International Journal of Coal Geology 75 (2008) 175–184

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International Journal of Coal Geology

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Texture, mineralogy, and rock strength in horizontal stress-related coalmine roof falls

S.E. PhillipsonMine Safety & Health Administration, Roof Control Division, Pittsburgh Safety & Health Technology Center, P.O. Box 18233, Pittsburgh, PA 15236, United States

E-mail address: phillipson.sandin@dol.gov.

0166-5162/$ – see front matter. Published by Elsevier Bdoi:10.1016/j.coal.2008.05.018

A B S T R A C T

A R T I C L E I N F O

Article history:

Geologic structures can repr Received 30 January 2008Received in revised form 29 May 2008Accepted 31 May 2008Available online 5 June 2008

Keywords:PetrographyHorizontal stressRoof fallCoal mine

esent planes of preferential weakness that, by dismembering the roof beam, maycontribute to the failure of roof spans. However, beam deflection and roof failure also occur in rocks where novisible geologic discontinuities are present. This suggests that roof failure may depend on rock strength,which in turn depends on intrinsic textural properties inherent to the rock. In this study, rock samples werecollected from horizontal stress-related roof fall material in coal mines for petrographic characterization andcompressive strength testing. Brittle, stress failure-prone rock types include thinly interlaminated siltstoneand shale, and black shale that had been lightly recrystallized. Samples exhibit a narrow range of densityvalues between approximately 2.5–3.0 g/cm3 but exhibit a wide range of unconfined compressive strengthvalues, between approximately 20–70 MPa. Results of laboratory observations suggest that for samples ofcoal mine immediate roof shale, compressive strength is not well correlated with density, grain size, suturedgrain boundaries, or quartz content. These results for shale are generally at odds with the results of similarstudies for sandstone. The great variability of strength, texture, and mineralogy documented in these samplesmay be an indication of their complexity and the need for specialized methodology in the study of shalestrength.

Published by Elsevier B.V.

1. Introduction

During the course of investigations by the Mine Safety and HealthAdministration's Roof Control Division (RCD), many roof falls wereencountered that did not appear to be controlled by obvious structuralgeologic discontinuities. Those falls exhibited characteristics ofhorizontal stress-induced failure, involving flexure or buckling of theroof beam without the presence of pre-existing structural geologicdiscontinuities to act as planes of preferential weakness. Horizontalstress is attributed to the transmission of forces through the crust bythe action of plate tectonics (Mark, 2001). Although the presence ofgeologic structural discontinuities can provide planes of preferentialweakness along which a roof span may fail, roof failure in the absenceof such structures may be influenced by some textural or miner-alogical property inherent to the rock.

Much work has been performed on determining the strengths ofvarious sedimentary rocks, and several engineering classificationsystems such as thewidely used RockMass Rating system (Bieniawski,1973, 1979) and newly developed Coal Mine Roof Rating (Molinda andMark, 1999; Mark, 1999; Mark and Molinda, 2005) have attempted torelate rock strength to mine roof stability. For purposes of rockclassification schemes, rock strength has been regarded as a mechan-ical property that can be quantified by such laboratory tests as the

.V.

triaxial or uniaxial compression tests, or the splitting tensile(Brazilian) strength test. Other index tests such as the SchmidtHammer rebound number or the point load test attempt to provide aproxy for uniaxial compressive strength (i.e. Rusnak and Mark, 2000).However, very little work has been performed to relate intrinsicmicro-textural properties, rock strength, and mine roof instability. Inthis study, the quartz and feldspar content, dominant grain size,degree of grain suturing, and grain shape are compared to values ofunconfined uniaxial compressive strength for coal mine immediateroof rocks that experienced failure by excess in situ horizontal stress.Samples of roof rocks and roof fall debris were collected from coalmines in the Appalachian and Illinois Basins. This study relatesquantitative and semi-quantitative petrographic descriptions of rockscollected from coal mine roof falls to rock strength, and assesseswhether microscopic rock textures may influence rock strength inhorizontal stress-related roof falls in underground coal mines.

2. Background

Although rock mechanics testing has become widespread as amethod to characterize the behavior of rocks in mining and civilengineering environments, the use of petrographic properties tocharacterize ground stability has found only limited applicability forcivil engineering projects, and is virtually absent in mining engineer-ing projects. The lack of petrographic information incorporated inmining engineering applications stands in contrast with the

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conclusions of several researchers, who indicate that a petrographiccharacterization is the essential core of a proper investigation into thesuitability of geological materials (Price, 1960; Fahy and Guccione,1979; Verhoef and Van DeWall, 1998; El Bied et al., 2002). Methods ofstability analysis of underground openings require input data on thegeomechanical behavior of rocks (Bieniawski, 1973; Fahy andGuccione, 1979; Ulusay et al., 1994; Molinda and Mark, 1999). Becausecore samples obtained from sedimentary strata are commonly of suchpoor quality that mechanical tests are not possible, mechanicalproperties may be estimated from simple index tests or from rockproperties that are known to correlate with the necessary mechanicalproperties. Petrologic properties, such as texture and mineralogy,influence rock strength and can be readily measured throughmicroscope inspection of thin sections (Fahy and Guccione, 1979). Inorder to obtain useful information from core samples unsuitable forstrength testing, some investigators utilized statistical techniques topredict a value of rock strength based on petrographic properties (Bell,1978; Fahy and Guccione, 1979; Shakoor and Bonelli, 1991; Richardsand Bell, 1995; Bell and Culshaw, 1998; Prikryl, 2001). However,techniques based on linear regression appear applicable to only anarrow range of properties from a specific location or rock type.

