The works comprised the construction of 4 No. discrete sections of 400mm diameter secant pile walls extending to depths of up to 90m from the dam crest with the lake level maintained and power generation unaffected. The associated works included dam strengthening, gallery infill, drainage, joint sealing and access track upgrade.

Arapuni Dam is a 64 metre high curved concrete gravity dam, with a crest length of 94 metres, across the Waikato River bed.  The dam, commissioned in 1929, forms the reservoir for a 186MW hydro-electric power station. There has been a history of periodic seepages, through fissures within the Ignimbrite, since the reservoir was refilled in 1932. In 2000 a trend of increasing pressure and flow under the dam was identified and solid particles of clay were observed in the drainage system. The actual washout of clay and the potential for further localised washout of clay-filled joints lead to the need to provide a positive and durable barrier.

The Alliance investigated numerous options, including grouting, jetting and wire sawing and found there was nothing available that would form the cut off with a sufficient level of confidence. Taking an holistic risk based approach, the use of secant piles was selected even though secant piles to 90m with a verticality tolerance of 1 in 3600 had not been done before. A methodology using a guide frame and forming a verifiable panel was developed and tested.

The method was successfully implemented to construct four discrete concrete cut-off walls under an operating dam, without adversely affecting dam safety or electricity generation.

The construction of overlapping / secant piles to such depths represented a significant extension to international technology and experience in this area.

The project has been widely reported and published around the world and received engineering and construction awards including:

• Shell Environmental Award 2007
• IPENZ Engineering Excellence Award 2008

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