CROWN PILLAR ON VENTILATION DRIFT AT DEEP MILL LEVEL ZONE (DMLZ) PT FREEPORT INDONESIA

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1 CROWN PILLAR ON VENTILATION DRIFT AT DEEP MILL LEVEL ZONE (DMLZ) PT FREEPORT INDONESIA Rian Nugroho, PT Freeport Indonesia, DMLZ Engineering, Papua, Indonesia Andrew Parhusip, PT Freeport Indonesia, DMLZ Construction, Papua, Indonesia William Teweng, PT Freeport Indonesia, DMLZ Geotechnical, Papua, Indonesia Abstract Crown pillar is a situation which two different levels of development tunnel overlaying with adjacent distance. This situation results small pillar between these two drift/tunnels. Many cases crown pillar cannot be avoided due to designated ore body position and existing facility drift such as ventilation drift, haulage drift. Crown pillar also been a case on Deep Mill Level Zone (DMLZ) PT Freeport Indonesia. This paper will discussed a geotechnical decision to state a crown pillar status on DMLZ, Consequence assessment start from Development phase, pre-production/ground support phase and steel set construction to fix Crown pillar issue on DMLZ, a calculation on economic/actual cost of all of these phases revealed on this paper. The DMLZ Mine is the fourth vertical lift in the East Ertsberg Skarn system (EESS) deposit (figure 1) which is located between the 2905 and 2605m elevations, hosted in diorite (vein-style mineralization) and in skarn (disseminated and vein style mineralization). The DMLZ mine initiated caving produce in September 2015 and is continuing with production ramp-up activities. A significant amount of development and preproduction work was required to get the DMLZ to its current state and to initiate the cave, where the operations require adequate ventilation system. It takes intake drift and exhaust drift, which is will be used access to develop the other level such as extraction drift, and haulage drift. Introduction PT. Freeport Indonesia (PTFI) is the one of mining companies operating in Indonesia and start production in 1972, currently Freeport Indonesia implementing two methods of mining, there is open pit mine at Grasberg and underground mine at Deep Ore Zone (DOZ) with block cave mine method and Big Gossan with open stope method. The Deep Mill Level Zone (DMLZ) is the part of underground expansion project that will be prepared with underground block cave mine and will replace the production from the DOZ Mine. Figure 2 Design of DMLZ Mine Layout (source: Vulcan Maps) Crown Pillar The DMLZ Mine have design of intake level will be used as main access for development haulage access, and it will have thin pillar with exhaust level. It with a distance of countless pillar from back of exhaust drift to floor of intake drift. It have 4.3 meter between both levels, more precisely the location of pillar at DMLZ Intake 1 Lateral 2 Drift and DMLZ Exhaust 4 Drift (figure 3). Figure 1 Future Underground Mine Layout at PTFI 1

2 Table 1 DMLZ Mine Crown Pillar Calculation Figure 3 DMLZ Mine Crown Pillar Location, Vulcan Maps The dimension of intake drift is 7 meter width by 7 meter height as same as at the exhaust drift. It means the pillar have block of probable failure and have not good safety factor from the geotechnical consideration, it shown in figure 4 and 5 with calculation on table 1. One of the challenges involved in this case study is cut the pillar or build the raise between these two drift with blasting sequence properly to maintain the stability of the pillar around. Blasting Pattern According to the cut the pillar needs, and with regard to the actual drift, the blasting pattern to cut the pillar has two sequences with calculation blasting controllable variables. The blasting pattern sequences are with staggered pattern, diameter of charge holes is 102 millimeter with 1 meter of burden and 1 meter of spacing, it shown in figure 6. Figure 4 DMLZ Mine Crown Pillar 3D Representation Figure 6 DMLZ Mine Crown Pillar Blasting Pattern GEOTECHNICAL The Map3D numerical model using for analysis the condition of pillar, following the development sequences. In the early stages of sequence the rock behaves more or less elastically. This means that deformations occur without causing any damage to the rock. At some point the loads get high enough that the rock begins to crack and the deformation are no longer entirely elastic. Due to damage to the internal structure, only part of the deformation is recoverable upon unloading. Figure 5 DMLZ Mine Crown Pillar 2

3 Figure 9 Strength envelope graph use linear distribution Figure 7 Sigma-1 response when the drift close and past the pillar (source: Map3d model) In elastic modelling, the proceed along a linear projection above the non-linear part of the stress-strain response, various levels of over stressing corresponding to levels of damage or non-linear strain. The borehole camera observation would like to define various criteria to represent different ground conditions and calibrate the strength envelope. From the model, the spalling indicated is in 4m before past the pillar and the damages would expect when the exhaust drift past the pillar. Figure 8 Damage prediction on the core pillar (Source: Map3D model) Average pillar stress compare to strength envelope Back-analysis of observed in situ response conducted to determine a representative rock mass strength envelope and corresponding coefficient of variation. This need not be restricted only to stress driven failure events such as pillar failures. Strength envelopes corresponding to a variety of stress induced responses the strength envelope graph was created from the borehole camera observation on several areas at DMLZ mine. Figure 10 Strength envelope graph use linear distribution 3