Rock strength is one of the most important parameters evaluatedin rock mechanics (Price, 1960; Rashed and Sediek, 1994; Prikryl,2001). The uniaxial compressive strength has been found to correlatewith mechanical properties such as point load index, Schmidthammer rebound number, and Los Angeles degradation abrasionloss (Kasim and Shakoor, 1996; Rusnak and Mark, 2000). Variation inrock strength is explained by a number of factors including grain size,grain shape, degree of grain interlocking, preferred orientation(fabric), quartz content, matrix content (generally regarded as silt orclay), mineral composition, density, porosity, texture, moisturecontent, and state of alteration (Hawkes and Mellor, 1970; Spry,1976; Dobereiner and De Freitas, 1986; Howarth and Rowlands, 1986;Hawkins and McConnell, 1990; Shakoor and Bonelli, 1991; Edet, 1992;Bell and Culshaw, 1993; Rashed and Sediek, 1994; Ulusay et al., 1994;Kasim and Shakoor, 1996; Bell and Culshaw, 1998; Bell and Lindsay,1999; Tugrul and Zarif, 1999). Petrographic features of a rock areintrinsic properties, which control themechanical behavior of the rockmass at the fundamental level (Singh et al., 2001). Merrian et al. (1970)defined texture as the relative amounts, sizes, and shapes ofconstituent grains, as well as the manner in which they interlock.

Although a number of studies have been conducted on therelationship between petrographic and mechanical properties, themajority of work has been conducted on sandstone and the author isunaware of similar work that has been published for shale. Many ofthe petrographic properties determined to be important for strengthin sandstone are not applicable to the study of shale. For example,finer-grained sandstone was found by a variety of studies to begenerally stronger than their coarser-grained counterparts (Spry,1976; Fahy and Guccione, 1979; Ulusay et al., 1994; Tugrul and Zarif,1999), although shale, which is by definition finer-grained thansandstone, exhibits lower values of unconfined compressive strength.Furthermore, although matrix volume has been correlated todecreasing compressive strength in sandstone (Howarth and Row-lands, 1986; Bell and Culshaw, 1998), the material that would beclassified as “matrix” or “grain coatings” in sandstone may constitutethe majority of many shale samples. Related to the volume of matrixmaterial, Kahn (1956) defined the packing density as the ratio of thesum of the length of grains in a traverse across a rock section to thetotal length of the traverse. Similarly, studies of sandstone in whichcompressive strength declined linearly with an increase in porosity(Howarth and Rowlands, 1986), and total pore volume was inverselycorrelated with unconfined compressive strength, tensile strength,and point load strength (Bell, 1978; Fahy and Guccione, 1979; Shakoorand Bonelli, 1991; Richards and Bell, 1995) are not applicable to shale,which is characterized by low porosity. Although the shape of grain

boundaries probably exerts a significant control on sandstone strength(Taylor, 1950; Spry, 1976; Fahy and Guccione, 1979; Hawkins andMcConnell, 1990), and studies of sandstone by Fahy and Guccione(1979) indicated that sphericity was inversely correlated withcompressive strength, shale is generally characterized by flat micaflakes, a shape that is not characteristic of rounded or angular grains ofquartz or feldspar. Finally, even though high quartz content has beencorrelated with higher uniaxial compressive strength and tensilestrength in sandstone (Fahy and Guccione, 1979; Shakoor and Bonelli,1991; Bell and Lindsay, 1999; Tugrul and Zarif, 1999), while Richardsand Bell (1995) reported a highly significant statistical correlationbetween quartz content and Brazilian tensile strength, and amoderatecorrelation between quartz content and unconfined compressivestrength, quartz content in shale is much lower than in sandstone.

Previous studies of different rock types have identified propertiesthat may be applicable to the study of shale textures, and some ofthese conventions are adopted in this study in the interests ofquantifying observations. Taylor (1950), Fahy and Guccione (1979),Howarth and Rowlands (1986), and Hawkins and McConnell (1990)recognized the influence of grain interlocking on sandstone strength,and Taylor (1950) assigned weights to classifications of grainboundaries, such that tangential contacts were weighted as 1×, longcontacts were weighted as 2×, concavo-convex contacts wereweighted as 3×, and sutured contacts were weighted as 4×. Howarthand Rowlands (1986) concluded that shear failure of crystals andcrystal grains is resisted by interlocking grains. Shakoor and Bonelli(1991) found that sandstones with higher percentages of suturedgrain contacts exhibited higher values of compressive strength, tensilestrength, and Young's modulus, while Richards and Bell (1995)reported a highly statistically significant correlation between thenumber of sutured contacts and the unconfined compressive strength,and a positive correlation between sutured contacts and Braziliantensile strength in sandstone. Additionally, Prikryl (2001) recognizedthe effect of preferred orientation (fabric) of minerals inmagmatic andmetamorphic rocks, and indicated that the maximum unconfineduniaxial compressive strength was oriented parallel to the lineationand to the preferential shape orientation of rock-forming minerals ingranites. Shale is similarly texturally anisotropic, with a stronglypreferential layering defined by thin bedding laminations and micaflakes. Finally, total rock strength in any rock is reduced by thepresence of macroscopic, microscopic, and sub-microscopic defectssuch as cavities, cracks, joints, foliations, and veins (Spry, 1976; Edet,1992; Tugrul and Zarif, 1999; Prikryl, 2001).

