Viability Index Method for Wedge Analysis in Gudang Handak Underground Mine PT. Antam Tbk

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1 Viability Index Method for Wedge Analysis in Gudang Handak Underground Mine PT. Antam Tbk Fithriyani Fauziyyah 1, Ryan Pratama 2 and Zufialdi Zakaria 3 1 Magister Program of Hydrogeological Engineering, Institute of Technology Bandung, Indonesia 2 Geotechnical Department, UBPE Pongkor PT. Antam Tbk, Bogor 16650, Indonesia 3 Laboratory of Geology Technic, Geological Engineering, Padjadjaran University, Bandung, Indonesia * fithriyani.uji.fauziyyah@gmail.com Abstract Common problems that are often found in an underground mine is unstable tunnel. Instability of tunnel based on the mechanism is divided into two, caused by structural controll and stress controll. The stability on Gudang Handak underground mine PT. Antam Tbk is more influenced by structural controll. The presence of these structures has potential to form Wedge failure. Wedge analysis both in terms of dimensions and the disaster will became the basis in determining the appropriate type of ground support. Wedge area and its possibility to fall can be predicted more accurately by using the Viability Index approach initiated by Diederich et al. This will be very beneficial, especially from the safety factor which can be improved and the cost can be reduced. Tunnel supporting that is needed, based on this research, is rock bolt with splitset types that has length more than 2,4 m. Because of splitset limitations that owned by UBPE Pongkor, so the supporting design can be added by using the shotcrete and H-Beam which suited with the needs of ground support. Keywords: structural geology, wedge, ground support 1. Introduction There are a lot off parameters that must be accounted for tunnel instability analysis, which are rock mass strength, geology structure, main stress and strain, rock mass classification, discontinuity, etc. These parameters must have same influence in stability and or in different action. Instability of tunnel based on mechanism is divided into two, caused by structural control and stress control (Eberhadrt 2006). According to historical tunnel collapse in UBPE Pongkor especially in underground mine Gudang Handak, mechanism instability tunnel more influenced by structural control. Common problems are often found in excavation tunnel is the fall of rock blocks formed by the intersection of the tunnel face and discontinuities plane in rock mass. This discontinuities can be found as structur likes bedding plane and joint which generally consists of several sets in different direction. The most common failure type is wedge that falling from the roof or sliding from the side walls of tunnel. This research was conducted to determine wedge formation (geometry and rock mass stress) in underground mine Gudang Handak using Wedge Viability Index approch by Diederich et al (2000). The research is located in several front production and development areas. Generally, divided into two block; central and south block. To make the research easier, each block divided into several domain according to distance of sampling areas. Central block consist of 6 domains with 3 primary vein and south block consist of 4 domains. Fig 1. 3D view Gudang Handak Mine L500, PT. ANTAM (PERSERO) Tbk.

2 1.1 Geological Setting in this Research Area Rock composer in research area is dominated by andesite breccia with its main component dark grey andesite with massive structure, afanitic-porphiritic texture, crystallization degree hipocrystalin, crystal shape subhedral and has light grey tuff as matrix, grain size fine-moderate sand, rounded-sub rounder, closed kemas with fine sorted. Other rocks that abundantly present here is tuff lapilli with claystone inset in some locations. This claystone presence is very disturbing the mining activity because it s characteristic that easily swell. Vein which contains gold reserve is consisted of quartz mineral that in some location, especially in south blocks, is inserted by the presence of manganic oxide layer, kaolinite, and calcite. Fig 2. Geological map according to Milesi et al, 1999 Geological structures which located in this area usually have dominant directions NW-SE and NE-SW. The most common geological structure type here is joints and fault. Joints can be filled by quartz mineral, calcite, clay, combination, or maybe be without any filling. While fault is always filled with clay materials with slickensided characteristic. 1.2 Geotechnic The rock mass which composing the research area is based on Bieniawski classification (1989) consisted of rock mass class III (medium rock) to class IV (poor rock). Those class characteristic is very influenced by the geological condition from each area. The geological technic condition summary in research area is contained in table 1,2,3 and 4. Table 1. RQD value of Gudang HandakCentral Block and South Block based on scanline method

3 Table 2. Discontinuity condition of Gudang Handak Central Block and South Block Table 3. Groundwater condition of Gudang Handak Central Block and South Block Table 4. RMR value of Gudang Handak Central Block and South Block 2. Theories and Experiment 2.1 Tunnel and Ground Support Generally, tunnel type in mining is divided into two; development access and production stope. Development access, like ramp and crosscut, has standard dimension 4m x 4m with flat roof, while production stope has dimension which following the vein width. The ground supports used by UBPE Pongkor are Splitset, Steel Set (H-Beam) and Shortcrete

