MERCURY MOBILE
The jet grouting trial work site was set up on the left abutment from July-September, at the Arapuni Hydro Dam.
We are assessing options to install a seepage cutoff wall in the left abutment, this is a concrete barrier that prevents seepage. We are working on geotechnical assessments which will help inform the remediation work design and construction.
As part of this process, we’ve begun discussions with our iwi partners, Raukawa, Ngaati Koroki Kahukura and Ngaati Hauaa, as well as the South Waikato District Council, Waikato Regional Council, Waipā District Council and other key stakeholders to better understand the impacts of the remediation work. We will keep the community informed as plans are developed.
The dam has had remediation treatment for leakage, dating back to its construction. Following the first lake fill in 1930, significant seepage of water traveling through the left abutment from the headrace to the dam face was observed. The lake was lowered, and seepage controls were retrofitted.
The Arapuni Hydro Power Station was constructed between 1924 and 1929.
Major remediation work was completed on the dam in the mid-2000s.
This involved remediation work to construct a concrete cut-off wall under the dam, without disrupting power generation or causing environmental damage. This was a pioneering effort in the field of dam engineering.
The project won multiple engineering awards, including the Shell Environmental Excellence Merit Award in 2007 and the New Zealand Engineering Excellence Award in 2008. The work was recognised for its innovative approach to dam remediation while maintaining operations and minimising environmental impact.
The project was led by an alliance group comprising Mighty River Power (now Mercury), Brian Perry Civil and Trevi Group, a specialist in underground engineering. The three have reformed to work together on the remediation of the left abutment at the Arapuni Dam.
The left abutment of the dam is where our current remediation work will be focused.
In 2024, we began investigating options to install a seepage cutoff wall in the left abutment, as a modern equivalent to manage the seepage.
We began early works in 2025, relocating old and installing new subsurface instruments to help us understand what’s happening under the left abutment. We now have more than 100 subsurface sensors providing real-time data. This information will help us respond to any changes in the left abutment as we carry out the remediation work.
This is a soil improvement technique used to strengthen and stabilise the ground. It has been used in several hydro dam remediation projects around the world. During September and October, we studied the results of the trial which helped inform and plan the main works for 2026.
Download our info sheet on jet grouting here.
For safety, we will close the western end of the swing bridge where it connects to Powerhouse Road.
We are making changes to Powerhouse Road so it is safe for Waikato River Trail users and safe to use as a detour route when the main works start.
Download the Powerhouse Road detour route for local essential travel here. Download the alternative route for heavy vehicles here.
We submitted a business case to our board which was approval in late 2025. We are now finalising the main works timeline and progressing towards the construction phase.
In early March we began upgrades to Powerhouse Road, between Arapuni Road intersection and the lower bridge across the Waikato River, to prepare it for use as a temporary detour route.
We hosted the South Waikato District Council at the dam site and led them on a tour of the Arapuni Hydro Station, to bring them up to speed on the project.
We submitted a business case for the main works programme to the Mercury board.
In February our relief drainage work started to make good progress, on track for completion in March.
In January we began setting up a new work site at the bottom of the dam for our relief drainage work.
December 2024 newsletter April 2025 newsletter July 2025 newsletter December 2025 newsletter
From mid-2025, we will be working on relief drainage at the bottom of the dam. We will also be working at the top of the dam, using a rig to relocate and introduce more subsurface instruments. This will help us monitor what’s happening underneath the left abutment.
From mid-2025, we will also be working on a jet grouting trial. The results of all of this work will help inform the main programme of work which we’re aiming to start in 2026.
In 2025, we may need to use temporary traffic management on Arapuni Road to move equipment, but the road will remain open throughout this stage of the project.
In 2026, Arapuni Road across the dam will be closed to allow us to complete the main work on the abutment. We will use a one-lane detour route, managed by traffic lights, along Power Station Road which reconnects to Arapuni Road, north of the village. More details for the closure timing and durantion will be provided once understood.
For safety, we will have to close the western end of the swing bridge which connects to Power Station Road, where people walking and cycling would normally join the road.
We are still developing options for the remediation work; we will complete technical assessments to better understand any impact on the local community. As we start to develop plans for the works, we will engage with the community to see how we can minimise disruptions.
We held our third community drop-in event on 4 October 2025 and we are planning our fourth event in March-April 2026.
While there will be no change to the electricity generation, doing the required works now ensures that future generations can continue to benefit from this integral hydro asset. The power station will continue to operate as normally during the work.
We want to better understand what’s important to local people and the groups we work with. As we start to develop plans for the remediation work, we will continue to engage with stakeholders and local community to let them know what is planned. If you would like to contact the project team, please email: project.arapuni@mercury.co.nz.
Our contractors, Avalon, began work on clearing vegetation from the left abutment in October 2024 and completed the work in November 2024. We needed to clear the vegetation so we could accurately geo-map the abutment to determine the best plan for relief drilling work, which we want to start in early 2025.