IntroductionThe Ok Tedi Open Pit Mine is located at the head-waters of the Ok Tedi River, in the Star Mountainsof the Western Province of Papua New Guinea (seeFigure 1).

Daily rainfall records in the project area areavailable from 1985 onwards. The average rainfallin the period 1985—2010 is between 8m and 10mper annum (depending on the position of the rain-fall gauge) with a recorded maximum of 10.7m(2005) and minimum of 4.7m (1997) as shown inFigure 2. Rainfall is relatively evenly distributedover the year with the maximum occurring dur-ing July and the minimum during November. Theactual average annual evapotranspiration for thepit area is estimated at 656 mm/annum (OTML,1999).

geology and HydrogeologyThe oldest sediments at Ok Tedi are sediments ofthe Ieru Formation. The Ieru Formation comprisesdark grey to black mudstones, shales, siltstonesand sandstones, laid down on a predominantlynear shore shallow marine shelf during the UpperCretaceous (Aptian – Seronian). The Ieru Forma-tion is overlain by the Darai Limestone (Eocene –Lower Miocene). The nature of the contact be-tween the two units is uncertain. The Darai Lime-stone consists of foraminiferal limestones,mudstones and wackestones of a shallow marinefacies. The Darai limestone exhibits widespreadevidence of solution enlargement of verticaljoints and cave (karst) systems are known in sev-eral locations, the most notable of which is theHarvey Creek Cave Spring (HCCS). This is devel-

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Mine groundwater control and slope depressurisation methods at Ok tedi in one ofthe highest annual rainfall areas in the world

Stefan Müller, Jayanathan Mylvaganam, Ian de Bruyn, Jeff Price

SRK Consulting (Australasia) Pty Ltd, Level 1, 10 Richardson Street, West Perth, Western Australia, 6005,Australia

abstract The Ok Tedi Mine is located in the Western Province of Papua New Guinea. The average rainfall(1985—2010) is between 8m and 10m per annum with a maximum of 10.7m (2005) and minimum of 4.7m(1997). From 2007, boreholes have been drilled and fully cemented vibrating wire piezometers with data log-gers were installed in these holes to monitor the piezometric head in the different lithologies. Packer tests toestimate hydraulic conductivity values were conducted. The software FEFLOW and Phase² were utilised tomodel the pore pressure distribution in the pit walls and the results were used to access the pit wall stability.

Key Words 8m rainfall, high walls, vibrating wire piezometers, drain holes, depressurisation

Figure 1 Ok TediLocality map.

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OK Tedi

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oped in the lower unit under the Moscow Sector.Larger sinkholes have been uncovered periodi-cally during mining, particularly in the upper uniton the west wall; one of which was large enoughto engulf an excavator (OMTL, 1999).

The Darai Limestone is overlain by the PnyangFormation (Middle Miocene). This Pnyang Forma-tion consists of mudstones, siltstones with subor-dinate limestone lenses.

The sediments in the Ok Tedi area were sub-jected to major thrust faulting and folding priorto the placement of the intrusive. Emplacementof the Pliocene Sydney Monzodiorite is believedto have produced the Ok Tedi skarns throughmetasomatic replacement of limestone/carbon-ate facies by reaction with magmatic fluids. TheSydney Monzodiorite is relatively unaltered andcontains limited economic gold and copper min-eralisation, although it is enriched near the Fubi-lan monzonite porphyry contact. The faultnetwork provided conduits for magma and intru-sion by multiple phases of variably mineralisedcalc-alkaline stocks with contemporaneous skarnformation.

Although numerous exploration, geotechnicaland dewatering boreholes have been drilled overthe past 30 years, only an extremely limited num-ber are currently being used to monitor the hydro-geological environment in and around the mine.From 2007 to February 2010, fifteen boreholeshave been drilled for the specific purpose of hydro-geological monitoring. Fully cemented vibratingwire piezometers with automatic data loggerswere installed in these holes to monitor the piezo-metric head in the different lithologies (see Figure3 and Figure 4). More than 30 packer tests to esti-mate hydraulic conductivity values were con-ducted in >10 boreholes over the past years.

Using groundwater level monitoring data thepiezometric surface throughout the mine was cre-ated using the contouring software Surfer and in-corporated into a wireframe surface using

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Figure 2 Annual Precipitation Data – Ok Tedi Mine.

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Figure 3 Geological section (W-E) through bore-hole PZ05.

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Mine2—4D. When the groundwater level wire-frame is overlain with the current pit develop-ment wireframe (see Figure 5) the seepage facecan be seen. Seepage outside this area (e.g. higherup the face) represents unsaturated seepage flowfrom recharge events. It is evident that most ofthe central and northern part of the pit is cur-rently in a state of equilibrium. Inflow into the sys-tem equals the outflow of the system. However,considering the planned increased dropdown rate

of the pit bottom, the passive depressurisationrates of the high wall might not be adequate andactive dewatering measures must be imple-mented to reduce the hydraulic head. The relation-ship was studied using a program of geotechnicaland hydrogeological modelling.