4 4m 3m 1m which combined with mesh and or trap installation. The splitset size is divided into 3; 1.4m length and 1.8m length which done by Jackleg, and 2.4m which done by Jumbo Drill. Tunnels inside Gudang Handak have span 3,5m in width and 4m in height. Tunnel size is very important against the wedge fall potential and which ground support technique will be used. Profil Standar 4m x 4m Profil Arching 3.5m x 4m 4m 3.5m Fig 3. Tunnel with standard span with arching opening (try outs) 2.2 Wedge Viability Analysis Diederichs (2000) stated that wedge failure which happened is experiencing wedge height and wedge volume reduction compared with the prediction result from full span analysis which because of joint persistence limitation, distance and stabilization effects from pressure to wedge sides (Diederichs and Kaiser 1999, Brady and Brown 1993, Sofianos 1986). Viability Index (Iv) is an effort to predict the possibility of wedge collapse potential more detail so the ground support used before, full-span demand (S) can be more effective by change it to Effective Span (S*). Viability index (Iv), become the ratio to determine wedge dimension (ex : Wedge Height, Bolt Demand, Liner Demand) that most possible to happen. Effective span value, or scalled span, can be obtain from equation (1) and viability Index value can be obtain from equation (2). S = I V S (1) I V = I O I S (2). Explanation : I V = Viability Index, I O = Occurence Index, I S = Instability Index, S* = Effective Span, S = Actual Span. Occurrence index (Io) is a potential value for the wedge occurrence which consist of several factors; joint set dominance factor, joint length factor, and joint spacing factor using equation (3) below I 0 = (F D F L F S ) (3) Explanation: I o = Occurrence Index, F D = Joint Set Dominance Factor, F L = Joint Length Factor, F S = Joint Spacing Factor. Joint set dominance factor (F D ), is a value based on the domination of each joint set which formed a wedge (3 joint sets), F D value is the total from each of those joint sets like in table 5. Joint length factor (F L ) is the value which based from the joint length that seen on the roof. F L is the total from each joint set (3 joint set),like in table 6

5 Table 5. Joint Set Dominance Factor Criteria (F D ) Joint Set Dominance Factor Criteria F D Most Dominance (1 set only) 4 Dominance (Always Present) 3 Intermediate (Frequently Present) 2.5 Minor/Random (Typically Absent) 1.5 Table 6. Joint Length Factor Criteria (F L ) Joint Length Factor Criteria F L Shears, Bedding 4 > 1.5 Span or 0 end visible Span or 1 end visible Span or 2 end visible 2 <0.2 Span 1 Joint spacing factor (F S ) is the value which based from the space between joint that formed wedge. F S value is the total from each joint set (3 joint set) like in table 7. Table 8. Joint Spacing Factor Criteria (F S ) Joint Spacing Factor Criteria <0.1 Span Span Span Span 2 >1.5 Span 1 F S Explanation : I S = Instability Index, F J = Join Factor, F G = Gravity Factor, F C = Clamping Factor. Instability index (I S ) characterized the collapse potential of a wedge which produced by 3 main factors; Joint Factor (F J ) Gravity Factor (F G ) and Clamping Factor (F C ). Instability Index is included in equation (4) below, I S = (F J F G F C )/ (4) Joint factor (F J ) is the value based on frictional force, dilation, and cohesion that owned by each joint that formed the wedge. Use the F J value from Jr/Ja (Barton et al. 1974) which owned the lowest value or the middle value from the joints that make the wedge like in table 8. Gravity factor (F G ) is the value that based on the collapse wedge type. The classification is included in table 9. Table 8. Joint Factor Criteria (F J ) Jr/Ja F J < >4 3 Table 9. Gravity Factor Criteria (F G ) Wedge Behavior F G Free Falling 10 Falling with 1 joint or edge vertical 9 Falling sliding with 1 near vertical joint 7 inverted Pure sliding no rotation 4 Clamping factor, (F C ) is the value that based on the clamping effect was produced by confinement strain on the roof. This factor is determined by the ratio between wedge height, span, and tangential stress that happened

