FRACTURE ANALYSIS AT THE TASIAST OPEN PIT MINE IN WESTERN AFRICA *A. C. Gagnon & H. Saroglou Department of Civil and Environmental Engineering, Imperial College London South Kensington, London, United Kingdom (*Corresponding author: andre.gagnon13@imperial.ac.uk) 1
FRACTURE ANALYSIS AT THE TASIAST OPEN PIT MINE IN WESTERN AFRICA ABSTRACT The focus of this paper is on the analysis of geometrical discontinuity properties for the rock mass located within an open pit mine located in Western Africa. Discontinuity data were collected using various techniques including oriented core logging, borehole televiewer surveys, surface pit wall mapping and photogrammetry. The geological setting was initially studied prior to the data analysis to produce threedimensional ground models of the regional and mine site geology along with block diagrams illustrating predicted fracture patterns. Stereographic and statistical discontinuity data analysis of the geometrical properties of the rock mass was subsequently performed to identify unique structural domains throughout the open pit area. The discontinuity data collected throughout the studied open pit area were primarily discontinuity orientation, frequency and spacing and the investigation focused on the reasons for observed differences between the various data collection techniques. Stereographic analysis was used to assess the preferred geometry of discontinuities filtered by lithology, data collection technique, borehole, discontinuity type and pit wall (footwall or hanging wall). The observed fracture pattern is explained adequately by the geological setting. The need for accurate ground models for the design of open pit slopes is highlighted. KEYWORDS Fracture, Open Pit, Stereographic, Statistical, Analysis INTRODUCTION The Tasiast Mine is an open pit operation located in north-western Mauritania, approximately 300 kilometres north of the capital Nouakchott. The Tasiast mine, which is owned and operated by Kinross Gold Corporation (Kinross), began commercial operations in 2008. The Tasiast mine is located in an area characterized by flat desert terrain within the western reaches of the Sahara Desert region. The current mine site includes mill buildings, shop facilities, waste dumps, tailings facilities and two main open pit operations (Piment and West Branch). The West Branch pit is the focus of this paper. The current open pit plan for West Branch indicates a final pit measuring approximately 2.5 km long by 1.4 km wide and 510 m deep. Rock slope stability is often associated directly with the presence of discontinuities or weak links in natural materials. Discontinuities have many geometrical and mechanic properties such as orientation, shape, and size, which often define the behaviour of the rock mass (Hudson & Harrison, 1997). Geological structures such as discontinuities can define potential rock slope failures. Therefore, the design of open pit slopes requires the development of a structural geology model, which divides the proposed pit into regions of homogeneous geological structure. Folds, major faults, or geological contacts may separate these regions. Design discontinuity sets can be interpreted for each structural region using oriented core, borehole televiewer surveys, photogrammetric mapping, and pit wall mapping data (Read & Stacey, 2009). The existing structural geology model for the open pit is based on geological and geotechnical data collected using various techniques. Techniques performed include surface pit wall mapping, oriented borehole drilling, and digital methods including televiewer surveying and photogrammetry. The study focused on field data collected 2
on orientation, fracture frequency and spacing, and identified structural patterns in the discontinuity data within the open pit. Field data were statistically analysed and corrected for sampling biases, where possible, to increase confidence in the data set. Regional Geologic and Tectonic Setting GEOLOGY AND FORMATION OF FRACTURES There is generally a good correlation between discontinuities and the geological environment in which they develop. The geometrical properties of discontinuities are typically very important when assessing slope stability of rock slopes in open pit mines. Therefore, a detailed understanding of the geological setting and formation of the discontinuities is paramount before any discontinuity analysis is conducted. The West Branch open pit is located in the western region of the Reguibat Shield, which spreads across the northern region of the West African Craton. The West African Craton can be described according to Goodwin (1996) as a tectonically stable body made up of Precambrian rocks of various types and ages. The rocks throughout the region have undergone at least two major