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COALTECH 2020

INTERIM PROGRESS REPORT

Task 1.4

Geotechnical factors affecting high- and low-wall stability in opencast coal mines

Sub-task 3b

Wireline logging applicability for the identification 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 of identifying geotechnical features. Primary geotechnical features include joints and faults. Secondary geotechnical features include weak bedding planes and weathered sedimentary bands and zones. A full suite of geophysical probes was used to wireline log six geological boreholes 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 appropriate data, constituting the basic input into the final predictive methodology for slope stability hazard rating. 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 afforded to 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 use of WellCAD for interpretation of wireline data. Marianne Maccelari, Business Area Manager, Orebody Information, CSIR Miningtek, is thanked for 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 7 2.1 Sonic (full wave form) Probe 7 2.2 Density (gamma-gamma) Probe 8 2.3 Optical and Acoustic Televiewer Probes 8 2.4 Formation Dip-meter 9 2.5 Summary 9

3. WIRELINE, LITHOLOGICAL AND GEOTECHNICAL LOGGING RESULTS 10 3.1 Lithological differentiation: Lithological Core Logging vs Density Probe 10

3.2 Identification of geotechnical features: Geotechnical Core Logging vs Optical and Acoustic Televiewer Probes 11

3.3 Summary 14

4. BENEFITS AND SHORTCOMINGS 14 4.1 Physical Core Logging: Lithological and Geotechnical 14 4.2 Geophysical Wireline Logging 14

5. CONCLUSION 15

6. REFERENCES 16

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LIST OF FIGURES

Page

2.1.1 Correlation plot between sonic transit time and unconfined compressive strength 7

2.2.1 Typical density probe response curve through a sedimentary package 8 2.4.1.a Modelled seam floor elevation contours 9 2.4.1.b Resultant vector map of strata azimuth and dip, based on modelled contours 9 3.1.1 Actual density trace from Borehole A and associated macro lithology 10 3.2.1 Joint traces indicated on optical televiewer data 12 3.2.2 Joint traces indicated on acoustic televiewer data 13

LIST OF TABLES

1.1 Geotechnical features and related geophysical techniques 6 2.1 Geological / geotechnical data obtainable from probes used at New Vaal

Colliery 7

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

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

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1. INTRODUCTION The applicability of geophysical wireline logging techniques has been investigated in terms of identifying geotechnical features. Field investigations conducted in terms of Sub-tasks 1 through 3a of Task 1.4 (Stewart and Letlotla, 2003; van Heerden, 2004) have indicated that highwall slope stability is primarily a function of the presence, frequency, and attitudes of geological discontinuities (geotechnical features) and their relationships with exposed opencast coal highwalls. The purpose of this exercise is to determine if geophysical wireline logging can be applied in the development of a predictive methodology for slope stability hazard rating in opencast coal mines. Before the applicability of the wireline technique can be assessed, it is necessary to first determine which of the numerous geophysical probes / tools are best suited to the identification of geotechnical features. Principle geotechnical features include joints and faults, while secondary geotechnical features include weak bedding planes and weathered sedimentary bands and zones. Table 1.1 shows various geotechnical and geological characteristics that can be 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, resistivity Spectral 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 to delamination), structural feature identification and orientation (dip and dip direction)

Density, sonic, neutron, resistivity, acoustic televiewer, optical televiewer

Rock strength (UCS, tensile strength and strength moduli)

Sonic, neutron, density, resistivity, acoustic televiewer, 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 televiewer

Water ingress Temperature To facilitate the identification of appropriate geophysical probes, six boreholes drilled at Anglo Coal’s New Vaal Colliery, as part of the mine’s routine drilling programme, were selected for geophysical wireline logging. Five probes were used to geophysically log the six selected boreholes. The probes included full wave form sonic, acoustic televiewer, optical televiewer, density, and formation dip-meter. In addition to geophysical logging, all core samples were lithologically and geotechnically logged for comparative purposes. The results from this phase of logging were used to select the necessary tools to adequately identify geotechnical features. In addition to probe selection, critical benefits and shortcomings were identified regarding the application of geophysical techniques to the identification of geotechnical 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 of geological 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

Density Lithological differentiation (stratigraphic interfaces) and strata thicknesses, elastic moduli

Acoustic Televiewer (water-filled holes only)

Geological discontinuities (including dip and dip direction), principle stress direction, lithology identification

