Rock Mechanics for Natural Resources and Infrastructure ISRM Specialized Conference 09-13 September, Goiania, Brazil CBMR/ABMS and ISRM, 2014 Borehole Camera And Extensometers To Study Hanging Wall Stability Case Study Using Voussoir beam - Cuiabá Mine Reuber Ferreira Cota Anglogold Ashanti, Sabará, Brazil, rfcota@angolgoldashanti.com.br Rodrigo Peluci de Figueiredo Federal University of Ouro Preto, Ouro Preto, Brazil, rpfigueiredo@demin.ufop.br SUMMARY: Cuiabá mine, owned by Anglogold Ashanti Córrego do Sítio Mineração, is located in Sabara-MG, with excavations more than 1100m below surface, is one of the most important underground gold mines in Brazil. In recent years, there have been significant problems of instability of the hanging wall (HW) in some stopes (production excavation). In order to understand and anticipate the problems of instability of the hanging wall, a monitoring system was implemented consisting of televised boreholes, in the walls of the excavation. This was in addition to the large number of Multiple Point Borehole Extensometers (MPBX) and SMART cables (Stretch Measurement to Assess Reinforcement Tension) installed in the mine. This paper presents an example of the identification from monitoring, in the Fonte Grande Sul orebody level 10.2 stope (about 680m below surface), with evidence of instability in the hanging wall. The observation of borehole cracks, shears, failures, and displacements, indicated the beginning of instability in the hanging wall, which allowed measures to be taken to stabilize this area. A detailed follow-up confirmed the stabilization after actions have been implemented. In order to exploit the data collected during the process of study and to attempt to validate a simple method for evaluating the stability of the hanging wall in schist, a stability study was performed using the voussoir arch theory. Despite the identification of the thickness of the beams formed within the hanging wall, the geological complexity, evidenced by interbedded rocks with different elastic characteristics and strength, folds and boudinage, which was beyond the simplification of the calculations, did not allow a proper assessment of the stability of the studied area using the voussoir arch theory. KEYWORDS: Underground mine, rock mechanics, borehole camera, extensometers, voussoir beam. 1 INTRODUCTION Cuiabá mine, owned by Anglogold Ashanti Córrego do Sítio Mineração, located in Sabara- MG, with excavations more than 1100m below the surface, is one of the most important underground gold mines in Brazil, with annual production of approximately 9 tons of gold. Following an increase in production in beginning of 2007, problems of instability, mainly in hanging wall, were identified in mining excavations (stopes). A study to understand and predict these problems was implemented comprising of constant monitoring with borehole cameras of boreholes with borehole cameras and extensometers MPBX (Multi Point Borehole Extensometer) and SMART (Stretch Measurement to Assess Reinforcement Tension) cables. A case study was conducted in Fonte Grande Sul (FGS), Level 10.2 stope, located between 700m and 665m below surface, which showed that working with the data collected by extensometers and the filming of boreholes, can be a powerful tool to recognize timely evidence of instability of the hanging wall, allowing that mitigation activities can be implemented to stabilize the area.