Geotechnical and hydrogeological modellingAn appropriate phased modelling approach wasdevised in order to provide the most suitable in-

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Figure 4 Pressure head monitoring data of borehole PZ-05.

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Figure 5 West-East section through the open pit viewed from the south.

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puts to the final UDEC (discrete element) analysesof the West Wall Cutback intended at OK Tedi. Thisapproach included the following steps:

1 Slope stability analyses of two sections bymeans of Phase² small-strain finite elementanalysis was conducted to determine the ap-proximate position (i.e. distance behind theface) of the groundwater level and the porepressure distributions required in order to ob-tain a Factor of Safety (FoS) of approximately1.3 or greater.

2 2-D and 3-D hydrogeological modelling bymeans of FEFLOW software to determine thedrainhole configurations required in order toachieve the groundwater level that the Phase 2modelling had indicated to allow for a stableWest Wall Cutback.

3 The results of the FEFLOW model were thanused as real pore pressure data in a subsequentPhase² slope stability analysis.

Rock mass strength, rock fabric and hydraulicinput parameters were based upon an extensivelaboratory and field test program between 1997and 2010 (OTML 1999, SRK 2006, 2007, 2010) incollaboration with OTML staff.

Variations in groundwater conditions wereused in both sections to study the requirementsfor dewatering/depressurisation for the pit wallstability. These are listed in Table 1 and the posi-tion is shown in Figure 6.

For each analysis, plots of total displacementand maximum shear strain were used to assessslope performance. The FoS in each case is deter-

mined by means of the strength reductionprocess and thus is expressed by means of theStrength Reduction Factor (SRF). The first-pass sta-bility results are summarised in Table 1. These re-sults show that a the piezometric surface needs tobe pushed back to around 150m behind the south-ern part of the pit wall and 200m behind the cen-tral/northern part of the pit wall, in order forsuitable FoS to be achieved.

Initial 2-D FEFLOW seepage and depressurisa-tion analyses were carried out to investigate thepassive drainage requirements (horizontal drain-holes) that must be employed to obtain piezomet-ric surfaces of approximately 150m and 200mbehind the West Wall as predicted by Phase 2analysis. Separate sets of analyses were carried outfor each section, in order to assess the effective-ness of various drainhole configurations. Verticaldrainhole spacings of 120m (i.e. drainholes at thebase of each bench stack) were assessed, withdrainhole lengths of 150m, 200m and 300m.

Numerous dewatering/depressurisation bore-holes hole have been drilled into the west wallover the past years and are still drilled. Based onthe the current monitoring data it seems to behaving a positive impact as the piezometric headis lowering on an annual basis. Based on all theseobservations there does not seem to be an exces-sive built up of water behind the west wall and thesaturated piezometric surface is ±200m away/be-hind the western high wall. This means that cur-rent dewatering and depressurisation measuresare resulting in reduced pore pressure and this isleading to an improved stability of the west wall.

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Figure 6 Positions of thesections.

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summary and preliminary ConclusionsFrom the hydrogeological and slope stability nu-merical analyses completed so far, the followingcan be concluded:

Several types, sets and phases of analyses wereconducted to build a logical and orderly sequenceof results (and to test sensitivities) in order for pre-liminary establishment of the most suitable pas-sive drainage requirements for ensuring suitablestability of the West Wall Cutback.

The slope stability analyses and 2-D hydrogeo-logical analyses have been conducted on selectedsections and the quasi 3-D hydrogeological analy-ses have been conducted on blocks of limited ex-tent.

A drainhole length of 300m, horizontal drain-hole spacing of 30m and vertical drainhole spac-ing of 120m is necessary to accomplish apushback of piezometric surface behind the wallof 150m and 200m (see Figure 7) to achieve a FoSof ≈ 1.3.

acknowledgementSRK thanks OTML especially Messrs Sheppard,Baczynski, and Kaegi for their support and for the op-portunity to work on this project.

referencesOk-Tedi Mining Ltd. (1999): Risk-based slope design

optimisation studies for the Ok Tedi copper-goldmine.

SRK (2006): Drainage Adit Geotechnical InvestigationSRK (2008): Ok Tedi Underground Advanced Concep-

tual Mining Study.SRK (2010): Presentation of preliminary results of hy-

drogeological and slope stability numerical analy-ses for the Ok Tedi west wall cutback, in advanceof final udec analyses.

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Table 1 Different groundwater conditions used for pit wall stability assessment.

Section Groundwater Conditions Critical SRF

1

Worst case groundwater level (piezometric surface almost at pit wall) 1.02 150m pushback of piezometric surface behind wall 1.36 200m pushback of piezometric surface behind wall 1.46 300m pushback of piezometric surface behind wall 1.55

2

Worst case groundwater level (piezometric surface almost at pit wall) 0.88 150m pushback of piezometric surface behind wall 1.21 200m pushback of piezometric surface behind wall 1.32 300m pushback of piezometric surface behind wall 1.41

Figure 7 Pore pressure dis-tribution with drainholes at120m vertical spacing, and

horizontal spacing 30m,comparing unsaturated

flow for 300m long horizon-tal drains.

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