6 Table 10a. Clamping Factor Criteria (F C ) Moderate to High Stress Wedge Wedge Cone Angle F C Height/Span Stress State >2 < <0.3 >60 10 Table 10b. Clamping Factor Criteria (F C ) Low to Zero Stress Wedge Wedge Cone Angle F C Height/Span Stress State >2 < <0.3 >60 10 Table 10c. Clamping Factor Criteria (F C ) Tensile/Open Joint Wedge Wedge Cone Angle F C Height/Span Stress State >2 < <0.3 >60 10 Wedge safety factor (FS) can be used on effective span (S*, which counted so the whole deciding ratio that will be the happened the most have calculated the worst case scenario, make the notation S* become S** (effective span after being applied by FS value). FS calculation using equation (5) and effective span after being applied in FS can be obtain from equation (6) FS = 1 I V... (5) S = FS S... (6) Explanation: I V * = Largest value of viability index, FS = Factor of Safety, S* = Effective Span, S** = Effective Span with safety factor. S** is used as the ratio for determining the wedge height (H*) that most likely to fall and used as reference for ground support needs, like what has been written in equation (7) below, H = (S S) H... (7) 2.3 Geometry Support Demand Wedge geometry which obtained through structure mapping and UNWEDGE programming (wedge height, weight, width etc) are used to determine the support demand for linear and bolt. Linear demand, which obtained from dividing result between wedge weight and wedge periphery, reflects shortcrete thickness value needed for tunnel ground support. For bolt demand, which obtained from dividing result between wedge weight and its surface area, reflects rock bolt UTS (ultimate tensile strength) value that can be used with H length. Fig 4. Bolt demand (wedge mass / wedge surface area) and Liner demand (wedge mass /wedge periphery), Diederichs et al (2000) Bolt Demand (BD)= weight wedge/face area wedge (ton/m²)...(11)

7 Liner Demand (LD)= weight wedge/perimeter(ton/m)...(12) Factor of Safety (FS) = 1 / Iv...(13) Span Effectife (S*)= S x Iv x FS...(14) Wedge Height Effectife (H*) = (S*/S) x H...(15) Bolt Demand Effectife ( BD*) = (S*/S) x B...(16) Liner Deman Effectife ( LD*) = (S*/S)² x LD...(17) with; Iv = Viability Index max from each domain S = full span, 3,5 meters 3. Data and Analysis Geological structure data from all over the research location is gathered and analyzed according to Wedge Viability Index method. Analysis is used to obtain the value of dominance factor parameter (F D ), length (F L ), spacing (F S ), and joint (F J ). The summary of all data on each location is included in table 11 to table 19. Table11. Viability parameter data on each individual joint location domain 1, central block. J8 R J1 J2 J3 J4 J5 J6 J7 39/ /2 12 9/6 159 /65 63 /66 /43 10/45 / FD 1,5 1,5 3 1,5 2,5 2,5 4 3 FL 2 2,5 2,5 2,5 2,5 2,5 2,5 2,5 FS 3 2, ,5 2,5 2,5 2 FJ Table 13. Viability parameter data on each individual joint location domain 3, central block. J8 R J1 J2 J3 J4 J5 J6 J7 224/ / /84 181/ /60 7/ / /30 FD 3 3 1, ,5 FL 2,5 2 2,5 2,5 2 2,5 2 2 FS 2, ,5 2,5 2,5 FJ Table 12. Viability parameter data on each individual joint location domain 2, central block.. R J1 J2 J3 J4 J5 J6 J7 340/ / / / / / /85 FD 1,5 2,5 1,5 4 1,5 2,5 1,5 FL 2, ,5 2,5 2,5 2,5 FS 2,5 2, ,5 2 FJ Table 14. Viability parameter data on each individual joint location domain 4, central block. R J1 J2 J3 J4 J5 151/ / /52 210/ /52 FD 4 2,5 3 1,5 3 FL 2,5 2,5 2,5 2,5 2,5 FS 3 2,5 2,5 2 2,5 FJ

8 Table 15a. Viability parameter data on each individual joint location domain 6, central block. R J1 J2 J3 J4 J5 8/ / 75 90/ /35 52/ 60 FD 2,5 2,5 1,5 1,5 1,5 FL 2,5 2 2,5 2 2,5 FS 3 3,3 3,3 3,3 3,3 FJ Table 17. Viability parameter data on each individual joint location domain 2, south block. R J1 J2 J3 J4 J5 J6 261/ / / / / 56 99/ 42 FD 1,5 1, ,5 3 FL ,5 2,5 FS ,3 3,3 FJ Table 15b. Viability parameter data on each individual joint location domain 6, central block. R J6 J7 J8 J9 J /60 212/ 72 46/ /48 125/ 33 FD 3 1,5 1,5 3 4 FL 2,5 2,5 2,5 2,5 2,5 FS 3 3 3,3 3,3 3,3 FJ Table 18. Viability parameter data on each individual joint location domain 3, south block.. R J1 J2 J3 J4 J5 J6 98/ / / / / / 56 FD 2, ,5 3 1,5 FL 2 2,5 2,5 2 2,5 2 FS 3,3 3,3 3 3,3 3,3 3 FJ Table 16. Viability parameter data on each individual joint location domain 1, south block.. R J1 J2 J3 J4 132 /24 323/ / /60 FD 2,5 1,5 3 4 FL 2 2 2,5 2,5 FS 3 3 3,3 3 FJ Table 19. Viability parameter data on each individual joint location domain 4, south block.. R J1 J2 J3 J4 J5 J6 301/ / / / / / 50 FD 1,5 1, ,5 1,5 FL 2,5 2,5 2,5 2,5 2,5 2,5 FS 3,3 3,3 3 3,3 3,3 3 FJ Where R is parameter, J1-Jn is joint set number, and is strike/dip. Wedge modeling is using UNWEDGE v.3.0 software from Rocscience. Wedge data taken is the joint combination data which formed wedge on the roof, failure mode, and wedge height (apex; H) 1. Domain 1, Blok Central

