Introduction
The township of Mangakino, New Zealand is at the northern end of the Taupo District and lies on the edge of Lake Maraetai on the Waikato River. The town was constructed in the 1950’s to house workers employed on the various dam construction projects on the Waikato River. While never intended to be a permanent settlement it has survived and is now a mix of owner-occupied and rental residences, beaches and vacant lots. It currently has some 660 houses on individual sites.
The Challenge
Several factors led the Taupo District Council (TDC) to believe that the wastewater reticulation system was nearing the end of its useful working life. The 19 km of wastewater reticulation for the 660 homes is all around 60 years old and mostly earthenware. Many of the pipes were laid beneath the houses, which gave rise to concerns about the potential for foundation problems if a pipe structurally failed. In storm events wash-through of solids from settling tanks within the network occurs and the treatment plant is not equipped to manage this. It was suspected that infiltration and inflow within the system was excessive and this could be largely attributed to the condition of the pipes. Approximately $7million would be required for a complete renewal. The prospect of this significant financial hurdle for both the council and the community was somewhat daunting.
Taupo District Council faced a dilemma, it did not have enough information to justify the replacement of the entire network, it could not identify all the specific lines requiring urgent replacement and it could not properly plan for a staged and prioritised renewal of the network.
The Assessment
Previous evidence indicating the poor condition of the network included a 2006 CCTV survey of 50% of the network, high levels of inflow and/or infiltration during rain events and the 2012 smoke testing of most of the town. However, much of this information was incomplete and of variable quality.
The 2006 CCTV survey was the most conclusive evidence that the pipework was on its last legs and the first step of the review was to take a sample of the surveys, and the associated assessments, to confirm that the information was valid and robust. It quickly emerged that the quality of the inspections, and associated log-sheets, was highly variable. It was also apparent that the assessment that followed was inconclusive in its consistency and generally very pessimistic about the condition, and likely life expectancy of the pipes.
The review determined that the 2006 survey and assessment could not be relied upon to provide robust evidence that would support the level of expenditure required to renew the entire network. Given that this assessment was now well over 10 years old it seemed reasonable to assume that the condition of the pipes could only be worse.
The Solution
Assetlife Alliance has extensive experience in asset management, CCTV inspections, condition assessment, the use of trenchless technology for pipeline rehabilitation and the development of sustainable strategies to ‘Optimise Pipeline Renewals’. Assetlife Alliance was engaged by the council to review the available information and determine if there was an alternative path forward that would be more affordable while still addressing the issues.
To remove the uncertainty and allow the development of a logical and fully justifiable solution, the council worked with Assetlife Alliance to develop a decision-making strategy. This included undertaking a complete CCTV survey of the network and then applying a structured set of Pass/Fail criteria to determine if a particular line could continue, required repair, or required rehabilitation/renewal.
It was recognised that the survey might reveal that the entire network was on its last legs but the evidence of that would then be clear and complete. There was an expectation that at least a portion of the network would have some useful life remaining and this would provide the opportunity to spread the cost of the renewals over a more affordable period, perhaps 10 years or more.
The approach adopted included:
- CCTV inspection of the entire 19 km of pipework
- Assessment of pipe condition and likelihood of failure
- Determination of ‘Pass/Fail’ using criteria agreed with TDC reflecting structural condition, criticality (e.g. under a building), infiltration, serviceability and capacity for intended purpose.
- For the ‘Fail’ pipes determining the appropriate response (maintenance, spot repair, rehabilitation or renewal)
- Determination of prioritisation and plan for staged works over several years
The survey determined that 34% of the network achieved a ‘Pass’ grade meaning TDC could be confident that these pipes will continue to provide an acceptable level of service for the next 10 years. However, 64% of the network received a ‘Fail’ grade and required a relatively urgent intervention.
Given the uncertainties that had been revealed, and the potential magnitude of the decisions that would have to be made, the overall strategy was intended to replace perception and uncertainty with high quality information and lead to a structured decision making process.
This allowed Taupo District Council to make well informed decisions and proceed with confidence to plan the works and make appropriate budgetary provisions about how to manage the renewal of the Mangakino wastewater reticulation.
The approach adopted for Mangakino aligns closely with the water industry’s desire to adopt sustainable
processes that are evidence driven and improve the economicefficiency of managing network renewals.
