Construction control, monitoring and long term management
Construction control and monitoring
Construction of the waste rock embankments is carried out by the mining contractor, under the supervision of OceanaGold personnel, with the assistance of the design engineer and surveyors as necessary. Our personnel assist the contractor in planning construction, observing all construction activities and undertaking control testing of fill placed in the embankments.
Regular surveys of the embankments are carried out to measure movements. The embankments are instrumented with a network of piezometers that measure pore water pressure and provide information necessary to assess the stability of the embankment. Water quality and flows are measured for both the surface water and underdrainage systems. Modelling and monitoring of the performance of the cap (zones G, H and topsoil) is an important component of the monitoring programme.
The consent conditions stipulate the requirements for the reporting of data to the appropriate regulatory authorities. The design engineer prepares an annual inspection report to confirm that the embankments are in a safe and stable condition. An independent peer review panel, consisting of experts in the fields of geotechnical engineering, geochemistry, hydrogeology and rehabilitation, also carry out inspections on a regular basis. They review data and reports, and they themselves provide reports to the regulatory agencies. They have a supervisory role over the construction of facilities.
Over time, it is expected that the water quality of individual drains within the underdrainage system will improve, and the flow will decrease. This has been demonstrated with the monitoring carried out at Storage 2.
Long term management
Monitoring of individual drain flows and quality will continue during the closure period. It is expected that individual drains will be piped to the Ohinemuri River and Ruahorehore Stream once monitoring indicates that the discharge will not have a significant adverse effect on the receiving waters.
OceanaGold will provide sufficient funds to the post closure Trust to allow it to carry out monitoring of the drains and the receiving waters in the long term. The water treatment facilities will be gifted to the Trust to allow water to be treated in the unlikely event that this becomes necessary.
A number of scenarios were looked at to determine what would happen if, for instance, the underdrains, upstream cutoff drains, initial toe drains and gully drains became blocked, and seepage was able to bypass the underdrainage system (bypassed seepage). A computer model was used to model seepage through the embankments and the underlying foundations. Modelling of the effects of seepage on both the Ohinemuri River and the Ruahorehore Stream was carried out. This took into account varying infiltration rates through the capping layer.
It was found that the upward movement of groundwater provides a significant constraint to seepage out of the embankments. In the event that bypassed seepage should occur in the long term, the modelling demonstrated that it would not have any significant adverse effects on the Ohinemuri River or Ruahorehore Stream.