9 2. Domain 1, South Block Fig 5. Viability Index graphic and the database for example Data shows the factor that has the biggest role to the unstable wedge condition in research area is the control of joint filler material, which is clay (kaolinite, weathered MnO 2 ), and falling pattern which mostly appear as falling wedge. 3.1 Ground Support Demand Ground support calculations using UNWEDGE v.3.0 software (Rocscience) with wedge height in scalled (H*) is suitable with wedge viability index calculation. Example of scalled wedge feature usage in UNWEDGE software is written in figure 8. Fig 6a.Example of wedge modeling using UNWEGE before given ground support in domain 4, central block Fig6b. Example of wedge modeling using UNWEGE after given ground support in domain 4, central block. Table 20. The example of wedge supporting possibility calculation in domain 4, central block, after analyzed with viability index

10 4. Conclusion According to the research result with the theme wedge analysis according to viability index method approach in Gudang Handak L.500 underground mine, we can conclude that: 1. Central block is composed dominantly by porphyritic andesite and andesitic breccia with andesite as its component and tuff lapilli as its matrix. South block is composed by breccia andesite with more claystone and black silt. And the rock mass in central block is more altered because the it is closer to the vein. 2. Rocks that present in central block is included in class III, and for south block is included in class III and IV according to Bienawski classification (1994) 3. Recommended ground support in central block is: Domain 1 is splitset with length 0,24 m and tensile strength max 0,2 ton/m² Domain 2 recommended ground support is spliteset with the length 2,7 m and tensile strength max 2,2 ton/m² Domain 3 recommended ground support is spliteset with the length 1,3 m and tensile strength max 1 ton/m² Domain 4 recommended ground support is spliteset with the length 1,5 m and tensile strength max 1,2 ton/m² Domain 5 recommended ground support is spliteset with the length 0,24 m and tensile strength max 0,2 ton/m² Domain 6 recommended ground support is spliteset with the length 2,4 m and tensile strength max 2 ton/m² 4. Recommended ground support in south block is: Domain 1 1 is splitset with length 0,56 m and tensile strength max 0,62 ton/m² Domain 2 recommended ground support is spliteset with the length 0,27 m and tensile strength max 0,22 ton/m² Domain 3 recommended ground support is spliteset with the length 0,4 m and tensile strength max 0,5 ton/m² Domain 4 recommended ground support is spliteset with the length 0,3 m and tensile strength max 1,3 ton/m² ACKNOWLEDGEMENT We want to thank you to Quality Control Biro, Mine Plan and Development, and Mine Operation that has entrusted and facilitated all the research activity. And we want to thank Mr. Budi Purwana as the head of Quality Control Biro, Mr. Yosep Purnama as the head of Grade Control and Geotechnic Department, Geological Engineering Department of Padjadjaran University that always supporting and guiding in the making of this paper. REFERENCE Brady, B. And Brown, E Rock Mechanics for Underground Mining. Chapman and Hall, 1993, 571p. Barton, N, Lien, R. And J, Lunde Engineering Classification of Rock Masses for Design of Tunnel Support. Rock Mech, 6 (4), Diederichs, M., Espley, S., Langille, C., Hutchinson, D.J A Semi Empirical Hazard Assessment Approach To Wedge Instability In Underground Mine Openings, GeoEng2000, pp. 1-8, Melbourne. Hoek, E, Kaiser, P.K, Bawden, W.F Support of Underground Excavations in Hard Rock. Rotterdam, Brookfield. A.A. Balkema. Kramadibrata, Suseno dan Watimena, Ridho Mekanika Batuan. Buku Diktat, Institut Teknologi Bandung Rachmad, Lufi Ground Support in Mining. Mining and Geotechnical Consultant, GEOMINE Sadisun, Imam A GL-3221 Pengantar Geologi Teknik : Aspek Keteknikan Batuan (power point). Eng Geo Laboratory. Institut Teknologi Bandung Singh, B and Goel, R Rock Mass Classification. Roorkee, India. Elsevier. Unit Geomin, Laporan akhir Pemboran Geoteknik Daerah Ciurug dan Gudang Handak Pongkor-Jawa Barat, pp. 28, Pongkor