Mountain Road — Cyclone Gabrielle Repair

Cyclone Gabrielle in February 2023 generated catastrophic slope failures across Mountain Road in the Waitākere Ranges — one of Auckland's key road corridors through the foothills — affecting nine separate sites along the route. GSI was engaged by Ventia as both designer and contractor — a design-build model that allowed the engineering team to adapt solutions in real time as ground conditions varied across the nine sites.

Each site had its own unique design challenges — weak soil, fractured rock, and steep inclines — requiring individual geotechnical investigation and solution. Multiple crews had to work simultaneously within a narrow road corridor, coordinating with civil works and operating under strict environmental rules to protect the area from Kauri Dieback. The solution mix combined high-capacity micropiles, soil nails, shotcrete, and erosion control matting matched to each site's failure mechanism.

GSI's staged remediation programme restored stability and resilience to all nine slopes along Mountain Road. The design-and-build approach allowed the team to deliver complex geotechnical solutions safely and efficiently despite the access constraints, environmental requirements, and variable ground conditions. Full road access was restored and long-term performance improved.

As both designer and contractor, GSI's single-team model delivered a faster and more integrated response than would have been possible through separate design and construction contracts. Full QA documentation including load test records was provided to Ventia on project completion. Mountain Road is a demonstration of what integrated design-build slip repair looks like at scale — nine sites, one contract, one team.