3. Methodology

Samples were collected from roof fall material in coal mines of thePennsylvanian-aged Appalachian and Illinois Basins so that petro-graphic properties and rock strength could be determined. Coal mineswere developed by the room-and-pillar mining method, and by thelongwall mining method. All roof falls had occurred after the groundhad been supported with roof bolts installed on a pattern, withsupport ranging from 4- to 6-foot (1.2–1.8 m), fully-grouted, headedrebar to 6- to 8-foot (1.8–2.4 m), mechanically-anchored, resin-assisted bolts. Rock samples were collected only from roof falls thatappeared to have failed in response to excess in situ horizontal stress,and where no visible structural geologic discontinuities had dis-membered the roof beam. Geologic mapping confirmed an absence ofvisible structural controls and documented the effects of horizontalstress in themanner proposed byMark (1999), inwhich stress damagefeatures are plotted on section maps to determine the orientation ofthe horizontal stress field.

Core extraction and rock strength testing was were performed atthe National Institute for Occupational Safety and Health's (NIOSH)Pittsburgh Research Laboratory. For each study location, 2–16 coresamples were extracted from multiple samples of roof fall material.

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Core was extracted both parallel to bedding and perpendicularly tobedding, when possible, for unconfined uniaxial compressive strengthtesting. Because of the non-uniform size of samples available from rooffall material, extraction of standard NX size core was not alwayspossible. In those cases, 1-inch (2.54-cm) diameter core samples thatwere 2.5 in (6.3 cm) long were extracted, preserving a 2.5 L/D ratio.Finished core samples were reported by the testing laboratory as0.990 in (2.515 cm) in diameter, with actual L/D ratios of 2.508 to 2.530.In limited instances, a L/D ratio of 2 was necessary due to small samplesize (Mine #2, Mine #3, Mine #5, Mine #7). In these samples, the L/Dratio ranged from 2.006 to 2.022, with finished diameters of 0.990 in.

Samples from Mine #6 did not yield standard sized core samples,because they separated along bedding partings after core extraction.Instead, core pieces were tested by the point load method, and discswere obtained from NX-sized core fragments for splitting tensile(Brazilian) strength testing at the Colorado School of Mines' EarthMechanics Institute. Core samples were prepared in accordance withASTM D-4543, and disc samples were prepared in accordance withASTM D-3967. Unconfined uniaxial compressive strength and tensilesplitting strength tests were performed in accordance with ASTM D-2938 and ASTM D-3967.

Hawkes and Mellor (1970) described three broad modes of failureobserved in compression testing: 1) cataclasis consists of a generalinternal crumbling by formation of multiple cracks in the direction ofthe applied load, such that when the specimen collapses, conical endfragments are left and long slivers of rock form around the periphery;2) axial cleavage, or vertical splitting, in which one or more majorcracks split the sample along the loading direction; 3) shearing of thetest specimen along a single oblique plane. The mode of failure inwhich the rock specimen crumbles by internal cracking and then isburst apart by conical or wedge-shaped end segments is generally

Fig. 1. Map of the eastern and central United States, showing locations of mines where samCompare to Table 1 for rock properties.

accepted as a valid mode of failure which represents the true behaviorof most rocks. Failure along a distinct single shear plane has beenwidely accepted as the normal mode of failure. The tested coresamples in this study generally exhibited failure along a single,inclined or nearly vertical plane.

Thin sections were studied by standard methods with a MeijiModel 9400 binocular microscopewith polarizing light capability, andeyepiece micrometer. Thin sections were examined for grain size,grain shape, packing density, the percent of void space, the percent ofsutured grain boundaries, andmodal composition. Textural propertieswere based on at least 400 counted points, using a Zeiss mechanicalstage mounted on a Zeiss binocular polarizing light microscope.Average grain size was determined by measurements along twoperpendicular axes passing through the center of each grain. Graincontacts were classified according to Taylor (1950). In contrast toworkperformed by others on sandstone, mica flakes were regarded asgrains, rather than matrix, due to the domination of shale by mica.

4. Textural and strength properties of roof fall rocks

Fig.1 shows the geographic location of the fourmines in the IllinoisBasin and eight mines in the Appalachian Basin from which sampleswere collected. Samples were collected from the upper MiddlePennsylvanian (Desmoinesian) Herrin and Springfield Seams in theIllinois Basin, and from a variety of Lower and Middle Pennsylvanianseams in the Central and Southern Appalachian Basin.

4.1. Locations and characteristics of horizontal stress-related roof falls

The roof falls documented in relation to horizontal stress in coalmines commonly occurred in intersections, or began in intersections

ple material was collected in relation to outlines of the Appalachian and Illinois Basins.

