Slope Stability Analysis of Hamata Tailings Dam, Hidden Valley Mine, Papua New Guinea

 ABSTRACT

Construction and management of Tailing dams in Papua New Guinea (PNG) is faced with many challenges such as high altitude with high rainfall (2000-5000 mm/yr), high seismicity and structurally controlled zones which pose threat to the slope stability of tailings dams. Therefore, slope stability analysis is necessary to give confidence to some extent to the stakeholders. The location for this study is at Hamata Tailings (dam) Storage Facility (TSF) at Hidden Valley Mine in PNG which has two rock/earth filled embankments, the main dam and the saddle dam with downstream construction method. Currently the TSF owner is planning to raise the dam height from RL 2000 to RL 2015 with extra 15 Mt storage capacity as the pond water approaching its designed capacity at RL 2000. The objective of this study is to analyse the slope stability of Hamata TSF using phase 2 based on the design basics for the crest expansion from RL 2000 to RL 2015 and beyond and recommend an ideal slope stability under various conditions in terms of shear strength reduction factor ((SSRF). The results obtained in this study is useful for PNG Mining Regulators in comparing company results in the appraisals for tailings dam development proposals and, it will be useful to future researchers in PNG and other similar tropical regions.

 Keywords: Tailings dam, slope stability analysis, Hamata TSF crest expansion, embankment, Shear Strength Reduction Factor, RL-Reduced Level(m).

 INTRODUCTION

Tailing dam construction in PNG are faced with natural factors such as high altitude with high rainfall (2000-5000 mm) coupled with high seismicity zones and geological/geotechnical conditions which pose threat to the stability of tailing dams. One of the learned experience is the case of Ok Tedi tailings dam failure in 1984 (Griffiths et al. 2004). After this incident, the PNG government allowed mining companies to discharge tailings into the river systems and on to the sea floor (deep-sea tailings placement (DSTP)) which pollutes the riverine and ecology within the vicinity of the mine impacted natural environment and communities downstream and the marine lives respectively.  However, the PNG Government amended the Environment Act to abolish riverine tailings discharge and encourage tailings dam construction in PNG.

In compliance with the PNG government’s intention, the Hidden Valley mine and the K92 mine have constructed tailings dams respectively and store their tailings in the facilities overcoming all odds. However, management of the tailings dam under challenging environment is one of the key concerns of the dam owners to make sure the dam is stable throughout the operation till closure and post closure. On the other hand, the mining regulators and the impacted communities downstream also concern about the stability of the dam as it will affect their livelihood in an unlikely event of failure.

 In this study, it is proposed to review and assume Hamata Tailings Dam designs and embankment material properties to evaluate the slope stability conditions of the TSF under various geotechnical/soil parameters.

 Study Location – Hidden Valley Mine

The Hidden Valley(HV) Mine (coordinates: 7⁰27’17” S,146⁰40’24” E) in PNG operates the Hamata Tailings dam.  Hidden Valle Mine is an Open pit gold-silver mine located in Morobe Province, about 210 km North West (NW) of Port Moresby. The Mining Lease was Granted in 2005 for 20 years and renewal upon expiry. The Lease holder is Harmony Gold Ltd. Mine development Construction started in 2007 and commercial production began in September 2010.

 Figure 1 PNG map (Courtesy of Mineral Resources Authority) showing location of Hidden Valley Mine (Circled).

 Mine Layout

The mining lease area has two main mine pits which are about 6 km apart and mining at three main ore bodies which are named as Hidden Valley- Kaveroi(HVK) and Hamata epithermal gold and silver deposits.  The Hidden Valley and Keveroi Ore deposits are close to each other while the Hamata ore body is on its own. Ore mined from HVK is transported via belt conveyor to the processing plant near Hamata pit. 

The mine has a total mineral resources of 68.776 Mt at Hidden Valley Kaveroi deposits with a metal content of 3.307 Moz Au and 57.270 Moz Ag while the Hamata deposit has a total mineral resource of 2.216 Mt ore with metal content of 0.133 Moz Au as of June 2019 (HV Annual Report-2020).

 Figure 2 Hidden Valley Mine Plan (Rynhoud et al ,2017)

 Hamata Tailings (Dam) Storage Facility (TSF)

The Hamata Tailings (Dam) Storage Facility is constructed using the downstream method with two earth and rock filled embankments, the saddle dam and the main dam. The dam Construction commenced in June 2007 and the starter embankment construction was completed in February 2009 (Rynhoud et al, 2017). The embankments are constructed using the waste rock/materials from the two mine pits at HVK and Hamata.

Klohn Crippen Berger Ltd (KCB) is the design engineer for the Hamata TSF, (Rynhoud et al, 2017). The main dam and the saddle dam is designed to a maximum crest elevation of RL 2000 with a storage capacity of about 40 Mt of tailings with a mill throughput of 4.2 Mtpa (Rynhoud et al, 2017). The height of the dam from the main dam is about 145 m at the RL 2000 crest.

Figure 3 Hamata Tailings Dam, Main dam at NW and Saddle dam at SE (Google Image-7°25'36.6"S146°38'32.0"E)

 Problem Statement

Tailings deposition and sedimentation at Hamata TSF result in ponded water approaching dam crest elevation at RL 2000, the miner proposed to raise the dam height to RL 2015 with extra 15 Mt tailings storage capacity.