After processing the data related specifically to filmed boreholes, it was possible to recognize the thicknesses of the beams formed by the shear and cracks identified inside the holes in schistose rock in the hanging wall, thus, together with the identification of other parameters, an assessment was done on the applicability of the analogy of voussoir arch theory for the stability assessment of the hanging wall for the study area. 1.1 Location Cuiabá mine is located close to Sabara city, Minas Gerais state (Figure 1), 35 km from Belo Horizonte. instability. 2 CASE STUDY 10.2 FONTE GRANDE SUL STOPE (FGS) For a better understanding of all the mechanisms involved in this study, site characterization is presented, covering location, geometric characterization of the mining panel as well as the geological and geotechnical characteristics. After describing the study case, the interpretation of data obtained mainly by the filmed holes, it was possible to define the thickness of the beam, caused by the separation of cracks and schistose shears located in the hanging wall. With the geomechanical characterization of the hanging wall, the geometry of the excavation and knowledge of the thicknesses of the beams, it was possible to evaluate the applicability of the method voussoir modified by Diederichs and Kaiser (1999a), for the study of stability of schistose hanging wall. 2.1 Case Study Location Figure 1. Cuiabá mine location. 1.2 Objectives Case study is located in Fonte Grande Sul (FGS) orebody, level 10.2 between 700 and 665m below surface (Figure 2). The main objectives of this work are listed as follows: Study the evolution of deterioration in the stability of the hanging wall during the process of mining activities; Assess the effectiveness of the monitoring systems used, extensometers and filming of boreholes, as tools to identify timeously, the indications of instability in the hanging wall; Evaluate the effectiveness of mitigation activities for the stabilization for area with evidence of instability; To study the applicability of the analogy of voussoir arch theory as a mechanism for the stability of the area with indications of Figure 2. Case Study location. 2.2 Characterization 2.2.1 Stope Geometry Stope 10.2 FGS has a vertical height of 35m, strike of 460m, with the predominant dip of the
orebody at 30. The mining method applied is cut and fill with backfilling of waste rock and hydraulic fill. all of the hanging wall exposures were classified in bad rock mass with scores between 1 and 4. 2.2.2 Geological and Geotechnical Characterization Fonte Grande Sul orebody is located in the normal limb of the large tubular fold in Cuiabá mine. Orebodies located in the normal limb, in general have the following lithological sequence, from bottom to top, schistose metabasalts and schistose meta-andesite, forming foot wall; banded iron formation with sulfides, defining the ore (in the study area the thickness range between 7 and 8m); a layer of graphite phylite and sericite schist, both forming the hanging wall. Two major discontinuity families have been identified in the area. One family is defined by schistosity that is the main structure of the mine; the other family is defined by discontinuities with the same direction of schistosity, but with a 180 o difference of plunge direction (Figure 3). Figure 4. Rock mass map for 10.2 FGS orebody. Laboratory tests were done to characterize hanging wall rocks. These results can be visualized in the table 1. Table 1- Laboratory test results for hanging wall rock. Lithology Amount of samples Uniaxial Compressive Strength (MPa) Minimun Average Maximum Poisson (ν) Elastic Module E (GPa) Grafite phylite 9 12 63 149 0,13-0,25 12-51 Sericite schist 52 36 66 141 0,15-0,26 39-78 2.2.3 Support Characterization Figure 3. Family of discontinuities (Software DIPS 5.0). Geomechanical classification of wall hanging was carried out for 10.2 FGS stope using the Rock Mass Rating (RMR) classification (Bieniawski, 1989) and Q - Rock Tunnelling Quality Index (Barton et al, 1974.). It can be noted that there is little variation in the quality of the rock mass along the wall hanging (See Figure 4). The average hanging wall RMR was rated between 60 and 41. Q values for almost Plain strand Cablebolting, 9.5m in length and maximum axial strength of 25t to 27t were used in the hanging wall on a 1.5x1.5m pattern (Figure 5). Plates and barrels were installed on the cables. Approximately 1500 cables are installed on a monthly basis at Cuiabá mine. Ore Foot Wall Hanging Wall Figure 5. Stope 10.2 FGS with visualization of support.
