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COALTECH 2020

INTERIM PROGRESS REPORT

Task 1.4

Geotechnical factors affecting high- andlow-wall stability in opencast coal mines

Sub-task 3b

Wireline logging applicabil ity for theidentif ication of geotechnical features

by

Grant van Heerden, Pr.Sci.Nat. CSIR Miningtek

Report Number: 2004 – 0175

May 2004



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EXECUTIVE SUMMARY

The applicability of geophysical wireline logging techniques has been investigated in terms ofidentifying geotechnical features. Primary geotechnical features include joints and faults.Secondary geotechnical features include weak bedding planes and weathered sedimentarybands and zones. A full suite of geophysical probes was used to wireline log six geologicalboreholes drilled at Anglo Coal’s New Vaal Colliery opencast mine. After interpretation of the

resulting data, it was concluded that three probes are needed to obtain sufficient appropriatedata, constituting the basic input into the final predictive methodology for slope stability hazardrating. The three probes are the density, optical televiewer and acoustic televiewer probes.



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ACKNOWLEDGEMENTS

The author would like to thank the following Anglo Coal personnel for their assistance affordedto the author during field work investigations carried out at New Vaal Colliery:

• Mark Mattushek (Divisional Geologist) and the geological staff

• Johan Fourie (Chief Mine Surveyor) and the survey staff

• Izak de Villiers (Senior Mine Planner) and the planning staff

Steve Lynch, Geophysicist, Anglo Technical Division, is thanked for his assistance with the useof WellCAD for interpretation of wireline data.

Marianne Maccelari, Business Area Manager, Orebody Information, CSIR Miningtek, is thankedfor her constructive criticism during the drafting of this final report.



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CONTENTS

Page

Executive Summary 2

Acknowledgements 3

List of Figures 5

List of Tables 5

1. INTRODUCTION 6

2. PROBE SELECTION 72.1 Sonic (full wave form) Probe 72.2 Density (gamma-gamma) Probe 82.3 Optical and Acoustic Televiewer Probes 82.4 Formation Dip-meter 92.5 Summary 9

3. WIRELINE, LITHOLOGICAL AND GEOTECHNICAL LOGGING RESULTS 103.1 Lithological differentiation: Lithological Core Logging vs Density Probe 10

3.2Identification of geotechnical features: Geotechnical Core Logging vsOptical and Acoustic Televiewer Probes

11

3.3 Summary 14

4. BENEFITS AND SHORTCOMINGS 144.1 Physical Core Logging: Lithological and Geotechnical 144.2 Geophysical Wireline Logging 14

5. CONCLUSION 15

6. REFERENCES 16



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LIST OF FIGURES

Page

2.1.1Correlation plot between sonic transit time and unconfined compressivestrength

7

2.2.1 Typical density probe response curve through a sedimentary package 8

2.4.1.a Modelled seam floor elevation contours 92.4.1.b Resultant vector map of strata azimuth and dip, based on modelled contours 93.1.1 Actual density trace from Borehole A and associated macro lithology 103.2.1 Joint traces indicated on optical televiewer data 123.2.2 Joint traces indicated on acoustic televiewer data 13

LIST OF TABLES

1.1 Geotechnical features and related geophysical techniques 62.1 Geological / geotechnical data obtainable from probes used at New Vaal

Colliery7

3.1.1 Borehole B: Coal seam depth and thickness measurements from lithological andgeophysical logging

11

3.1.2 Borehole C: Coal seam depth and thickness measurements from lithological andgeophysical logging

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1. INTRODUCTION

The applicability of geophysical wireline logging techniques has been investigated in terms ofidentifying geotechnical features. Field investigations conducted in terms of Sub-tasks 1through 3a of Task 1.4 (Stewart and Letlotla, 2003; van Heerden, 2004) have indicated thathighwall slope stability is primarily a function of the presence, frequency, and attitudes ofgeological discontinuities (geotechnical features) and their relationships with exposed opencast

coal highwalls. The purpose of this exercise is to determine if geophysical wireline logging canbe applied in the development of a predictive methodology for slope stability hazard rating inopencast coal mines.

