SH1 Mangamuka Gorge Slip Repairs

Through close collaboration between NZ Transport Agency Waka Kotahi, WSP, local hapū, and contractors, extensive geotechnical investigations, design, and construction were carried out despite additional weather challenges and expanding slip numbers.

Location

Northland, New Zealand

Client

NZ Transport Agency Waka Kotahi

Project Status

Complete: December 2024

Reconnecting Northland through resilience and collaboration

Nestled in the heart of Te Tai Tokerau Northland, the Mangamuka Gorge is a vital corridor for communities, businesses, and visitors alike. However, its steep terrain and complex geology make it vulnerable to extreme weather.

In August 2022, a major storm triggered 15 significant slips—six of which were deemed critical—forcing the closure of State Highway 1 (SH1) through the gorge. The damage was severe, and the engineering challenges ahead were formidable.

WSP was part of a multi-disciplinary team led by NZ Transport Agency Waka Kotahi, contributing geotechnical expertise, design support, and construction services. Initial investigations were carried out between August and November 2022 to understand the extent of the damage and the underlying ground conditions. These insights informed the decision to reinstate SH1 through the gorge, rather than pursue alternative routes.

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Engineering though adversity

The project adopted an Early Contractor Involvement (ECI) model, enabling close collaboration between designers, contractors, and the client. WSP worked alongside the Head Contractor, CLL, to develop responsive design solutions tailored to the gorge’s challenging terrain. As the project progressed, further weather events and record rainfall in 2023 compounded the damage, increasing the number of slips to 36—16 of which were critical. This led to an expanded scope and extended programme.

Despite these setbacks, the team worked at pace to restore the highway. Repairs included reinforced concrete piles anchored deep into the maunga (mountain), improved drainage systems, and environmental protections to safeguard local wildlife. WSP’s design team remained embedded onsite throughout construction, providing real-time solutions and MSQA services to match the accelerated pace of delivery.

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A community led recovery

From the outset, the project prioritised engagement with local hapū, who played a key role in both planning and delivery. At its peak, around 80% of the workforce and many suppliers were drawn from the local community, reflecting the project’s deep connection to the region.

The road officially reopened at 12:01am on Friday 20 December 2024, restoring a critical lifeline for whānau, friends, and businesses across Northland. The reopening marked the end of long detours and re-established essential access to services, bringing relief and renewed resilience to the region.