tectonic deformation events. The first event included east-west compression resulting in north-south folding, shearing and thrust faulting. The second major event included strong northwest-southeast compression. This second event produced tighter folding and shearing, brittle north-south strike slip faulting, low angle reverse faults and second order faulting and shearing. The rocks were then later transected by discontinuous east-west and northeastsouthwest mafic dykes, which are understood to have probably formed during the relaxation period following the northwest-southeast compression where the dykes followed the fractured fault zones. The open pit is located in an intraplate region where, according to Zoback (1992), the maximum principal stress is horizontal and compression is dominant. The major principal horizontal stress is estimated to trend approximately east-west based on the historic tectonic setting. The stress regime of the area can be classified as mixed mode faulting category T/SS (predominantly thrust faulting with strikeslip components) based on the definitions of Zoback (1992). Mine Site Geology Rocks in the region where the open pit is located generally consist of alternating volcanic and sedimentary units. They have been separated by mafic dykes and have been metamorphosed to midgreenschist to lower amphibolite peak-grade and have undergone multiple deformation events. The overall stratigraphic sequence includes meta-sedimentary rocks that were deposited within rift basins that formed on top of meta-volcanic rocks which are underlain by Precambrian basement rocks. Main lithologies include the greenschist zone (diorites and basalts) (GST), felsite (FVC), banded iron and magnetite formation (BIM), siliciclastic meta-sedimentary rocks (SVC), and mafic dykes (MDO/MGO). The dominant orientation of foliation within the open pit is moderately to steeply dipping towards the east to northeast. Structural Geology The West Branch pit is hosted within a package of strongly folded and sheared rocks in the hanging wall block of the Tasiast thrust system. The Tasiast thrust is the lowermost structure and higher level shear zones are interpreted to by splays. It is likely that the Tasiast thrust is linked to a system of deeply rooted ductile faults that would have formed as normal faults during the basin-forming rift event. According to Price and Cosgrove (1990), many reverse faults likely originate as normal faults and are subsequently reactivated by changes to the stress field. This reactivation due to a changing stress field aligns well with the historical tectonic setting at West Branch. Therefore, reverse faults commonly dip with similar angles (approximately 60 to 70 ) as normal faults but exhibit the reverse sense of movement. Fold repetition is evident within the Tasiast thrust system. Folds are associated with many of the major thrust systems (Price & Cosgrove, 1990). When the fault movement occurs in the basement rocks, folds form an important component of deformation within the overlying rocks. The dominant orientation of 3
foliation within the West Branch pit is moderately to steeply dipping towards the east to northeast. Overall, the dip angle of the thrusts and of the foliation steepens from the southern part of the West Branch pit toward the north. The Tasiast thrust and its splays are commonly situated at the contacts of lithological units, suggesting that the thrusts are the result of strain localisation at these boundaries. Ground Models and Fracture Block Diagrams Three-dimensional ground models illustrating the aforementioned regional geology and mine site scale geology are illustrated in Figure 1 and 2. The regional-scale conceptual ground model includes the main rock assemblages and post-tectonic granites along with major thrust and strike-slip faulting patterns characteristic of the historical tectonic history and stress regime. Precambrian orthogneiss basement rocks greatly underlie the major rock types and are therefore not shown at the chosen scale. Figure 1. Regional-scale conceptual ground model 4
Figure 2. Mine site-scale conceptual ground model Block diagrams are helpful in presenting orientation measurements qualitatively and can help illustrate the relationship between the excavation and the structure of the rock mass (ISRM, 1978). The maximum principal stress orientation is also presented on the block diagrams to aid in structure orientation evaluation. Block diagrams for minor and major structures expected based on the geological environment and historical tectonic setting within the open pit area are shown in Figure 3 and Figure 4. The block diagrams illustrate the main parameters that characterise the rock mass including discontinuity characteristics such as orientation, frequency and spacing, and the dimensions and shape of the blocks of rock. Figure 3. Predicted rock mass block diagram of major structures 5