The foundation of expansion (RL 2015) embankment is likely to begin at RL1960 to RL 1970 of the RL 2000 design. With the pond water seeping through the embankments coupled with high rainfall, the geotechnical parameters are altered over time and displacement of embankment is anticipated under wet conditions and/or seismic conditions and potential dam slope failure is anticipated in a worse case scenario.

The focus of this study is to review available options to minimize significant displacement of embankment under various stress conditions.

 Significance of Study and Research Advancement

Related literatures of slope stability analysis of tailings dam in PNG is rarely available online except the design basics of dam published by Rynhoud et al 2017 and Murray et al 2010. There are also publications of tailings dam about Frieda River Mining Project and Ok Tedi Mine of which most of the data from this study is obtained from all these publications.

Further research can be done beyond this study in terms of slop stability analysis of tailings dam under various geotechnical and seismic conditions in similar tropical regions.

 Objective Of The Study

The objective of this study is to analyze the slope stability of the Hamata Tailings dam construction and operation of crest expansion from RL 2000 to RL 2015 and recommend an ideal risk factor of safety under various stress conditions. The study adopts Finite Element Analysis in Phase2 software to analyze the slope stability conditions of the dam for RL 2015 based on the design basics and material properties of the dam embankment. Design basics are modified for the purpose of modelling and may not represent construction design specifications.

Slope Stability conditions are expressed in terms of Shear Strength Reduction Factor (SSRF or SRF) and the corresponding displacement under stress conditions.

 METHODOLOGY

Study methodology is designed in a way to review related literatures of the past and collect field data including design parameters and proceed with modelling. Results from the model are interpreted to make conclusion and necessary recommendation is anticipated.

In the case of field research which is impossible at hand, related data from other projects in both PNG and abroad are borrowed for the purpose of modelling in this study.

General information regarding mining in PNG are reviewed and adopted some scripts in this paper. Most of the data at hand is obtained from both unpublished and published literatures related to the Mining in PNG and off-course Hamata TSF.

Most of the material property data is expected to be borrowed from Frieda River Mining Project and other publications and reports which are referenced in this paper.

 Design Basics

The design basics are adopted from the published papers by Murray et al, 2010 and Rynhoud et al, 2017 for RL 2000 and assumptions are made for RL 2015 in terms of construction methodology. A combination of upstream, centerline and downstream method of construction is assumed to analyze the slope stability condition of the dam using the phase 2 software. The maiden modified design in the model is shown in Figure 4.

Figure 4 Proposed expansion design (model-cross section, main dam) for RL 2015 (modified from Murray et al, 2010)

 The foundation of expansion embankment is at RL 1960 and RL 1970 in the model design. The expansion (RL 2015) design for the model is modified from the RL 2000 design published by Murray et al, 2010.  

This study adopted material property data from the proposed Frieda River Tailings Dam in PNG which has similar embankment fill materials to that of Hamata TSF. 

 Modeling And Results

Based on the borrowed embankment material property/parameters and design basics data, a maiden model was built in Phase2 and computed to observe the behavior of the TSF embankment. The material strength parameters used in the model are shown in the Table 1. Mohr-Coulomb failure criteria is used for all plastic materials type computed in the model.

 Table 1 Material strength parameters

Material	Unit Weight (MN/m3)	Cohesion  (MPa)	Friction angle(o)
Bedrock	0.02	0.25	45
Boulder Colluvium	0.018	0	30
Lacustrine(clay-assume)	0.02	0.03	20
Alluvium(Organic)	0.018	0	15
Oxide	0.018	0	25
Random Fill (Oxide)	0.018	0	25
Fresh rock fill	0.023	0.6	25
Gravel filter drain	0.021	0	35
Tailings	0.02	0.001	34

 The results obtained from the maiden model shows that potential failure is anticipated. At the critical SRF of 1.4, the total displacement is 0.153 m at the embankment. Ground water conditions and permeability are not computed in the maiden model but will consider in the preceding models.

  

Figure 5(a) Shear Strain, underground water seepage flow rate at main dam embankment in model. (b) Total Total Displacement. (c) Shear Strength Reduction Curve.

Figure 6 Model Results at various SRF in terms of Shear Strain and Displacement Progression at increasing SRF.

 DISCUSSION and CONCLUSION

Results indicate that shear strain is concentrated along the chimney drain and almost steady at all stages of SRF. Displacement is significant and vary at all stages.  At Critical SRF of 0.48, Maximum Displacement is 0.251m.   Maximum displacement is observed at the foundation of RL2015 expansion.  The    Toe of TSF has insignificant shear strain and is stable but weight of displaced materials can induce stress at the toe to be unstable over time. Thus, it requires more attention in this regard.

The Weight of RL2015 foundation cause the maximum shear strain at the chimney drain/ channel in the model and thus displacement at the crest of RL 2015 and along the slope of the downstream embankment.  Seepage water might cause the saturation of embankment materials and failure is anticipated during wet conditions and/or seismic activity.

Therefore there following measures will be taken in the next phase of this study:

q  Model

ü  Variation of model parameters and input data

ü  Analyze other Sections of the TSF.

q  Review Counter Measures to stabilize the unstable slope conditions:

ü  Construction Method

ü  Fill material variations

ü  Geotechnical support systems –i.e. geogrid

ü  Design parameter variations etc..

 

Note:

 This publication is a work in progress and several articles will be published in the future. If you want full paper of this publication and the advanced information regarding Slope Stability Analysis of  Tailings dam then contact  us via contact form.

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