DRAW POINT CONSTRUCTION IMPROVEMENT AT DEEP MILL LEVEL ZONE (DMLZ) PT FREEPORT INDONESIA USING THE SIX SIGMA METHOD

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1 DRAW POINT CONSTRUCTION IMPROVEMENT AT DEEP MILL LEVEL ZONE (DMLZ) PT FREEPORT INDONESIA USING THE SIX SIGMA METHOD Andrew Parhusip, Freeport-McMoRan Inc., Papua, Indonesia Fikky Hartono, Freeport-McMoRan Inc., Papua, Indonesia Mario Siray, Freeport-McMoRan Inc., Papua, Indonesia Rakhmat Mulyanto, former Freeport-McMoRan Inc., Papua, Indonesia Abstract PT Freeport Indonesia is preparing the Deep Mill Level Zone (DMLZ) mine to be the successor of the currently operating Deep Ore Zone (DOZ) mine. The DMLZ is block cave mine situated 500 m below the DOZ mine and approximately 1,600 m depth from surface. the 2570 level with column heights of roughly 500 m. The Undercut is on 2590 level, some 20 meters above the extraction level. Both of the block caving mines require draw point construction in the extraction level to extract the ore from caved area above. Learning from the experience in DOZ, the construction & maintenance of draw points in the production panels is a significant challenge in supporting consistent production. The average DOZ draw point construction duration is approximately 46 shifts per draw point. The time frame for construction is dependent on several variables; availability of the conventional forming system, material availability, and development tolerances. The repair of concrete floors and lintel sets causes production downtime. Reviewing the old DOZ draw point design, there is an opportunity to increase constructability, durability, and reliability of DMLZ draw points. This paper discusses how the DMLZ has improved draw point construction productivity and is on track to significantly reduce costs whilst increasing the quality by using the Six Sigma Improvement Method. Introduction The DMLZ Mine is part of the PT. Freeport Indonesia (PTFI) future expansion to its underground mining complexes. It is the fourth vertical lift in the East Ertsberg Skarn System (EESS) deposit (figure 1). Figure 1 Future Underground Mine Layout at PTFI The DMLZ mine will replace the production from the DOZ mine, which is planned to be complete in The DMLZ mine initiated caving in September 2015 and is continuing with production ramp-up activities. A significant amount of pre-production development and construction work was required to get the DMLZ to its current state and to initiate the cave. Development access began in September 2004 with the initial blast of the AB Adit portals. Looking at historical DOZ costs, it can be seen in Figure 2 below that the largest percentage of Currently, it is under development phase. The DMLZ mine will be a block cave mine using an advanced undercutting method. The DMLZ extraction level will be approximately 1,600 meters below the surface on 1

2 expenditures is associated with draw point construction and shotcreting activities. approximately 60% of the DOZ extraction level footprint. With an identified issue around drawpoint availability, a Six Sigma approach was applied in the DOZ and the draw point design was improved to create a more efficient construction method, at a lower total cost, and a stronger and more durable product. Figure 2 Detail cost per activity on Production Sequences Draw Point Construction The draw points are the critical link in connecting the caved material to the extraction level and allowing for LHD s to extract ore. These draw points consist of double steel lintel sets and formed concrete that are required to protect the drawpoint and keep it viable over its planned life. Definition Stage Improvement Project This first stage of the project charter, and describes the background and actual data of the DOZ draw point design, the working method and actual duration plus cost of Draw point Construction. The graph below collected from actual data , indicates that based on historical data the DOZ draw point constructed an average of 9.2 DP s/month,with maximum achievement 14.9 DP s a month and a minimum of 6.2 DP s. This variation in achievement was due to several factors including the draw point construction process employed and production rate. At that time DOZ production rate 80K per day on average. Figure 3 Active Draw Point Learning from the experience in the DOZ, the construction & maintenance of draw points in production panels can have a significant impact to production. The construction and repair durations for the DOZ draw points is dependent on availability of key materials and logistical support. In addition, roadway repair from ground movement and water damage can further impact draw point and panel availabilities. Repeated secondary blasting activities can damage lintel sets and lead to major repair activities being undertaken. Non-uniform roadways, drift profiles and lintel set damage make it difficult for the tele-remote and automated LHD operations to be carried out in Figure 4 DOZ Accomplishment for Draw Point Construction Reviewing the DOZ drawpoint construction data for revealed that an average of DP s per month were constructed that year from a plan of 12 per month. The average time taken to construct a DP was measured at 1,432 man-hours per DP versus a plan of 1,292 man-hours, or a 10% actual increase versus the plan. A review of the work method with the goal of a significant improvement in productivity was undertaken as part of this improvement program. The graph below in Figure 5 shows the actual distribution of time taken to construct a drawpoint in the DOZ. The graph indicates that there is quite a spread in times taken per draw point and is indicative 2