Optical Televiewer (dry holes only) Geological discontinuities (including dip and dip direction), 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 data sets are considered to be sufficient to allow for a preliminary assessment of highwall hazard regarding slope stability. The suitability of each probe is discussed in more detail below. 2.1 Sonic (full wave form) Probe Correlations between sonic velocity and uniaxial compressive strength (UCS) may provide a useful means of assessing rock strength (Pers. Comm., Campbell, 2004). In fact, an empirical formula was derived by McNally (1990) based on Australian coals and the relationship is shown in Figure 2.1.1. Some workers (Lindsay et al., 2001; Hack, 2002), however, are of the opinion that UCS is not a valid parameter in slope stability assessment, mainly because most rockmasses will, in reality, be stressed under conditions similar to those of triaxial tests rather than 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 Density (gamma-gamma) Probe Coarse lithological differentiation, in the present context, implies accurate (to within 10 cm) differentiation between coal and non-coal stratigraphic horizons. Non-coal stratigraphic horizons 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-coal strata within the coal-bearing Ecca sediments of South Africa, thus allowing for lithological differentiation 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 probe response 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 (modified after Reeves, 1981).

2.3 Optical and Acoustic Televiewer Probes These 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 dry holes, 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 core samples. Since any observed structural discontinuities (faults etc.) cannot be oriented with any certainty, no meaningful structural interpretations can be made. The optical and acoustic televiewer probes produce a 360° orientated image of the borehole wall. When used in conjunction with appropriate software, such as WellCAD, accurate and oriented structural information can be obtained, allowing for meaningful structural interpretations to be made. At most mine sites, boreholes will be dry in the upper sections and water-filled in the lower sections, 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-meter The mine-scale geological model is based on information from exploration and routinely drilled geological boreholes, and is updated on a regular basis. The model is accurate enough to allow for sedimentary strata azimuth and dip calculations to be made. Figure 2.4.1.a is a plot of seam floor elevation contours modelled from borehole data. Figure 2.4.1.b is a resultant vector map of 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 Summary Considering the geological and geotechnical characteristics that can be identified with these five probes and the data needed that cannot otherwise be accurately obtained from the geological model, only three probes are considered necessary. Therefore, data from three probes (density and 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 lithological and geotechnical logging results. Results from selected boreholes are presented here in order to illustrate comparisons, explain discrepancies, and to show that the use of these three probes for geotechnical investigations of this kind is sufficient. 3.1 Lithological differentiation: Lithological Core Logging vs Density Probe Macro lithological interpretation is achieved by processing the density probe data. Figure 3.1.1 is an actual density trace with the associated macro lithology indicated.

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

In many cases, the wireline depths and interpreted lithological thicknesses are considered to be more accurate than those obtained from standard lithological logging. This is mainly because there is a greater chance of core loss during drilling and core recovery than there is of winch slippage, both resulting in incorrect depth and associated thickness measurements. Although the 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 two boreholes 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 and

geophysical logging. Lithological Geophysical Seam

From To Thick From To Thick Top 28.00 38.06 10.06 28.00 37.99 9.99

Middle 52.08 59.16 7.08 52.10 59.68 7.58 Bottom 64.00 71.06 7.06 63.97 N/R N/R

N/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 no Bottom Seam thickness was determined from geophysical data. The Middle Seam thickness discrepancy is ascribed to core loss. The absence of a Bottom Seam base from geophysical data is due to silting up at the bottom of the borehole thus preventing the density probe from reaching the true end of hole. Table 3.1.2: Borehole C: Coal seam depth and thickness measurements from lithological and

geophysical logging. Lithological Geophysical Seam

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 to core losses and poor depth corrections rather than wireline logging inaccuracies. 3.2 Identification of geotechnical features: Geotechnical Core Logging vs Optical

and Acoustic Televiewer Probes Due to the nature of routine production drilling, which is typically a high monthly metreage, the resulting core is often highly broken due to high drilling rates, handling and transport. Thus, a comparison needed to be made between results of geotechnical core logging and computer assisted interpretation of televiewer data. Figure 3.2.1 is a section of optical televiewer data (black and white image) from a borehole where 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 dip angles determined. Figure 3.2.2 is a section of acoustic televiewer data (colour image) from the same 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. What is not visible in these images are discontinuities resulting from bedding plane separation and washouts (weak and/or weathered zones), although these types of discontinuity are readily discernable. A reasonable amount of exposure to interpreting televiewer data is, however, necessary before the interpreter can be confident when assigning discontinuity types to observed anomalies in televiewer data. Geotechnical core logging is complicated by the fact that the condition of the core is greatly influenced 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 samples result in additional breaks in the core which are, by definition, not geological discontinuities. In addition, exposure of the core to ambient atmospheric conditions will result in varying degrees of 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 from horizontal (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 is able to ‘pick’ only geological discontinuities. The relevance and accuracy, therefore, of information derived from geotechnical logging of core from routinely drilled geological boreholes is considerably less than that obtained from televiewer data. Geotechnical logging of non-oriented core samples can really only supply information useful for calculating rock quality designation (RQD) values. Since the televiewer data excludes breaks resulting 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. The logger cannot always be certain as to the origin / cause of observed breaks in core samples and

