Arapuni Dam, located on the Waikato River 55 km upstream from Hamilton New Zealand, has a history of seepage incidents, relating to its rock foundation, since first filling in 1927. The latest seepage event led to a comprehensive investigation to determine the extent of the foundation features requiring treatment to prevent further incidents from developing. A targeted cost effective solution involving drilling and concreting overlapping vertical piles from the dam crest through the dam and underlying rock formation to a total depth of 90m was selected to form four separate permanent cut-off walls at discrete locations beneath the dam. The discrete cut-off solution removes the risk of further piping incidents at Arapuni, restores the foundation seepage condition to that typical of a concrete dam and, with coverage of approximately half the crest length of the dam, is considerably cheaper than providing a cut-off over the full length of the dam crest. This solution was enabled by an extensive exploratory drilling and testing investigation before hand.
Investigation drill holes and weirs connecting to foundation underdrains were instrumented and monitored providing near real-time pressure and flow data. Safety of the dam was thus monitored during cut-off construction and cut-off effectiveness was verified subsequently.
NZGS Symposium