Progress towards establishing relationships between the mineralogy and physical properties of coal measures rocks

Int. J. Rock Mech. Min. Sci. & Geomech. Abstr. Vol. 19, pp. 81 to 89, 1982

Printed in Great Britain, All rights reserved

0148-9062/82/020081-09103.00[0

Copyright © 1982 Pergamon Press Ltd

Progress Towards Establishing Relationships Between the Mineralogy and Physical Properties of Coal Measures Rocks B. G. D. SMART*:~ N. ROWLANDSt A. K. ISAAC*

The paper reaffirms both the applicability of standard X-ray diffraction techniquesfor the mineralogical analysis of argillaceous Coal Measures rocks and the use of such analyses in the assessment of physical properties of samples that can only be obtained from the field in a form unsuitable for normal uniaxial and triaxial compression testing procedures. Laboratory techniques which enable compression test samples to be prepared from weak sedimentary rocks are described, and results are presented for compression and swelling tests on samples of known mineralogy. Finally, the influence of mineralogy on compressive strength, cohesive strength and swelling is examined for a number of rocks.

INTRODUCTION Excessive loss of section in longwall gateroads due to the deformation of weak floor strata continues to impair the economic extraction of numerous coal seams. Figure 1 illustrates the occurrence of rapid and severe floor-heave behind an advancing longwall face in the lower 4ft seam, this being typical of gateroads in several seams in the South Wales Coalfield I-1]. Given that the major controls on the deformation of floor strata are geological, knowledge of the thicknesses, mineralogy and in-situ properties of these strata are pre-requisites of any thorough field investigation aimed at developing the understanding and subsequent control of floor-heave. Accordingly, it was decided to attempt to provide a rapid means of determining rock mineralogy from powdered samples, with the following rationale: (1) It is generally considerably easier in a production environment to drill open rather than cored boreholes. Analysis of the flushings from such open holes could enable the mineralogy and thicknesses of floor strata to be determined, thus extending the number of sites at which it would be feasible to conduct investigations. (2) By examining the correlation between mineralogy

* Department of Mineral Exploitation, University College, Cardiff CFI 1XL, U.K. t Formerly Department of Mineral Exploitation, now Scientific Officer, Occupational Health and Safety Institute, MeGill University, Montreal, Quebec, Canada H3A 2TS. :~Present address: Department of Mining and Petroleum Engineering, University of Strathclyde, Glasgow G1 1XJ, Scotland. R.M.M.S.

19/2 c

81

and physical properties of strata from which cores could be obtained, the information obtained from open holes could be extended to include an assessment of physical properties. The mineralogical analysis of Coal Measure rocks has however proved difficult due to the fact that the overwhelming majority of these rocks are argillaceous in nature with grain size in the order of 2-3/am. Traditional petrographic analysis is rarely possible as the image of any crystal is obscured by the diffraction patterns originating on its facets and the facets of underlying and adjacent crystals. An attempt [2] made to quantify the minerals present in such samples using differential thermal analysis, concluded that such an analysis would become an extensive investigation instead of a rapid check on the mineralogy of rocks whose physical properties were being measured. Despite certain operational difficulties discussed later, X-ray diffraction (XRD) appeared to offer promise as a rapid semi-quantitative method of mineralogical analysis, and it was decided to attempt to calibrate an XRD machine against more exacting chemical analyses obtained using an energy-dispersive analyser (EDA) attached to a transmission electron microscope. The particular relevance of analytical techniques in the study of the rocks has been demonstrated with arenaceous rocks for which it has been shown that, other factors being comparable, quartz content is directly proportional to their compressive strength I-3,4] and it was decided to extend this work into a range of argillaceous coal measure rocks.

B . G . D . Smart et al.

82

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Fig. 1. Behaviour of the lower 4 ft underclay floor along the L3 roadways, Britannia Colliery, S. Wales (after Afrouz and Harvey).

