Erosion-driven slope instabilities

  • By Robert Parker, Neil Bar, Paul Kuira and John Semi

Managing progressive erosion and chasm enlargement risks in a high rainfall environment at Ok Tedi copper-gold mine

Introduction

Ok Tedi is a large open pit copper-gold mine situated at an altitude of 2000 m above sea level in the Star Mountains of the Western Province of Papua New Guinea. Current excavated slopes range from 500 m to 800 m in height and planned future slopes will be 1000 m high. Annual precipitation in excess of 12 000 mm has been recorded adjacent to the mine site. Rainfall is monitored at eight locations around the pit with average annual rainfall being 10 000 mm. Rainfall can be intense and localised, with the Southern Dumps typically receiving 10 per cent more rainfall than the Northern Dumps (four km apart). Although rainfall occurs relatively steadily throughout the year, June to August typically receive rain in the order of 200 mm or 20 per cent more rainfall than the other months of the year (Bar, Kuira & Semi, 2014).

Surface water runoff generated during high intensity rain events initiates localised erosion on benches and bench faces on a daily basis. In zones of friable and highly fractured rock, the erosion quickly leads to undercutting geological structures, enabling small-scale block toppling, planar and wedge sliding mechanisms to occur. When uncontrolled, the problem progressively worsens leading to the development of large chasms along major geological structures and within weak shear zones on pit slopes.

Chasms of up to 240 m high and 220 m wide have developed at Ok Tedi as illustrated in Figure 1. It is important to emphasise that in the context of this article, each chasm formed over a significant period of time from months to several years and was not a catastrophic, single-event slope failure. For example, chasm W3 in Figure 1 was initiated as a localised single bench failure and progressively enlarged to its present size over a ten year period, even after Ok Tedi invested considerable remedial effort and expense in attempts to control its growth. The remedial efforts included sophisticated ground control monitoring, establishment of new access ramps down the slope face, construction of new drains for diversion of surface water runoff away from the chasm footprint, ground stabilisation and chasm crest, toe and slope face dewatering. All of this remedial activity had to be accommodated within the daily framework of other pit mining activity and completed in often extremely poor weather conditions.

Chasms on West Wall of Ok Tedi (November 2014).
Figure 1. Chasms on West Wall of Ok Tedi (November 2014).

Geological setting

Ok Tedi comprises several lithologies ranging from weak sedimentary rocks, such as mudstone, to strong igneous intrusions, such as monzonite porphyry. The Ok Tedi copper-gold deposit is structurally complex with several phases of structural deformation and intrusive activity. Figure 2 shows an indicative geological cross-section. Major geological features include:

  • The Taranaki and Parrot’s Beak Thrust Faults had displaced strata over considerable regional distances and resulted in the observed repetition of limestone and siltstone units in the Ok Tedi pit area. The thrust faults were later domed during the intrusion emplacement.
  • Gleeson Shear Zone (or fracture zone) is a portion of the west wall which is host to weak, highly sheared and fractured rock masses.
Figure 2. Simplified geological cross-section superimposed with groundwater profile.
Figure 2. Simplified geological cross-section superimposed with groundwater profile.

The groundwater regime comprises of two notable perched water tables above the major thrust faults as illustrated in Figure 2. Both perched water tables are recharged by rainfall and downslope surface water runoff. Historically, the west wall slope has been depressurised with the use of horizontal drain holes. However, due to adverse drilling conditions in the Gleeson Shear Zone, almost no horizontal drains were drilled in the vicinity of chasm W3.

Chasm enlargement mechanisms and historic remedial efforts

The chasms have enlarged by a combination of different modes of instability. However, in all circumstances, uncontrolled surface water flow had initiated and progressively eroded and undercut the walls of the chasms. Wall failure modes include block toppling, planar and wedge sliding along pervasive structures and slip circle failure through weak rock and residual soil masses. By way of example, Figure 3 illustrates the localised wedge failure mechanism which contributed to the enlargement of chasm S3 below the haul road:

  1. Friable material in upper benches had been shotcreted to minimise erosion; horizontal weep holes of up to 15 m depth were drilled into the face to reduce pore pressures in the benches.
  2. Combination of surface water runoff from rainfall and groundwater from weep holes progressively eroded cohesive elements within wedge defect planes on unsupported lower bench.
  3. Small wedge on lower bench fails after a high intensity rainfall event and undercuts the shotcreted bench above which failed shortly thereafter.
  4. From the location in Figure 3, this mechanism progressed vertically upwards 45 m, and down a further 15 m for a period of approximately nine months before it was arrested by competent rock mass conditions in the slope.

