The Christchurch ocean outfall - From design to commissioning
Annual Conference
❄
New Zealand’s longest ocean outfall pipeline was officially opened on March 24th 2010. This pipeline transfers treated wastewater from the Christchurch wastewater treatment plant to diffusers located 3 km offshore. As Christchurch City’s largest project ever undertaken, the NZ$85 M project comprises a new pump station and ~5 km of 1800 mm diameter pipeline with a design capacity of 6.5 m 3 /s. The pipeline comprises 2.33 km of concrete pipe installed by microtunnelling and 2.53 km of HDPE pipe installed by dredge and lay.
An Interactive Tender Process (ITP) was successfully used as a mechanism to transfer knowledge from the owner and designers to tenderers, with the objective of reducing unknowns, bid contingency allowance and the risk of contract claims.
The microtunnel was constructed in three sections, using a Herrenknecht AVN 1800T/DB Microtunnel Boring Machine (MTBM); firstly an 874 m section beneath a tidal estuary, secondly a 604 m drive along a highly sensitive residential street, and thirdly a 830 m section beneath the dunes and surf zone out to the connection point with the HDPE marine pipeline. A ‘wet recovery’ of the MTBM took place at this connection point. During the tunnelling operation, 7,000 year old shells and timber from an ancient estuarine area, as well as layers of sediment with higher than anticipated silt content impacted on the contractors progress.
During the construction of the 2.53 km HDPE marine pipeline issues arose which resulted in the project being delivered approximately 18 months behind schedule. These included; adverse sea conditions, HDPE weld failure, connection of the marine works to the microtunnel works, and serious health and safety issues during construction. The resolution of these issues along with the lessons learned will feature as a part of this paper.
This paper describes the contractual framework, ITP, partnering, microtunnel advance, adverse sea conditions, marine pipeline construction issues, safety issues and commissioning of the system including initial dilution testing.