2.3 Visual Scale of Cracks and Shears Borehole camera monitoring was started in Cuiabá mine for the identificiation and classification of cracks and shears. Following the collection of a large dataset, it was possible to make a visual scale of cracks and shears (Figure 6). This scale allow different levels of cracks and shears. Borehole camera monitoring of holes in the hanging wall (HW), with mining evolution, assisted to identify intense shearing in the schistose mainly when the holes were more further from stope face. 8). For stable areas, in general, a typical displacement is of ± 1.5cm near the HW face after blasting. Therefore this high displacement was a first indication of instability in this area. Figure 7. Vertical cross section in 10.2 FGS.. Figure 6. Visual scale of cracks and shears for holes with a diameter of 5 cm. 2.4 Monitoring and Actions Evolution For more adequate understanding of the case study, it is necessary to know the chronology of events that occurred, from the identification of signs of instability to the rehabilitation of the area with mitigation actions. The development of research and interventions are listed as follow: 15/11/2007 - A SMART cable (9.5m long) and an extensometer MPBX (15m long) were installed, about 2m far from each other (along the direction of the layer) on the hanging wall. In this moment the vertical height of the mining panel was about 15.5 m. The geometry of the stope can be seen in Figure 7. 05/12/2007 - Significant displacement was recorded (2.8 to 2.5 cm near the surface of the hanging wall) after the production blast. The behavior of the displacements recorded by the SMART cable and MPBX was similar (Figure Figure 8. Displacement graph (A) SMART cable and (B) MPBX. 29/01/2008 - Monitoring with borehole camera allowed the captuirng of images inside the borehole in the hanging wall. The inspected hole had a length of 15.5m and it was almost perpendicular to the schistosity. Shears of level 1 were identified approximately 7.0, 1.1, 0.5 and 0.05 m, measured from the face of the HW (Figure 9). 07/02/2008 - Another important displacement was recorded by the SMART cable and MPBX (about 3cm near the face of the HW - Figure 10). There was no activity at this location during this period. It indicated, again, that this area was with serious stability problems in the
hanging wall (HW). After the identification of the deterioration of rock mass conditions, cablebolting were installed plus the installation of plates and barrels as reinforcement. Hydraulic filling was also done in this stope. 14/03/2008 Borehole 1 was again filmed. Progresses in shears were identified when compared to the previous recording (Figure 12). Figure 9. Vertical section with cracks and shears inside hole 1 on 29/01/08. Figure 12. Vertical section with cracks and shears inside hole 1 on14/03/08. Figure 10. Displacement graph A) SMART cable and B) MPBX. 29/03/2008 Production blasting was done at the study area. 09/04/2008 - Borehole 1 was again filmed to evaluate the condition of the HW after blasting. High level of shearing was identified 7m (measured from the surface) blocking the hole (Figure 13). The extensometer MPBX apparently reached the limit of measurement, not detecting further displacement (Figure 14). 14/02/2008 The borehole camera was used in borehole 1 and level 1 shears were identified approximately 8.0, 7.5, 7.0, 6.1, 4.5, 4.0, 3.5, 3.0, 2.5, 1.1, 0.8., 0.75, 0.6 and 0.5 m measured from the surface of the HW (Figure 11). Figure 11. Vertical section with cracks and shears inside hole 1 on14/02/08. Figure 13. Vertical section with cracks and shears inside hole 1 on 09/04/08.
Extensive mechanical rock scaling was required to remove broken rock material. A brow of ± 2.5m was formed in the HW with a length of ± 10m, along the direction of schistosity, and 13m along the dip (Figure 16). Figure 14. MPBX displacement graph with televised hole information. Recordings with the borehole camera allowed the study to gradually evaluate the evolution of shearing over time. Some images of the development of shears can be viewed as a function of time for certain positions within the rock mass in Figure 15. Figure 16. Vertical section with cracks and shears and HW picture with brow after scaler machine working. A new MPBX was installed and other holes were drilled for borehole camera monitoring. The evolution of shear and crack and the identificiation by means of borehole camera filming in holes 1, 2, 3, 4, 5 and 6 can be seen in Figure 17. The quantity and magnitude of cracks and shears decreased after reinforcement with cableblolting was done. This is the most important indicator with respect to the borehole camera information, indicating the improvement of the HW geomechanical rock mass condition. Figure 15. Shear evolution over time for 1.1 and 7 m into the HW. In order to continue monitoring this site more holes were drilled for monitoring of with the borehole camera. Cablebolting were installed on denser pattern of 1.0 x 1.0 m and 9.6m length. Figure 17. Cracks and shear evolution. In the hole 6 there was no cracks or shears.
