CONTRIBUTION OF THE GEOMECHANICAL CLASSIFICATIONS FOR OPTIMIZATION OF BENCH IN KIMBERLITE ROCK MASS (CASE IN STUDY CATOCA MINE)

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CONTRIBUTION OF THE GEOMECHANICAL CLASSIFICATIONS FOR OPTIMIZATION OF BENCH IN KIMBERLITE ROCK MASS (CASE IN STUDY CATOCA MINE) ABSTRACT. Patrícia Alexandra Silva Sebastião patricia.sebastiao@tecnico.

1 CONTRIBUTION OF THE GEOMECHANICAL CLASSIFICATIONS FOR OPTIMIZATION OF BENCH IN KIMBERLITE ROCK MASS (CASE IN STUDY CATOCA MINE) ABSTRACT. Patrícia Alexandra Silva Sebastião This dissertation addresses a relevant issue that occurs frequently in mine engineering. The instability of rock slopes, often result in landslides and falls of blocks, being responsible for material losses, and in loss of lifes. For the analysis of slope stability, the dominant factor is the discontinuities and their geometric characteristics. The criteria and types of rupture that can be observed in the rock mass are discussed. This paper use a case study from Lunda Sul, Angola, Africa. The case study is the diamond mine of the Catoca mining company located in Angola in the province of Lunda Sul. The systematic discontinuity of the ten slopes of exploitation in the Kimberly massif were studied systematically. The DIPS program, version 6.0, was used to analyze the existing families in the rock mass and the Rocplane and Roctopple for the determination of the safety factor developed by Rocscience. Some considerations on slope safety are made based on the safety factors obtained. 1. INTRODUCTION The main objective of this dissertation is to study the influence of mining works on the stability of rock masses using the parameters currently used in the geomechanical classifications of the rock masses. The "DIPS" program, version 6.0 for analysis of the families in the massif, and the Rocplane and Roctopple for the determination of the safety factor, developed by Rocscience, was used to address the importance of geomechanical classifications as measuring instruments of the rock mass quality during exploitation operations. To this end, a review of state of art done. The geomechanical classifications were developed with the purpose of characterizing the quality of the rock mass in a fast and inexpensive way using the information acquired with the surveys carried out in situ, with the attribution of a quality index. A classification that has been recognized as universal in this field has not yet been reached, although several authors (Bieniawski 1989) (Barton et al. 1974) (Hoek 1994) (Hoek et al. 1995). Provide some proposals for the classification of the rock mass that will be presented here. For the evaluation of slope stability some geomechanical classifications, such as that of Bieniaswlki that calculates the RMR Rock Mass Rating (Bieniawski, 1989), and Romana, which determines the SMR, Slope Mass Rating (Romana, 1993), have been used. 1

2 2. Geomechanical Classifications of Rock Mass The field of characterization of the rock mass has been experiencing considerable progress in recent years. In rock masses for some revisions in geomechanical classification have been carried out, the use of empirical methodologies, such as the RMR systems (Bieniawski, 1989), SMR Romana (1985) Q (Barton et al., 1974) and GSI Hoek et al., 2002), is common in slope stability. Table 1 Methods of rock mass classification RQD- Rock quality designation The rock quality designation (RQD) was developed by Deere (1963) to provide a quantitative estimate of rock mass quality from drill core logs. The Rock Mass Rating (RMR) System is a geomechanical classification system for rocks, developed by Z. T. Bieniawski between 1972 and It combines the most significant geologic parameters of influence and represents them with one overall comprehensive index of rock mass quality Q-system The Q-value determines the quality of the rock mass, but the support of an underground excavation is based not only on the Qvalue but is also determined by the different terms in the above equation. This leads to a very extensive list of classes for support recommendations. SMR - Slope Mass Rating by Romana In the evaluation of slope stability in rocky masses, Romana (1985, 1993, 2003) developed a classification system called SMR, a modification of the RMR system developed by Bieniawski. The SMR index is obtained by adding two adjustment factors to the RMR, one that depends on the relative orientation between discontinuities and the slope and another depending on the excavation method. RQD = Σi(>10cm) x 100 L 1. Uniaxial compressive strength of rock material 2. Rock quality designation(rqd) 3. Spacing of discontinuities 4. Condition of discontinuities 5. Groundwater conditions 6. Orientation of discontinuities Q = RQD Jn Jr X Ja X Jw SRF SMR = RMR89 + (F1 F2 F3) + F4 3. CASE STUDY KIMBERLITICAL SLOPES OF THE CATOCA MINE 3.2 Location of study area In administrative and geographic terms, the kimberlite chimney of Catoca is located to the Northeast of the Republic of Angola, in the Northwest part of the Province of Lunda Sul to 30 km of its Capital Saurimo. The territory of the SMC concession area is located within the topographic sheet 121-SG34 (scale 1: 1,000,000) of the Topographic Cadastre of the Angolan State, in a region located in the boundaries between the North and South Lundas, and occupies an area of 340 km2, delimited by coordinates of 20o15'00 "-20o24'15" of longitudes East and 9o18'00 "- 9o29'20" of South latitudes. The geographical coordinates of the Catoca chimney are: 20o18 'East longitude and 9o25' Of South latitude. 2

