - Use of anchored me chan ica l bo lts in iension to s ec ure fractu red rock in quarry highwa lf . FIGURE 33. - Setup for rockbolt pull t est. rotated to mi x the r e sin; a f t e r hardening Te ns ion bleedoff on f our mecha n i c al of the r esin, the nut could be rotated bolts is shown in figure 36, which indi­ fur ther to s he a r the pin, tighte n t he cates that losses of 700 to 2 , 000 lb t en­ nut , and put the rebar in t ension. sion occurred within 24 h after installa­ For the pul l test, after e ach bolt was tion and additional losses of 300 to 900 pretensioned to a 1 , 000-lb load wi th a lb occurred over the following 36 days . hand torque wrench, the load on the hy­ About 6 weeks later, hole 3 showed a n draulic ram was increased i n 1,000-lb i n­ unexplained increase in tension of about crements, while the bolthead displ acement 1,200 lb. was recorded at each increment until the Four load-displacement curves f or anchorage capacity was exceeded. resin-anchored bolts are illustrated in Four representat ive load-displacement figure 37. Three of t he bolts showed curves for the mechani cal bolts tested only 0. 115 i n displacement or les s , even are i llustrat e d in figure 35; the d i s­ under 20 , 000 lb load . The fourth bolt tinctive porti ons of a curve are label ed (No. 2) began to fail at s lightly over on the top- left panel. An chorage ca­ 20, 000 lb. Probabl y a third o f the total pacity ranged from 14, 500 t o 16,600 lb. displacement is accounted f or by bolt Di splacement ranged f rom 0 . 0 19 to 0 .025 stretch and the remainder by act ual an­ in pe r 1 ,000-lb loa d . chorage d ispl a cement.

SUMMARY AND CONCLUSIONS

An investigation WhS conducted of the Penns ylvania to determine the nature of slate quarrying operations in eastern ground control hazards and geotechni cal 33

FIGURE 34. - Measuring rockbolt tension bleedoff. methods by which these hazards might be ~. Collapse of highwall, or portion reduced. The study was somewhat handi­ thereof, weakened by bedding plane faults capped by the inaccessibility of quarry and fractures. The presence of spar, wa­ highwalls, the absence of outcrop outside ter, and weathered material in the fault the quarry walls, and the severity of planes greatly increases the probability winter weather, which disrupted instru­ of highwall failure. mentation. Nonetheless, the geologic 2. Falls of large ice accumulations character of the quarry operations was from highwalls, especially during the examined, and specific ground control spring thaw. hazards were identified. Several geo­ Highwall collapse is the foremost cause technical methods were employed to detect of injuries and fatalities in quarries. the se hazards, and rockbolts were as­ Ma jor h ighwall failures have occurred in­ sessed as to their anchorage capacity in frequently, but without warning or any slate. obvious antecedent events. They have oc­ The principal ground control hazards curred, however, in proximity to known or that were identified as a result of this suspected geologic discontinuities, and study and from the history of accidents have sometimes been facilitated by a that have occurred over many years of highwall overhang or l ack of artificial operations are as follows: restraint. I t is essential to identify 34

18 20 A B A Anchorage capac ITy B 16 - 14 I Anchor load = 12 T Ckl OOd 10 Resrn 2

Slack zone tronsfer of lood 10 anchor

Ancho rage d isplacemenT = Anchoroge dlsplacemenr = 0.019 in pe r 10 3 Ib 0.023 In pcr 103 Ib

0 E ~ Q 0" 0"

Res in 3

Anchorage di splacemen t Anchorage displacement = 0. 022 In per 10 3 Ib 0.0 25 in per 103 Ib

o 0.1 0.2 0.3 0.4 0.5 0 0.1 0.2 0.3 0.4 0.5

HE AD DISPLACE MENT, In

0.2 0 0 .1 0.2 FIGURE 35. - Representative load-di splacement o 0.1 HEAD DI SPL ACEMENT, In curves for mechanical bolts. FIGURE 37. - Representative load-displacement curves for full-column resin-anchored bolts, using pu II te sts.

