The Nakan Dam, at the Kelian Equatorial Mining gold mine, in East Kalimantan, Indonesia was constructed to store 165 million tonnes of acid waste rock under 1 m of water. The project comprised four earth and rockfill embankments. The high level diversion conduit (HLDC) was built through one of these to provide runoff diversion. The HLDC included an inclined decant system up the upstream face of the saddle embankment with inlets at 10 m intervals to allow for incremental filling of the reservoir during waste dumping. A flow of approximately 50 L/s was observed, when the bellmouths were closed, indicating a leak had developed within the HLDC.

PSM was engaged to permanently seal the leak in the HLDC.

The Nakan Dam is located in a remote region of Borneo, approximately 2 km south of the equator. It experiences high annual rainfall with an operating reservoir level of approximately 30 m head.

Permanent sealing (grouting) of the leak could not be undertaken without temporarily halting or reducing the flow. Working conditions were risky with a potentially low oxygen atmosphere and/or explosive environment within the HLDC and subsequent rise in reservoir level following sealing. The location of the leak within a pipe buried beneath 70 m height of the earth and rockfill embankment was unknown.

Other engineering and technical opportunities included:

• The HLDC horizontal alignment was dog-legged to best follow the natural ground, further complicating the task of inserting and positioning the packer
• Neutralisation of the acidic waters in the Nakan Dam reservoir, with the addition of lime, had resulted in a precipitate in the HLDC potentially risking the packer operation
• Achieving grout length within the HLDC before the onset of initial set in the HLDC or supply lines.

Following a rigorous assessment, PSM’s tailored approach included:

• Development of several potential solutions to manage the leak in the HLDC, including construction of a scaled model to assess whether the placement of graded gravel filter within the HLDC would sufficiently reduce flow
• Use of flexible corrugated Helcor pipe joined with compression couplings in a gravel surround beneath the embankment’s upstream and downstream rockfill zones
• Construction of a welded steel pipe in a rigid concrete surround through the central clay core and transition zones
• Development of a bespoke 1 m diameter packer to temporarily seal the HLDC within the HLDC’s steel pipe section downstream of the supply lines, including testing of two full sized prototypes
• Preparation of trial grout mixes to assess possible mix designs
• Mobilisation of a team of construction divers to initially investigate the location of the leak, clean the steel pipe with scabbling hammers and position the packer immediately downstream of the supply lines 1 and 2
• Mobilisation of grout agitator, high-flow grout pumps and ancillary equipment, materials and labour.

PSM successfully permanently sealed a leaking 1 m diameter pipe located beneath a 70 m high earth and rockfill embankment. The design solution incorporated a bespoke 1 m diameter inflatable packer to seal the pipe outflow before displacing the water via a high-flow grouting operation.

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