Physical property testing generally requires a cylinNow while the angles of diffraction depend only on drical test specimen produced from a core. In order to the geometry of the crystal lattice, the actual intensity improve core recovery from bulk samples of weak of the X-rays diffracted depends ideally on: rocks with relatively high clay content, the commonly (i) The way in which the effects of individual atoms used thin-walled diamond bit was replaced with a combine to give additive diffraction. These effects have double-walled core barrel and saw-tooth diamond bit, been quantified and relative intensities of diffraction for the flushing medium being compressed air. The ends of the characteristic angles (0) of most minerals have been the cores obtained were also finished 'dry' to produce tabulated [5]. test specimens. (ii) The amount of mineral present. After describing the development of the analytical and testing techniques, results are presented for some Thus although it would appear that quantification rocks of the South Wales Coalfield indicating that mincould be accomplished with relative ease, it is rather eralogy does appear to influence certain physical more difficult in practice for the following reasons: properties, and conclusions are made regarding extension of the work. (a) Preferred orientation of some minerals during sample preparation affect peak intensities. (b) Non-crystalline (e.g. carbonaceous) material present in a sample is not detected by the diffractMINERALOGICAL AND CHEMICAL ANALYSIS ometer. (c) The degree of crystallinity of the material affects X-ray diffraction (XRD) peak intensities. X-ray diffraction is widely used in the identification (d) Overlap of reflections from different minerals in a of crystalline minerals. Briefly, the phenomenon applied sample affects peak intensities (e.g. 7 A reflections of is that of the diffraction of X-rays of known wavelength kaolinite and chlorite). (2) through precise angles 0 as they pass through crysConsequently, analysis can only be performed at talline material, angle 0 being related to the spacing d between planes of atoms in the crystal lattice. 2, 0 and present on a semi-quantitative basis. Such investigations have been carried out by a number of workers on (d) are related by the Bragg equation, Coal Measure rocks in the U.K. [6-10]. n2 = 2d sin 0, In order to overcome some of the difficulties associated with quantitative analysis, it was decided to estabwhere n is an integer defining the order of reflection. Minerals have characteristic spacings d, and therefore lish a standard sample preparation method as a basis by monitoring over a suitable range of 0 the diffracted for producing consistent results, and 'calibrate' these X-ray intensities emitted by a prepared sample of rock results against results from accurate but laborious when subjected to a collimated beam of X-rays, either energy dispersive chemical analyses performed on a by allowing the X-rays to expose a strip of film located transmission electron microscope. An X-ray diffraction spectrometer was used as this around the specimen, the powder-camera method, or by effectively traversing a counter round the specimen, gave greater resolution of 'd' values, and intensities the spectrometer method, the angles at which 'peak' were easier to observe. In addition, symmetry of difintensities occur, and therefore the crystalline minerals fracted peaks could be studied and the time taken for analysis was shorter. present, can be detected.

Mineralogy and Physical Properties of Coal Measures Rocks

83

X RD sample preparation

below floor levels at Merthyr Vale, Taft Merthyr, Cwm, Approximately 50 g of the rock sample under investi- and Marine Collieries. The material was scanned from gation was placed in a disc grinder for I rain; 0.5 g of 5° = 20 to 34 ° = 20 so as to include the major diffracthis was removed and placed in a micro-ballmill for tion peaks for chlorite, illite, kaolinite and quartz--the 5 min. Precisely 0.2 g of the resultant powder was then major rock-forming minerals in Coal Measure rocks. removed and placed in a clean test tube with 20 ml of For example, Fig. 2 demonstrates differences in minerdistilled water and dispersed in an ultrasonic bath. alogy in four samples of underclay taken from different After allowing 30 sec for flocculated material to settle, collieries, and Fig. 3 illustrates changes of mineralogy 2 ml of the suspension was removed and smeared on a with depth in the floor of the B10 District, Merthyr glass plate. After heating at 60°C until all the water had Vale Colliery. It can be seen from comparing intensities evaporated, the sample was ready for diffraction analy- of the quartz 3.34 A reflection peak in Fig. 3, that sis. The clay minerals in samples prepared in such a quartz content decreases with depth, contrary to the manner are preferentially orientated and this can be general situation previously described in a study of used to advantage in identification as preferred orienta- underclays in the South Wales Coalfield I-7]. The repeatability of the XRD results, and hence their tion intensifies basal reflection peaks. suitability for analyses, were assessed by comparing Analysis and results peak intensities for different samples of the same rock Diffraction traces were recorded for forty floor and and also repetitive analyses of one sample. Thus, for roof samples from different collieries in South Wales. example, the variation in the 3.34 A peak for quartz was More detailed studies were performed on material col- found to be of the order of + 2% in different samples of lected from dry flushings taken at selected intervals the same rock, and + 1% in repetitive analysis of the same sample, showing the XRD results to be suitable for semi-quantitative analyses. Semi-quantitative analysis of mineral content was o<[ ~<[ performed by comparing specific peak intensities with percentage estimates of corresponding mineral content •E .c a~ derived from energy dispersive chemical analyses as deo ~ _= scribed in Section 2.3. N