------events that could warn of impending t-'------'-=-=------failure. The in situ stress field in a quarry could be a factor in the instability. Lateral rock pressures are manifest in the quarry floors as slot closure and the

40~LLLLLL~~~~~UL~2~O LLLLLL~~3~0~~~~\,~ULLL~60· so-called pressure breaks during extrac­ TIME, days tion of slate. Although stress measure­ ments were conducted in two quarries, a FIGURE 36. - Tensi on bleedoff graph for four ful l assessment of their significance mechan ical bolts. must await additional measurements and the development of an appropriate finite geologic discontinuities both within and element analysis, a procedure by which beyond existing highwalls ; this can be stresses are calculated for the individ­ accomplished through a geotechnical i n­ ual elements of a continuous medium. vestigation utilizing mapping p roc e­ The season for ice falls is limited dures, resistivity surveys , test drill­ and can be anticipated , and some pre­ ing, and otheL techniques not covered i n cautions can be exercised. However, in­ this report . Once hazardous conditions j uries from falls of ice occur almost are identified. instrumentation such a s annual ly , and a fatality occurred as re­ strai n gauges to measure the movement o f ce ntly as 1984. Wa ter diver sion is bas ic loose rock masses or tension-load indi­ to the prevention of ice accumulation and cators on rockbolts can be useful in de­ can be accomplished only through deter­ tecting incipient movement and antecedent mining the courses that the water follows 35 in reaching the brink and upper reaches in the level of geotechnology could lead of the quarry highwall. Once these to a substantial reduction in potential courses are established through a hydro­ ground control hazards. New quarries logic study. the water inflow can be re­ especially should be planned to take ad­ duced at least partially through the use vantage of improved methods of site in­ of diversion ditches and drains, dewater­ vestigation, quarry design, and hazard ing wells, surface barriers, or grouting. detection. Modern slate extraction de­ Unless water diversion is accomplished, vices such as rock chain saws, jet cut­ expedient ice control measures will be ting, or hydraulic splitting, to name a largely ineffective in preventing ice few, should lead to reduced hazards, re­ falls < duced waste product, and an increase in Slate quarrying operations in eastern productivity. This will require support Pennsylvania are unique , mostly old, and from outside the industry gnd a more con­ tradition bound, and a modest increase certed effort from the industry. REFERENCES 1. Bowles, o. Slate Mining in Maine. Harrisburg, PA, 4th Ser., Bull. M-9, BuMines RI 2181, 1920, 5 pp. 1927, 400 pp. 2. The Technology of Slate. 14. Behre, C. H. Geologic Factors in BuMines B 218, 1922, 132 pp. the Development of the Eastern Pennsyl­ 3. Drilling and Broaching in vania Slate Belt. Trans. AlME, v. 76, Slate Quarries. BuMines RI 2532, 1923, 1928, pp. 407-408. 6 pp. 15. Stickler, C. W., W. F. Mullen, and 4. The Wire Saw in Slate A. W. Bitner. Industrial Studies of Quarrying; Preliminary Report. BuMines Pennsylvania Slate Production. Sch. RI2820, 1927, 10pp. Miner. Ind., PA State Univ., State Col­ 5. The lUre Saw in Slate lege , PA , Bull. 58, 1951, 43 pp. Quarrying; Second Supplementary Report. 16. Mullen, W. F., and C. W. Stickler. BuMines RI 2918, 1929, 8 pp. Operational Studies in the Pennsylvania 6. A System of Accounts for Slate Industry. Min. Eng. (N.Y.), v. 3, the Slate Industry. BuMines RI 2971, No. 12, 1951, pp. 1097-ll00. 1929, 33 pp. ;.7. Hoyt, F. D. What's New in the 7. Slate. BuMines IC 7719, Pennsylvania Slate Industry. ColI. 1955, 12 pp. Miner. Ind., PA State Univ., University 8. Thoenen, J. R. The Wire Saw in Park, PA, Bull. 66, (1956), 49 pp. Slate ~Jarrying; Supplementary Report. 18. Miller, B. L., D. M. Fraser, and BuMines RI 2851, 1928, 8 pp. R. L. Miller. Northampton County Penn­ 9. Watson, W. I., E. Ohlsson, C. E. sylvania, County Report 48. PA Geol. Shorey, R. J. Miller, and A. J. Whittier. Sur v • , Earrisburg, PA, 4th Ser., 1939, Study of the Slate Mining Industry of ~eprinted 1973 , 496 pp. Vermont-New York (contract J0199075, Ar­ 19. Davis, R. E., A. A. Drake, Jr., thur D. Little, Inc . ) . BuMines OFR 125- a nd J . B. Epstein. Geologic Map of the 80, 1980, 186 pp.; NTIS PB 81-178936. Bangor Quadrangle , Pennsyl vania-Ne~v Jer­ 10. Butt, J., and I. Donnachie. In­ sey. J.S. Geol. Surv. Map GQ-665, 1967. dustrial Archaeology in the Briti sh 20. Epstein, J . B. Geologic Map of Isles. Harper & Row, 1979, 307 pp. the Stroudsburg Quadrangle , Pennsylvania­ ll. Earney, F. C. The Slate Industry New J ersey. U. S . Geol. Surv. Map GQ- of Western Vermont. J. Geogr., v. 62, 1047 , 1973. No.7, 1963, pp. 200-310. 