Before the applicability of the wireline technique can be assessed, it is necessary to firstdetermine which of the numerous geophysical probes / tools are best suited to the identificationof geotechnical features. Principle geotechnical features include joints and faults, whilesecondary geotechnical features include weak bedding planes and weathered sedimentarybands and zones. Table 1.1 shows various geotechnical and geological characteristics that canbe detected with the geophysical techniques / probes indicated.

Table 1.1: Geotechnical features and related geophysical techniques (after Jeffrey, 2003).

Geotechnical / geological characteristic Technique / probe

Lithology identification

Clay identification (montmorilonite &bentonite)

Acoustic televiewer, density, neutron,resistivitySpectral gamma, resistivity

Stratigraphic interfaces Seismics, density, resistivity

Seam identification Sonic

Seam/strata thickness Density, neutron

Formation dip and dip direction Dip-meter

Seam dislocations Seismics

Fractures, parting planes (leading todelamination), structural feature identificationand orientation (dip and dip direction)

Density, sonic, neutron, resistivity, acousticteleviewer, optical televiewer

Rock strength (UCS, tensile strength andstrength moduli)

Sonic, neutron, density, resistivity, acousticteleviewer, natural gamma

Porosity/moisture content Sonic, neutron, resistivity, density

Abrasiveness Natural gamma

Weathering, presence of burnt/oxidised coal Resistivity, magnetic susceptibility

Temperature Temperature

Stress field maximum direction Acoustic televiewer, optical televiewerWater ingress Temperature

To facilitate the identification of appropriate geophysical probes, six boreholes drilled at AngloCoal’s New Vaal Colliery, as part of the mine’s routine drilling programme, were selected forgeophysical wireline logging. Five probes were used to geophysically log the six selectedboreholes. The probes included full wave form sonic, acoustic televiewer, optical televiewer,density, and formation dip-meter. In addition to geophysical logging, all core samples werelithologically and geotechnically logged for comparative purposes.

The results from this phase of logging were used to select the necessary tools to adequatelyidentify geotechnical features. In addition to probe selection, critical benefits and shortcomings

were identified regarding the application of geophysical techniques to the identification ofgeotechnical features.



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2. PROBE SELECTION

The five probes used to log the six boreholes are best suited to assist with the identification ofgeological and geotechnical characteristics as indicated in Table 2.1 below.

Table 2.1: Geological / geotechnical data obtainable from probes used at New Vaal Colliery.

Geophysical probe Geological / geotechnical characteristics

Sonic (full wave form) Seam identification, rock strength, porosity

DensityLithological differentiation (stratigraphic interfaces) andstrata thicknesses, elastic moduli

Acoustic Televiewer (water-filledholes only)

Geological discontinuities (including dip and dipdirection), principle stress direction, lithologyidentification

Optical Televiewer (dry holes only)Geological discontinuities (including dip and dipdirection), principle stress direction

Formation dip-meter Strata dip direction and dip

The data that need to be gathered by geophysical techniques must allow for coarse lithological

differentiation and the identification of planes of discontinuity within the rockmass. These datasets are considered to be sufficient to allow for a preliminary assessment of highwall hazardregarding slope stability. The suitability of each probe is discussed in more detail below.

2.1 Sonic (full wave form) ProbeCorrelations between sonic velocity and uniaxial compressive strength (UCS) may provide auseful means of assessing rock strength (Pers. Comm., Campbell, 2004). In fact, an empiricalformula was derived by McNally (1990) based on Australian coals and the relationship is shownin Figure 2.1.1. Some workers (Lindsay et al., 2001; Hack, 2002), however, are of the opinionthat UCS is not a valid parameter in slope stability assessment, mainly because mostrockmasses will, in reality, be stressed under conditions similar to those of triaxial tests ratherthan uniaxial test conditions. It is, therefore, not necessary to determine sonic velocity by

geophysical methods, or otherwise.