Energy dispersive chemical analysis (EDA)

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The useful working range of the transmission electron microscope is in the study of particles with an upper size limit of 50/am and a lower limit defined only by the resolution of the microscope. Conventional electron microscopy of such particles only reveals morphology while some structural details may be determined from the application of electron diffraction techniques. The attachment of an energy dispersive analyser to the electron microscope enables chemical analyses of single particles to be made and the bulk chemistry of polyminerallic phases to be determined. The energy dispersive analyser (EDA) uses the principle that when a material is bombarded by a high energy beam of electrons (in this case in the electron microscope) secondary X-radiation is emitted from the constituent atoms of the material. The energy values of these secondary X-rays are dependent upon the different elements present in the material. The EDA system collects secondary X-radiation emitted by the specimen under investigation, and amplifies and differentiates the signal to produce a string of pulses, each one corresponding to an X-ray and with a height proportional to its energy. The pulses are measured and counted by a multi-channel analyser, and the result is the accumulation of a spectrum of counts against X-ray energy. Relative peak heights can be compared so as to give a semi-quantitative determination of the various elements present. The electron microscope with EDA attachment was used to analyse the individual mineral particles in the

84

B. G. D. Smart et al. TABLE l. MINERAL CONTENT DETERMINED BY EDA FOR A NUMBER OF SAMPLES

Sample

Illite, %

Kaolinite, %

Quartz, %

Fe, Ti, %

9 14 18 19 23 24 30 40 41 60 64 S101 S102 S103 S 104

44.0 -3.0 47.0 28.0 90.0 95.0 48.0 55.0 58.0 4.0 80.0 72.0 74.0 71.0

30.0 87.0 88.0 15.0 10.0 -2.0 33.5 19.5 7.0 81.0 5.0

24.0 5.0 3.0 31.0 55.0 5.0 -14.5 25.0 28.0 11.0 14.0 19.0 20.0 20.0

2.0 5.0 3.0 6.0 6.0 2.0 1.0 1.0

-

-

2.0 --

rocks under investigation and for bulk analysis of powdered rock samples [11]. Sample preparation Samples of finely powdered material were dispersed in distilled water to form a suspension. Two different concentrations of suspension were prepared for each sample, a thin dispersion for single particle analysis and a thicker dispersion for bulk analysis. Small amounts of these suspensions were withdrawn, placed on a carboncoated electron microscope grid and allowed to dry. The samples were then ready for electron microscope micro-analysis. Analysis and results Bulk chemistry and single particle chemistry analyses of the rocks under investigation were performed. Knowing the chemistry of individual mineral phases in the rock from single particle analysis, it was possible to establish the relative amounts of these minerals from the bulk analysis on an elemental proportion basis [11]. Table 1 shows the results for the samples analysed. Carbonaceous and H 2 0 content were determined by thermal gravimetric techniques of bulk analysis, whilst the single particle analyses of clay minerals were adjusted for theoretical H 2 0 content. Correlation of XRD and EDA results Semi-quantitative analysis of all the rocks under investigation for quartz content was made possible by producing correlation curves for the 3.34 and 1.82 A XRD peak intensities with the available EDA results as shown in Fig. 4. Allowance was made for the overlap of the 3.31 A illite peak with the 3.34 A quartz peak by subtracting the height of the 5.03 A illite peak scaled to allow for the different relative intensities, i.e. the 3.34 A quartz peak was assumed to be given by:

Carbonaceous matter, % 1.0 3.0 3.0 1.0 1.0 3.0 2.0 3.0 -2.0 2.0 0.5 8.0 2.0 6.0

5.0 2.0 0.5 1.0 2.0 3.0

fracted peak to background noise ratios, the results produced the best correlation of all the minerals examined, i.e. to within 4-59/0 quartz by weight at 80% confidence limits. Similar correlation curves were produced for kaolinite by considering both the 3.58 and 7.18 A XRD peaks. Due to the variation in the peak widths recorded, the measurement of half-height peak width x peak height was used to quantify peak intensity, the curves produced being shown in Fig. 5. The correlation was not good, due to an enhanced XRD response at low kaolinite contents. Of the two peaks 120

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actual 3.34 A quartz peak height = apparent height -0.8 × 5.03 A illite peak

~O~'//e 23

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Fig. 4. Correlation of XRD and EDA results for the 3.34 and 1.82 quartz peaks.