Figura 1 Location of study área 3.3 Exploitation Method The Exploitation includes the direct removal of rock material with excavators from 5-18 m 3 that is transported in trucks of 40-100 tons.

The main geometrical parameters of the mine are: Height of slopes (10-20 m); Height of workbenches (10 m); Height of non-operational temporary benches (10-30 m from the +960 elevation); slope angle

3 Figura 1 Location of study área 3.3 Exploitation Method The Exploitation includes the direct removal of rock material with excavators from 5-18 m 3 that is transported in trucks of tons. Hard rocks like gneiss are exploited with blasting produced at the Catoca mine. The main geometrical parameters of the mine are: Height of slopes (10-20 m); Height of workbenches (10 m); Height of non-operational temporary benches (10-30 m from the +960 elevation); slope angle of slopes (45º, 60º, 65º, 75º) for consistent gneisses (30º) for triple benches and (35º) for double benches; Width of the transport ways (20-30 m) Width of work fronts (30-60 m) Safety edge width (2-3 m) 3.4 Location of slopes The zone selected to analyze the instability of kimberlite rock slopes is located in the North, South, south-west and southeast of the mine (Fig. 2). Ten slopes were selected for measurements and tests as well as for sample collection. Figura 2 Location of slopes studied. 3

4 3.5 Methodology For this work a set of observations was made for each slope selected to determine the mechanical and geometric parameters of the rock masses. All measurements were performed using tape measure (aperture, spacing). In the case of roughness and weathering, as well as filling, an electronic compass was used for the orientation of the slopes, the necessary information was obtained through observation in the field. The classifications proposed by ISMR (1981) and By Bieniawski (1989), were applied. In order to determine the strength of the material (rock), a set of samples collected in the field were tested in laboratory. The values obtained are presented in table 2: Sample (cubos) (A1) (A2) (A3) (A4) (placas) (a1) (a2) (a3) (a4) Width (cm) Height (cm) Area cm 2 Tabela 2 Test results Length (cm) Strength (t) σ c (MPa) σ c média (MPa) σ t R.á tração (MPa) 4,78 4,75 4,77 4,77 33,61 115,6 128, ,8 4,67 4,74 4,74 45,76 157, ,77 4,74 4,77 4,77 41,09 141, ,79 4,74 4,76 4,76 28,24 97,1 - σ t. média á tração (MPa) - 1, ,88 5,39-1, , , , , , , ,54-4,83 1, ,9 - Granito 4,77 4,74 4,98 4,98 41,34 134,8 140,2 - - (cubos)a5 Granito A6 4,81 4,79 4,79 4,79 32, Granito 4,84 4,81 4,8 4,8 62,99 213, Granito - 4,8 4,79 4,79 30,09 101,9 - - Granito (placas) (a5) Granito (a6) Granito (a7) Granito (a8) - 1, ,44 7,87-2, , , , , , DATA ANALISIS Data analysis was performed, based on laboratory data and field observations that, later allowed the determination of the existing families in the rock mass, the material strength, and other parameters necessary for the use of Dip's, Roctopple and Rocplane software. 4

5 4.1. SMR calculation With the data collection of all the elements, it was possible to evaluate the SMR index for the different slopes studied,. It was done are this evaluation into two parts: determination of the RMR index, whose results are presented in Table 4 followed by the determination of the SMR index (Table 5). The RMR calculation are derived from the average observed / measured characteristics along the slopes studied by the author. Table 3 presents the values to be used in the calculation of the RMR index for each slope. Frequency values were obtained by the expression suggested by Priest and Hodson (1976) through the average frequency of discontinuities per linear meter (λ). A theoretical equivalent of the parameter RQD is calculated which quantifies the total of spacings between discontinuities equal to or greater than 0.1 m: RQD = e-0,1 λ(0,1 λ+1) 100 tal que: λ= 1 S [m -1 ] (4.1) Thus according to this procedure the RQD was determined for the slopes studied. Na exemple of the procedure for calculating the frequency of discontinuities, λ, for slope 1: λ= = 8.33 (4.2) Table 3 - Calculation of frequency of discontinuities Talude estudado Espaçamento (m) Frequência das descontinuidades Talude Talude Talude Talude Talude Talude Talude Talude Talude Talude

