Our Critical Assets are Aging - A Story of Renewal Over Replacement

Anna Gibbs (Stantec NZ)

The Featherston Street wastewater rising main is a critical pipeline that conveys wastewater flows from the Wellington Central Business District (CBD) to the main Interceptor on The Terrace. The rising main is part of a wider pressure network that services three pump stations in total. Laid in 1947, the aged rising main system had been identified by Wellington Water Limited as being in poor condition and susceptible to failure. At more than 75 years old, the Featherston Street rising main was reaching the end of its useful life and required replacement. However, renewing a 180-metre-long pipeline in the Wellington CBD comes with unique challenges. To manage the impact on businesses, traffic, and the public, the trenchless technology of slip lining was utilised to carry out a successful renewal.

This paper outlines the investigations into trenchless renewal options, the methodology selection process, the material selection process, and the successful management of trenchless installation-related risks. Trenchless technologies are also an effective method of reducing waste and minimising carbon emissions during pipeline construction. This paper explores the benefits of slip lining from an environmental perspective, including calculations that directly compare the carbon emissions associated with slip lining to those produced by the traditional dig-andlay renewal methodology. 

Redundancy had recently been built into the existing rising main system, allowing an online renewal as opposed to needing a new pipe alignment in an area where underground services are densely populated. This paper describes how the renewal methodology was selected, taking into account design criteria, pipe diameters, surrounding services, disruption, as well as contractor technology and experience in the Wellington Region.

Despite its advantages, an online, trenchless pipe renewal posed several technical issues. At more than 75 years old, the existing pipe was cast-iron and therefore subject to tuberculation. Tuberculation are mounds of corrosion that can develop on the inside of a metallic pipe over time. A major concern for the project team was that the new pipe would undergo damage when pulled across these rigid tuberculation nodules during installation. This paper discusses the various mitigation measures the project team explored and successfully implemented to manage the risk of damage during installation and ensure that the final product is free of any defects that could result in a failure before its 100-year design life is complete.