21 . Map Showing Slate Quarries 12. Robertson, J. L. I s Underground and Dumps in t he Stroudsburg Quadrangle, Mining in Your Future Expansion Plans? Pennsylvania-New Jersey, With a Discus­ Rock Prod. , v , 36, No . 6 , 1983 , pp . 29- sion of Their Env i ronmental Significance. 34. U.S. Geol. Surv. , Misc. Field Stud. Map 13. Behre, C. H. Slate in Northampton MF- 578- A, 1974, 2 sheets. County, Pennsylvania. PA Geol. Surv., 22. Parsons Brothers Slate Co. Slate 34. Adams, J. Stress-Relief Buckles Roofs. Natl. Slate Assoc., New York, 3d in the McFarland Quarry, Ottawa. Can. ed., 1953. 84 pp.; Am. Inst. Architects J • Ea r t h Sc 1. , v • 1 9 , No • 10 , 1 982 , File No. 12-D. pp. 1883-1887. 23. Mackenzie, J. D. Development of 35. Coates, D. F. Some Cases of Re­ Substitutes for Asbestos (contract sidual Stress Effects in Engineering J0199056, Univ. CA). BuMines OFR 124-82, Work. Sec. in State of Stress in the 1981, 26 pp.; NTIS PB 82-252016. Earth's Crust, ed. by W. R. Judd. Else­ 24. Chemical & Engineering News. Al­ vier, 1964, pp. 679-688. kali Resistance of Slate-Limestone Fibers 36. Smith, A. C. In Situ Rock Probed. V. 61, No.3, 1983, p. 47. Stresses and Small Anticlinal Features in 25. Meade, L. P. Several Industrial Eastern North America. M.S. Thesis, Cor­ Minerals Post Production Gains; Some nell Univ., Ithaca, NY, 1977, 136 pp. Don't. Dimension Stone. Min. Eng. (Lit­ 37. Sbar, M. L., and L. R. Sykes. tleton, CO), v. 34, No.5, 1982, p. 559. Contemporary Compressive Stress and Seis­ 26. Dickson, T. Slate--Rebuilding the micity in Eastern North America: An Ex­ Markets. Ind. Miner. (London), No. 145, ample of Intra-Plate Tectonics. Geol. 1979, pp. 45-53. Soc. America Bull. 84, 1973, pp. 1061-- 27. McBride, E. F. Flysch and Associ­ 1882. ated Beds of the Martinsburg Formation 38. Ferguson, H. F. Valley Stress Re­ (Ordovician), Central Appalachians. J. lease in the Allegheny Plateau. Eng. Sediment. Petrol., v. 32, No.1, 1962, Geol. (Sacramento), v.4, No.1, 1967, pp. 39-91. pp. 6~68; available from Assoc. Eng. 28. Drake, A. A., Jr., and J. B. Ep­ Geol., Brentwood, TN. stein. The Martinsburg Formation (Middle 39. Geologic Observations and and Upper Ordovician) in the Delaware Geotechnical Effects of Valley Stress Re­ Valley, Pa.-N.J. U.S. Geol. Surv. Bull. lief in the Allegheny plateau (Paper 1244-H, 1967, 16 pp. pres. at Am. Soc. Civil Eng. (ASCE), Wa­ 29. Behre, C. H., Jr. Bedding-Plane ter Resources Engineering Meeting, Los Faul ts and Their Economic Importance .. Angeles, CA, June 22-27, 1974). ASCE Ch. in Mining Geology, ed. by W. C. Lacy. preprint, 1974, 31 pp. Hutchinson Ross Publ. Co., v. 60, 1983, 40. Anderson, J. G. C., J. Arthur, and pp. 241-258. D. B. Powell. The Engineering Geology of 30. Yegulalp, T. M., and S. H. Bosh­ the Dinorwic Underground Complex and Its kov. Improved Ground Control in the Approach Tunnels. Paper in Proceedings, Stone, Sand, and Gravel Industries (con­ Conference on Rock Engineering (CORE 77), tract JOI00027, H. H. Aerospace Design ed. by P. B. Attewell. Univ. Newcastle­ Co., Inc.) BuMines OFR 60-83, 1981, 296 Upon-Tyne, England, 1978, pp. 491-510. pp.; NTIS PB 83-183111. 41. U.S. Code of Federal Regulations. 31. U.S. Mine Safety and Health Admin­ Title 3Q--Mineral Resources; Chapter I-­ istration. Injury Experience in Stone Mine Safety and Health Administration, Mining, 1982. Dep. Labor, IR 1146, 1984, Department of Labor; Subchapter N--Metal 343 pp. and Nonmetal Mine Safety and Health; Part 32. Brown, E. T., L. R. Richards, and 55--Safety and Health Standards-Metal and M. V. Barr. Shear Strength Characteris­ Nonmetal Open Pit Mines; July 1, 1982, tics of the Delabole Slates. Paper in pp. 309-340. Proceedings, Conference on Rock Engineer­ 42. Stahl, R. L. Detection and Delin­ ing (CORE 77), ed. by P. B. Attewell. eation of Faults by Surface Resistivity Univ. Newcastle-Upon-Tyne, England, 1978, Measurements--Gas Hills Region, Fremont pp. 33-51. and Natrona Counties, Wyo. BuMines RI 33. Paterson, T.~ and L. J. Arthur. 7824. 1973, 28 pp. Tunnel Portals at Dinorwic, North Wales. 43. Detection and Delineation Paper in "Tunnelling 79" Conference. of Faults by Surface Resistivity Me a­ Inst. Min. and Metall., London, 1979, surements--Schwartzwalder Mine, Jefferson pp. 3-14. 31