Figure 4. Predicted rock mass block diagram of minor structures Formation and Types of Fractures According to ISRM (1978), the term discontinuity can be applied to any fracture in a rock mass that has zero or low tensile strength. It is a general term for most types of joints, bedding planes, foliation and schistosity planes, broken or shear zones, and faults. The significance of discontinuities is that they form planes of weakness within a rock mass such that failure tends to occur preferentially along these surfaces (Wyllie & Mah, 2004). The main discontinuity types found at the open pit include joints, foliation, bedding, faults and shear zones. In metamorphic rocks, such as those found throughout the open pit area, one joint set typically forms parallel to the foliation and two or more joint sets form at approximately right angles to this orientation (Terzaghi, 1946). Foliation is expected to be the dominant discontinuity in terms of minor structures within the rock mass at the open pit along with secondary joint sets oriented approximately perpendicular to the foliation as described above. In the footwall of the thrust system, planar block failures are expected to occur due to the orientation of the foliation. However, in the hanging wall, where the rock mass is expected to be more fractured, toppling failure is expected due to the pit wall orientation relative to the foliation. Major structures including thrust faults and shear zones oriented approximately parallel to the main lithological contacts, and dominant foliation angle are expected throughout the open pit. Mafic dykes and strike-slip faults with a southwest strike are also expected within the rock mass. Strike-slip faults are a result of shearing due to east-west compression and created fractured zones that acted as conduits for mafic dykes to reach the surface. GEOTECHNICAL INVESTIGATION FOR DETERMINATION OF FRACTURE PATTERN Geotechnical Investigation 6
The geotechnical site investigation comprised eight geotechnical boreholes drilled to collect additional geotechnical data within the West Branch pit (Golder Associates, 2013) in order to augment the data used in a previous study (URS Scott Wilson, 2012). The data collected previously by URS were not used in this study. The locations and orientations of the 2013 geotechnical boreholes in relation to the pit outline are shown in Figure 5. Parameters and observations logged during the 2013 geotechnical campaign were collected for each drill run interval were used to assess discontinuity orientation and spacing data and include the following: a) Tore core recovery (TCR), b) Solid core recovery (SCR), c) Rock Quality Designation (RQD), d) Fracture Count (per run), e) Fracture Type, and f) Alpha and beta angles (dip and dip direction) for discontinuity orientation. Additional discontinuity data were collected by Kinross from exploration holes holes between 2011 and 2013. The geotechnical data, combined with orientation data collected by televiewer and photogrammetric surveys along with surface pit wall mapping, provide the basis for the discontinuity analysis. The fractures were distinguished in the following types: a) Axial Plane, b) Bedding, c) Fault, d) Fold Axis, e) Foliation, f) Joint, and g) Shear. Fracture Orientation Data Sources Figure 5. Geotechnical Borehole Plan (modified from Golder, 2014) Gathering structural data on discontinuities and estimating how their orientation and spatial distribution characteristics vary across the walls of an open pit mine is one of the most important activities in open pit slope design because the data are used as the main input for slope stability analyses (Read & Stacey, 2009). The orientation of discontinuities relative both to each other within the same set and to any structure or excavation face is vital in geotechnical engineering design. Four data collection methodologies were used to determine the orientation of rock mass structures encountered within the West Branch pit including: 1. Core orientation using the Reflex ACT II core orientation device on eight oriented boreholes. 2. Borehole televiewer (BHTV) acoustic and optical surveys performed on eight boreholes, 3. Surface photogrammetric surveys on exposed pit walls, and 4. Surface pit wall mapping The geotechnical data used in this study only includes a subset of the extensive greater data set that has been compiled for the West Branch pit area. 7