Fig. 2. Location map of sample mines in the Illinois and Appalachian Basins, shown in relation to horizontal stress directions (yellow lines) determined by RCD mapping inunderground mines. Directions in the southern Illinois Basin are consistent at approximately N 80° E; directions in southern West Virginia are consistent at approximately N 80° W;directions in northern West Virginia/southwestern Pennsylvania are consistent at approximately N 70° E. (For interpretation of the references to colour in this figure legend, thereader is referred to the web version of this article.)

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and then worked down the adjacent entry or crosscut. The falls werecommonly steep-sided and bounded by a steep rib cutter whendeveloped in shale that appeared to have been lightly to moderatelyrecrystallized, or in the thinly bedded alternating shale/siltstone rockknown commonly in the U.S. coal mining community as “stackrock.”Underground mine mapping indicated the presence of excess in situhorizontal stress by documenting the presence of aligned cutters andpreferentially-oriented, shallow pot-outs in the immediate roof in themanner suggested by Molinda and Mark (1999). Mark (1999) suggeststhat documenting the orientation of many shallow pot-outs that arecommonly localized in the immediate roof can yield an inferred

Fig. 3. Photo of pot-out (alternately known as a cutter) interpreted to have formed byhigh in situ horizontal stress. Thinly laminated shale immediate roof has been buckleddownward along a trend that is interpreted to be perpendicular to the orientation of themaximum horizontal stress, which is indicated by converging white arrows. Roofsupport consists of fully-grouted, headed rebar with wooden header boards.

orientation of maximum in situ horizontal stress, because the stressdirection is expected to be oriented perpendicularly to the long axis ofpot-outs. Thus, the shallow, shale-hosted pot-outs are analogous to astress ellipse. Fig. 2 indicates the inferred orientations of horizontalstress determined by RCD personnel during the course of ground

Fig. 4. Photo of brow at a mine in southern Indiana attributed to failure by horizontalstress (Mine #9), showing buckled strata and cutter formation that bounds one side ofthe fall cavity. Note presence of thinly interbedded, alternating siltstone (light) andshale (dark). Ground support consists of fully-grouted, headed rebar with wood headerboards, and wood cribs to support dead load.

Fig. 5. Brow of roof fall cavity attributed to failure by horizontal stress in southeasternKentucky (Mine #3).

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stability investigations, in relation to the mines where study sampleswere collected.

Horizontal stress-related roof falls have different cavity profiles ascompared to those associated with structural geologic weaknesses(Fig. 3). In the variety of thinly interlaminated siltstone and shale thatis commonly referred to by some miners as “stack rock,” fall cavitiesare commonly bounded by a cutter at the roof line at one rib. Althoughthe term “cutter” has been attributed to a variety of different features,it is herein used to describe a high-angle, stress-related fracture that isexpressed at the roof–rib interface, and extends nearly vertically intothe roof above the ribline (Fig. 4). When fall cavities allow observationof the immediate roof, it appears that cutters represent a fold axialplane that extends nearly vertically through buckled, commonly

Fig. 6. Map of a mining section in Mine #9, southern Indiana, showing the trends of cuttedevelopment, and preferential trend of roof fall locations across the section. Cutter orientatiindicated by dark black converging arrows.

thinly laminated sedimentary strata arranged in a narrow zone ofstacked chevron folds. Cutters are commonly expressed at the visibleroof line by “guttering,” or shallow potting that extends along theroof–rib interface and may extend across the roof adjacent to pillarsthrough crosscuts. The fall cavities are therefore steep-sided, and fallmaterial is represented by large slabs that appear to have experiencedlittle rotation when falling out of the roof, landing in roughly correctstratigraphic order, right-side-up. In other mines, where the roof rockis characterized by shale that appears more dense than normal,exhibiting a ringing or crystalline sound and evoking the informalcomparison of porcelain dinner plates, horizontal stress-related rooffalls are characterized by slabs of dense, apparently recrystallizedshale (Mine #2, Figs. 1 and 2) Fig. 5. Fall cavities are commonly not assharply defined, and fall material appears to have been dumped out ofthe roof in a jumbled pile.

An indication of the presence of horizontal stress can beascertained by underground mapping of elongated, ellipsoidal pot-outs and cutters (Fig. 6). In the example shown in Fig. 6, thepreferential direction of elongation of cutters toward the northwest,as well as the development of roof falls across the section in anorthwesterly direction, is suggestive of a horizontal stress orienta-tion directed between the northeast–southwest.