A comparison between the displacements collected by the 1 st MPBX, the phase of instability of HW, and the 2 nd MPBX installed subsequently after installation of cablebolting reinforcement can be seen in Figure 18. Figure 19. Study area after mining activities. 2.5 Voussoir Arch Theory Figure 18. Displacement graphs (A) MPBX with instability indication and (B) MPBX 2 after reinforcement installation. Through the analysis of Figure 18, we can identify significant difference in displacement between the instruments. It is noteworthy that the measuring time for MPBX 2 is much larger than for MPBX 1, furthermore the larger amount of blasting that occurred in the study area. The MPBX 2 installed after the application of reinforcement show typical displacement for areas without signs of instability. Mining activities has been successfully completed in the area. This was due to the identification of signs of instability, the installation of reinforcement and constant monitoring using extensometers and borehole camera. A current photo of the study area can be seen in Figure 19. In order to exploit the data collected during the process of study and to attempt to validate a simple method for evaluating the stability of the hanging wall in schist, a stability study was performed using the voussoir arch theory. First of all it was necessary to find the rigid limit below hydraulic/rockfill floor to be considered. For trying to solve this problem, 6 displacement graphs from MPBX and SMART cables were analyzed after they were covered with hydraulic/rockfill in the same stope; five extensometers did not identify displacement after blast number 3 (Figure 20) after being covered, or 6m along ore dip below the floor. Then span considered was normal span plus 6m. Figure 20. Displacement graph with blasting and moment of covered extensometer. In order to minimize errors related to lithologic recognition in all televising of holes in the study area, the interbedded graphite phylite was recognized immersed in sericite /chlorite schist (Figure 21).
Figure 21. Graphite phylite immersed in sericite/chlorite schist. The thickness of the beam to be used is 2.5 m on the graphite phylite which was removed with the scaler equipment. Another simulation used a beam thickness of 4.7 m because of the crack/shear identified in the first and second hole at the same depth within the hanging wall (Figure 22). allowed measurements could be done to stabilize the site. The success achieved after the implementation of stabilization measures, attested by monitoring, demonstrates the efficiency of extensometers and borehole camera as important tools to minimize the risk of fall of ground in the hanging wall. The analysis performed by calculation using the analogy of voussoir indicated a very stable situation. This result disagrees with the data obtained by monitoring. The difference between the stability analyzes, voussoir and monitoring, may be associated with frequent intercalations of rocks with different elastic properties and strengths that were identified within the hanging wall (Figure 23), beyond the possibility of improper choice of the beam thicknesses studied. Figure 21. Beam thickness used. All parameters used in the voussoir analogy e results for both simulation can be visualized in the table 2. The results do not indicate instability for both simulation. Table 2- Parameters used in the voussoir analogy to the simulation 1 and 2 and the results obtained. Parameter Simulation 1 Simulation 2 Span (m) 25 25 Thickness (m) 2.5 4.7 Rock Mass Elastic Modulus (GPa) 13.3 13.3 Specific Weight (KN/m 3 ) 28 28 Bedding angle 30 30 UCS (MPa) 63 63 Support pressure (KPa) 87.1 109 Buckling Limit (<35%) 2% 2% Crush Safety Factor 369.7 80.8 3 RESULTS The timely detection by the extensometers and the images obtained by borehole camera monitoring in the area with signs of instability, Figure 22. Example of graphite phylite intercalation. REFERENCES Diederichs, M.S., Kaiser, P.K. (1999a). Stability of Large Excavations in Laminated Hard Rock Masses: The Voussoir Analogue Revisited, International Journal of Rock Mechanics and Mining Sciences, Canada, v. 36, 97-117p. Diederichs, M.S., Kaiser, P.K. (1999b). Tensile Strength and Abutment Relaxation as Failure Control Mechanisms in Underground Excavations, International Journal of Rock Mechanics and Mining Sciences, Canada, v. 36, 69-96p. Hutchinson, D.J, Diederichs, M.S. (1996). Cablebolting in Underground Mines, Bitech Publishers Ltd, British Columbia, Canadá, 406p.