Mineralogy and Physical Properties of Coal Measures Rocks

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85

flushing medium increases the likelihood of core fracture. The major structural defects occurring in these rocks are slickensides, listric surfaces, microfissures, jointing and preferred orientation of platy minerals. Stress imparted to the core by the drill barrel during the drilling operations initiates fracture along these planes and the presence of water lubricates the fracture surfaces. The above factors were considered in the design of a drilling system for preparation in the laboratory of cores from soft materials. The system comprised a heavy duty workshop drill, on to which was fitted an A.W.M. double-tube core barrel capable of accepting cores of up to 200 mm in length. The drill speed, although variable was generally maintained at 800 rpm. A saw-toothed diamond bit was fitted to the barrel and this proved capable of drilling through hard bands of ironstone or quartzitic material as well as the softer seat-earths. The bit was cooled using compressed air at 0.56 M N / m 2 pressure, thus eliminating deterioration of the rock from water flushing, and dust was removed from the atmosphere using a vacuum extractor and filter arrangement. Annular eccentricity and vibration of the bit during operation were reduced by the

, / / ~

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40 60 80 % Kaolinite (EDAX)

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Fig. 5. Correlation of XRD and EDA results for the 7.2 and 3.58 A kaolinite peaks.

examined, the 3.58 A gave marginally better results. Although reasonable correlation curves were obtained, these appear to be non-linear due to an enhancement of the XRD response at low values of kaolinite content. Finally, correlation curves were produced for illite by comparing the 5.03 and 9.98 A peak intensities with the appropriate EDA results. The measurement of halfheight peak width x peak height was used to quantify the 5.03 A peak while the area under the diffuse 9.98 A peak with associated background shift was calculated by drawing an equivalent triangle, the correlation for the 9.98 A peak being the better of the two. Thus correlation curves have been obtained enabling the quartz, kaolinite and illite contents of samples analysed only by XRD, to be estimated.

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Specimenpreparation Test specimens are normally produced by coring from blocks and then finishing the ends of the cores to produce cylinders of acceptable standard [12]. Preparation of cores from soft Coal Measure rocks is, however, difficult due to the structures within these rocks that form planes of weakness. The use of water as a

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86

B . G . D . Smart et al.

application of a 'steady' placed above the dust extractor box. As the bit penetrated the rock, the core passed into a non-rotating inner barrel and, in consequence, the torsional stress usually encountered during drilling was relieved. Samples were removed by unscrewing the drill bit and retrieving the core from the inner barrel, thus reducing the chance of core breakage. It was essential that each rock specimen was securely clamped and that the core barrel was correctly aligned as any 'throw' of the bit during drilling could have caused fracture of the core. Samples for testing were cored as soon as possible after collection to reduce the effects of variation in humidity, i.e. the swelling and ultimate disintegration of some rocks. Due to the fragile nature of cores, conventional lapping and grinding techniques were not employed in end preparation. Instead, the core ends were prepared by rotating the core in the chuck of a lathe at 200 rpm and traversing a silicon carbide disc, rotating at 1420 rpm across the rough ends. The disc was powered by a motor mounted on the carriage of the lathe. If further finishing was required this was effected with silicon carbide paper mounted on a steel backing disc and powered in the same way as the disc. Application of the above coring and end preparation techniques extended the range of Coal Measures rocks which could be tested, enabling the deformation characteristics to be determined for all but the weakest underclays which contained a high density of structural discontinuities. Radiographic examination of cores The compressive strength of a material is related to the structures within that material; in some cases structure orientation can have a dramatic effect upon compressive strength. In fine grained Coal Measure rocks it is often difficult to observe with the naked eye structural components such as bedding and slickensiding. The use of thin sections may provide some structural information but in the case of soft, friable rocks, preparation is difficult and does not always supply maximum information for large samples. X-radiography provides a method of non-destructive structural analysis of cores prepared for compressive testing and is quick and simple when compared with conventional petrographic techniques. Radiographic analysis has been used for the investigation of sedimentary struc-

