|Owner||Mighty River Power|
|Main Contractor||Alliance Trevi - Brian Perry Civil Eng. - Mighy River Power|
|Duration of works||2005 - 2007|
Arapuni Dam is a 64 m high curved concrete gravity dam, with crest length of 94 m, across the Waikato River bed. The dam, commissioned in 1929, forms the reservoir for a 186 MW hydroelectric power station.
The key original features of the dam include concrete cut-off walls and a network of porous (no-fines) concrete under drains at the dam/ foundation interface. The under drain is the main uplift control at the dam/foundation interface. The original cut off walls extend to a depth of 65 m below the dam crest and also into the left and right abutments of the dam. No grout curtain was constructed during the original construction.
In June 1930 the reservoir was completely dewatered, due to the development of a large crack in the headrace channel near the powerhouse, whilst this was being repaired; a grout curtain was constructed along the upstream heel of the dam and along the front of both abutment cut-off walls. This grout curtain was constructed just upstream of the dam and cut-off walls, using a single row of grout holes at 3 m centres, but it was not physically connected to the dam. At the steep gorge walls a bitumen plug was constructed and the grout curtain extended radially from the crest abutments.
There has been a history of periodic seepages, through fissures within the Ignimbrite, since the reservoir has been refilled in 1932. Observed flow rates of up to 750 litres per minute have been managed using targeted grouting works.
In 2000 a trend of increasing pressure and flow was identified and solid particles of clay and bitumen, together with lake life, were observed exiting the drainage system. Investigations by Dam Watch showed that the very weak Nontronite clay infill was eroded from within a fracture forming a ‘pipe’ connecting the reservoir and the dam drainage system; this was successfully grouted in 2002.