Lined drainage systems have been implemented to enable efficient water flow, reduce scouring and prevent surface water flowing down slopes.

  • Chasm wall failures damaging lined drains above the crest of Chasm W3 have been the prime driver for the enlargement observed between 2013 and 2015 (Figure 4).
  • Reinstatement of lined drains diverting surface water away from Chasm W3 in 2013 and 2014 assisted in reducing the rate of chasm enlargement.
Figure 3. Localised wedge failure mechanism in Chasm S3.
Figure 3. Localised wedge failure mechanism in Chasm S3.

Over the last decade, numerous ground support methods have been trialed to arrest chasm enlargement with varying degrees of success. These include:

  • Grouted piles (typically 15 m deep) at the crest of benches and access ramps above and below chasms. These temporarily reduced the rate of enlargement in Chasm W3 for a period of approximately 12 months (Figure 4).
  • Mesh and shotcrete of highly friable bench faces which temporarily reduced the rate of erosion above Chasm W3. However, this was unsuccessful in portions above Chasm W3 where shotcrete failed from excess pore pressure build up behind the face, even with the installation of weep holes.
  • Rock-filled gabion baskets and large concrete blocks to reduce toe erosion. Gabion baskets temporarily reduced the rate of toe erosion at Chasm W3 before being undercut and failing due to excess surface water flow. Large concrete blocks were successfully used for 2-3 years to temporarily arrest Chasm W3 toe erosion and enlargement.
Figure 4. Examples of failed grouted piles, mesh and shotcrete at Chasm W3.
Figure 4. Examples of failed grouted piles, mesh and shotcrete at Chasm W3.

Risk management

The major risks associated with the chasms are:

  • safety risks associated with uncontrolled ground movement resulting from progressive failures in the chasms could result in major consequences such as serious injuries, fatalities and equipment damage
  • economic risks involving short-term production losses due to haul road blockage following failure events, particularly associated with Chasm W3
  • economic risks involving permanent loss of access to the ore body through the continued enlargement and potential joining of Chasms W3 and S3.

The economic risks are being managed through ongoing review and maintenance of surface water drainage and the construction of secondary and tertiary haul roads to access the ore body. Chasm W3 enlargements in 2013 and 2014, which substantially impaired mine production for several weeks, have significantly increased the priority of these remedial efforts.

Safety risks are being managed with a combination of operational and slope monitoring controls:

  • Implementation geotechnical hazard awareness and daily inspections and hazard reviews.
  • Debris catchment bunds and traffic management on haul road (lighting towers during night shift and spotters to divert traffic if debris flows are initiated).
  • Slope monitoring systems are used for the early detection of slope instabilities and are currently capable of detecting movements of less 10 m2 of the slope face. Two levels of deformation alarm thresholds have been set to prompt geotechnical engineers to:
    • carry out inspections and rigorously review the monitoring data
    • instigate critical monitoring restricting or pit access altogether.

Conclusion

High rainfall coupled with a highly-friable rock mass has created a challenging mining environment where erosion-driven instabilities have been observed to progressively enlarge from a 15 m high bench failure to a 200 m plus high chasm.

While in their infancy (less than 50 m high), the enlargement of chasms has been greatly reduced or arrested with the use of ground support and effective surface and ground water management. Once chasms have enlarged beyond than 50 m high, surface water management has been considered the most economic method of reducing the rate of chasm growth until the slope is cut back.

Without these challenges and lessons learnt over the last decade, the operation would be less prepared for the 1 000m high west wall cutback which is currently in its early stages of mining. The following strategies are planned and currently being implemented for this cutback to protect the mine schedule:

  • slope dewatering and depressurisation using horizontal drilling methods in weak, friable rock.
  • ground support of benches and bench faces in weak and friable rock using grouted piles, mesh and shotcrete to reduce erosion and the likelihood of initiating chasms.
  • development of ramp networks for surface water drainage and additional access to the ore body.

References

Bar, N., Kuira, P. & Semi, J. 2014, Managing risk associated with erosion-driven slope instabilities with ground support and surface water management in a high rainfall environment at Ok Tedi Copper-Gold Mine, Proceedings of Engineers Australia Convention 2014: Mastering Complex Projects 24-28 November 2014, Melbourne, Australia.

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