4.2. Petrographic properties

Common properties of samples generally associated with rockstrength by other researchers are summarized in Table 1, and includethe unconfined compressive strength, density, quartz content, matrixvolume, void space volume, the amount of sutured grain boundaries,the packing density, qualitative sphericity, and grain size with therange of sizes observed. Most of these parameters have been appliedto study of sandstone, and may not be completely applicable to shale.For instance, the fine-grained mica that would be characterized asmatrix material in sandstones represents the framework grains in

rs (short red lines) and roof falls (crosshatched). Note preferential direction of cutteron and trend of roof falls indicate an overall north N 75° E horizontal stress orientation,

Table 1Summary of rock type with selected mechanical and petrographic properties

Mine no. Loc. Rock type Coal seam UCS (MPa) σ (g/cm3) %Q %M %V %S PD (%) Sphericity Grain size range (mm) Dom. grain size (mm)

#2-par WV sh-r Beckley Crystal 52.19 2.95 2 0 0 86 100 subangular. b .01–0.04 0.02#2 WV sh-r 35.58 2.95 2 0 0 86 100 subangular. b .01–0.04 0.02#3-a KY sh-r Hazard No. 4 66.90 2.55 12 1 0 49 100 angular 0.01–0.3 0.042b sr-r 32.91 2.51 8 0 0 64 100 angular b .01–0.1 0.035c sh-r NR 2.66#4 KY sh-r Blue Gem NR 2.79#5 KY sh-r Harlan 21.31 2.70 8 0 0 82 100 angular b .01–1.0 0.035#6 KY sh Upper Harlan 38.5⁎ 2.94 5 35 1 26 99 angular b .01–0.06 0.01#7 KY sr Kellioka 45.57 2.64 17 0.4 0 78 100 angular 0.01–0.2 0.07#8 IL sr Herrin NR Extreme water degradation#9 IN sr Springfield 60.54 2.57 20 1.7 0.7 12 100 subangular b .01–0.1 0.04#10-b IN SS Springfield 33.81 2.51 28 19 0 26 100 angular .02–0.3 0.09#10-c SS 45.26 2.43 30 14 0 18 100 angular .02–0.5 0.1#11 IL sh Springfield 56.37 2.52 12 7 0 7 100 subangular .01–0.1 .04#12 WV sh Kittanning 63.6 – 7 0 0 37 100 angular b .01–0.08 0.01#13 WV sh No. 2 Gas 92.86 – 14 24 0 1 100 angular 0.02–0.2 0.07

Percentages are based on 400+ point counts. UCS = unconfined uniaxial compressive strength; ⁎ denotes UCS value based on point load test; %Q = percent volume of quartz; %M =percent volume of matrix; %V = percent volume of void space; %S = percent of sutured grain boundaries; PD = packing density; grain size = average value of major plus minor axes ofgrain viewed in a single plane; SS = sandstone; sr = thinly interlaminated siltstone and shale, “stackrock”; sh-r = recrystallized shale; par = strength determined parallel to beddingplanes; all values reported for strength perpendicular to bedding planes unless otherwise noted. NR = core sample non-recoverable.

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shale and therefore mica flakes are characterized as grains rather thanmatrix. The term “matrix” was only used in these samples to refer tomaterial that was too fine-grained to be positively identified with apolarizing light microscope at a magnification of 400×, even thoughsuch material most likely represents a mixture of muscovite and ironhydroxide. Using this convention, the packing densities for the coalmine immediate roof rocks are nearly 100%. Furthermore, thecharacterization of sutured grain boundaries is somewhat differentbetween sandstone and shale, in that the micas dominating shale arenot minerals that readily exhibit the sutured boundaries noted inquartz and feldspars. However, suturing is interpreted to have takenplace in mica where grain boundaries protrude into neighboringboundaries, rather than sharing long, straight boundaries, or whereragged ends of mica flakes interfinger with corresponding raggededges of neighboring grains, exhibiting a heavy, dark grain boundaryanalogous to the sutured quartz and feldspar boundaries.

Mineralogy and rock names are summarized in Table 2. Althoughthe samples of coal mine immediate roof in the field appeared as dark,fine-grained sedimentary rock that would commonly be referred to asshale or shale with sand laminations, petrographic examinationcommonly indicated a coarser grain size than that can be applied toshale. The rock names are based on grain size andmineral content. For

Table 2Summary of mineralogy and rock names for individual rock samples

Mine no. Loc. %Qtz. %Plag. %Micro. %Musc. %Biotite

#2-par WV 2 0 0 78 19#2 WV 2 0 0 78 19#3-a KY 12 10 0 25 52b 8 4 0 32 56c – – – – –

#4 KY – – – – –

#5 KY 8 10 0 55 27#6 KY 5 2 0 80 11#7 KY 17 21 0 43 17#8 IL – – – – –

#9 IN 20 15 0 44 4#10-b IN 28 21 b1 42 8#10-c 30 23 0 23 9#11 IL 12 18 b1 55 5#12 WV 7 3 0 76 9#13 WV 14 6 0 42 7

Rocks are named according to Potter et al.'s (1980) classification scheme for shale, with thethat rocks are named according to Dott's (1964) classification of immature sandstone; ⁎ denobarely distinguishable; matrix is interpreted as material that is indiscernible at 400× magn

samples with grain sizes in the Wentworth scale of fine silt, Potter etal.'s (1980) classification scheme for shale is used, which resultsgenerally in the name of “laminated siltstone”. Although the term“siltstone” often has connotations regarding quartz and feldsparcontent, it should be noted that in this classification, it is meant only toindicate a size fraction, such that the siltstones are dominated bymica,with only minor quartz. For samples that exhibit abundant quartz andfeldspars, with grain sizes in the Wentworth scale of fine or very finesand, Dott's (1964) classification scheme for immature sandstone isutilized, resulting in names such as mudstone and wacke.

