Abstract

The Ok Tedi Copper-Gold Mine is located in the remote and rugged Star Mountains of Papua New Guinea with the crest of the pit wall at 2,200m above sea level. Annual rainfall is 8,000mm to 13,000mm. The main pit is oval shaped and covers a surface area of 3km by 2km. Current slope heights are 500m to 850m; but some of these will approach 1000m in the future. Surface water in the current pit is discharged through vertical shafts into a 3km long drainage tunnel beneath the base of the pit, which then discharges into a creek lower in the mountains.

In order to achieve stable and economic future pit slopes, the planned 1000m high cutback needs to be dewatered 250m behind the designed slope, or about 500m behind the existing slope. Investigations were undertaken to design a drainage gallery network, which would start at the base of the existing pit and dewater the slope. The ground conditions behind the current and proposed slopes are extremely variable and often unfavorable, causing several large instabilities on the pit slopes. The ground conditions are defined by two regional scale thrust faults and a major shear zone. This paper discusses the challenges faced in characterizing ground conditions and designing the drainage gallery tunnel network.

1.
Introduction

Ok Tedi is a remote mine is situated at 2000m elevation in the rugged Star Mountains of the Western Province of Papua New Guinea (Fig. 1). Open pit mining of the copper-gold deposit began in 1984 (England, 1993). The current pit has 500m to 850m high slopes; the West Wall Cutback slope will be 1000m high (de Bruyn et al. 2013). Areas around the mine are named after major world cities, including New York, Paris, Moscow, Berlin, Edinburgh and several others (Bar & Baczynski, 2017).

Annual rainfall varies around the pit footprint and ranges from 8,000mm to 13,000mm. Monthly rainfall is similar throughout the year, although June to August typically has 20% more rainfall than the other months (Fig.2).

This paper examines several geotechnical and hydrogeological issues associated with mining at Ok Tedi and the West Wall Cutback project, but concentrates on rock mechanics investigations, site challenges and design for the proposed drainage gallery.

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