Figure 2.1.1: Correlation plot between sonic transit time and uniaxial compressive strength(source: Jeffrey, 2003).



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2.2 Densi ty (gamma-gamma) ProbeCoarse lithological differentiation, in the present context, implies accurate (to within 10 cm)differentiation between coal and non-coal stratigraphic horizons. Non-coal stratigraphichorizons are typically represented by inter-seam sandstones, siltstones, shales and mudstones.There is typically a marked density variation across the contact between coal and non-coalstrata within the coal-bearing Ecca sediments of South Africa, thus allowing for lithologicaldifferentiation to be based on density alone. The density probe measures electron density and

this is related to the bulk density of the material. Figure 2.2.1 illustrates a typical density proberesponse curve through a hypothetical sedimentary sequence. The density probe is, therefore,considered necessary for geotechnical investigations.

Figure 2.2.1 Typical density probe response curve through a sedimentary package (modifiedafter Reeves, 1981).

2.3 Optical and Acoustic Televiewer ProbesThese two probes are discussed under the same section since the major difference between

the two is their respective mode of operation. The optical televiewer can only be used in dryholes, while the acoustic televiewer can only be used in water-filled holes (Table 2.1).

Routine, on-mine, geological borehole drilling almost always produces non-oriented coresamples. Since any observed structural discontinuities (faults etc.) cannot be oriented with anycertainty, no meaningful structural interpretations can be made. The optical and acousticteleviewer probes produce a 360° orientated image of the borehole wall. When used inconjunction with appropriate software, such as WellCAD, accurate and oriented structuralinformation can be obtained, allowing for meaningful structural interpretations to be made. Atmost mine sites, boreholes will be dry in the upper sections and water-filled in the lowersections, necessitating the use of both probes in a single borehole. The two televiewer probes

are, therefore, considered necessary to provide structural and geotechnical information.



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2.4 Formation Dip-meterThe mine-scale geological model is based on information from exploration and routinely drilledgeological boreholes, and is updated on a regular basis. The model is accurate enough to allowfor sedimentary strata azimuth and dip calculations to be made. Figure 2.4.1.a is a plot of seamfloor elevation contours modelled from borehole data. Figure 2.4.1.b is a resultant vector mapof strata azimuth and dip. The use of the formation dip-meter is thus unnecessary.

Figure 2.4.1: a) Modelled seam floor elevation contours.

Figure 4.2.1: b) Resultant vector map of strata azimuth and dip, based on modelled contours(Dip, measured in degrees below horizontal, = ASIN {Vector Magnitude}).

2.5 SummaryConsidering the geological and geotechnical characteristics that can be identified with these fiveprobes and the data needed that cannot otherwise be accurately obtained from the geologicalmodel, only three probes are considered necessary. Therefore, data from three probes (densityand two televiewers) will be used to assess the wireline technique in terms of identifying

geotechnical features.

a

b



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3. WIRELINE, LITHOLOGICAL AND GEOTECHNICAL LOGGING RESULTS

Data obtained from the three selected probes were processed and compared with lithologicaland geotechnical logging results. Results from selected boreholes are presented here in orderto illustrate comparisons, explain discrepancies, and to show that the use of these three probesfor geotechnical investigations of this kind is sufficient.

3.1 Lithological differentiation: Litho logical Core Logging vs Density ProbeMacro lithological interpretation is achieved by processing the density probe data. Figure 3.1.1is an actual density trace with the associated macro lithology indicated.

Figure 3.1.1: Actual density trace from Borehole A and associated macro lithology (thelithological nomenclature applied is specific to New Vaal Colliery).

In many cases, the wireline depths and interpreted lithological thicknesses are considered to bemore accurate than those obtained from standard lithological logging. This is mainly becausethere is a greater chance of core loss during drilling and core recovery than there is of winchslippage, both resulting in incorrect depth and associated thickness measurements. Althoughthe core logger is, in all cases, expected to perform core recovery and depth adjustment

calculations prior to logging, human error outweighs mechanical error.