Mine #2 experienced severe degradation of the immediate roofshortly after mining, which resulted in the abandonment of thesubmain. The submain was affected by a series of large intersectionroof falls in which large (N2 m), detached blocks of shale andsandstone fell out in approximately original stratigraphic order.Observations indicated the presence of numerous, elliptical pot-outsthat were elongated along a preferential bearing, indicating aninferred in situ horizontal stress direction of N 90° E (Fig. 2). Muchof the immediate roof in the abandoned submainwas characterized byhanging, cantilevered slabs of shale. Although apparently susceptibleto degradation by horizontal stress, a sample of the mine roofexhibited a relatively high unconfined compressive strength parallel

%FeOx % Void % Matrix Rock name

0 0 0 Laminated siltstone0 0 00 0 1 Laminated quartzo-feldspathic siltstone

0 0 Laminated quartz-bearing siltstone– – –

– – –

0 0 1 Laminated quartzo-feldspathic siltstone1 1 35⁎ Laminated quartz-bearing siltstone0.2 0 0.4 Feldspathic mudstone†

– – – Mudshale15 0.1 2 Feldspathic mudstone†

1 0 19⁎ Feldspathic wacke†

1 0 14⁎ Feldspathic wacke†

2 0 7 Laminated quartzo-feldspathic siltstone5 0 0 Quartz-bearing laminated siltstone7 0 24⁎ Quartz-bearing laminated siltstone

term “silt” used as a size classification rather than mineralogical connotation; † denotestes that matrix material is represented by very fine-grainedmuscovite and biotite that isification.

Fig. 7. Microscopic texture of sample from Mine #2, dominated by muscovite flakes.Muscovite flakes are interpreted to exhibit suturing where ragged-edged, mutuallyimpinging boundaries interlock with each other. Field of view 1 mm at 100×, takenunder crossed polars.

Fig. 9. Microscopic texture of “stackrock” from Mine #7. Abundant, large quartz grainsoccur in layers that alternate with fine-grained muscovite and biotite mica. Quartzcontent is much higher than sample fromMine #2, but UCS value is lower. Field of view1 mm at 100×, taken under crossed polars.

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to bedding of 52.19 MPa (7567 psi), and a relatively high dry density of2.95 g/cm3. Core samples were also obtained perpendicularly tobedding, to maintain consistency with other samples, and yielded anunconfined compressive strength of 35.58 MPa (5159 psi). Theseresults indicate a +30% greater compressive strength parallel tobedding that than perpendicular to bedding. Despite the relativelyhigh strength of the rock, thin section observation indicated a very lowoccurrence of quartz (3%), and instead revealed nearly completedomination by fine-grained muscovite and biotite flakes (Fig. 7).

A sample fromMine #5 has significantlymore quartz (8%), and alsoexhibits a larger average grain size of 0.035 mm, which is nearlydouble that of the Mine #2 sample (Fig. 8). However, the compressivestrength of the Mine #5 sample is nearly 40% lower at 21.31 MPa(3090 psi) than documented in the sample fromMine #2, which has acompressive strength of 35.58 MPa (5159 psi) perpendicular tobedding (Table 1). Although both samples exhibit a very high numberof sutured contacts between neighboringmica flakes, which dominatethe mineralogy, the texture of the Mine #2 sample appears morehomogenous. In contrast, the Mine #5 sample is characterized bynumerous, discontinuous micro-partings that are characterized byiron hydroxide-stained bedding laminations (Fig. 8).

A sample collected from roof fall material at Mine #7 ischaracterized by much more quartz (17%) than either sample fromMine #2 or Mine #5 (Fig. 9). Although its compressive strength of45.57 MPa (6603 psi) is more than double that of the sample fromMine #5, and much higher than the value of compressive strength

Fig. 8. Microscopic texture of sample from Mine #5, southeastern Kentucky. Quartzcontent is higher than sample from Mine #2 in Fig. 5, but the UCS is much lower. Notethe presence of iron hydroxide-stained partings. Field of view 1 mm at 100×, takenunder crossed polars.

obtained for the sample from Mine #2 perpendicular to bedding, it isstill lower than the very quartz-poor sample from Mine #2 that wastested parallel to bedding (Table 1). Similarly to the sample fromMine#5, the sample from Mine #7 is characterized by abundant, yetdiscontinuous iron hydroxide-stained partings developed alongmuscovite-rich bedding laminations (Fig. 9). Observation of texturesfrom the sample from Mine #7 indicates the abundance of angular,relatively coarse quartz grains, many of which touch each other alongcorners or are more commonly bounded by muscovite lathes. Ironhydroxide-stained bedding laminations that are rich in biotite aredistributed through the rock, and bound thicker areas that arecharacterized by the presence of coarse-grained, angular quartz.

