Emergency response process for slip remediation along the State Highway 4, New Zealand

State Highway 4 (SH4) is an approximately 240 km long, two-lane highway connecting Whanganui in the lower North Island to Te Kuiti in the Waikato, New Zealand. A major storm event in June 2015 lead to over 43 landslips affecting an approximately 85 km section of highway between Whanganui and Raetihi. Twelve of these slips resulted in considerable damage to the highway corridor. As part of the Network Outcomes Contract for the Manawatu-Whanganui region, Beca Ltd was commissioned by Higgins Group Holding Ltd and the NZ Transport Agency to provide engineering services, including emergency response, geotechnical investigation, design and construction monitoring of the remedial measures at the slip sites.

Initial emergency response was undertaken to restore access to the affected communities. This was followed by site prioritisation to identify the sites which required in-depth investigation and design for remediation (referred to as ‘Category B’ sites). Site-specific ground investigations were undertaken at these sites to assess the ground and groundwater conditions and likely failure mechanisms. Remedial options in the form of reinforced, embedded and gravity retaining structures, earth fill buttresses and road re-alignment were developed. Detailed design and construction of the preferred options were undertaken over an approximately 18-month period.

This paper presents the processes adopted to remediate the slip sites through close collaboration between the client, consultant and contractors as well as the local stakeholders. It discusses the geotechnical aspects of the design solutions, and presents the main lessons learnt and recommendations for further improving the collaboration between all parties involved in a NOC delivery model.