First New Zealand Installation of DN375 Iplex Restrain® PVC‑U Pipe
Case Study Summary
Following extreme weather events that caused flooding and ground instability, Auckland Council identified the need to replace a failing stormwater pipeline servicing a residential area. The objective was to convey stormwater away from the road corridor and adjacent properties, with discharge to a downstream creek.
This project marked the first installation in New Zealand of DN375 Iplex Restrain® PVC‑U pipe, expanding the locally available options for trenchless stormwater pipeline construction on steep, constrained sites.
Project Context
The replacement pipeline alignment ran down a steep private residential driveway at an approximate gradient of 18 degrees. Site constraints included limited working space, installation depths exceeding three metres, ongoing property access requirements and the need to preserve the existing driveway surface. These factors made conventional open‑cut trenching impractical and increased reinstatement risk.
Design Considerations
The design needed to address the challenges associated with steep alignment, high ground loads and the potential for differential ground movement in an area with a history of flooding and instability. Material selection, joint restraint capability and pipe stiffness were key considerations, alongside compatibility with trenchless installation methods and integration with the existing stormwater network.
This project provides a reference example for stormwater upgrades where access constraints and alignment conditions limit the feasibility of traditional trenching approaches.
Solution
The project team selected New Zealand‑manufactured Iplex Restrain® PVC‑U pipe in DN375, produced in accordance with AS/NZS 1260:2017. The mechanically restrained rubber ring joint system enables axial loads to be transferred through the pipeline, supporting installation on steep grades where reliance on surrounding ground conditions alone may be limited.
The pipe system had a stiffness class of SN16, suitable for anticipated installation depths and ground loading associated with the site conditions.
Construction Delivery
Installation was completed using Horizontal Directional Drilling (HDD) by Kerry Drainage & Civil. The trenchless approach allowed the pipeline to be installed between access pits without excavating the driveway surface, supporting continued access during construction and reducing surface disruption.
Iplex provided project‑specific technical support during installation, including guidance on joint assembly and installation sequencing, particularly given the steep alignment and first‑use DN375 configuration.
Outcome
The completed stormwater upgrade delivered a standards‑compliant pipeline consistent with the project’s access, constructability and performance requirements. The installation demonstrates the application of mechanically restrained PVC‑U pipe in a steep, constrained residential environment using trenchless construction methods.
As the first New Zealand installation of DN375 Iplex Restrain® pipe, the project establishes a reference point for future stormwater applications where trenching is not feasible and controlled joint restraint is required.