4.3. Mechanical properties

Because unconfined compressive strength is such an importantindex property for engineering classifications, graphs were con-structed for the study samples in which the unconfined compressivestrength was plotted against various petrographic properties so thattheir degree of relation could be assessed. Fig. 10 simply displays therange of unconfined compressive strength values for samples ofhorizontal stress-related roof fall material collected during this study.Values of unconfined compressive strength range between approxi-mately 20 and 70 MPa. Sample 2 was collected from the Pennsylva-nian-aged Appalachian coalfield of southern West Virginia; Samples3–7, 12, and 13 were collected from the Pennsylvanian-aged southern

Fig. 10. Graph of unconfined compressive strength values for samples collected fromroof falls interpreted as horizontal stress related. Samples displayed a wide variety ofUCS values, mainly between 20 and 70 MPa. Points labeled with sample no. from Tables1 and 2; A denotes Appalachian Basin, I denotes Illinois Basin.

Fig. 11. Density of immediate roof samples, compared to unconfined compressivestrength, collected from horizontal stress-related roof falls in underground coal mines.For these samples of immediate roof, there is virtually no correlation between UCS andDensity. Density values are as expected for rocks dominated by muscovite and biotite,and range between approximately 2.5–3 g/cm3. Points labeled with sample no. fromTables 1 and 2; A denotes Appalachian Basin, I denotes Illinois Basin.

Fig. 13. Unconfined compressive strength compared to the combined content of quartzand feldspars. Compare to Fig.10, inwhich quartz content alone has virtually no relationto unconfined compressive strength. UCS appears to actually increase with acorresponding decrease in combined quartz and feldspar content above 30 MPa.

182 S.E. Phillipson / International Journal of Coal Geology 75 (2008) 175–184

Appalachian coalfield of eastern Kentucky, and; Samples 8–11 werecollected from the Pennsylvanian-age coalfield of the Illinois Basin(Fig. 1 and 2). Based on this limited data population, it is not clear thatthere are significant strength differences between shale roof rocks ofdifferent regions.

Fig. 11 portrays the value of unconfined compressive strength forrecrystallized shale compared to the dry density of samples. Allrecrystallized shale samples fall within a relatively narrow range, fromslightly below 2.5 g/cm3 to slightly below 3.0 g/cm3 as might beexpected for rocks dominated by mica and exhibiting essentially novoid space. There does not appear to be a clear relationship betweenrock density and unconfined compressive strength for these samples.The highest-density samples have only moderate values of compres-sive strength, whereas the samples with the highest values forcompressive strength have relatively low values for density. Thusthere appears to be no correlation between unconfined compressivestrength and density for the samples of shale immediate roof collectedduring this study.

Fig. 12 portrays the relationships between unconfined compressivestrength for samples and the volume of quartz grains documented ineach sample. There does not appear to be a clear relationship betweenquartz content and compressive strength in the samples of immediate

Fig. 12. Values of unconfined compressive strength are plotted against the percentquartz content for samples of interlaminated sandstone/shale involved in horizontalstress-related roof falls. The unconfined compressive strength of these rocks does notappear to be related to quartz content, and the two samples with the highest quartzcontent exhibit only moderate unconfined compressive strength values. Points arelabeled with sample no. from Tables 1 and 2; A denotes Appalachian Basin, I denotesIllinois Basin.

roof rock. This observation contrasts with a commonly assumedrelationship among underground mining personnel between higherquartz content and a stronger roof horizon. In practice, muchemphasis in the mine may be placed on whether the roof “drillshard” regarding the assumed strength of the immediate roof. It iscommonly assumed that slower penetration rates exhibited by drillsteel on roof-bolting machines can be correlated to a higher sand, orquartz content even for low amounts of quartz that may occur as thinsand streaks. It does not appear that quartz content alone is ameaningful indicator of the strength of the samples.

When feldspar content is included with quartz, the relation withunconfined compressive strength changes somewhat. At least in therange above 30 MPa, there appears to be an increase in the unconfinedcompressive strength associated with a decrease in combined quartzand feldspar content (Fig. 13). This is interpreted to be a result of theaddition of feldspar, and may be related to the observations of TugrulandZarif (1999),who reported that increasing feldspar canbeassociatedwith a reduction in strength because of cleavage and microfissuresinherent in feldspar. Below the point of 30 MPa, the two samples withvery low combined quartz and feldspar also exhibit the lowestunconfined compressive strength values. However, it should be notedthat the sample population is very small, and is not sufficient to drawconclusions based on such widely separated geographic locations.

Fig. 14 portrays the relationships between the unconfinedcompressive strength and the percent of sutured grain contactsdocumented in samples of recrystallized shale. The characterization of

Fig. 14. Values of unconfined compressive strength are plotted against the percent ofsutured grain contacts for samples of interlaminated sandstone/shale involved inhorizontal stress-related roof falls. Samples from the Illinois Basin exhibit very fewsutured contacts that are restricted to quartz and feldspar grains within quartzo-feldspathic bands, but display similar UCS values to samples from the AppalachianBasin.

Fig. 15. Values of unconfined compressive strength are plotted against the average grainsize for samples of interlaminated sandstone/shale involved in horizontal stress-relatedroof falls. The average grain size in these rocks does not appear related to the unconfinedcompressive strength.