4 Refer to another damage criteria created from damage model DMLZ for the baseline of calibrations (Source: Bewick.R & Teweng.W, 2015). The average pillar stress was increase during the development sequences, indicated by the σ1 σ3 above the spalling initiation line. stable, the second option is break through the pillar and build the bridge for the option. Construction as a solution To answer this problem, raise between these two drift must be connected by some structure. As the Intake will be used as main access for development haulage access, future structure required to meet the determined load. Biggest equipment with full capacity of load is aimed as the determined load. Articulated Dump Truck Series AD 60 is chosen as the designated load with full muck capacity assumption. From table 2 and Figure 13 below indicated that structure has to bear more than 110 ton moving load. Open raise condition left two options for this situation. Steel sets or to build a structure as bridge. These options required to be taken to connect Intake drift above. Steel Sets options will be constructed on below level or Exhaust Level, otherwise Bridge option will be constructed to connect intake level. Absolutely these two options will bear designated load as mentioned before. Figure 11 Damage calibration graph (Source: Bewick.R & Teweng.W, 2015) Figure 13 AD60 Truck full load distribution Table 2 Weight specification on Haulage Equipment (Caterpillar Handbook) Figure 12 Average Pillar stress on damage calibration graph (Source: Bewick.R & Teweng.W, 2015) The damage or strength criteria graphs shown the pillar will be have damages when the drift pass on the below of pillar. The pillar have a potential failure or major damage on the future. Base on the geotechnical analysis, first option is if the drift will be use for the main access on life of mine, steel beam is required for keep the pillar 4

5 Structural Loadings Commonly structural loads is defined as several types; Permanent load, traffic load caused by heavy equipment s and others. Permanent loads consist of Self weight of the structures and other non-structural element. Meanwhile Traffic load for bridge design consist of D lane load and T truck load. The D lane loading is applied across the full width of the bridge roadway and produces effects in the bridge equivalent to a queue of real of vehicles. The total amount of D lane loading applied depends upon the width of the bridge roadway. The T truck loading is single heavy vehicle with three axels which are intended to simulate the effects of the wheels of heavy vehicles. Only one T truck may be applied per design traffic lane. D Lane Load Uniform Distribution Load (UDL) with intensity q kpa, q depends on the bridge loaded length: L < 30m; q = 8.0 kpa L > 30m; q = 8 * (5 + 15/L) kpa UDL may be places in discontinue length to get the maximum influence. In this case L is amount from discontinue length of each D Lane load. D Lane load place vertical to traffic direction. T Truck Load Only one truck has to be placed in every traffic lane. T truck has to be placed in the middle of traffic lane. The load is applied on the structure as point moving load which produces the maximum effect on moment Steel set is designed with steel beam combinations with addition on connector between each set, proposed gap between each set is 1.2 meter. Seen figure 15 below, maximum bending moment will happen on the center of the cross beam. Additional stiffener between cross beam and the column reduce maximum moment significantly. Figure 15 Steel set 3D design Using steel set answer the solution for this crown pillar problem, proven can handle designated loads. With some recommendation like all supports for the frame of steel sets are fixed, steel set spacing must be lower than 1200 mm. Figure 16 shows bending moment in each frame caused by the loads. Steel Sets Option Figure 14 shows steel set design as a structure to bear maximum load. As illustrated on below Figure. Intake level and exhaust level separated by concrete fill. Steel sets hold concrete load and moving load generated by AD 60. Figure 16 Bending Moment on steel set design arrangement Figure 14 Steel Set arrangement design Steel bridge Option Figure 17 shows steel frame fabricated as bridge to connect Intake level. Using profile Welded beam 900 as main girder and Universal beam 410 as connector, bridge designated to hold and connect intake level as 5

6 well as steel set option. Bridge will be constructed on Intake level, while this option will consume less concrete and less steel consumption rather than steel sets. Applied loads on bridge will be same as applied on steel set. stress, UB 410 as connector beam reduced this stress significantly. Figure 19 Bending Moment Distribution on Steel bridge design ACKNOWLEDGEMENT Figure 17 Steel Bridge 3D design Figure 18 show Plot Bridge unto actual drift on intake level. This bridge designated to serve heavy equipment access on underground. Figure 18 Bridge design plotted on actual drift on plan view Table 3 Comparison of material consumption based on two proposed design The authors would like to thank to management teams of PTFI for permission to prepare and publish this paper, also thank to Mr. Timothy Casten for final Review and Underground DMLZ Construction team for support and sharing knowledge. REFERENCES 1. AISC-ASD (American Standard for Steel Design), (2010). 2. Nurdi, Herry. Various Design, DMLZ Martin CD, Kaiser PK, McCreath DR. Hoek-Brown parameters for predicting the depth of brittle failure around tunnels. Can Geotech J C.D. Martina,W.G. MaybeebThe strength of hard-rock pillars ; International Journal of Rock Mechanics & Mining Sciences 37 (2000) 1239± W. G. Maybee ;Pillar Design In Hard Rock, Hoek, E., Kaiser P.K. and Bawden W.F Support of underground excavations in hard rock 7. Hoek, E Estimating Mohr-Coulomb friction and cohesion values from the Hoek-Brown failure criterion. Intnl. J. Rock Mech. & Mining Sci. & Geomechanics Abstracts. 12 (3), Kaiser and Kim (2008), Rock Mechanics Challenges in Underground Construction and Mining 9. P. K. Kaiser1 M.S. Diederichs2, C. D. Martin3, J. Sharp4, and W. Steiner5, UNDERGROUND WORKS IN HARD ROCK TUNNELLING AND MINING The comparison of material consumption briefly showed on table 3. Steel bridge could be alternative solution to face crown pillar. Structural analysis showed on Figure 19 indicated biggest moment happen on the center girder and on the abutment, further analysis on concrete abutment is required to ensure the foundation strong enough. Bridge structure also receive torsion and shear 6