6 Slope Frequency of discontinuities (λ) (m -1 ) RQD index value Weigth RQD Weight resistance in uniaxial compression Weight spacing of discontinuities Weight of presence of water Table 4 RMR index calculation RMR calculation Discontinuity conditions RMRbasic Weight of persistence Aperture Weight Roughness Weight Filling weigh Weigth of the weathenig state of weathering Classe RMR II II II III II III III III III III RQD calculation example (T1): e- 0,1 λ (0,1 λ+1) 100 ; e-0,1 8.3(0, ) 100 = 78.6% **RMR calculation example (T1): RMR = = 75. Although the recommendations proposed by Romana (1993) are not directly applicable to the type of stabilization to be carried out, and a more careful study is need to the correct design of suport solution, a synthesis of possible measures to be taken is proposed for each slope studied table 5. Table 5 Calculation of the SMR index and stability works proposed by Romana (1993) Slope RMR F1 F2 F3 SMR SMRFinal Containment techniques proposed by Romana (1993) T (Bom) T (Bom) T (Bom) T (Bom) T (Razoável) T (Razoável) T (Razoável) T (Razoável) T (Razoável) T (Muito fraco) *F4= 0 For all families studied** Family studied for landslides. None; Slope foot ditches; Flexible seals; Sporadic clutches Slopes of slope or metal grids Systematic grooves or anchors Systematically designed concrete Wall support at the foot of the slope and / or concrete filling Slopes of foot of slope and / or metallic networks Systematic grooves Sporadically designed concrete 6

7 4.2 Stereographic analysis With the elements obtained, a study of stability analysis was carried out with the aid of two computer programs, the first one of which, Dips 6.0 of Rocscience, allowed to define the orientations of the main families of discontinuities, based on the existing families in Mina (Margarida, 2012) defined 4 main families existing in the rock mass. The results stability analisis obtained with were presented Dip's software. The Software to understand the orientation of the main families of discontinuities that contributed to the instabilization of the ten slopes studied at the Catoca mine. The types of instabilization analyzed were: Planar sliding; Wedge sliding; Toppling. 4.3 Safety Factor Evaluation of the slope stability can not be performed without the phenomena knwledge that can induce critical situations. The values of the safety factor determined by the Rocplane software are presented for the case under study. 1.5 was considered the lowest permissible stability value. The table below summarizes the values of safety factors for each slope. Table 6 - Summary of safety factor values calculated by Rocplane slope Safety factor T T T T T5 1 T6 1 T T8 1 T9 1 T Tabela 7 - Summary of safety factor values calculated by Roctopple Slope Safety factor T T T3 0.7 T T T6 0.7 T T T T

8 4.4 Analysis and discussion of results From the analysis of the results we can verify that the SMR (Slope Mass Rating) allowed the clear identification of three classes for the slopes studied, according to Romana (1993), slopes 1,2,5,9 that present an SMR between 61 and 72 are classified as Good, in the slopes 3,4,6,8, and 10 are classified according to Romana (1993), as reasonable, with an SMR between 41-58, the slope / which presents in this criterion as unstable, presents The SMR value of 19. With the help of Dip's 6.0 it is possible to observe the families of discontinuities that contribute most to instability of the slopes. With respect to slopes 1,2,5 and 9 it is observed that families do not produce instability regarding wedge formation. As regards slopes 5 and 9, still within the class of slopes of good quality, the possibility of slippage by toppling resulting mainly from the contribution of the 2m family is observed. With regard to slopes classified as reasonable according to SMR (3,4,6,8 and 10), Dip's allowed to conclude that all have at least one type of instability either by wedge or by toppling. The slopes 4,5,6, and 8 with respect to the probability of falling of blocks (toppling). There is unstable slope 4, 8 and 10 with probability of wedge generation. With regard to slope 8, families 2m and 3m act by allowing the formation of wedges, which is why it is attributed to the instability of this slope to this type of slip. The same analysis is performed for slope 7, as expected, given its low SMR value (19), the predominant break is by toppling with the largest contribution of the 2m family. The Rocplane was used for the study of the most feasible sliding plane in the stability analysis of wedges, the values of safety factor vary from , showing that only slope 1 and 2 could be considered stable, with respect to this Type of slip (FS> 1.5), all other slopes, show that its geometry is not the most adequate to guarantee the stability of this massif. This means that even class II embankments should be resized or reduced in height or lowering the slope angle. The Roctopple was used for toppling stability analysis and safety factors, ranging from , showing susceptibility of all slopes to landslide sliding, are susceptible. Systematic use of fire handles in the exploitation of the material promoting the increase of the existing fracturing. This fragmented aspect is just visible in figure 5.39, where it is also observed, the material placed for stabilization of the foot of the slope. Romana (1993), recommends in its classification the adoption of systematic corrective measures, for slopes of poor quality as is the case of slope 7. As can be observed in figure 5.30 this slope is very unstable, with the fall of blocks (Toppling). About measures for resizing slopes, such as changing their height or changing their angle of friction, did not deepen since they would need to be studied considering the mining. 8