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sutured grain contacts in shale may be more difficult than in coarser-grained clastic rocks, where sutured quartz and feldspar boundariesaremore readily apparent, as typified by indistinct grain boundaries orblue lines as might be observed in Fig. 7. The flat, sheetlike crystalhabit of muscovite and biotite, with their strongly developed naturalcleavage, would not normally be considered to exhibit stronginterlocking characteristics. However, many of the samples studiedappeared similar in texture to low grade metamorphic rock such asphyllite or slate, with grain boundaries of mica flakes apparently fusedwith no interstitial matrixmaterial. Although the samples of immediateroof had largely been assumed to represent a single population, ins-pection of the graph of sutured boundaries and unconfined compressivestrengthappears to indicate that twodistinct populationsof rocks canbedefined. There appears to be one population of rocks with fewer than50% sutured boundaries, and a second population of rocks with greaterthan60% sutured boundaries. It is interesting to note that thepopulationof samples with fewer than 30% sutured boundaries were dominantlycollected from the intracratonic Illinois Basin,whereas the population ofsamples with greater than 60% sutured boundaries were dominantlycollected from the southern Appalachian Basin in the vicinity of the PineMountain thrust (Figs. 1 and 2). However, the sample with the highestunconfined compressive strength value (Mine #13, with 14% quartz,92.86 MPa) exhibits the lowest number of sutured boundaries (1%) andconsists of the finest-grained material.

Fig. 15 portrays the relationship, or in this case lack of relationship,between theunconfined compressive strength and the average grain sizeof recrystallized shale samples collected for this study. There appears tobe no correlation between the unconfined compressive strength andaverage grain size for these samples. Because the average grain sizes ofthe studied samples are very small, ranging between 0.01 mm and0.07 mm, they may not provide a sufficiently wide spectrum formeaningful comparisons, and general changes in unconfined compres-sive strength may not be sensitive to minute changes in average grainsize, especially with a range of values that is already very small.

5. Discussion and conclusions

Horizontal stress-related ground failures commonly occur in coalmines that are characterized by an immediate roof composed of thinlyinterlaminated, alternating brittle and soft layers. The brittle layers arerepresented by well-indurated or lightly metamorphosed (or diag-enetically compacted) shale and very fine sandstone along theKentucky/West Virginia/Virginia border, adjacent to the AppalachianMountains, or by well-indurated shale within the Illinois Basin.

Perhaps surprisingly, in light of the importance placed on this factorby some formal and informal rock quality rating systems, the quartz

content of immediate roof shale does not appear to be related to rockstrength. For the generally fine-grained samples of immediate coalmineroof collected in this study, there is virtually no correlation between thequartz content and the unconfined compressive strength. It is suggestedthat quartz content is not correlated to rock strength in shalebecause theobdurate quartz grains are not in contact with each other to form asupporting framework, so do not lend strength to the rock. Also sur-prisingly, there is virtually no correlation between unconfined com-pressive strength and the percent of suturedmica grain boundaries. It issuggested that sutured mica grains do not affect rock strength in thesame way that sutured quartz grains do, because the shear strength ofthe planar mica grain contacts may be much lower than that of quartzgrains. Similarly, there is virtually no correlation between grain size andcompressive strength. These observations are generally at variancewiththe results reported forprevious studies of sandstone, although it shouldbe noted that the constituents of shale would simply be classified asmatrix material in a study of sandstone, and that sandstone with somuch “matrix” material would be expected to be weak.

The effect of delamination could not be adequately constrained ordocumented. Several samples exhibited partings defined by stronglyaligned and concentrated mica lathes, which represent planes ofpreferential weakness developed parallel to bedding. Unconfinedcompressive strength testing might not account for this preferentialweakness because samples were tested perpendicularly to bedding.This was necessitated by the extreme difficulty of obtaining coresamples parallel to bedding, and was also required by the ASTMstandard for testing. Thus, the UCS value can only be considered as anindex property. It could be more appropriate for future work to devisea sample extraction and testing procedure that would allowdetermination of compressive strength parallel to bedding, or conductan assessment of bending or flexural strength, similar to the outerfiber tensile strength (i.e. Merrill, 1957). The strongly parallel fabricrepresented by thin bedding laminations and aligned mica flakescould be analogous to the metamorphic fabric described by Prikryl(2001), and could account for the greater than 30% difference in UCSvalues obtained parallel and perpendicular to bedding in Sample #2.

Although the sample population is too small to draw broadconclusions, plots of unconfined compressive strength compared tothe percent of sutured boundaries tend to be separated, with samplesthat display a high number of sutured grain boundaries represented byrocks from the southern Appalachian Basin, and rocks with a lownumber of sutured grain boundaries represented dominantly by rocksfrom the intracratonic Illinois Basin. Additional samples should bestudied to determine if rocks from the respective basins can beseparated based on the content of sutured grain boundaries. A greaternumber of samples could also determinewhether some coal seams areassociated with roof shale that has a greater degree of grain boundarysuturing than others within the same basin. The results could haveimplications for the influence of burial compaction or tectonic activityon rock strength.

Acknowledgements

The author gratefully acknowledges the assistance of SteveSchatzel, Steve Tadolini, Dennis Dolinar, and the entire rockmechanicsstaff at the NIOSH Pittsburgh Research Laboratory for performingsample preparation and strength testing, and for providing the use ofequipment. Special thanks are also extended to those with MSHA andindustry who assisted in obtaining rock samples for this study.

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