Oriented Core and Televiewer Surveys The discontinuity orientation data obtained from the borehole core for each of the eight geotechnical boreholes were evaluated in Rocscience s stereonet analysis program, DIPS (version 6). Orientation data were first analysed using stereographic projections to identify the main discontinuity sets. To account for orientation data bias, the Terzaghi (1965) correction was applied to allow for the bias in sampling orientation, and to more accurately represent the populating of discontinuities (Wyllie & Mah, 2004). A convenient method for inspection of error in orientation data is to plot the orientation data assuming a vertical borehole. This assumption centres the alpha angle contours relative to the centre of the stereonet. The orientation data were filtered in DIPS by borehole, lithology, discontinuity type, and pit wall (footwall or hanging wall). Blind zones for each borehole were assessed for each stereonet type to illustrate those orientations where structures would likely not be identified in the boreholes. BHTV orientation data for the eight boreholes were processed using data files provided by Kinross including discontinuity type, depth, dip, dip direction and aperture. These data were imported into DIPS for analysis and compared with structural data obtained from the oriented core and surface pit wall mapping techniques. Surface Mapping and Photogrammetry Discontinuity orientation data obtained from surface pit wall mapping were analysed in DIPS to determine representative concentration of poles for various discontinuity types. A total of 444 dip and dip direction measurements were included in the pit wall mapping data set. Discontinuity orientation data obtained from photogrammetric digital surveying on exposed pit walls was analysed in DIPS to determine representative foliation and joint sets. A total of 163 dip and dip direction measurements were included in the photogrammetry orientation data set. DISCONTINUITY ANALYSIS Initial data filtering identified poor quality orientation data collected from the oriented core logging method. A comparison plot between the oriented core data and televiewer data for one of the boreholes is presented in Figure 6. The orientation data obtained from the televiewer surveys matched the expected discontinuity patterns as described in Section 2.4 and were considered representative of site Large cluster of foliation measurements dipping towards the NW, which does not match the dominant foliation dip direction within the pit. Large concentration of poles for foliation aligns very well with the known dominant foliation orientation within the pit area. conditions. Figure 6. Orientation data collected in borehole H4 using oriented core (left) and televiewer (right) The remaining three collection techniques were deemed to be of higher quality than the oriented core method due to various sources of error. The orientation data analysis indicates that the predominant orientation of discontinuities within the West Branch pit moderately to steeply dip towards the northeast following the orientation of the main lithological contacts. One predominant joint set was identified that is oriented approximately perpendicular to the dominant foliation set, which aligns with the expected fracture pattern. The large scatter in the joint data measurements from all collection methods does not allow for a second joint set to be identified with a reasonable amount of confidence. 8
The frequency and total spacing of discontinuities were calculated using Rock Quality Designation (RQD) and Fracture Index (FI) values obtained from core logging for each borehole, lithology, and pit wall. Total discontinuity frequency was calculated using RQD values for each drill run following the method outlined in Hudson (1987) and Priest (1993), which assumes a negative exponential distribution for discontinuity frequency. A negative exponential distribution is applicable for properties of discontinuities such as their size and spacing (Wyllie & Mah, 2004). No frequency or spacing measurements were available from the surface pit wall mapping and photogrammetric methods. The recorded FI (per run) values were highly variable and inconsistent throughout the borehole data. Therefore, total spacing values calculated using FI were not considered reliable. Discontinuity frequency and spacing analysis indicate that total spacing values calculated from RQD measurements made during borehole core logging do not show major differences between the main lithological units and pit walls (footwall and hanging wall) as shown in Figure 7. One important consideration to point out is that uncertainty exists in the spacing data analysis since it is often difficult to obtain accurate estimations of joint set spacing from oriented core data (Park & West, 2002). Joint set spacing estimations from oriented drilling are typically wider than those obtained from surface mapping. Figure 7. Total spacing (m) calculated from RQD for the footwall (left) and hanging wall (right) Stereographic analyses of the recorded discontinuities were performed for the various lithologies, discontinuity types, pit walls, and collection methods. Based on the results of the comparative data analysis on the orientation data of the main lithological units, there doesn t appear to be a large difference in the foliation orientations. All of the lithologies show foliation dipping towards the northeast with the exception of the BIM unit, which dips more towards the east. However, this orientation is only based on 40 individual measurements, most of which are from the less reliable oriented core method. The same applies to the GST unit, which again has very few orientation measurements because of the previously selected borehole locations and orientations. The main joint set identified in all lithologies dips towards the southwest. For the GST and MDO units, limited data measurements reduce the confidence level in the identified orientation set. Based on the current data set, this comparative analysis indicates that differences in discontinuity orientation are not related to lithology. Orientation data collected from oriented core logging was deemed mostly unreliable after data filtering. Three main joint sets were identified using stereographic projections from 120 data measurements including two sets moderately to steeply dipping towards the west to northwest and one set moderately dipping towards the east to southeast. The number of measurements does present some uncertainty in the set orientations since a minimum of 150 is considered adequate for a reasonable confidence level in the data. The eight bedding measurements show a similar orientation to the first joint 9
set and were probably incorrectly logged as bedding. The orientations of the faults align well with known faults in the deposit area with one trending east-west and the other two trending west to northwest. Two clusters of poles for foliation measurements were identified which do not align with the expected dip angle of the known foliation set, particularly the first set, which has a dip angle of 17 degrees. The other identified foliation set dips more to the east than the known foliation orientation and does not align well with the data collected by other methods. Only joint and foliation discontinuity types were logged during the televiewer image processing and thus no measurements exist for the remaining main discontinuity types including faults, bedding, and shear zones. Two main joints sets were identified using stereographic projections from 417 data measurements. Both sets dip towards the west to southwest but with very different dip angles (shallow and steeply dipping). The steeply dipping joint set (71/247) aligns very well with the expected fracture pattern presented in Section 2.4 as it is oriented approximately perpendicular to the foliation. The foliation data measurements indicate a very high confidence in the data quality as they very accurately match the known foliation set orientation throughout the deposit area. Surface pit wall mapping measurements were made throughout both the footwall and hanging wall zones of the West Branch pit. Orientation data collected by surface pit wall mapping performed by Kinross Tasiast s technical personnel indicate a dominant foliation set with a dip and dip direction of 47/074, which is consistent with the known dominant foliation set orientation. Both identified joint sets appear consistent with the expected fracture pattern and match the measurements made by other collection methods. Orientation data collected by photogrammetric surveying indicate a dominant foliation with a dip and dip direction of 47/078, which is consistent with the known foliation set throughout the West Branch pit area. When filtering out the foliation orientation data measurements, one representative steeply dipping joint set exists with an orientation of 88/004, along with two lesser joint sets with orientations of 59/242 and 64/080. This steeply dipping joint set that dips toward the north has been observed in pit wall photographs and is consistent with the current geological model prediction presented in Section 2.9. This joint set is oriented approximately perpendicular to the main foliation set, which is expected for metamorphic rocks such as those found throughout the West Branch deposit. For the photogrammetric data set, no distinct difference in orientation patterns was observed when filtering the data for the western side (footwall) and the eastern side (hanging wall) of the pit. The orientation of the main foliation set within the hanging wall zone is very similar to the one measured in the footwall zone, which dips towards the northeast at 50/075. The orientation of the foliation set