3 of the impact of external influences such as logistical support, actual drawpoint opening requirements, development tolerances and variations in construction methodologies. The average time take was 16 days per drawpoint. Figure 7 Average hour duration per Draw point activity Figure 5 DOZ duration completion for 1 DP The chart below in Figure 6 describes the detail workflow of draw point construction work method that was used in the DOZ mine. Figure 8 Pareto chart for DOZ Draw point Construction Figure 6 DOZ Drawpoint Construction Work Method Measurement Stage Target vs Plan The graph below (Figure 7) indicates average completion hours and standard deviation per sequence for each activity. From this data it can be concluded that the wall and roof meshing and forming is the longest duration activity, followed by the steel erection. These activities account for 41% of the total draw point construction time as can be seen in Figure 8. Both of these activities also exhibit the highest swings in standard deviation indicating potential process issues to be improved. Actual Cost of DOZ draw point construction Cost is an important factor to be improved upon. The table and pie chart shown in Table 1 and Figure 9 shows the distribution of costs for the different actives in a percentage of the total. From this data it is clear that the concrete and steel works are the bulk of the expenditure per drawpoint. Table 1 Detail cost of DOZ Draw point Figure 9 Detail cost for Draw point DOZ 3

4 Analysis Stage Root Cause Finding Based on the DOZ historical data an analysis was conducted to identify the root causes behind the DOZ drawpoint duration and cost drivers. From the analysis it was concluded that the wall & roof meshing and forming was the longest sub process of draw point construction. In addition, any significant development over break at a drawpoint also caused delays as the larger excavation required the installation of more materials and concrete. A further delay factor was the variable logistical supply efficiency in providing materials to the work place to be used in the manually intensive forming process The DOZ drawpoints utilize a double lintel set with each set having a double cap. This resulted in erection issues when the excavation was tight in the shoulder areas and required chipping by crews to make room for sets to fit into the correct position. In some cases the construction crews would need to burn holes in the steels sets to fit tie rods to hold the plywood forms in place. At certain periods, rail was installed in the floor of the drawpoints to help reduce wear in the concrete from the LHD mucking action. Over time the rail was exposed from excessive wear and lifted out of place, requiring time consuming repair activities. Other areas of concern identified were the abrasion resistance of the concrete, the application of high strength concrete products and the impact of water on the roadways. Figure 10 Trial Draw point Modular forming Figure 11 Trial Draw Point Modular forming at jobsite (Figure 12). Improvement Stage Reviewing and Selecting Options Drawpoint Construction and Formwork Once the issues with the current drawpoint construction process were identified then several options to help improve the process were selected for review. It was decided to move away from the manual forming process that utilized plywood and go to a modular and re-usable forming system. This would reduce wastage of materials, provide a consistent and uniform product, and simplify and accelerate the construction process. A re-usable formwork system from PERI was adopted in the DMLZ and is shown in Figures 10 and 11. Figure 12 DMLZ Drawpoint Concrete Slab design Lintel Set Re-Design The DMLZ lintel sets designs are significantly different from the sets used in the DOZ. Several key changes were adopted to help overcome some of the problems encountered in the DOZ construction process and for the higher stress levels anticipated in the DMLZ, 500m below the DOZ. The lintel sets are installed in pairs and the DMLZ system utilizes four connector beams between the two sets to provide additional stiffness and improve load shedding. This will help the lintel set survive anticipated higher ground stress, hang-up blasting 4

5 damage and loader impacts. Figure 13 illustrates the results of FEM analysis by Beck Engineering on the existing DOZ sets. The results of the analysis indicated that the main failure mode was experienced in the columns. Figure 15 DMLZ Lintel Set Structural Analysis using Staad Pro V8i The other concern raised during the lintel set redesign process was the impact of secondary blasting in the drawpoints to bring down hang-ups and break up boulders. This blasting process impacts the lintel sets and can move them laterally. In order to stiffen the dual lintel set package four connector beams were added (Casten, 2015), to connect the two lintel sets together, as can be seen in Figure 16. Figure 13 Beck Engineering FEM analysis on DOZ lintel set To get a better analysis on the failure modes, the DOZ and DMLZ lintel sets were compared using STAAD Pro V8i, a structural steel analysis software. This resulted in failures occurring in the columns and reinforced the work by Beck Engineering. Figures 14 and 15 show the STAAD analysis. An important factor for increasing the shear capacity of the columns is the cross sectional area of the web. By increasing the web size the lintel set capacity increased from KPa/lintel (in DOZ) to be KPa/lintel (in DMLZ), an increase of 72%. The redesigned DMLZ lintel set has a smaller actual ratio overall (ratio of total load to structural capacity of the steel member). A smaller actual ratio indicates a stronger structural capacity. The DOZ has a ratio of while the DMLZ linte set has a ratio of 0.389, over twice the shear stress capacity. Figures 16 and 17 show the DMLZ lintel set with the following notable changes in the design: The DMLZ set uses a single 350 mm beam for the cap versus two 327 mm stacked caps in the DOZ. This cap has a 20 mm web versus a 10.5 mm web thickness and provides much greater strength in a more compact system. This change increased the capacity of the sets from 260 KPa in the DOZ to 450 KPa in the DMLZ. This also reduces overall height and helps avoid development tights which are a source of construction delay. Figure 14 DOZ Lintel Set Structural Analysis using Staad Pro V8i Figure 16 Comparison of DOZ lintel set vs DMLZ lintel set The lintel sets were designed to work with the PERI formwork and incorporate holes for tie rods and connectors (Figure 18). 5