tures, fossils, ores and has also been applied to studying shear in soils [13] and fracture in rocks [14]. Radiographs were taken in an industrial X-ray unit. An applied voltage of 60 kV and current of 5 mA were found most useful for the size of samples under investigation, and the X-ray source was situated at a distance of 1 m from the sample. Prints of some of the radiographs are shown in Fig. 7, illustrating various structures. Triaxial compression tests The standard procedure for triaxial testing of rock and soil is to form the Mohr's envelope for a rock by ascertaining one point on the failure surface per specimen. However, the multiple failure state test [15! allows for several single failure states to be obtained from one specimen. Testing was performed using an Avery 500 kN universal testing machine in conjunction with a triaxial cell of the type developed by Hock & Franklin [16]. A variable speed pump powered a constant pressure unit capable of developing confining pressures of up to 70 MN/m 2. The specimen was initially loaded until the predetermined confining pressure equalled the axial pressure of the specimen. The axial load was then increased until reaching peak strength, this being characterized by the slight arrest of the load indicator of the compressive machine, or in the case of the more brittle rocks studied by Singh [17] by cracking sounds. At this stage the confining pressure was abruptly increased by a finite increment and the corresponding maximum stress was noted, thus obtaining up to 5 values per specimen. The effect of multiple failure on the subsequent peak strengths was studied by Kovari & Tisa [ 15] and Moretto & Bolognesi [16] who concluded that the loss of strength when establishing further maxima is practically negligible. After the sample had failed it was possible to obtain residual strengths for the samples at different confining pressures. Triaxial compression testing results Triaxial compression tests were performed in conjunction with Aziz [19] and Knight [20] upon seven different Coal Measure rocks, four obtained from the roof strata and the remaining three from floor strata. Multi-failure state testing was employed and between

i

(a)

(b) Fig. 7. Prints of core radiographs.

(cl

87

Mineralogy and Physical Properties of Coal Measures Rocks TABLE 2. TRIAXIALTESTRESULTSLISTEDAGAINSTQUARTZ ;CONTENTOFROCK Uniaxial compressive strength determined from triaxial results, MN/m 2

Cohesive strength, MN/m 2

Angle of internal friction

Quartz content %

Markham M21 (root)

75.7

18.0

25

32.0

Markham M21 (floor)

40.9

12.2

28

40.0

Taft Merthyr B3 (floor)

69.0

16.6

30

30.0

Merthyr Vale B20 (roof)

50.0

10.0

28

25.0

Cwm 713 (roof)

63.0

19.6

27

32.0

Marine BLII (roof)

29.2

10.5

26

22.0

Marine BLI1 (floor)

25.9

8.0

27

20.0

Sample

two and five results were obtained from each core at different confining pressures. The parameters evaluated from the data were peak and residual strength envelopes, cohesive strength and angle of internal friction. Uniaxial compressive strength was derived by extrapolation of the Mohr's envelopes. Computation of results was performed using a computer program TRIAXL [21] which obtains the least squares best fit for the Mohr envelope. The derived parameters are listed in Table 2, while typical graphical presentation of results in the form of a Mohr's envelope is given in Fig. 8. The failure envelope plotted by computer was not always representative, this occurring most often where results displayed a large degree of scatter. In such cases it was necessary to draw the failure envelope manually and often it could not be represented by a single line but by a probable zone of failure. Envelopes tended to display curvilinear relationships between principal stresses at failure, indicating plastic deformation at high confining pressures. Scatter in results for different rock types reflected the amount of structural discontinuities present in core samples under test. In certain cases, investigation of output from TRIAXL demonstrated that the best fitting model did not always provide the most realistic results. In such cases the second highest order of fit was generally found to represent the input data more realistically.

