Waihola Underpass

Donald Stewart approached Isaac Construction to examine the feasibility of constructing an underpass under State Highway 1 (SH1) south of Waihola, Otago. This project was a complete contractor-led Design and Build offering encompassing all elements (excluding Resource Consents) of the build. Our local team, led by Dane Meek and Chris Moore, identified risk elements and mitigations, sought approvals from New Zealand Transport Agency – NZTA Waka Kotahi (NZTA) and Clutha District Council (CDC), ensured a local supply chain, and coordinated with other subcontractors to deliver a high-quality project.

Establishing a comprehensive traffic plan and obtaining approval from all stakeholders included creating a bypass road for SH1, mitigating the impact on motorists, and creating a safe working site for the construction team. This full bypass meant we had flexibility with our work programme, allowing us to alternate works on either side of the culvert as required.

Working alongside Hynds and our in-house surveyor to model the site, we established a methodology that resulted in a box culvert that was ‘drip-free’ and ensured the environmental challenges in the region did not impact Lake Waihola and the surrounding environment. Hynds took care of the manufacturing of the bespoke culvert locally, resulting in a reduction of emissions.

Collaboration with a contractor on a neighbouring site provided opportunities to recycle materials during our project. We also looked to the existing road material while developing the temporary road around the construction site and identified that the materials from within this road could be recycled into constructing the new lanes/roads and approaches to the underpass. Using the aggregates from the temporary road in the lanes required sourcing the correct material and staging our works so it could be reused. Additionally, rather than buying new culvert pipes for the temporary road, we recycled culvert pipes that we already had from previous temporary works.

The final scope included excavation, installation and backfill of the box culvert from wingwall to wingwall, and a 450 mm diameter civil boss pipe under the road for future services, concrete approach aprons, roading and topsoiling of the site, building the two-lane bypass temporary road, and the provision of traffic management. The overall planning process, our relationship with the other contractors on site, and our flexible approach to the construction tasks ensured that all available construction space and time was fully utilised. This ensured that there was a “one-pass” construction approach to the works.