County. Colo. BuMines RI 7975 , 1974, 51 . Mooney, H. M. Depth Determina­ 27 pp. tions by Electrical Resistivity. Min. 44. Stahl, R. L. Detection and Delin­ Eng. (N • Y • ), v. 6, No.9, 1954, pp. 915- eation of Faults by Surface Resistivity 918. Measurements--Conda Mine, Caribou County, 52. Robertson, J. L. Site Geology Key Idaho. BuMines RI 8072, 1975, 20 pp. to Quarry Plan. Rock Prod., v. 86, No. 45. Hubbert, M. K., and J. M. Weller. 8, 1983, pp. 33-35. Location of Faults in Hardin County, Il­ 53. Hooker, V. E., J. R. Aggson, and linois, by the Earth-Resistivity Method. D. L. Bickel. Improvements in the Three­ Trans. AIME, v. 110, 1934, pp. 40- 48. Component Borehole Deformation Gage and 46. Lee, F. W., and S. J. Hemberger. Overcoring Techniques. BuMines RI 7894, A Study of Fault Determinations by Geo­ 1974, 29 pp. physical Methods in the Fluorspar Areas 54. Merrill, R. H., and J. R. Peter-­ of Western Kentucky. BuMines RI 3889 , son. Deformation of a Borehole in Rock . 1946, 27 pp. BuMines RI 5881, 1961, 32 pp. 47. Wenner, F. A. Method of Measuring 55. Merrill, R. H. Three-Component Earth Resistivity. U.S. Bur. Stand. Borehole Deformation Gage for Determining Bull., v. 12, 1915, 632 pp. the Stress in Rock. BuMines RI 7015, 48. Telford, W. M., L. 1'. Geldart, 1967, 38 pp. R. E. Sheriff, and D. A. Keys. Applied 56. Hooker, V. E. Stress Fields--What Geophysics. Cambridge Univ. Press, 1976, Is Known About Them. Paper in Ground 860 pp. Control Aspects of Coal Mine Design. 49. Moore, R. W. An Empirical Method Proceedings: Bureau of Mines Technology of Interpretation of Earth-Resistivity Transfer Seminar; Lexington, Ky.; March Measurements. AIME Tech. Publ. 1743, 6, 1973. BuMines IC 8630, 1973, pp. 22- 1944, 18 pp. 27. 50. Earth-Resistivity Tests 57. Hooker, V. E., and C. F. Johnson. Applied to Subsurface Reconnaissance Sur­ Near-Surface Horizontal Stresses Includ­ veys. ASTM Spec. Tech. Publ. 122, 1952, ing the Effects of Rock Anisotropy. Bu­ 228 pp. Mines RI 7224~ 1969, 29 pp. 38