Azimuth = 175.8 Dip = 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 resultant RQD 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 not be used as a critical input factor for slope stability analysis, rather, it can be used in a more qualitative 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 a discontinuity spacing of 9 cm will have an RQD of 0%, while the same rockmass with a discontinuity spacing of 11 cm will have an RQD of 100%. Will a 2 cm difference in discontinuity 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 lower RQD values.

Azimuth = 194.9 Dip = 38.1

Azimuth = 179.1 Dip = 56.2

Azimuth = 190.1 Dip = 41.7

DEPTH

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3.3 Summary The three selected probes are considered suitable to provide the necessary minimum data in order to describe a rockmass in terms of lithological and geotechnical characteristics. The processed data from these probes comprise the basic inputs into a predictive methodology for highwall slope stability hazard rating. 4. BENEFITS AND SHORTCOMINGS There are both positive and negative aspects to geophysical wireline and physical core logging practices. It is not the purpose of this investigation to veto one practice in favour of the other since both have their place in geological and geotechnical investigations, and there is no single reason why both cannot be applied to one borehole. 4.1 Physical Core Logging: Lithological and Geotechnical Some of the shortcomings regarding the logging of physical core samples have been mentioned in 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 core breaks, core condition is heavily influenced by drilling method and technique, drilling rates, core recovery (out of core barrel and hole), handling, transport and storage;

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

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

Not only do some of these shortcomings adversely affect the integrity of geological models, they also 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 surface condition (smooth, rough, slicken-sided etc.).

In terms of rockmass behaviour and slope stability assessment, some of the benefits of core logging are critical and therefore necessary. 4.2 Geophysical Wireline Logging For an interpretation to be meaningful, the data acquired must be of good quality. This implies that, not only must the geophysical probes and accessory equipment (winch, cable etc.) be maintained in good working order, the borehole to be surveyed must be drilled to a high standard in order for data to be reliable and meaningful. Winch slippage and cable stretch will result in incorrect depth and thickness measurements, introducing errors in interpretation. Data resolution is primarily a function of the rate at which the probe is raised from the borehole as well as the recording interval. Depending on the type and intended use of the data being acquired, inappropriate data resolution may adversely affect subsequent interpretation. Another factor affecting depth, and in some cases, thickness measurements, is silting-up at the bottom 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 result in incorrect depth and thickness measurements.

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Televiewer data quality, particularly optical televiewer data, is highly susceptible to borehole wall condition, specifically in terms of cleanliness. Cleanliness of the borehole wall is a function of drilling methods and techniques and these will need consideration before embarking on a wireline logging programme. In certain instances it is necessary to flush and clean the borehole prior to geophysical logging, and in a production environment, this may introduce certain logistical difficulties involving water cartage and pumping. Perhaps the most significant benefit of geophysical wireline logging is the fact that optical and acoustic televiewer data is oriented. The structural interpretation that can be done from good quality optical and acoustic televiewer data is incomparable (Pers. Comm., Campbell, 2004). As previously mentioned, it is the relative orientations and attitudes of geological discontinuities and 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 spent carrying out structural interpretations from televiewer data. If required, however, RQD can be determined from the televiewer data. 5. CONCLUSION The suite of probes available (Table 1.1) for the identification of geological and geotechnical features allows for a detailed characterization of a rockmass. The scope of this investigation, however, requires that only certain geological and geotechnical characteristics need to be identified and quantified, these being macro-lithology (coal and non-coal) and geological discontinuities (faults, joints etc.). Five geophysical probes were utilized during this investigation. Of the five, only three were considered necessary to provide the required geological and geotechnical information in terms of 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 wet

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

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6. REFERENCES Campbell, R. (2004). Personal Communication (rcampbell@sctaust.com). 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, 25-28 November, 2002. pp. 1-32.

Jeffrey, L.S. (2003). A preliminary investigation into the geotechnical interpretation of

geophysical 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, New

Zealand. International Journal of Coal Geology (45). pp. 127-145. McNally, G.H. (1990). The prediction of geotechnical rock properties from sonic and neutron

logs. 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-wall

stability. COALTECH 2020 Task 1.4, Sub-task 1, Report Number: 2003-0190. CSIR Miningtek, 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.