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Table 3.1.1 and Table 3.1.2 compare depth and thickness measurements of coal seams for twoboreholes where, in one case, there is good correlation and the other, correlation is poor.

Table 3.1.1: Borehole B: Coal seam depth and thickness measurements from lithological andgeophysical logging.

Lithological GeophysicalSeam

From To Thick From To Thick

Top 28.00 38.06 10.06 28.00 37.99 9.99Middle 52.08 59.16 7.08 52.10 59.68 7.58

Bottom 64.00 71.06 7.06 63.97 N/R N/RN/R = Not Recorded

Overall, the correlation between the two logs for Borehole B is good, with two exceptions:lithological Middle Seam thickness is 0.50 m less than the geophysical equivalent; and noBottom Seam thickness was determined from geophysical data. The Middle Seam thicknessdiscrepancy is ascribed to core loss. The absence of a Bottom Seam base from geophysicaldata is due to silting up at the bottom of the borehole thus preventing the density probe fromreaching the true end of hole.

Table 3.1.2: Borehole C: Coal seam depth and thickness measurements from lithological andgeophysical logging.

Lithological GeophysicalSeam

From To Thick From To Thick

Top 24.65 33.97 9.32 25.31 35.17 9.86

Middle 46.81 53.76 6.95 48.44 56.13 7.69

Bottom 58.34 64.52 6.18 60.70 66.95 6.25

The discrepancies observed in the data pertaining to Borehole C are more than likely due tocore losses and poor depth corrections rather than wireline logging inaccuracies.

3.2 Identification of geotechnical features: Geotechnical Core Logging vs Opticaland Acoustic Televiewer Probes

Due to the nature of routine production drilling, which is typically a high monthly metreage, theresulting core is often highly broken due to high drilling rates, handling and transport. Thus, acomparison needed to be made between results of geotechnical core logging and computerassisted interpretation of televiewer data.

Figure 3.2.1 is a section of optical televiewer data (black and white image) from a boreholewhere two geological discontinuities have been identified. With the use of WellCAD software,the two joint traces have been delineated and their respective azimuth (dip direction) and dipangles determined. Figure 3.2.2 is a section of acoustic televiewer data (colour image) from thesame borehole, somewhat deeper and water-filled, also showing identified discontinuities and

their respective azimuth and dip data.

It should be clear from these images that only geological discontinuities are discernable. Whatis not visible in these images are discontinuities resulting from bedding plane separation andwashouts (weak and/or weathered zones), although these types of discontinuity are readilydiscernable. A reasonable amount of exposure to interpreting televiewer data is, however,necessary before the interpreter can be confident when assigning discontinuity types toobserved anomalies in televiewer data.

Geotechnical core logging is complicated by the fact that the condition of the core is greatlyinfluenced by: drilling method and technique; condition of drilling equipment; rate of penetration;core diameter, recovery, handling and transport. All of these aspects of producing core samplesresult in additional breaks in the core which are, by definition, not geological discontinuities. Inaddition, exposure of the core to ambient atmospheric conditions will result in varying degreesof slaking, also resulting in core breaks that are not strictly geological discontinuities.



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Figure 3.2.1: Joint traces indicated on optical televiewer data. Azimuth, in degrees, measured

clockwise from True North (0°) and Dip, in degrees, measured down-dip fromhorizontal (0°).

Since televiewer data is obtained in situ and the image produced is that of the borehole wall,none of the issues mentioned relating to core condition are applicable, and the interpreter isable to ‘pick’ only geological discontinuities. The relevance and accuracy, therefore, ofinformation derived from geotechnical logging of core from routinely drilled geological boreholesis considerably less than that obtained from televiewer data.