9 CONCLUSION The statistical analysis that the Dip's program allows, was not performed because only the mean values of the main discontinuity families were used, which were measured in the field, because the company did not use all this information. Therefore, the final considerations of this work should be taken as qualitative and non-quantitative analyzes. Based on the results obtained from the determination of the SMR index (Romana, 1993), half of the ten slopes studied were classified as reasonable, four were classified as good, one (already at the lower limit, close to reasonable) and one as weak. The values of the SMR index allow us to make some recommendations for the stabilization of the slopes under analysis. Thus, for reasonable slopes, it is possible to create sloping foot ditches, for slopes with good flexible seals and / or sporadic bolts. For bad slope, slope foot ditches and metal nets, systematic bolts, sporadic projected concrete. Finally, it can be concluded that the applied methodology was adequate, the software Dip's 6.0 allowed to understand the contribution of the discontinuities that compartmentalize the mass. The Rocplane and Roctopple software s were used to calculate the safety factors, thus providing important indicators for the stability analysis of the Catoca mine slopes. According to the Roctopple software, all slopes have safety factors lower than unity, so they are unstable. This observation is corroborated by Dip's, except for slopes 1, 2, 8 and 10. It can thus be concluded that in these cases the slope geometry is the most determinant factor for stability. In these latter cases the contribution of the families that intercept the slope is irrelevant. For Rocplane only slopes 1, 3 and 10 have safety factors greater than 1.5. Other recommendations such as those foreseeing the less stable slopes are not presented because they interfere with mining aspects that are not the object of study in this work that is intended only of a geomechanical nature. As future work, it is recommended to use the methodology used, but with the inclusion of all the information available to the Mine, regarding the discontinuity surfaces. It is further proposed that the results obtained should be integrated into the method of exploitation in order to ensure economically viable operating conditions with maximum safety. 9

10 References 1. BARTON, N.,LIEN, R. & LUNDE,J. (1974). EngineeringClassification of RockMasses for the Design of Tunnel Support. Rock Mechanics, BIENIAWSKI, Z. T. (1979). The geomechanics classification in rock engineering applications. proc. 4th Int. Congress Rock Mechanics, ISRM, Montreux, 2, HOEK,E.,CARRANZA-TORRES,C.&CORKUM,B.(2002). Hoek-Brown Failure Criterion 2002 Edition. Proceedings of the NARMS-TAC Conference,Toronto,1, ISRM (1981). Basic geotechnical description of rock masses. Int. Society of Rock Mechanics, Commission on the classification of rocks and rock masses. Int. J. Rock Mechanics Min. Sci. Geomech. abstr., 18, MARGARIDA, N. D (2012) Caraterización geotécnica de los taludes del kimberlito Catoca. Tese de Doutoramento em Engenharia de Minas, especialização em geomecânica. Instituto superior minero metalúrgico. Moa-Houlguín. 6. ROMANA, M. (1985). New adjustment ratings for application of Bieniawski classification to slopes. International Symposium on the Role of Rock Mechanics, VALLEJO, L. I. G. D., FERRER, M., ORTUÑO, L. & OTEO, C. (2002). Ingeniería Geológica. Prentice Hall. 744 p. 8. WYLLIE, D. C. & MAH, C. W. (2004). Rock Slope Engineering. 4 Ed. Spon Press 10

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