Test procedure and results The swelling, i.e. longitudinal elongation, of air-dried rock specimens as they became saturated with water was measured on the apparatus shown in Fig. 10, this being seen as an easily derived index of one of the effects of water on a given rock. Air drying was used in preference to oven drying as it was found that specimens tended to break up along bedding planes when heated at 102°C. The specimens used were prepared as for compression testing and placed dry in the apparatus. On immersion of the specimen in distilled water, the longitudinal elongation was recorded against time until maximum elongation was reached. Typical recordings are shown in Fig. 1l(a), while a plot of maximum elongation against the quartz content for the samples tested is shown in Fig. 1l(b). CORRELATION OF MINERALOGY WITH COMPRESSION AND SWELLING TEST RESULTS Table 2 presents a comparison of compression test and mineralogical properties of rock specimens, while correlation curves for uniaxial compressive and cohesive strength values vs quartz content are presented in Fig. 9. Values for uniaxial compressive strength compare well with quartz content with the exception of

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Micro-laminated mudstone (compact) Silty underclay with rootlets Silty underclay with siderite nodules Laminated mudstone (compact) Silty underclay with siderite nodules Laminated soft mudstone with siderite bands Soft underclay

SWELLING TEST

120

=

Description of rock

)O0

150

200

Norrnol s t r e s s , MN / m e Fig. 8. Typical graphical presentation of triaxial test results.

88

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uo

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Fig. 9. (a) Uniaxial compressive strength determined from triaxial results vs quartz content for seven rocks. (b) Cohesive strength vs quartz content for seven rocks.

floor material obtained from the M21 District, Markham Colliery. X-radiographs indicated that this material contained a large amount of rootlets and this probably explains the low value for compressive strength when compared with quartz content. Cohesive strength showed good correlation for 6 samples, whereas Markham floor material displayed low values. The relatively low value for the floor material can again be attributed to structural discontinuities (i.e. rootlets). Relatively constant values were obtained for the internal friction of the samples tested. This appears to indicate that even at quartz content percentages up to 40%, quartz particles do not abrade against each other and that frictional properties are largely a function of clay minerals. The angle of internal friction did, however, exhibit a tendency to decrease with increasing confining pressure. This is in accordance with the theory that soft rocks undergo plastic deformation at high confining pressures. Investigations upon underclay samples from the floor of the Marine BLI1 District presented difficulties in estimating the actual point of failure of some cores at high confining pressures. Examination of these cores after testing demonstrated the presence of multiple shear planes and a large amount of lateral deformation of samples. Such phenomena indicate a change in nature of the rock at high confining pressures to a semi-fluid state, associated with the loss

of internal friction caused by plastic deformation of clay minerals which form the greater bulk of the rock. The laboratory investigations showed that swelling and decomposition of the core samples was initiated almost immediately after water immersion, and this may be attributed to capillary action along microcracks, slickensides, etc. Further gradual disintegration may be due to absorption of water by clay minerals, LOr

(O)

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Time ~ hr

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" 'I 0

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Quartz content ,%

Fig. 11. Swelling test results. (a) Typical plot of longitudinal elongation against time. (b) Plot of maximum elongation against quartz content.

Mineralogy and Physical Properties of Coal Measures Rocks but this appeared to be evident only in the samples with high illite and low quartz content. Generally, however, the m a x i m u m elongations of the samples were found to be related inversely to quartz content as shown in Fig. l l(b). CONCLUSIONS

AND RECOMMENDATIONS

89

heave and floor mineralogy within the South Wales Coalfield. While the above recommendations are m a d e with regard to a particular Coalfield, the same approach could be investigated elsewhere, provided a local XRD calibration is established. Acknowledgements---The authors gratefully acknowledge the financial

and material support provided by the National Coal Board for the An X-ray diffractometer has been calibrated against work described in the paper. more exacting bulk chemical analyses to enable the The authors also wish to thank colleagues in the Department of semi-quantitative determination of the mineral content Mineral Exploitation, in particular the late Professor John Platt for his continual encouragement and Dr F. D. Pooley for providing of argillaceous rocks from the Coal Measures of South XRD and EDA facilities. Wales. The rock samples were collected in an unweathered state from underground sites and then subjected to Received 16 January 1981; revised 25 June 1981. standardized techniques of sample preparation and X R D analysis. The minerals quantified were quartz REFERENCES kaolinite and illite, while the presence of other minerals, e.g. siderite, could be detected. The best calibration was 1. Afrouz A. & Harvey J. M. 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