Geotechnical logging of non-oriented core samples can really only supply information useful forcalculating rock quality designation (RQD) values. Since the televiewer data excludes breaksresulting from, inter alia, core recovery and handling, RQD values obtained from televiewer data

can be expected to be more accurate that those obtained from logging the actual core. Thelogger cannot always be certain as to the origin / cause of observed breaks in core samples and

Azimuth = 175.8Dip = 31.2

Azimuth = 182.9

Dip = 43.0

DEPTH



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thus the room for error to incorrectly ‘code’ observed breaks is much higher and the resultantRQD will be somewhat erroneous.

Figure 3.2.2: Joint traces indicated on acoustic televiewer data. Azimuth, in degrees,

measured clockwise from True North (0°) and Dip, in degrees, measured down-dip from horizontal (0°).

In addition, there are several issues regarding RQD determination that indicate that it should notbe used as a critical input factor for slope stability analysis, rather, it can be used in a morequalitative or descriptive capacity. Some of these issues (Lindsay et al., 2001; Hack, 2002) are:

• The value of 10 cm of unbroken core to measure RQD is arbitrary. A rockmass with adiscontinuity spacing of 9 cm will have an RQD of 0%, while the same rockmass with adiscontinuity spacing of 11 cm will have an RQD of 100%. Will a 2 cm difference indiscontinuity spacing actually have a 100% impact on rockmass behaviour?

• RQD determination by core logging is influenced by drilling method and equipment,operators and core handling etc.

• Core diameters are not standardized and smaller core diameters tend to result in lowerRQD values.

Azimuth = 194.9

Dip = 38.1

Azimuth = 179.1

Dip = 56.2

Azimuth = 190.1Dip = 41.7

DEPTH



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3.3 SummaryThe three selected probes are considered suitable to provide the necessary minimum data inorder to describe a rockmass in terms of lithological and geotechnical characteristics. Theprocessed data from these probes comprise the basic inputs into a predictive methodology forhighwall slope stability hazard rating.

4. BENEFITS AND SHORTCOMINGS

There are both positive and negative aspects to geophysical wireline and physical core loggingpractices. It is not the purpose of this investigation to veto one practice in favour of the othersince both have their place in geological and geotechnical investigations, and there is no singlereason why both cannot be applied to one borehole.

4.1 Physical Core Logging: Lithological and GeotechnicalSome of the shortcomings regarding the logging of physical core samples have been mentionedin previous sections and these are summarized as follows:

• Depth and thickness measurements: errors may be introduced due to core loss by

grinding and incomplete core recovery (this is often the case with pulling the “final run”)in the absence of accurate core recovery and depth adjustment calculations;

• Core condition (mostly applicable to geotechnical logging): in terms of additional corebreaks, core condition is heavily influenced by drilling method and technique, drillingrates, core recovery (out of core barrel and hole), handling, transport and storage;

• Exposure: core exposed to ambient atmospheric conditions influences the degree ofslaking and additional core breaks may thus be introduced;

• Orientation: the majority of geological boreholes drilled on mines produce non-orientedcore samples.

Not only do some of these shortcomings adversely affect the integrity of geological models, theyalso preclude the use of certain geotechnical observations and measurements in structural

modelling.

Key benefits of obtaining core samples from geological boreholes are:

• Core samples may be subjected to mechanical and chemical analysis;

• Observed geological discontinuities may be described in detail regarding, for example, joint condition (intact or broken), presence and type of in-fill material, and joint surfacecondition (smooth, rough, slicken-sided etc.).

In terms of rockmass behaviour and slope stability assessment, some of the benefits of corelogging are critical and therefore necessary.

4.2 Geophysical Wireline LoggingFor an interpretation to be meaningful, the data acquired must be of good quality. This impliesthat, not only must the geophysical probes and accessory equipment (winch, cable etc.) bemaintained in good working order, the borehole to be surveyed must be drilled to a highstandard in order for data to be reliable and meaningful. Winch slippage and cable stretch willresult in incorrect depth and thickness measurements, introducing errors in interpretation. Dataresolution is primarily a function of the rate at which the probe is raised from the borehole aswell as the recording interval. Depending on the type and intended use of the data beingacquired, inappropriate data resolution may adversely affect subsequent interpretation.