6 and the LHD buckets were able to catch and move the rail. Figure 17 3D Design of DMLZ Lintel set. In the DMLZ mine the combination of AnvilTop and embedded rail were substituted for high strength, abrasion resistant concrete.. The high strength concrete planned for the DMLZ draw point roadways underwent testing at the University of Indonesia in Jakarta. It was shown that 80 MPa concrete with fiber (6kg/m3 plastic fiber) had same performance on abrasion resistance to AnvilTop and concrete in the MPa range (Table 1). One of the key challenges in producing the high strength fibrecrete is ensuring that the batch plant can provide the correct level of quality on a consistent basis based on the required concrete recipe (Table 2). Table 1 Laboratory test on abrasive resistance of HPC Figure 18 Actual Lintel set on DMLZ Concrete Works In the DOZ, the draw point floor was reinforced concrete with a minimum thickness of 30 cm, a compressive strength of 50 MPa with the addition of 4 cm of an iron aggregate mortar topping known asanviltop for increased abrasion resistance. Based on the vendor specifications from BASF, AnvilTop is a heavy duty cement-based metallic-aggregate floor topping. It provides heavy-duty protection in key areas subject to abrasion and impact. Its energy absorbing capacity is significantly greater than plain concrete, integral fiber concrete, and high-strength naturalaggregate toppings The road gradient was also kept at a minimum of 3% in order to minimize water ponding. It was noted that the presence of standing water greatly decreases the durability of the road concrete. In some areas of the DOZ mine used rail was installed as part of the concrete process to provide additional wear protection against bucket loading. This proved to be problematic one the concrete wore down past the rail embedment Table 2 Mix Design and trial result for HPC DMLZ Draw point Construction Performance The DMLZ mine has applied all of the improvements discussed to the draw point construction process. Figure 19 shows a picture of a completed draw point in the DMLZ mine. 6

7 Table 3 Improvement List on DMLZ Draw point Category Description Lintel set already proven to hold static load with overall static load max (452.5kPa or452.5kn/m2) Lintel set wall & roof forming Rail Installation & Anvil top Air blast effect must be include on design calculation (covering concrete) - add connector beam on cross beam and column beam put additional hole on cross beam and column for PERI modular system Eliminate tight risk on lintel installation by improving design reduce height of double beam Improving material procurement and reducing 4 column different marking and 2 beam different marking to be 1 column and 1 beam marking to eliminate factor lack of material Find alternate design which enable to apply modular system which have standard work methodsubtituted by PERI modular forming system Find concrete technologies to increase water, impact & abrasive resistance on concrete floorsubtituted with High Perfomance concrete (HPC) with minimum requirement UCS test 80 Mpa Figure 20 Comparison of histogram DOZ vs DMLZ (Duration completion) The reduced construction duration in DML and the redesign has resulted in a reduction in cost and manhours needed per draw point as shown in Figure 21. The DMLZ has achieved an average of 14 draw points constructed per month which equates to annual savings of several millions dollars in materials and labor. Figure 19 Complete draw point construction on DMLZ A comparison of draw point construction durations based on the draw points constructed in DMLZ in 2015 and DOZ data is shown in Figure 20. In summary the DMLZ average time to construct one draw point is 410 hours compared to 444 hours for the DOZ. Figure 20 also shows the standard deviation between the two statistical distributions. The DMLZ mine has a lower variation on construction timing than DOZ which is attributable to the new method and also the impact to construction in DOZ from other activities such as production and caving activities, currently not present in DMLZ. Figure 21 Cost Comparison DMLZ vs DOZ 7

8 ACKNOWLEDGEMENT The authors would like to thank to management teams of PT Freeport Indonesia for permission to prepare and publish this paper,and Underground DMLZ Construction team for support and sharing knowledge. REFERENCES 1. Mulyanto, Rakhmat. Six Sigma Method for Draw point Construction. MBA-ITB Thesis, (2011). 2. Manggala, D. Mengenal Six Sigma Sederhana., (2013). 3. Tarigan, A., Widiyanto, F., Ramirez, M., & Osborne, K., (2010). The Deep Ore Zone (DOZ) Mine Cave Sequence Modifications. SME Annual Meeting Beck Engineering, Beck Engineering Analyis on Lintel set Drawpoint DOZ BASF. MasterTop 300 Technical Data Guide. (2014). 6. Casten, Tim, personal discussion, 2015, 8