Project overview
The Bundle 6 project was a technically complex undertaking, part of a series of emergency works funded by NZTA Waka Kotahi and Buller District Council. These works addressed severe flood damage across the Buller region caused by the July 2021 and February 2022 flooding events. Sites requiring more extensive repairs were grouped into bundles based on location. Bundle 6 focused on five critical sites along Denniston Road, located 15 minutes north of Westport.
The project faced a range of challenges due to its location within the rugged and steep terrain of the West Coast bush. These conditions required innovative engineering solutions and highly skilled professionals with Isaac Construction to address real-time challenges and ensure the road’s long-term stability against environmental pressures. Geotechnical complexity was a significant factor, as the project involved constructing five retaining walls to stabilise the road and prevent further slope failures. One damaged road section featured a 2m high steep head scarp (angled at 60° to 80°) above a 15m debris slope, with ongoing erosion caused by an exposed culvert. Environmental constraints, such as heavy rainfall, posed risks to construction schedules and the structural integrity of the road. Logistical challenges arose from limited access to the steep, remote terrain, necessitating meticulous planning for the transport of materials and equipment. Traffic management was also critical, as maintaining partial road access during repairs was essential for community and tourism connectivity and emergency services, requiring carefully designed management systems.
The rehabilitation work included several key components. Earthworks preparation involved stabilising slopes and removing uphill slip material to prevent further erosion. Retaining wall construction required installing engineered hardfill, geogrids, and culverts to reinforce the roadbed and improve drainage. Damaged culverts were replaced to upgrade drainage infrastructure and manage water flow effectively, which helped prevent further scarp erosion. The road reinstatement resulted in restoring the road’s width and provided safe access for vehicles.
To address these challenges, innovative engineering solutions were employed. Adaptive machinery, such as long-reach excavators, was used from safer vantage points to manage instability on steep terrains. On-site recycling of excavated materials reduced environmental impact and logistical costs. Advanced drainage systems, including modern culverts and table drains, were installed to minimise the risk of future erosion. Reinforcement structures, such as rockfall safety fences, were also implemented to improve slope stability and protect against future hazards.
The completion of the Bundle 6 works successfully restored full access to Denniston Road, ensuring safe and reliable connectivity for residents, tourists, and emergency services. The project stabilised the road against geotechnical risks, enhancing its resilience to the challenging West Coast weather. It highlighted the effectiveness of adaptive and sustainable engineering solutions in addressing complex issues in remote areas. By overcoming significant physical, environmental, and logistical challenges, the project demonstrated the value of technical innovation and community-focused approaches in protecting critical infrastructure.
Denniston Road serves as the sole access route to the Denniston Coal Mining Heritage Village and a residential community on the Denniston Plateau. As an essential lifeline for locals and a significant driver of regional tourism, restoring full access was prioritised by the Buller District Council.