Another factor affecting depth, and in some cases, thickness measurements, is silting-up at thebottom of the borehole, mainly due to water ingress (Table 3.1.1). Silting-up is a function of the

rate of water ingress and the time window between completing drilling and wireline logging.Core remaining at the bottom of the hole, as a result of incomplete core recovery, will also resultin incorrect depth and thickness measurements.



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Televiewer data quality, particularly optical televiewer data, is highly susceptible to borehole wallcondition, specifically in terms of cleanliness. Cleanliness of the borehole wall is a function ofdrilling methods and techniques and these will need consideration before embarking on awireline logging programme. In certain instances it is necessary to flush and clean the boreholeprior to geophysical logging, and in a production environment, this may introduce certainlogistical difficulties involving water cartage and pumping.

Perhaps the most significant benefit of geophysical wireline logging is the fact that optical andacoustic televiewer data is oriented. The structural interpretation that can be done from goodquality optical and acoustic televiewer data is incomparable (Pers. Comm., Campbell, 2004). As previously mentioned, it is the relative orientations and attitudes of geological discontinuitiesand the exposed highwall that control the rockmass behaviour.

Since geotechnical logging of non-oriented core can only assist with RQD determinations,which, as has been mentioned, not to be of major importance, resources can better be spentcarrying out structural interpretations from televiewer data. If required, however, RQD can bedetermined from the televiewer data.

5. CONCLUSION

The suite of probes available (Table 1.1) for the identification of geological and geotechnicalfeatures allows for a detailed characterization of a rockmass. The scope of this investigation,however, requires that only certain geological and geotechnical characteristics need to beidentified and quantified, these being macro-lithology (coal and non-coal) and geologicaldiscontinuities (faults, joints etc.).

Five geophysical probes were utilized during this investigation. Of the five, only three wereconsidered necessary to provide the required geological and geotechnical information in termsof identifying geotechnical features applicable to the formulation of a predictive methodology for

slope stability hazard rating. The three probes thus identified are:• The density probe: to allow for macro-lithological differentiation;

• The optical televiewer: to allow for identification of geological discontinuities in dry holes;

• The acoustic televiewer: to allow for identification of geological discontinuities in wetholes.

The data acquired by these geophysical probes in geotechnical investigations will providesufficient information to allow for reasonable assessments to be made, after appropriateinterpretation and modelling, regarding slope stability of coal opencast highwalls.



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6. REFERENCES

Campbell, R. (2004). Personal Communication ([email protected]).

Hack, R. (2002). An evaluation of slope stability classification. In: C. Dinis da Gama and L.Ribeira e Sousa (Eds.). Proc. ISRM EUROCK’2002, Portugal, Madeira, Funchal, 25-28November, 2002. pp. 1-32.

Jeffrey, L.S. (2003). A preliminary investigation into the geotechnical interpretation ofgeophysical logs. COALTECH 2020 Task 2.15, Sub-task 1a, Report Number: 2004-0069. CSIR Miningtek, Johannesburg. 20p.

Lindsay, P., et al. (2001). Slope stability probability classification, Waikato Coal Measures, NewZealand. International Journal of Coal Geology (45). pp. 127-145.

McNally, G.H. (1990). The prediction of geotechnical rock properties from sonic and neutronlogs. Exploration Geophysics (21). pp. 65-71.

Reeves, D.R. (Ed.). (1981). Coal Interpretation Manual. BPB Instruments Limited,Loughborough, England. 100p.

Stewart, R.S. and Letlotla, S. (2003). The impact of geotechnical factors on high- and low-wallstability. COALTECH 2020 Task 1.4, Sub-task 1, Report Number: 2003-0190. CSIRMiningtek, Johannesburg. 43p.

Van Heerden, G. (2004). The impact of geotechnical factors on high- and low-wall stability.COALTECH 2020 Task 1.4, Sub-task 3a, Report Number: 2004-0174. CSIR Miningtek,Johannesburg. 13p.

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