APPENDIX.--GLOSSARY OF QUARRY WORKERS ' TERMS

Broach.--Break and remove the narrow predominantly thick beds of pure, soft rib of rock between closely spaced paral­ commercial slate, and commonly given ro­ lel drill holes. mantic names such as Acme, Phoenix, Dia­ Channeling (Drilling and Broaching).-­ mond, and Albion. Cutting a narrow channel or slot by Sculp.--Break slate along a cross grain drilling closely spaced parallel holes perpendicular to both cleavage and and then broaching the intervening rock. bedding. Eighter.--A block of slate sculped and Sink.--The lowermost level in a benched split to a thickness equal to eight roof­ quarry. A sump. ing slates each 3/16 inu in thickness. Slate.--A fine-grained metamorphic rock Grit.--A bed consisting of soft slate with a pronounced fissility along planes mixed with a high percentage of abrasive, independent of original bedding. fine-grained quartz sand. Soft Slate.--A smooth, even-textured, Hard Roll (Hard End).--A thin, hard si­ light-colored. thick- bedded slate con­ liceous bed of slate. sisting of very fine-grained sericite, Loose (Silver) Ribbon.--A very thin quartz, and chlorite, as opposed to bedding plane layer of calcite, usually hard slate, which is more siliceous and indicating movement along bedding. calcareous. Pressure Break.--An irregular fracture Spar.--Vein-filling material of calcite that develops in a bench of slate during and quartz. extraction. The fracture trends across Square.--Sufficient roofing slate to cleavage and bedding and is attributed to cover 100 ft2 with a 3-in overlap, and abutment pressure from an adjacent bench weighing about 650 lb. or quarry wall. Wedging.--Splitting of stone by driving Ribbon.--A dark-colored thin bed of wedges into planes of weakness. slate, usually high in carbonaceous W4.re- Saw- Me-thod.--A method of cutting material. stone by immersing a twisted, multi­ Rotten Ribbon.--A highly weathered bed­ strand, tensioned wire in a slurry of ding plane fault. abrasive material and then passing it Run.--A particular group or sequence over the stone. of---beds usually characterized by

INT.-BU.OF MINES,PGH . ,PA. 28192