does not appear to change in dip direction for the two zones of the West Branch pit. However, the dip angle in the footwall zone is slightly less steep than the foliation dip angle within the hanging wall zone. This could likely be due to the hanging wall rock mass being subjected to more deformation and shearing than the footwall rock mass. Discrepancies can exist in data quality and accuracy between the various collection methods. Discontinuity sets obtained from oriented core drilling are usually more dispersed than the same sets obtained through surface mapping methods. This is due to a scale effect introduced when measuring the orientation of a discontinuity from a piece of core. Other sources of error include human error associated with the physical measurement of the alpha and beta angles. Measurement errors in surface mapping techniques have been reported to be as much as ±10 degrees for dip directions and ±5 degrees for dip angle (Brown, 2007). However, for digital photogrammetry, differences of only ±1 degree are now possible for measurements of dip and dip direction. CONCLUSIONS The geological environment in which the West Branch pit at the Tasiast Mine is situated includes heavily sheared and deformed rock masses. Discontinuities form as a result of geological processes such 10
as compression or cooling and thus are typically directly related to the geological setting. Despite the complex conditions, dominant fracture patterns exist within the rock mass including the east-northeast oriented foliation. Major structures such as faults and shear zones mostly parallel the main lithological contacts and dominant foliation orientation. Data analysis on the geometrical properties (orientation, frequency and spacing) of the discontinuities initially included bias corrections and data filtering to produce a more reliable and accurate data set. Orientation measurements made using the oriented core method were mostly poor quality with five out of the eight boreholes being discarded prior to subsequent stereographic and statistical analyses. The majority of the total spacing histograms produced for each borehole exhibited an exponential distribution, which is expected for discontinuity spacing values according to previous research findings (Wyllie & Mah, 2004). The main findings of the data analyses indicate that the orientation and spacing for joints and foliation do not vary significantly between the main lithological units within the West Branch pit. ACKNOWLEDGMENTS The authors wish to thank Jerry Ran and Gillian Gardhouse at Kinross Gold Corporation who contributed their efforts to make this research study a success. REFERENCES Brown, E. T. (2007) Block Caving Geomechanics, 2nd Edition. JKMRC, Brisbane. Golder Associates (2014) Geotechnical Study in Support of West Branch Expansion. Golder Associates (UK) Ltd., Report Number: 12514150422.502/A.0. Goodwin, A. M. (1996) Principles of Precambrian Geology, London, Academic Press. Hudson, J. A. (1987) The understanding of measured changes in rock structure, in situ stress and water flow caused by underground excavation, 2nd International Symposium on Field Measurements in Rock Mechanics, Kobe, Japan. Hudson, J. A. & Harrison, J. P. (1997) Engineering Rock Mechanics An Introduction to the Principles, Oxford, UK, Elsevier Science Ltd. ISRM (1978) International Society for Rock Mechanics, Commission on Standardization of Laboratory and Field Tests. Suggested methods for the quantitative description of discontinuities in rock masses. International Journal of Rock Mechanics and Mining Sciences and Geomechanics Abstracts, 15, 319-368. Park, H. J. & West, T. R. (2002) Sampling bias of discontinuity orientation caused by linear sampling technique, Engineering Geology, 66, 99-110. Price, N. J. & Cosgrove, J. W. (1990) Analysis of Geological Structures, New York, Cambridge University Press. Priest, S. D. (1993) Discontinuity Analysis for Rock Engineering. London, Chapman and Hall. Read, J. & Stacey, P. (2009) Guidelines for Open Pit Slope Design, Melbourne, CSIRO Publishing. Rocscience Software, DIPS program, Version 6.0 11
Terzaghi, K. T. (1946) Rock defects and loads in tunnel supports. Rock tunneling and steel supports. R.V. Proctor and T.L. White, eds., Youngstown, Ohio, The Commercial Shearing and Stamping Co. Terzhagi, R. D. (1965) Sources of error in joint surveys. Geotechnique, 15, 287-304. URS Scott Wilson (2012) Tasiast Expansion Project, Mauritania: West Branch Open Pit Slope Stability Analysis. Ashford, UK Wyllie, D.C. & Mah, C.W. (2004) Rock Slope Engineering, Fourth Edition, Frances and Taylor. Zoback, M. L. (1992) First- and Second-Order Patterns of Stress in the Lithosphere: The World Stress Map Project. Journal of Geophysical Research, 97 (B8), 11,703-11,728. 12